FQM 1212 Basics in Food Science PDF

FQM 1212 Basics in Food Science THE B.SC. FOOD QUALITY MANAGEMENT STUDY MATERIAL IN PRINT Published by The Wayamba University of Sri Lanka Faculty of Livestock, Fisheries & Nutrition...

FQM 1212 Basics in Food Science THE B.SC. FOOD QUALITY MANAGEMENT STUDY MATERIAL IN PRINT Published by The Wayamba University of Sri Lanka Faculty of Livestock, Fisheries & Nutrition 1  FQM 1212 Basics in Food Science Publisher Name : Wayamba University of Sri Lanka Book Details with ISBN numbers: FQM 1212 Basics in Food Science ISBN 978-624-5564-02-6 2  FQM 1212 Basics in Food Science All rights are reserved, and no part of this publication may be reproduced or distributed in any form or by any means or stored in a database or retrieval system without the prior written permission of Wayamba University of Sri Lanka. Author Prof. C. V. L. Jayasinghe Professor in Food Science and Technology (Chair) Department of Food Science and Technology, Faculty of Livestock, Fisheries and Nutrition, Wayamba University of Sri Lanka Assistant Author Ms. N. K. S. Kithmini Temporary Lecturer Department of Food Science and Technology, Faculty of Livestock, Fisheries and Nutrition, Wayamba University of Sri Lanka Editor Dr. P. M. H. D. Pathiraje Senior Lecturer Department of Food Science and Technology, Faculty of Livestock, Fisheries and Nutrition, Wayamba University of Sri Lanka 3  FQM 1212 Basics in Food Science Introduction Welcome to the course of study FQM 1212 – Basic in Food Science Program title: B.Sc. in Food Quality Management (External) Department offering the programme(s): Food Science & Technology Department responsible for the course: Food Science and Technology Course Code: FQM 1212 Year/Level: 02 No. of hours/units: 20 (hrs) number of direct contact hours 5 (hrs) of home assignments 4 (hrs) examination 71 (hrs) of independent learning No. of chapters/units: 06 Units Authorization year of study material in-print: 2022 Course Overview Course aims & objectives 1. To introduce the fundamental concepts on the behavior of food constituents, food processing and preservation technologies 2. To enable the students to understand the basics of food science. 3. To expose learners to basic practical aspects of food science. Intended Learning Outcomes of the Course On successful completion of the course, students will be able to; 1. Describe the theoretical foundation of the properties of the foods 2. Identify the unit operations involved in food technology 3. Explain the theoretical and practical aspects of food product processing 4. Analyse major food constituents qualitatively and quantitatively. 5. Identify the quality and safety assurance principles 4  FQM 1212 Basics in Food Science Organization of the material: The study material for this course is presented as one book. The handouts required for practical sessions will be given to you during the practical sessions. Teaching and learning methods: The course will consist of lectures, discussion, tutorials, lab practice, videos, demonstrations, self-study and group learning activities (SCLs) Assessments: Assessment method Time schedule Weight Formative/S ummative Mid-term examinations After 45% Formative completing the Performa/Tutorials contact sessions Final examinations 55% Summative Further reading 1. Potter NN and Hotchkiss JH. (1996) Food Science, 3rd edn., New Delhi.: CBS Publishers. 2. Mayer LH. Food Chemistry. 1st edn. CBS Publishers, New Delhi. 3. Fennema OR. Food Chemistry. 3rd edn. Marcel Dekker, Inc. New York Facilities required for teaching and learning Reading materials provided to the students, access to book chapters assigned for self- reading, computers with internet facilities and necessary software for watching videos Features for easy learning To help you in the learning process, we have introduced the following in the text. Contents at the beginning of each chapter will give you an overview of the chapter. The introduction in each chapter will indicate the information you could expect from the chapter. Within the text of each chapter, key words are highlighted in bold 5  FQM 1212 Basics in Food Science and are indexed at the back of each volume. Activities have been introduced in the text. These will help you to immediately assess your comprehension of what you have just read. Review questions are provided at the end of each chapter. Answering them will also serve as a permanent record and would facilitate revision. A list of books for further reading is recommended at the end of the module. This provides additional information. A glossary of technical terms is given at the end of the module. Graphic signals (icons) have been used in the margin to tell you what sort of material/activity you are about to deal with. The icons are indicated below. Learning Contact An Activity Outcomes Session Laboratory Field Visits Audio Sessions Materials Video Materials Further Review Reading Questions 6  FQM 1212 Basics in Food Science If you have any questions regarding this course, you can speak to module writer at the Faculty of Livestock Fisheries & Nutrtion/ Sri Lanka Standard Institute. You will be meeting him/her at the contact session and practical class. The contact information of the module writer is given below: E-mail address: [email protected] 7  FQM 1212 Basics in Food Science Unit 01- Introduction to Basics in Food Science Content Introduction to Basics in Food Science Intended Learning outcomes Define what is Food Science Identify the importance of studying Food Science Introduction to Basics in Food Science Definition of Food Science The scientific study of raw materials and their behavior(physical, chemical, and biochemical nature of foods) during formulation, processing, packaging, storage and evaluations consumer food products. Definition for the Food Technology The application of food science to the selection, preservation, processing, packaging, distribution, and use of safe, nutritious and wholesome food Food Science can be defined as the application of the basic sciences and engineering to study the fundamental physical, chemical, and biochemical nature of foods and the principles of food processing. Food technology is the use of the information generated by food science in the selection, preservation, processing, packaging, and distribution, as it affects the consumption of safe, nutritious and wholesome food. As such, food science is a broad discipline which contains within it many specializations such as in food microbiology, food engineering, and food chemistry. Because food interacts directly with people, some food scientists are also interested in the psychology of food choice. These individuals work with the sensory properties of foods. Food engineers deal with the conversion of raw agricultural products such as wheat into more finished food products such as flour or baked goods. Food processing contains many of the same elements as chemical and mechanical engineering. Virtually all foods are derived from living cells. Thus, foods are for the most part composed of "edible biochemicals," and biochemists often work with foods to understand how processing or storage might chemically affect foods and their 8  FQM 1212 Basics in Food Science biochemistry. Likewise, nutritionists are involved in food manufacture to ensure that foods maintain their expected nutritional content. Other food scientists work for the government in order to ensure that the foods we buy are safe, wholesome, and honestly represented. Food scientists are engaged in different activities such as, developing palatable, nutritious, low-cost foods. Dried milk can supply the needed calories and protein but is relatively expensive and is not readily digested by all. Fish "flour" prepared from fish of species not commonly eaten can be a cheaper source of protein. Incaparina, a cereal formulation containing about 28% protein, is prepared from a mixture of maize, sorghum, and cottonseed flour. Incaparina and similar products were developed to utilize low-cost crops grown in Central and South America. Yoghurt was developed from ingredients-like hydrolyzed starch syrup, and cow or buffalo milk-that are readily available in India. As food losses during storage and processing can be enormous, food scientists are involved in adapting and developing preservation methods appropriate and affordable to various regions of the world. Also, food scientists have developed thousands of food products including those used in the space shuttle program. Food scientists today are often involved in altering the nutrient content of foods, particularly reducing the caloric content or adding vitamins or minerals. Reducing the caloric content is accomplished in several ways, such as replacing caloric food components with low or non-nutritive components. The caloric content of soft drinks is reduced by replacing the nutritive sugar sweeteners (e.g., sucrose) with aspartame or saccharin. An important application of food science technology is in food packaging such as the controlled-atmosphere (CA) storage of fruits and vegetables. Fruits such as apples, after they are harvested, still have living respiring systems. They continue to mature and ripen. They require oxygen from the air for this continued respiration, which ultimately results in softening and breakdown. If the air is depleted of much of its oxygen and is enriched in carbon dioxide, respiration is slowed and ripening processs can be suppressed. There are active packaging which tells the consumer the freshness/ ripening status/ gas composition of the commodity it contains. One of the most important goals of the food scientist is to make food as safe as possible. The judicious application of food processing, storage, and preservation methods helps prevent outbreaks of food poisoning. Food scientists are involved in establishing international food standards to promote and facilitate world trade and at the same time to assure the wholesomeness and value of foods purchased between nations. 9  FQM 1212 Basics in Food Science Unit 02: Water Content 1. Introduction 2. Importance of water as a food constituent 3. Structure of water molecule 4. Physical properties of water & ice 5. Water Activity 5.1. Definition 6. Moisture Sorption Isotherm 6.1. Definitions and zones 6.2. Types of water 6.3. Sorption water in foods 6.4. Applications of moisture sorption isotherms 6.5. Moisture sorption isotherm measurement techniques 6.6. Influence of temperature on moisture sorption isotherm 6.7. Isotherm models 7. Water activity and reaction rate 8. Phase diagram 8.1. Crystal growth and nucleation 8.2. The glass transitions Intended Learning Outcomes Identify the importance of water as a food constituent Describe physical properties of water Describe interaction of water with ionic, nonionic, and neutral substances Discuss drying & hydration concepts. Define water activity Demonstrate the importance of water activity in food stability Describe Moisture Sorption Isotherms 10  FQM 1212 Basics in Food Science 1. Introduction The need for water exists at the molecular level, the cellular level, and at the metabolic and functional levels. Water is the major solvent for the organic and inorganic chemicals involved inthe biochemical reactions that are essential to life. The chemical formula for water is H2O. Water contains strong covalent bonds that hold the two hydrogen atoms and one oxygen atom together. The oxygen can be regarded to be at the center of a tetrahedron (figure 1), with a bond angle of 104° 30° between the two hydrogen atoms in liquid water and a larger angle of 109° 28° between the hydrogens in ice (figure 2). Figure 1: Structure of water molecule Figure 2: H2O molecule arrangement in liquid water and ice 11  FQM 1212 Basics in Food Science 2. Importance of water as a food constituent Water is an essential constituent of many foods. It may occur as an intracellular or extracellular component in vegetable and animal products, as a dispersing medium or solvent in a variety of products, as the dispersed phase in some emulsified products such as butter and margarine, and as a minor constituent in other foods. The amount, location, and orientation of water influence the structure of food such as appearance - colour, taste, texture and mouth feel. The control of water level in foods is an important aspect of food quality, as water content affects the shelf life and bacterial quality of food. When processing, a food product should maintain optimum water content that can maintain desirable characteristics in the product. 3. Structure of water molecule Water is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Water is a "polar" molecule, and there is an uneven distribution of electron density. Water has a partial negative charge near the oxygen atom due to the unshared pairs of electrons and partial positive charges near the hydrogen atoms. An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond. Three-dimensional hydrogen bonding provides, Large value for heat capacity, Low Melting point, High Boiling point Enthalpies of various phase transitions The reason for the unusual behavior of water lies in the structure of the water molecule and in the molecule's ability to form hydrogen bonds. In the water 105°C, the distance between the nuclei of hydrogen and oxygen is 0.0957 nm. The water molecule can be considered a spherical quadrupole with a diameter of 0.276 nm, where the oxygen nucleus forms the center of the quadrupole. To form a molecule of water, two hydrogen atoms approach the two sp3 bonding orbitals of oxygen and form two covalent sigma(s) bonds (40% partial ionic character), each of which has a dissociation energy of 4.6×102 kJ/mol (110 kcal/mol). The two negative and two positive charges form the angles of a regular tetrahedron. Because of the separation of charges in a water molecule, the attraction between neighboring molecules is higher than normal 12  FQM 1212 Basics in Food Science with van der Waals forces. In ice, every H2O molecule is bound by four such bridges to each neighbor. The binding energy of the hydrogen bond in ice amounts to 5 kcal per mole. 4. Physical properties of water & ice By comparing water's properties with those of molecules of similar molecular weight and atomic composition (CH4, NH3, HF, H2S, H2Se, H2Te), it is possible to determine if water behaves in an abnormal fashion. Based on this comparison, water is found to melt and boil at unusually high temperatures; to exhibit unusually large values for surface tension, permittivity (dielectric constant), heat capacity, and heats of phase transition (heats of fusion, vapourization, and sublimation); to have a moderately low value for density; to exhibit an unusual attribute of expanding upon solidification. In addition, the thermal conductivity of water is large compared to those of other liquids, and the thermal conductivity of ice is moderately large compared to those of other nonmetallic solids (Table 1) Table 1: physical properties of water Property State of water 20⁰ C (Liquid 0⁰ C (Liquid 0⁰ C (Ice) -20⁰ C (Ice) water) water) Density 0.99821 0.99984 0.9168 0.9193 (g/cm3) Viscosity 1.002˟10-3 1.793 ˟10-3 - - (pa.sec) Surface tension 72.75 ˟10-3 75.64 ˟10-3 - - against air (N/m) Vapour 2.3388 0.6113 0.6113 0.103 pressure (kPa) Heat capacity 4.1818 4.2176 2.1009 1.9544 (J/g K) Thermal 0.5984 0.5610 2.240 2.433 conductivity (liquid) (W/mK) 13  FQM 1212 Basics in Food Science Thermal 1.4 ˟10-7 1.3 ˟10-7 11.7 ˟10-7 11.8 ˟10-7 diffusivity (m2/s) Permittivity 80.20 87.90 ~90 ~98 (dielectric constant) 5. Water Activity It has been observed that various types of food with the same water content differ significantly in perishability. Thus, water content alone is not a reliable indicator of perishability. This situation is attributable, such as the intensity with which water associates with nonaqueous constituents—water engaged in strong associations is less able to support degradative activities, such as growth of microorganisms and hydrolytic chemical reactions, than is weakly associated water. The term “water activity” (aw) was developed to account for the intensity with which water associates with various nonaqueous constituents. 5.1 Definition The ratio of the vapour pressure of a solution and pure water at the same temperature a w =𝑃ൗ𝑃𝑜 Where, P= Partial vapour pressure of water in food Po= Vapour pressure of pure water at the same temperature According to Raoult's law, the lowering of the vapour pressure of a solution is proportional to the mole fraction of the solute: aw can then be related to the molar concentrations of solute (n1) and solvent (n2): 𝑃 𝑛1 aw = = 𝑃0 𝑛1+𝑛2 The extent to which a solute reduces aw is a function of the chemical nature of the solute. 14  FQM 1212 Basics in Food Science The equilibrium relative humidity (ERH) of a food product is defined as the relative humidity of the air surrounding the food at which the product neither gains nor loses its natural moisture and is in equilibrium with the environment. 𝑃𝑒𝑞𝑢 ERH= 𝑃𝑠𝑎𝑡 Pequ = partial pressure of water vapour in equilibrium with the food at temperature T and 1 atmosphere total pressure P sat = the saturation partial pressure of water in air at the same temperature and pressure A relationship exists between ERH and aw, since both are based on vapour pressure 𝑎𝑤 ERH= 100 Table 2: Typical water activities of food 6. Moisture Sorption Isotherm (MSI) 6.1 Definition and Zones A plot of water content (expressed as mass of water per unit mass of dry material) of a food versus water activity (P/P0) at constant temperature is known as a moisture sorption isotherm (MSI). Information derived from MSIs are useful; (a) for concentration and dehydration processes, (b) for formulating food mixtures to avoid moisture transfer among the ingredients, (c) to determine the moisture barrier properties needed in a packaging material, 15  FQM 1212 Basics in Food Science (d) to determine what moisture content will curtail the growth of microorganisms of interest, and (e) to predict the chemical and physical stability of food as a function of water content (see next section). Figure 3: Moisture Sorption Isotherm and different zones Sorption isotherms usually have a sigmoid shape and can be divided into three areas that correspond to different conditions of the water present in the food. The first part (A) of the isotherm, which is usually steep, corresponds to the adsorption of a monomolecular layer of water; the second, flatter part (B) corresponds to adsorption of additional layers of water; and the third part (C) relates to condensation of water in capillaries and pores of the material. There are no sharp divisions between these three regions. 16  FQM 1212 Basics in Food Science Figure 4: Sorption isotherm for a typical food product showing hysteresis phenomena Generally, adsorption isotherms are required for the observation of hygroscopic products. and desorption isotherms are useful for investigating the process of drying. A steeply sloping curve indicates that the material is hygroscopic (curve A, Figure 5); a flat curve indicates a product that is not very sensitive to moisture (curve B, Figure 5). Many foods show the type of curves given in Figure 6, where the first part of the curve is quite flat, indicating a low hygroscopicity, and the end of the curve is quite steep, indicating highly hygroscopic conditions. Such curves are typical for foods with high sugar or salt contents and low capillary adsorption. Such foods are hygroscopic. The reverse of this type of curve is rarely encountered. These curves show that a hygroscopic product or hygroscopic conditions can be defined as the case where a small increase in relative humidity causes a large increase in product moisture content. Hygroscopicity is a term used to describe how readily a material will take up moisture when subjected to a given shift (change) in relative humidity 17  FQM 1212 Basics in Food Science Figure 5: Sorption Isotherms of Hygroscopic Product (A) and Nonhygroscopic Product (B) *Note: in Figures 5 and 6,HUM% denotes ERM, which is the same as the water activity Figure 6: Sorption Isotherms for Foods with High Sugar or Salt Content; Low Capillary Adsorption 18  FQM 1212 Basics in Food Science Figure 7: Exposure to four materials exhibiting different degrees of hygroscopicity to a shift in water activity from aW(A) to aW(B). The same shift causes different amounts of water uptake ∆X To help better understand this, consider the four different sorption isotherms shown in Figure 7. Each isotherm belongs to a different material, and all four materials exhibit different degrees of hygroscopicity. Let us assume that we shift the equilibrium relative humidity (water activity) from water activity aW (A) to water activity aW(B) on all four materials. Then, we can observe the different quantities of water uptake experienced by each material as an indicator of different degrees of hygroscopicity. It can be clearly seen that material 2 will have greater water uptake than material 1. The slope of the secant in the segment of the curve between the starting point and final point of the shift in water activity can be taken as the water uptake potential (Wp value) of the given material. The Wp value serves only as an indicator of the moisture difference ∆x, which occurs on a given change ∆aW. It does not take into account the level of moisture already absorbed into the material earlier. Therefore, when we examine Figure 7 once again, we would have to note that although material 1 is a very good sorbent, when we make our shift in the water activity aW(A) → aW(B) experiment, material 2 will show higher water uptake. Therefore, we can say that material 2 has a greater Wp than material 1 in the range of water activities of our experiment. When we use Wp as an indicator of hygroscopicity, points A and B have to be clearly specified. Additionally, a constant temperature was assumed throughout this example. Labuza (1918) proposed various ways in which isotherms can be explained. The kinetic approach is based on the Langmuir equation, which was initially developed for the adsorption of gases and solids. This can be expressed in the following form: 19  FQM 1212 Basics in Food Science C - Constant M0 - monolayer sorbate content aw – water activity 6.1.Types of water The sorption isotherm indicates that different forms of water may be present in foods. It is convenient to divide the water into three types: Langmuir or monolayer water, capillary water, and loosely bound water. The bound water can be attracted strongly and held in a rigid and orderly state. In this form the, water is unavailable as a solvent and does not freeze. It is difficult to provide a rigid definition of bound water because much depends on the technique used for its measurement. Two commonly used definitions are as follows: 1. Bound water is the water that remains unfrozen at some prescribed temperature below O⁰C, usually -2O⁰C. 2. Bound water is the amount of water in a system that is unavailable as a solvent. 6.2.Sorption of water in foods Sorption processes can occur at the interface between solids and fluids (gases or liquids) or between fluids (gas–liquid, liquid–liquid). However, in the fields of food technology and engineering, an understanding of the sorption of water vapour in foods is of paramount importance in the design and specification of many food processing, packaging, storage and handling systems. In the case of water vapour adsorption (rehydration) in foods, we have sorption at a solid–fluid interface with water molecules being adsorbed onto the boundary surface of a dry solid material. 20  FQM 1212 Basics in Food Science Figure 8: Sorption of water vapour to a solid food surface. 1: gaseous water molecules, 2: adsorbed water (adsorbate), 3: food (adsorbent) Once monolayer saturation is achieved at the boundary surface and multilayers begin to form, the initially dry solid material will begin to absorb the water molecules through the porous structure into the interior of the material, just as a dry towel will absorb water from a wet surface. Thus, the initially dry solid material is called the “adsorbent”. When the process comes to an end at equilibrium, the resulting adsorbed water is called the “adsorbate” Figure 9: Examples of water vapour sorption isotherms. 1: silica (strong sorbent), 2: milk powder (multilayer forming material), 3: black tea (monolayer forming material), 4: fructose (nearly impervious to water, then rapid uptake of water until a syrup is formed) Figure 9 shows some examples of different sorption isotherms. Curve 1 shows a strong sorbent, which takes up large amounts of water, although in a region of very low water activity or equilibrium relative humidity. After rapid water uptake, we see a plateau, which is the result of the monolayer that has formed becoming stable. Silica for example would have a curve of this shape. Curve 3 shows a similar behavior but not with such extreme water uptake. When a material does not stop adsorbing water when a monolayer is formed and continues to form second, third and additional multilayers without pausing, we see a shape similar to that shown in curve 2. Some 21  FQM 1212 Basics in Food Science materials, such as PVC powder, do not adsorb very large amounts of water, and they behave like a substance impervious to water. Curve 4 shows crystalline fructose, which, in regions of lower water activity, shows nearly impervious behavior, but exceeding a certain level of water activity starts to dissolve and form syrup. The isotherm in this region is a dashed line because the original sample (in the form of dry crystals) no longer exists. 6.3.Applications of moisture sorption isotherms In drying operation, the removal of water is important, and hence, the desorption equilibrium moisture relationship is required to determine the lowest attainable moisture content at the process temperature and relative humidity. Labuza and Hyman applied the changing water activity of food ingredients and effective diffusivity to control moisture migration in multidomain foods when temperature changes occurred. The moisture sorption isotherms of food and agricultural products are therefore of special interest in the design of storage and preservation processes such as packaging, drying, mixing, freeze-drying and other processes that require the prediction of food stability, shelf life and glass transition and estimation of drying time, texture and deteriorative reactions in agricultural and food products. The precise determination of the equilibrium moisture contents of dehydrated foods provides valuable information for the accurate computation of thermodynamic energies from existing theories. 6.4.Moisture sorption isotherm measurement techniques Several methods of determining the moisture sorption isotherm of food products have been employed by investigators. The basic techniques include gravimetric, hygrometric, vapour pressure manometric and inverse gas chromatography and special methods involving the use of AquaLab. 6.5.1 Gravimetric method There are two common gravimetric methods of determining the moisture content of food products at different temperatures and water activities. One of these methods is the static gravimetric method, which involves the placement of the product in an atmosphere with which it then comes into equilibrium (weight loss or gain stops) without mechanical agitation of the air or product. For this method, several weeks may be required for the product to reach equilibrium, and because of the long period of time, mold usually develops on high- and intermediate-moisture foods at water activities above 0.8. For data obtained at water activities above 0.8 to be reliable, mold growth must be prevented during equilibration. Theoretically, at equilibrium, the water activity of the sample is the same as that of the surrounding environment. However, in practice, a true equilibrium is never attained because that would require an infinitely long period of time. 22  FQM 1212 Basics in Food Science 6.5.2 Hygrometric method Electric hygrometers are widely used for obtaining the MSIs of food products. There are many commercially available and specially designed hygrometers that are in use. The instrument (Figure 7) consists basically of a sensor, sample chamber and potentiometer. The sensor could use a hygroscopic chemical such as lithium chloride or an ion-exchange resin such as sulfonated polystyrene, the conductivity of which changes according to the water activity above the sample. The sensor could be a humidity sensor based on capacitance changes in a thin film capacitor. Electric hygrometers give rapid, relatively precise results and are easy to operate. The main problems involved with the use of hygrometers are. i. Evaluation of the equilibration time between the sample and sensor ii. Proper temperature control iii. Need for recalibration for some instrument Figure 10: Diagram of the moisture sorption isotherm apparatus utilizing the hygrometer 6.5.3. Vapour pressure manometric (VPM) method The vapour pressure manometric method involves bringing air to equilibrium with the food product at a fixed temperature and moisture content, and the relative humidity of the air is measured as the equilibrium relative humidity (ERH). In this method, the vapour pressure exerted by the moisture in the product is directly measured. As a result, it is taken as one of the best methods of determining the MSI of food. 23  FQM 1212 Basics in Food Science 6.5.4. Inverse gas chromatography (IGC) It is particularly suitable for the study of the lower region of water activity and for products with very low equilibrium moisture contents. 6.5.5. AquaLab instrument AquaLab is the fastest, most accurate and most reliable instrument available for measuring water activity, giving readings in 5 min or less 6.5.Influence of temperature on moisture sorption isotherms Temperature affects the mobility of water molecules and the dynamic equilibrium between the vapour and the adsorbed gases. If water activity is kept constant, an increase in temperature causes a decrease in the amount of sorbed water (Figure 11). This indicates that the food becomes less hygroscopic. An increase in temperature represents a condition unfavorable to water sorption. An exception to this rule is shown by certain sugars and other low molecular weight food constituents, which become more hygroscopic at higher temperatures because they dissolve in water. Temperature shifts can have an important practical effect on the chemical and microbiological reactivity related to quality deterioration of a food in a closed container. An increase in temperature at constant moisture content causes increases in water activity. Figure 11: Changes in water activity at constant moisture content and in moisture content at constant water activity with changes in temperature 24  FQM 1212 Basics in Food Science 6.6.Isotherm models Scientists have developed four isotherm models and equations that describe each isotherm. With some semiempirical equations with two or three fitting parameters to describe moisture sorption isotherms, the most common ones used in describing sorption on foods are the Langmuir equation, the Brunauer–Emmett–Teller (BET) equation, Oswin model, Smith model, Halsey model, Henderson model, Iglesias- Chirife equation, Guggenheim-Anderson-de Boer (GAB) model, and Peleg model. The oldest and best known model is that of Brunauer, Emmett, and Teller (BET). Figure 12: Four main types of isotherm models 7. Water Activity and Reaction Rate Water activity has a profound effect on the rate of many chemical reactions in foods and on the rate of microbial growth. Enzyme activity is virtually nonexistent in monolayer water (aw between 0 and 0.2). Not surprisingly, the growth of microorganisms at this level of aw is also virtually zero. Molds and yeasts start to grow at aw between 0.7 and 0.8, the upper limit of capillary water. Bacterial growth takes place when aw reaches 0.8, the limit of loosely bound water. Enzyme activity increases gradually between aw values of 0.3 and 0.8 and then increases rapidly in the loosely bound water area (aw 0.8 to 1.0). Hydrolytic reactions and nonenzymatic browning do not proceed in the monolayer water range of aw (0.0 to 0.25). However, lipid oxidation rates are high in this area, passing from a minimum at aw 0.3 to 0.4 to a maximum at aw 0.8. 25  FQM 1212 Basics in Food Science Table 3: Minimum water activity for microbial growth As the water activity is brought below 1 and lowered further in a food, the rates of these reactions begin to slow down. They proceed more slowly with further lowering water activity, which translates into longer and longer shelf lives. The actual shelf life of any specific food product at a given water activity may vary depending on the structure and composition of the food material and the spoilage mechanism of concern. The shelf life of a food will be limited by any one of a series of spoilage mechanisms that can destroy the food. These include microbial activity (bacteria, yeasts and molds), enzyme activity, browning reactions, and lipid oxidation (rancidity). Of all these reactions, microbial activity is by far the most sensitive to water activity, having the greatest need for available free water. Figure 12: Reaction rate vs Water activity 26  FQM 1212 Basics in Food Science 8. Phase Diagram The phase diagram (Figure 13) indicates the existence of three phases: solid, liquid, and gas. The conditions under which they exist are separated by three equilibrium lines: vapour pressure line TA, melting pressure line TC, and sublimation pressure line BT. The three lines meet at point T, where all three phases are in equilibrium. Figure 10 shows that when ice is heated at pressures below 4.58 mm Hg, it changes directly into the vapour form. This is the basis of freeze drying. It is possible to supercool water. When a small ice crystal is introduced, the supercooling is immediately terminated, and the temperature rises to O⁰C. Normally the presence of a nucleus is required. Generally, nuclei form around foreign particles (heterogeneous nucleation). It is difficult to study homogeneous nucleation. A homogeneous nucleus forms from the chance agglomeration of water molecules in the ice configuration. Usually, such nuclei disintegrate above a critical temperature. The probability of such nuclei forming depends on the volume of water; they are more likely to form at higher temperatures and in larger volumes. In ultrapure water, 1 mL can be supercooled to -320°C, droplets of 0.1 mm diameter to -350°C, and droplets of 1 µm to -410°C before solidification occurs. Figure 13: Phase diagram of water 27  FQM 1212 Basics in Food Science 8.1 Crystal Growth and Nucleation Crystal growth, in contrast to nucleation, occurs readily at temperatures close to the freezing point. It is more difficult to initiate crystallization than to continue it. The rate of ice crystal growth decreases with decreasing temperature. A schematic graphical representation of nucleation and crystal growth rates is given in Figure 14. Solutes of many types and in quite small amounts will greatly slow ice crystal growth. Figure 14: Schematic Representation of the Rate of Nucleation and Crystal Growth 8.2 The Glass Transition In aqueous systems containing polymeric substances or some low molecular weight materials, including sugars and other carbohydrates, lowering the temperature may result in the formation of glass. Glass is an amorphous solid material rather than a crystalline solid. Glass is an undercooled liquid of high viscosity that exists in a metastable solid-state. A glass is formed when a liquid or an aqueous solution is cooled to a temperature that is considerably lower than its melting temperature. This is usually achieved at high cooling rates. The normal process of crystallization involves the conversion of a disordered liquid molecular structure to highly ordered crystal formation. In a crystal, atoms or ions are arranged in a regular, three- dimensional array. In the formation of glass, the disordered liquid state is immobilized into a disordered glassy solid, which has the rheological properties of a solid but no ordered crystalline structure. The relationships among the melting point (Tm), glass transition temperature (Tg), and crystallization are schematically represented in Figure 16. At a low degree of supercooling (just below Tm), nucleation is at a minimum, and crystal growth predominates. As the degree of supercooling increases, nucleation becomes the dominating effect. The maximum overall crystallization rate is approximately halfway 28  FQM 1212 Basics in Food Science between Tm and Tg. At high cooling rates and a degree of supercooling that moves the temperature to below Tg, no crystals are formed, and a glassy solid result is obtained. During the transition from the molten state to the glassy state, the moisture content plays an important role. This is illustrated by the phase diagram of Figure 16. When the temperature is lowered at sufficiently high moisture content, the system goes through a rubbery state before becoming glassy. The glass transition temperature is characterized by very high apparent viscosities of more than 105 Ns/m2. The rate of diffusion-limited processes is more rapid in the rubbery state than in the glassy state, and this may be important in the storage stability of certain foods. Figure 15: Relationships Among Crystal Growth, Nucleation, and Crystallization Rate between Melting Temperature (Tm) and Glass Temperature (Tg) Figure 16: Phase Diagram Showing the Effect of Moisture Content on Melting Temperature (Tm) and Glass Transition Temperature (Tg) 29  FQM 1212 Basics in Food Science Activity Discussion of the importance of moisture sorption isotherms and their applications in the food industry Review Questions 1)Why measure water activity? Water activity is a critical factor in determining product safety and quality. It affects the microbial action, shelf life, safety, texture, flavor, and smell of foods and other products. When you know the water activity of your product, you can maximize safety, quality, and profit 2) What is the difference between water activity and water content? Water content is only a measure of how much water is in a product. Water activity measures what the water will do: susceptibility to microbial growth, clumping, texture issues, moisture migration, and more. 30  FQM 1212 Basics in Food Science Unit 03: Carbohydrates Content 1.Introduction 2.Monosaccharides 2.1 Glucose 2.2 Fructose 2.3 Galactose 3. Chemical Reactions of Monosaccharide sugars 3.1 Caramelization 3.2 Crystallization 3.3 Hydrogenation: Polyols 3.4 Oxidation to Aldonic Acids and Aldonolactones 3.5 Reduction of Carbonyl Groups 3.6 Uronic Acids 3.7 Hydroxyl Group Esters 3.8 Hydroxyl Group Ethers 3.9 Millard Browning 4. Oligosaccharides 4.1 Maltose 4.2 Sucrose 4.3 Lactose 5. Polysaccharides 5.1 Starch 5.2 Cellulose 5.3 Guar and Locust Bean Gums 5.4 Xanthan 5.5 Carrageenan 5.6 Algins 5.7 Pectins 5.8 Gum arabic Intended learning outcomes Describe the structure & nomenclature of Carbohydrates Differentiate Monosaccharides, Disaccharides and Polysaccharides Discussion of the functions of monosaccharides, disaccharides and polysaccharides and their role in food and reactions Analyse carbohydrates in food quantitatively and qualitatively. 31  FQM 1212 Basics in Food Science 1. Introduction Carbohydrates (from "hydrates of carbon") are organic compounds with the basic structure Cx(H2O)y. Among the most important types of carbohydrates in foods are sugars, dextrins, starches, celluloses, hemicelluloses, pectins, and certain gums. In animal organisms, the main sugar is glucose, and the storage carbohydrate is glycogen; in milk, the main sugar is almost exclusively the disaccharide lactose. In plant organisms, a wide variety of monosaccharides and oligosaccharides occur, and the storage carbohydrate is starch. The structural polysaccharide of plants is cellulose. The gums are a varied group of polysaccharides obtained from plants, seaweeds, and microorganisms. Carbohydrates can be categorized into three groups based on their structure: Monosaccharides, Oligosaccharides Polysaccharides 2. Monosaccharides It is monomeric in nature and cannot be further broken down. The general formula is Cx(H2O)x, and monosaccharides often exist in cyclic hemiacetal forms. Monosaccharides can be divided into two major groups according to whether their acyclic forms possess an aldehyde or a keto group Aldoses (Can be oxidized in to carboxylic acids) Ketoses 2.1 Glucose: Glucose is known as an aldose sugar because it contains an aldehyde group (CHO) located on the first carbon atom of the chain. It is conventional to number the carbon atoms along the chain so that the carbon atom with the highest number is farthest away from the aldehyde (or functional) group. The aldehyde group is therefore located on carbon one in glucose (and in all other aldose sugars). The numbering of the carbon atoms in glucose is shown in figure 1. 32  FQM 1212 Basics in Food Science Figure 1: Structure of glucose Two isomers of glucose exist, which are mirror images of each other, D-glucose and L-glucose. D-Glucose is the isomer that occurs naturally. In fact, there are two series of aldose sugars, known as the D-series and the L-series, each formed by adding CHOH groups to build the carbon chain, starting from the smallest aldose sugar, which is D- or L-glyceraldehyde (Figure 2). Figure 2: Mirror images of glycraldehyde In glucose, the highest-numbered asymmetric carbon atom is carbon-5. This is termed the reference carbon atom because its configuration determines whether the sugar belongs to the D-series or to the L-series. The hydroxyl group attached to it is called the reference hydroxyl group. This group is always on the right side in a D-series sugar. The most common configurations for glucose are pyranose structures, drawn according to the Haworth convention in Figure 3. These are anomers and are designated alpha (α) and beta (β). They are formed when the hydroxyl group on the fifth carbon reacts with the carbonyl group on the first carbon atom. (located on the first carbon, designated as Cl). As the ring closes, a new hydroxyl group is formed, termed the anomeric hydroxyl group, and the carbon atom to which it is attached is 33  FQM 1212 Basics in Food Science termed the anomeric carbon atom. For glucose and the other aldoses, the anomeric carbon atom is always the first carbon atom of the chain. The anomeric hydroxyl group can project towards either side of the ring, as shown in Figure 4. Figure 3: The D-glucopyranose anomers, drawn according to the Haworth convention Figure 4: The isomers of D-glucose The α-anomer has an anomeric hydroxyl group on the opposite face of the ring to carbon-6 (i.e., pointing in the opposite direction to carbon-6) when drawn according to the Haworth convention, whereas the β-anomer has an anomeric hydroxyl group on the same face of the ring as carbon-6 (i.e., pointing in the same direction as carbon- 6). 2.2. Fructose: Fructose is a six-carbon sugar, like glucose, but is a ketosesugar, not an aldose, because it contains a ketone (Figure 5). Similar to aldose sugars, there is a D- series and an L-series of ketose sugars, but D-fructose is the only ketose of importance in foods. In fructose, the ketone group is located on the second carbon of the chain. The second carbon atom is therefore the anomeric carbon in fructose. Fructose occurs mainly in the α- and β-furanose or five-membered ring configurations, as shown in Figure 6. 34  FQM 1212 Basics in Food Science Figure 5: Structure of fructose Figure 6: α and β Configurations of D-fructose 2.3 Galactose: This is a monosaccharide and has the same chemical formula as glucose, i.e., C6H12O6. It is similar to glucose in its structure, differing only in the position of one hydroxyl group. This difference, however, gives galactose different chemical and biochemical properties to glucose. The major dietary source of galactose is lactose, a disaccharide formed from one molecule of glucose plus one of galactose. Lactose is found only in milk; after weaning, significant quantities of dietary lactose are found only in dairy products. Figure 7: α and β configurations of galactose 35  FQM 1212 Basics in Food Science Glycosidic Bonds A glycosidic bond is formed when the carbonyl group of one monosaccharide reacts with a hydroxyl group of another molecule and water is eliminated (Figure 8). Figure 8: A glycosidic bond between two monosaccharides Properties of Sugars Such sugars as glucose, fructose, maltose, sucrose, and lactose all share the following characteristics in varying degrees: (1) They are usually used for their sweetness; (2) They are soluble in water and readily form syrups; (3) They form crystals when water is evaporated from their solutions (this is the way sucrose is recovered from sugar cane juice); (4) They supply energy; (5) They are readily fermented by microorganisms; (6) They prevent the growth of microorganisms in high concentrations, so they may be used as a preservative; (7) They darken in color or caramelize on heating; (8) Some of them combine with proteins to give dark colors, known as the browning reaction; and (9) They give body and mouthfeel to solutions in addition to sweetness. A very important advance in sugar technology has been the development of enzymatic processesfor the conversion of glucose to its isomer, fructose. Fructose is sweeter than glucose or sucrose. This has made possible the production of sugar syrups with the sweetness and certain other properties of sucrose starting from starch. Commonly, corn starch is hydrolyzed to provide the glucose, which is then isomerized. The United States produces enormous quantities of corn and with this 36  FQM 1212 Basics in Food Science technology has become less dependent on imported sucrose, the availability and price of which can fluctuate greatly. 3. Chemical Reactions of Monosaccharide sugars 3.1 Caramelization: The formation of the caramel pigment can be considered a nonenzymatic browning reaction in the absence of nitrogenous compounds. When sugars are subjected to heat in the absence of water or are heated in concentrated solution, a series of reactions occurs that finally leads to caramel formation. The initial stage is the formation of anhydro sugars. Heating of both reducing (e.g., glucose) and nonreducing sugars (e.g., sucrose) without any nitrogenous compounds results in a complex group of reactions leading to a dark brown caramel. Caramelization is the browning of sugar, a process used extensively in cooking for the resulting sweet nutty flavor and brown color. Brown colors are produced by three groups of polymers: caramelans (C24H36O18), caramelens (C36H50O25), and caramelins (C125H188O80). As the process occurs, volatile chemicals such as diacetyl are released, producing the characteristic caramel flavor. However, if the caramelization reaction is not controlled, it will create bitter, burned, and unpleasant tasting products. Intense heat and low pH trigger the reactions 37  FQM 1212 Basics in Food Science Figure 9: Caramelization 3.2 Crystallization: An important characteristic of sugars is their ability to form crystals. In the commercial production of sugars, crystallization is an important step in the purification of sugar. The purer a solution of a sugar, the easier it will crystallize. However, nonreducing oligosaccharides crystallize relatively easily. The fact that certain reducing sugars crystallize with more difficulty is due to the presence of anomers and ring isomers in solution, which makes these sugars intrinsically "impure" (Shallenberger and Birch 1975). Mixtures of sugars crystallize less easily than single sugars. In certain foods, crystallization is undesirable, such as the crystallization of lactose in sweetened condensed milk or ice cream. 38  FQM 1212 Basics in Food Science 3.3 Hydrogenation: Polyols Polyols or sugar alcohols occur in nature and are produced industrially from the corresponding saccharides by catalytic hydrogenation. Sorbitol, the most widely distributed natural polyol, is found in many fruits, such as plums, berries, cherries, apples, and pears. It is a component of fruit juices, fruit wines, and other fruit products. It is commercially produced by catalytic hydrogenation of D-glucose. Mannitol, the reduced form of D-mannose, is found as a component of mushrooms, celery, and olives. In recent years, disaccharide alcohols have become important. These include isomalt, maltitol, lactitol, and hydrogenated starch hydrolysates (HSHs). Maltitol is hydrogenated maltose with the structure shown in Figure 14. It has the highest sweetness of the disaccharide polyols compared to sugar. Lactitol is a disaccharide alcohol, 1,4- galactosylglucitol, produced by hydrogenation of lactose. It has low sweetness and a lower energy value than other polyols. It has a calorie value of 2 kcal/g and is noncariogenic. Figure 10: Structure of maltitol These sugar alcohols are used in chewing gum, breath mints, and other products that may be kept in the mouth for a while. Although products containing sugar alcohols may be labeled “sugar-free,” it is important to realize that sugar alcohols are not free of calories. They are not metabolized as efficiently as sugars and have a lower caloric value (between 1 and 3 kcal/g). Sugar alcohols may be used as a low-energy bulk ingredient (in place of sugar) in many food products. Since sorbitol is mostly transformed to fructose in the body rather than glucose, it is tolerated by diabetic patients. Hence, it can be used to replace sugar in diabetic foods. 39  FQM 1212 Basics in Food Science 3.4 Oxidation to Aldonic Acids and Aldonolactones Aldoses are readily oxidized to aldonic acids. The reaction is commonly used for quantitative determination of sugars. One of the earliest methods for quantitative measurement of sugars employed Fehling solution. Fehling solution is an alkaline solution of copper(II) that oxidizes an aldose to an aldonate and in the process is reduced to copper(I), which precipitates as brick-red Cu2O. Variations, the Nelson- Somogyi and Benedict reagents, are still used for determining the amounts of reducing sugars in foods and other biological materials. 3.5 Reduction of Carbonyl Groups Hydrogenation is the addition of hydrogen to a double bond. When applied to carbohydrates, it most often entails the addition of hydrogen to the double bond between the oxygen atom and the carbon atom of the carbonyl group of an aldose or ketose. Hydrogenation of D-glucose is easily accomplished with hydrogen gas under pressure in the presence of Raney nickel. The product is D-glucitol, commonly known as sorbitol, where the -itol suffix denotes a sugar alcohol (an alditol). Alditols are also known as polyhydroxy alcohols and as polyols. Figure 11: Oxidation of D-glucose catalyzed by glucose oxidase 3.6 Uronic Acids The terminal carbon atom (at the other end of the carbon chain from the aldehyde group) of a monosaccharide unit of an oligoor polysaccharide may occur in an oxidized (carboxylic acid) form. Such an aldohexose with C-6 in the form of a 40  FQM 1212 Basics in Food Science carboxylic acid group is called a uronic acid. When the chiral carbon atoms of a uronic acid are in the same configuration as they are in D-galactose, for example, the compound is D-galacturonic acid, the principal component of pectin. Figure 12: reduction of fructose 3.7 Hydroxyl Group Esters The hydroxyl groups of carbohydrates, such as the hydroxyl groups of simple alcohols, form esters with organic and some inorganic acids. Reaction of hydroxyl groups with a carboxylic acid anhydride or chloride (an acyl chloride) in the presence of a suitable base produces an ester. b. Hydroxyl Group Ethers The hydroxyl groups of carbohydrates, such as the hydroxyl groups of simple alcohols, can be from ethers as well as esters. Ethers of carbohydrates are not as common in nature as esters. However, polysaccharides are etherified commercially to modify their properties and make them more useful. Examples are the production of methyl (-O-CH3), sodium carboxy methyl (-O-CH2-CO2¯Na+), and hydroxypropyl (- O-CH2-CHOH-CH3) ethers of cellulose and hydroxypropyl ethers of starch, all of which are approved for food use. c. Millard Browning Under some conditions, reducing sugars produce brown colors that are desirable and important in some foods. Other brown colors obtained upon heating or during long- term storage of foods containing reducing sugars are undesirable. Common browning of foods upon heating or storage is usually due to a chemical reaction between reducing sugars, mainly D-glucose, and a free amino acid or a free amino group of an 41  FQM 1212 Basics in Food Science amino acid that is part of a protein chain. This reaction is called the Maillard reaction. It is also called nonenzymatic browning to differentiate it from the often rapid, enzyme-catalyzed browning commonly observed in freshly cut fruits and vegetables, such as apples and potatoes. When aldoses or ketoses are heated in solution with amines, a variety of reactions ensue, producing numerous compounds, some of which are flavors, aromas, and dark-colored polymeric materials, but both reactants disappear only slowly. The flavors, aromas, and colors may be either desirable or undersirable. They may be produced by frying, roasting, baking, or storage. Maillard browning products, including soluble and insoluble polymers, are found where reducing sugars and amino acids, proteins, and/or other nitrogen-containing compounds are heated together, such as in soy sauce and bread crusts. Maillard reaction products are important contributors to the flavor of milk chocolate. The Maillard reaction is also important in the production of caramels, toffees, and fudges, during which reducing sugars also react with milk. Activity 1: Select three food products that favorably support the Maillard reaction and discuss how its sensory attributes improved due to the reaction 4. Oligosaccharides An oligosaccharide contains 2 to 20 sugar units joined by glycosidic bonds. When a molecule contains more than 20 units, it is a polysaccharide. Disaccharides are glycosides in which the two monosaccharide units are bound. A compound containing three monosaccharide units is a trisaccharide. Structures containing 4 to 10 glycosyl units, whether linear or branched, are tetra-, penta-, hexa- , octa-, nona-, and deca saccharides, and so on. Because glycosidic bonds are part of acetal structures, they undergo acid-catalyzed hydrolysis, that is, cleavage in the presence of aqueous acid and heat. Only a few oligosaccharides occur in nature. Most are produced by hydrolysis of polysaccharides into smaller units. 4.1 Maltose: Maltose is 4-α-D-glucopyranosyl-β-D-glucopyranose. It is the major end product of the enzymatic degradation of starch and glycogen by α-amylase and has a characteristic flavor of malt. Maltose is a reducing disaccharide, shows mutarotation, is fermentable, and is easily soluble in water (Figure 9) 42  FQM 1212 Basics in Food Science Figure 13: Maltose 4.2 Sucrose: Sucrose or ordinary sugar occurs in abundant quantities in many plants and is commercially obtained from sugar cane or sugar beets. Since the reducing groups of the monosaccharides are linked in the glycosidic bond, this constitutes one of the few nonreducing disaccharides. Sucrose, therefore, does not reduce Fehling solution or form osazones and does not undergo mutation rotation in solution. Because of the unique carbonyl-to-carbonyl linkage, sucrose is highly labile in acidic media, and acid hydrolysis is more rapid than with other oligosaccharides. The structure of sucrose is shown in Figure 14. When sucrose is heated to 210°C, partial decomposition takes place, and caramel is formed. Figure 14: Sucrose. 4.3 Lactose: Lactose is a disaccharide of D-glucose and D-galactose and is designated 4-O-β-D-galactopyranosyl-D-glucopyranose (Figure 15). lactose is the only sugar in the milk of all species and does not occur elsewhere. Lactose is the major constituent of the dry matter of cow's milk, as it represents close to 50 percent of the total solids. 43  FQM 1212 Basics in Food Science Figure 15: Glactose Figure 16: Structure of Some Important Disaccharides Legumes contain several oligosaccharides, including raffinose and stachyose. These sugars are poorly absorbed when ingested, which results in their fermentation in the large intestine. This leads to gas production and flatulence, which present a barrier to wider food use of such legumes. 5. Polysaccharides Polysaccharides are polymers of monosaccharides. Like oligosaccharides, they are composed of glycosyl units in linear or branched arrangements, but most are much larger than the 20-unit limit of oligosaccharides. The number of monosaccharide units in a polysaccharide, termed its degree of polymerization (DP), varies. Only a few 44  FQM 1212 Basics in Food Science polysaccharides have a DP less than 100; most have DPs in the range of 200–3000. Larger compounds, such as cellulose, have a DP of 7000–15,000. 5.1 Starch Starch is a polymer of D-glucose and is found as a storage carbohydrate in plants. It occurs as small granules with a size range and appearance characteristic of each plant species. Starch is the predominant food reserve substance in plants and provides 70– 80% of the calories consumed by humans worldwide. Starch and starch hydrolysis products constitute most of the digestible carbohydrates in the human diet. Commercial starches are obtained from cereal grain seeds, particularly from corn, waxy corn (waxy maize), high-amylose corn, wheat, and various rice, and from tubers and roots, particularly potato, sweet potato, and tapioca (cassava). Starches and modified starches have an enormous number of food uses, including adhesive, binding, clouding, dusting, film forming, foam strengthening, anti-staling, gelling, glazing, moisture retaining stabilizing, texturizing, and thickening applications. Starch is unique among carbohydrates because it occurs naturally as direct granules. Starch granules are relatively dense ad insoluble and hydrate only slightly in cold water. Most starch granules are composed of a mixture of two polymers: an essentially linear polysaccharide called amylose and a highly branched polysaccharide called amylopectin. Amylose: Amylose is essentially a linear chain of (1,4)-linked α-D- glucopyranosyl units (Figure 17). Figure 17: Amylose Amylopectin: amylopectin contains branches. For every 15–30 glucose residues, there is a branch joined to the main chain by an α-1,6-glycosidic link (Figure 18) 45  FQM 1212 Basics in Food Science Figure 18: Amylopectin 5.1.1 Starch Granules Starch granules are made up of amylose and/or amylopectin molecules arranged radially. They contain both crystalline and noncrystalline regions in alternating layers. The clustered branches of amylopectin occur as packed double helices. The packing together of these double-helical structures forms the many small crystalline areas comprising the dense layers of starch granules that alternate with less dense amorphous layers. Because crystallinity is produced by ordering the amylopectin chains, waxy starch granules, that is, granules without amylose, have approximately the same amount of crystallinity as normal starches. Rice granules, on average, are the smallest of the commercial starch granules (1.5–9 μm), although the small granules of wheat starch are almost the same size. Many of the granules in tuber and root starches, such as potato and tapioca starches, tend to be larger than those of seed starches and are generally less dense and easier to cook. Potato starch granules may be as large as 100 μm along the major axis. 5.1.2 Gelatinization and Pasting Undamaged starch granules are insoluble in cold water but can imbibe water reversibly; that is, they can swell slightly and then return to their original size upon drying. When heated in water, starch granules undergo a process called 46  FQM 1212 Basics in Food Science gelatinization. Gelatinization is the disruption of molecular order within granules. Evidence for the loss of order includes irreversible granule swelling, loss of birefringence, and loss of crystallinity. Leaching of amylose occurs during gelatinization, but some leaching of amylose can also occur prior to gelatinization. The apparent temperature of initial gelatinization and the range over which gelatinization occurs depend on the method of measurement and on the starch:water ratio, granule type, and heterogeneities within the granule population under observation. These are the initiation temperature (first observed loss in birefringence), the midpoint temperature, the completion or birefringence endpoint temperature (BEPT, the temperature at which the last granule in the field under observation loses its birefringence), and the gelatinization temperature range. Figure 19: Representative Brabender Visco/amylo/graph curve showing viscosity changes related to typical starch granule swelling and disintegration as a granule suspension is heated to 95⁰C and then held at that temperature. (The instrument imparts moderate shear to the system). Tp is the pasting temperature, that is, the temperature at which a viscosity increase is recorded by the instrument. Under normal food processing conditions (heat and moisture, although many food systems contain limited water as far as starch cooking is concerned), starch granules quickly swell beyond the reversible point. Water molecules enter between chains, break interchain bonds, and establish hydration layers around the separated molecules. This plasticizes (lubricates) chains so they become more fully separated 47  FQM 1212 Basics in Food Science and solvated. The entrance of large amounts of water causes granules to swell to several times their original size. When a 5% starch suspension is gently stirred and heated, granules imbibe water until much of the water is absorbed by them, forcing them to swell, press against each other, and fill the container with a highly viscous starch paste. Such highly swollen granules are easily broken and disintegrated by stirring, resulting in a decrease in viscosity. As starch granules swell, hydrated amylose molecules diffuse through the mass to the external phase (water), a phenomenon responsible for some aspects of paste behavior. The results of starch swelling can be recorded using a Brabender Visco/amylo/graph, which records the viscosity continuously as the temperature is increased, held constant for a time, and then decreased. 5.1.3 Modified Food Starch Food processors generally prefer starches with better behavioral characteristics than provided by native starches. Native starches produce particularly weak-bodied, cohesive, rubbery pastes when cooked and undesirable gels when the pastes are cooled. The properties of starches can be improved by modification. Modification is performed so that the resultant pastes can withstand the conditions of heat, shear, and acid associated with particular processing conditions and introduce specific functionalities. Specific property improvements that can be obtained by proper combinations of modifications are reduction in the energy required to cook (improved gelatinization and pasting), modification of cooking characteristics, increased solubility, either increased or decreased paste viscosity, increased freeze–thaw stability of pastes, enhancement of paste clarity, increased paste sheen, inhibition of gel formation, enhancement of gel formation and gel strength, reduction of gel syneresis, improvement of interaction with other substances, improvement in stabilizing properties, enhancement of film formation, improvement in water resistance of films, reduction in paste cohesiveness, and improvement of stability to acid, heat, and shear. Modified waxy maize starches are especially popular in the U.S. food industry. Pastes of unmodified common corn starch will gel, and the gels will generally be cohesive, rubbery, long textured, and prone to syneresis (that is, to weep or exude moisture). However, pastes of waxy maize starch show little tendency to gel at room temperature, which is why waxy maize starch is generally preferred as the base starch for food starches. Properties that can be controlled by combinations of crosslinking, stabilization, and thinning of corn, waxy maize, potato, and other starches include but are not limited to the following: adhesion, clarity of solutions/pastes, color, emulsion stabilization ability, film-forming ability, flavor release, hydration rate, moisture holding capacity, stability to ac-ids, stability to heat and cold, stability to shear, temperature required to cook, and viscosity (hot paste and cold paste). 48  FQM 1212 Basics in Food Science Activity 2 List the importance of starches and their derivatives in the food industry to maintain product specificity. 5.2 Cellulose Cellulose is the most abundant organic compound and therefore the most abundant carbohydrate on earth because it is the principal cell wall component of higher plants. (It can be argued that D-glucose is the most abundant carbohydrate and organic compound if we consider cellulose as a combined form of this monomeric building block.) Cellulose is a high-molecular-weight, linear, insoluble homopolymer of repeating β-D-glucophyranosyl units joined by (1,4) glycosidic linkages (Figure 20). Cellulose and its modified forms serve as dietary fiber because they do not contribute significant nourishment or calories as they pass through the human digestive system. Dietary fiber does, however, serve important functions. Purified cellulose powder is available as a food ingredient. High-quality cellulose can be obtained from wood through pulping and subsequent purification. Chemical purity is not required for food use because cellulosic cell wall materials are components of all fruits and vegetables and many of their products. Carboxymethylcellulose - Carboxymethylcellulose (CMC) is widely and extensively used as a food gum. 49  FQM 1212 Basics in Food Science Figure 20: Structure of cellulose 5.3 Guar and Locust Bean Gums Guar and locust bean gums are important thickening polysaccharides for both food and nonfood uses. Guar gum produces the highest viscosity of any natural, commercial gum. Both gums are the ground endosperm of seeds. The main component of both endosperm is a galactomannan. Galactomannans consist of a main chain of β-D-mannopyranosyl units joined by (1, 4) bonds with single-unit α-D- galactopyranosyl branches attached at O-6 (Figure 5). Guar gum provides economical thickening to numerous food products. It is used frequently in combination with other food gums, such as in ice cream, where it is used in combination with carboxymethylcellulose, carrageenan, and xanthan 50  FQM 1212 Basics in Food Science Figure 21: A representative segment of a galactomannan molecule 5.4 Xanthan Xanthomonas campestris, a bacterium commonly found on leaves of plants of the cabbage family, produces a polysaccharide, termed xanthan, that is widely used as a food gum. The polysaccharide is known commercially as xanthan gum. Xanthan has a backbone chain identical to that of cellulose, which binds with trisaccharide units. Xanthan is widely used as a food gum because of the following important characteristics: solubility in hot or cold water; high solution viscosity at low concentrations; no discernible change in solution viscosity in the temperature range from 0 to 100°C, which makes it unique among food gums; solubility and stability in acidic systems; excellent compatibility with salt; interaction with other gums such as Locust Bean Gum; ability to stabilize suspensions and emulsions; and good solution stability when exposed to freezing and thawing. Figure 22: Structure of xanthan gum 51  FQM 1212 Basics in Food Science 5.5 Carrageenan Carrageenans are mixtures of several related galactans having sulfate half-ester groups attached to the sugar units. They are extracted from red seaweeds with a dilute alkaline solution; the sodium salt of carrageenan is normally produced. Carrageenans are linear chains of D-galactopyranosyl units joined with alternating (1,3)-α-D- and (1,4)-β-D-glycosidic linkages, with most sugar units having one or two sulfate groups esterified to a hydroxyl group at carbon atoms C-2 or C-6. Figure 23: Idealized unit of kappa-, -iota-, and lambda-type carrageenan 5.6 Algins Commercial algin is a salt, most often the sodium salt, of a linear poly (uronic acid), alginic acid, obtained from brown seaweeds. Alginic acid is composed of two monomeric units, β-D-mannopyranosyluronic acid and α-L-gulopyranosyluronic acid units. Figure 24: Sodium alginate molecule 5.7 Pectins Commercial pectins are galacturonoglycan poly α-D-galactopyranosyluronic acids with various contents of methyl ester groups. Native pectins found in the cell walls and intercellular layers of all land plants are more complex molecules that are 52  FQM 1212 Basics in Food Science converted into commercial products during extraction with acid. Commercial pectin is obtained from citrus peel and apple pomace. Pectin from lemon and lime peel is the easiest to isolate and is of the highest quality. Pectins have a unique ability to form spreadable gels in the presence of sugar and acid or in the presence of calcium ions and are used almost exclusively in these types of applications. By definition, preparations in which more than half of the carboxyl groups are in the methyl ester form (-COOCH3) are classified as high-methoxyl (HM) pectins; the remainder of the carboxyl groups will be present as a mixture of free acid (COOH) and salt (-COO-Na+) forms. Preparations in which less than half of the carboxyl groups are in the methyl ester form are called low-methoxyl (LM) pectins. The percentage of carboxyl groups esterified with methanol is the degree of esterification (DE). Figure 25: Simplified scheme of a pectin molecule with possible attached methyl and acetyl groups Activity 3 Pectin is commonly use in Jam processing. Explain the action of pectin on the physical stability (texture) of Jam 5.8 Gum Arabic When the bark of some trees and shrubs is injured, the plants exude sticky material that hardens to seal the wound and protects against infection and desiccation. Such exudates are commonly found on plants that grow in semiarid climates. Gum arabic is a heterogeneous material but generally consists of two fractions. One, which accounts for approximately 70% of the gum, is composed of polysaccharide chains with little or no nitrogenous material. The other fraction contains molecules of 53  FQM 1212 Basics in Food Science higher molecular weight that have protein as an integral part of their structures. The protein-polysaccharide fraction is itself heterogeneous with respect to protein content. Gum arabic is both a fair emulsifying agent and a very good emulsion stabilizer for flavor oil-in-water emulsions. It is the gum of choice for emulsification of citrus and other essential oils. An important characteristic is its compatibility with high concentrations of sugar. Therefore, it finds widespread use in confections with a high sugar content and a low water content. Figure 26: Structure of gum arabic (gap represents galactopyranose residues) The major dietary source of galactose is lactose, a disaccharide formed from one molecule of glucose plus one of galactose. Lactose is found only in milk; after weaning, significant quantities of dietary lactose are found only in dairy products. Figure 27: α and β configurations of galactose 54  FQM 1212 Basics in Food Science Unit 04: Proteins Content 1. Introduction 2. Structure and Classification 3. Categories of Amino Acids 4. Stereochemistry of Amino Acids 5. Properties of Amino acids 5.1 Acid-Base Properties of Amino Acids 5.2 Hydrophobic Properties of Amino Acids 6. Protein Structure 6.1 Primary Structure 6.2 Secondary Structure 6.3 Tertiary Structure 6.4 Quaternary Structure 7. Forces Involved in the Stability of Protein Structure 7.1 Steric Strains 7.2 van der Waals Interactions 7.3 Hydrogen Bonds 7.4 Electrostatic Interactions 7.5 Hydrophobic Interactions 7.6 Disulfide Bonds 8. Protein Classification 8.1 Simple Proteins 8.2 Conjugated Proteins 8.3 Derived Proteins 9. Reactions and Properties of Proteins 9.1 Amphoteric 9.2 Water-Binding Capacity 9.3 Salting-in and Salting-out 10. Protein Denaturation 11. Millard Browning 12. Functional Properties of Protein Intended Learning Outcomes Classify Proteins based on their structure Identify the functions of proteins in food systems Discuss uses of proteins in food industry 55  FQM 1212 Basics in Food Science 1. Introduction Proteins play a central role in biological systems. Every protein has a unique structure and conformation or shape, which enables it to carry out a specific function in a living cell. Proteins comprise the complex muscle system and the connective tissue network, and they are important as carriers in the blood system. All enzymes are proteins; enzymes are important as catalysts for many reactions (both desirable and undesirable) in foods. All proteins contain carbon, hydrogen, nitrogen, and oxygen. Most proteins contain sulfur, and some contain additional elements; for example, milk proteins contain phosphorus, and hemoglobin and myoglobin contain iron. Copper and zinc are also constituents of some proteins. Proteins are made up of amino acids. There are at least 20 different amino acids found in nature, and they have different properties depending on their structure and composition. When combined to form a protein, the result is a unique and complex molecule with a characteristic structure and conformation and a specific function in the plant or animal where it belongs. Small changes, such as a change in pH or simply heating a food, can cause dramatic changes in protein molecules 2. Structure and Classification α-Amino acids are the basic structural units of proteins. These amino acids consist of an α-carbon atom covalently attached to a hydrogen atom, an amino group, a carboxyl group, and a side-chain R group (Figure 1). Figure 1: Structure of amino acid Natural proteins contain up to 20 different primary amino acids (Table 1) linked together via amide bonds. These amino acids differ only in the chemical nature of the side chain R group. The physicochemical properties, such as net charge, solubility, chemical reactivity, and hydrogen bonding potential, of the amino acids are dependent on the chemical nature of the R group. Amino acids can be classified into several categories based on the degree of interaction of the side chains with water. Amino acids with aliphatic (Ala, Ile, Leu, Met, Pro, and Val) and aromatic side chains (Phe, Trp, and Tyr) are hydrophobic, and hence, they exhibit limited solubility in water. 56  FQM 1212 Basics in Food Science Table 1: Names, symbols, chemical structures and hydrophobicity indices of the 20 amino acids found in proteins 57 FQM 1212 Basics in Food Science 58  FQM 1212 Basics in Food Science Activity 1 What are the Sulfur containing amino acids? Draw the structures 59  FQM 1212 Basics in Food Science 3. Categories of Amino Acids Amino acids can be divided into four categories according to the nature of their side chains, as shown in Table 1. The first category includes all the amino acids with hydrophobic or nonpolar side chains. Hydrophobic (water-hating) amino acids contain a hydrocarbon side chain. Alanine is the simplest one, having a methyl group (CH3) as its side chain. Valine and leucine contain longer, branched, hydrocarbon chains. Proline is an important nonpolar amino acid. It contains a bulky five- membered ring, which interrupts ordered protein structure. Methionine is a sulfur- containing nonpolar amino acid. Nonpolar amino acids are able to form hydrophobic interactions in proteins; that is, they associate with each other to avoid association with water. The second group of amino acids includes those with polar uncharged side chains. This group is hydrophilic. Examples of amino acids in this group include serine, glutamine, and cysteine. They either contain a hydroxyl group (OH), an amide group (CONH2), or a thiol group (SH). All polar amino acids can form hydrogen bonds in proteins. Cysteine is unique because it can form disulfide bonds (—S—S—). A disulfide bond is a strong covalent bond, unlike hydrogen bonds, which are weak interactions. Two molecules of cysteine can unite in a protein to form a disulfide bond. A few disulfide bonds in a protein have a significant effect on protein structure because they are strong bonds. Proteins containing disulfide bonds are usually relatively heat stable and more resistant to unfolding than other proteins. The presence of cysteine in a protein therefore tends to have a significant effect on protein conformation. The third and fourth categories of amino acids include charged amino acids. The positively charged (basic) amino acids include lysine, arginine, and histidine. These are positively charged at pH 7 because they contain an extra amino group. When a basic amino acid is part of a protein, this extra amino group is free (in other words, not involved in a peptide bond) and, depending on the pH, may be positively charged. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid. These are negatively charged at pH 7 because they both contain an extra carboxyl group. When an acidic amino acid is contained within a protein, the extra carboxyl group is free and may be charged, depending on the pH. Oppositely charged groups are able to form ionic interactions with each other. In proteins, acidic and basic amino acid side chains may interact with each other, forming ionic bonds or salt bridges. 4. Stereochemistry of Amino Acids With the exception of Gly, the α-carbon atom of all amino acids is asymmetric, meaning that four different groups are attached to it. Because of this asymmetric center, amino acids exhibit optical activity, that is, they rotate the plane of linearly 60  FQM 1212 Basics in Food Science polarized light. In addition to the asymmetric α-carbon atom, the β-carbon atoms of Ile and Thr are asymmetric, and thus, both Ile and Thr can exist in four enantiomeric forms (similar molecular arrangements but positions can be changed). All proteins found in nature contain only L-amino acids. Figure 2: D- and L- configuration of amino acid This nomenclature is based on D- and L-glyceraldehyde configurations and not on the actual direction of rotation of linearly polarized light. That is, the L-configuration does not refer to levorotation as in the case of L-glyceraldehyde. In fact, most of the L-amino acids are dextro-rotatory, not levorotatory. 5. Properties of Amino acids 5.1 Acid-Base Properties of Amino Acids Since amino acids contain a carboxyl group (acidic) and an amino group (basic), they behave both as acids and bases; that is, they are ampholytes. For example, Gly, the simplest of all amino acids, can exist in three different ionized states, depending on the pH of the solution. At approximately neutral pH, both the α-amino and α-carboxyl groups are ionized, and the molecule is a dipolar ion or a zwitterion. The pH at which the dipolar ion is electrically neutral is called the isoelectric point (pI). When the zwitterion is titrated with an acid, the COO- group is protonated. The pH at which the concentrations of COO- and COOH are equal is known as pKa1 (i.e., negative logarithm of the dissociation constant Ka1). Similarly, when the zwitterion is titrated with a base, the group becomes deprotonated. 61  FQM 1212 Basics in Food Science Figure 3: Titration curve of a typical amino acid 5.2 Hydrophobic Properties of Amino Acids One of the major factors affecting physicochemical properties, such as structure, solubility, and fat-binding properties, of proteins and peptides is the hydrophobicity of the constituent amino acid residues. Hydrophobicity can be defined as the excess free energy of a solute dissolved in water compared to that in an organic solvent under similar conditions. The most direct and simplest way to estimate the relative hydrophobicity of amino acid side chains involves experimental determination of free energy changes for dissolution of amino acid side chains in water and in an organic solvent, such as ethanol. Activity 2 List down the hydrophobic amino acids? What special property you have observed from them? 6. Protein Structure Proteins are described as having four types of structures—primary, secondary, tertiary, and quaternary structures—and these build on each other. The primary structure determines the secondary structure and so on. 6.1 Primary Structure: The primary structure (protein primary structure) of a protein is the specific sequence of amino acids joined by peptide bonds along the protein 62  FQM 1212 Basics in Food Science chain. This is the simplest way of looking at protein structure. In reality, proteins do not exist simply as straight chains. 6.2 Secondary Structure: The secondary structure (protein secondary structure) of a protein refers to the periodic spatial arrangement of amino acid residues at certain segments of the polypeptide chain. The twist of the polypeptide chain is driven by near-neighbor or short-range noncovalent interactions between amino acid side chains. Important secondary structures include the following: Alpha helix Beta-pleated sheet The alpha (α) helix is a corkscrew structure, with 3.6 amino acids per turn. It is shown in Figure 2. It is stabilized by intrachain hydrogen bonds; that is, hydrogen bonds occur within a single protein chain rather than between adjacent chains. Hydrogen bonds occur between each turn of the helix. The oxygen and hydrogen atoms that comprise the peptide bonds are involved in hydrogen bond formation. The α-helix is a stable, organized structure. Proline cannot be formed if proline is present because the bulky five-membered ring prevents the formation of the helix. The beta (β)-pleated sheet is a more extended conformation than the α-helix. It can be thought of as a zigzag structure rather than a corkscrew. It is shown in Figure 2. The stretched protein chains combine to form β-pleated sheets. These sheets are linked together by interchain hydrogen bonds. (Interchain hydrogen bonds occur between adjacent sections of the protein chains rather than within an individual chain.) Again, the hydrogen and oxygen atoms that form the peptide bonds are involved in hydrogen bond formation. Figure 4: α- helix and β- pleated sheet 63  FQM 1212 Basics in Food Science 6.3 Tertiary Structure Tertiary structure refers to the spatial arrangement attained when a linear protein chain with secondary structure segments folds further into a compact three- dimensional form. The tertiary structure is shown in Figure 3. Figure 5: Tertiary structure of protein involved with various forces There are two types of protein tertiary structure: Fibrous proteins Globular proteins Fibrous proteins include structural proteins such as collagen (connective tissue protein) or actin and myosin, which are the proteins that are responsible for muscle contraction. The protein chains are extended, forming rods or fibers. Proteins with a fibrous tertiary structure contain a large amount of ordered secondary structure (either α-helix or β-sheet). Globular proteins are compact molecules and are spherical or elliptical in shape, as their name suggests. These include transport proteins, such as myoglobin, which carry oxygen to the muscle. Whey proteins and caseins, both of which are milk proteins, are also globular proteins. The global tertiary structure is favored by proteins with a large number of hydrophobic amino acids. 6.4 Quaternary Structure Protein quaternary structure, or the quaternary protein structure, involves the noncovalent association of protein chains. The protein chains may or may not be 64  FQM 1212 Basics in Food Science identical. Examples of quaternary structures include hemoglobin in blood, the actomyosin system of muscles and casein micelles in milk. Figure 6: The four levels of structures in protein Activity 3 Draw a hemoglobin structure and mark alpha chains, beta chains and different monomers using different colors 7. Forces Involved in the Stability of Protein Structure The protein primary structure involves only peptide bonds, which link the amino acids together in a specific and unique sequence. Secondary and tertiary structures may be stabilized by hydrogen bonds, disulfide bonds, hydrophobic interactions, and 65  FQM 1212 Basics in Food Science ionic interactions. Steric or spatial effects are also important in determining protein conformation. The space that a protein molecule occupies is determined partially by the size and shape of the individual amino acids along the protein chain. For example, bulky side chains such as proline prevent the formation of α-helices and favor random coil formation. This prevents the protein from assuming certain arrangements in space. The process of folding a random polypeptide chain into a unique three-dimensional structure is quite complex. The basis for the biologically native conformation is encoded in the amino acid sequence of the protein. In the 1960s, Anfinsen and coworkers showed that when denatured ribonuclease was added to a physiological buffer solution, it refolded to its native conformation and regained almost 100% of its biological activity. A majority of enzymes have been subsequently shown to exhibit similar propensity. The forces that contribute to protein folding may be grouped into two categories: (a) intramolecular interactions emanating from forces intrinsic to the protein molecule and (b) intramolecular interactions affected by the surrounding solvent. van der Waals and steric interactions belong to the former, and hydrogen bonding, electrostatic,