Food Chemistry PDF

# Chapter 1: Food Chemistry ## 1.1 Desirable and Potentially Undesirable Food Constituents and Their Importance - Food constituents or nutrients are used for growth, maintenance, tissue repair and reproduction, and foods are vehicles for them. - An individual food may contain only a few nutrients,...

# Chapter 1: Food Chemistry ## 1.1 Desirable and Potentially Undesirable Food Constituents and Their Importance - Food constituents or nutrients are used for growth, maintenance, tissue repair and reproduction, and foods are vehicles for them. - An individual food may contain only a few nutrients, or it may supply many, but no single food provides all nutrients in amounts and proportions necessary for adequate health. - Carbohydrates, fats, proteins, vitamins and minerals comprise the general class of desired nutrients. - The first three classes serve as major source of fuel. ### Protein - Protein supply essential amino acids and serve as a source of utilizable nitrogen for cell and tissue protein synthesis. - Amino acids may also serve as precursor for metabolically active compounds. - Examples: tryptophan and tyrosine for neurotransmitter; tryptophan for niacin; tyrosine for iodothyronines; lysine for carnitine formation. - These uses of the amino acids may be of little quantitative importance, in terms of total daily intake, but they have considerable physiological significance. ### Carbohydrates - Dietary carbohydrates are important source of energy intake. - The nutritionally important carbohydrates are: - Monosaccharides (glucose or fructose) - Disaccharides (sucrose, lactose) - Polysaccharides (starch, glycogen) - In human nutrition the celluloses and other plant polysaccharides are not used for energy because they are not hydrolyzed by secretions of the human digestive tract. - However, they contribute to dietary fibre intake (unavailable carbohydrates and lignin, a phenyl-propane derivative), which now appears to play an important role in the maintenance of normal gastrointestinal function, metabolism and health. - Available carbohydrates such as sucrose, lactose and starch are not specifically required for human health as far as is known. - They contribute about 40% of energy intake. ### Fats - Fats are concentrated source of dietary energy also serve as carriers of the fat soluble vitamins. - Lipids play essential roles in many enzyme reactions and in the maintenance of cell membrane structure and function. - Dietary fats are also source of essential fatty acids, such as linoleic (18:2n-6) C16H3202, linolenic C18H3202 (18:3n-3), and arachidonic (20:4-6) C20H3202 acids which are involved in the maintenance of normal membrane structure and function. - They are precursors of class of "local" hormone, the prostaglandins involving secretion, blood platelets, smooth muscle metabolism and modulation of nervous system activity. - It has recently been shown that an increase in dietary linoleic acids can influence prostaglandins biosynthesis, decrease the thrombotic tendency of blood platelets and improve heart function. - Although dietary fats are important in the maintenance of normal metabolism and health, excessive intake can promote disease. - It is found from epidemiological studies that excessive intake of saturated fats is one of the risk factor associated with the development of coronary heart disease (CHD) and possible with various types of cancer in human body. ### Vitamins and Minerals - Vitamins and minerals comprise the remaining two classes of essential nutrients. - Both vitamins and minerals appear in foods in a variety of forms, the chemical nature of which may have considerable nutritional importance, since it determines whether the nutrient is made available to the body during the process of digestion and absorption. - Thus, a measure of the total amount of various nutrients in a food may not be useful information for the nutritionist; it is important to know the fraction of total nutrient content available to the body. ### Vitamins - There are 13 vitamins necessary for normal growth, reproduction, and maintenance of health. - Their metabolic functions are diverse for example thiamine, pyridoxine, riboflavin, and niacin act as coenzyme, and a low dietary intake of these vitamins causes reduced activities of enzymes associated with energy transfer and utilization in amino acid metabolism. ### Minerals - The mineral elements constitute the final class of nutrients to be discussed. - The so-called macro-inognanic elements (sodium, potassium, chlorine, calcium, magnesium, phosphorous) are present in the body in the significant quantities, their combined mass being about 3 kg in adult men. - The micro-nutrients or trace minerals are required only in small amounts usually less than 30 mg per day. - Their body content is about 30g only. ## 1.1.1 Anti-nutrients - They are natural and synthetic compounds that interfere with the absorption of nutrients. - The three board classes of antinutrients are antiproteins, antiminerals, and antivitamins. ### Antiproteins - Antiproteins occur in many plants and some animals. - Various protease inhibitors inhibit proteolytic enzymes of the gut, usually by binding to the enzyme's active site. - Lectins are antiproteins that have binding sites for cell receptors similar to what antibodies have. - Consequently, lectins, also called hemaggulutinins, can agglutinate red blood cells. - Ovomucoids and ovoinhibitors are found in eggs and inhibit proteolytic enzymes. - Egg whites contain chymotrypsin inhibitors. - Many are sensitive to heat treatment and are inactivated at boiling temperatures for 15 min. - Trypsin and chymotrypsin inhibitors are found in legumes, vegetables, milk, wheat, and potatoes. - The trypsin inhibitor found in milk is only destroyed after heating to 85 °C for at least I h. - Soybean has pancreatic enzyme inhibitors that have been shown in rodents to reduce growth. - Elastase inhibitors have been isolated from soybeans, beans, and potatoes. - Lectins have been isolated from the legumes soybean, peanut, lima beans, kidney beans, fava beans, lentils, and pea, and potatoes, banana, mango, and wheat germ. - These compounds bind avidly to intestinal mucosal cells and interfere with amino acids, thyroxine, and fat absorption; therefore, such lectins are goitrogenic. - Ricin is a toxic lectin isolated from castor bean, and is notorious for causing deaths of children who eat raw castor beans as well for being a substance of concern as an instrument of bioterrorism. - Ricin's mode of action is through intestinal cell necrosis. - Most lectins are inactivated by moist heat; hence, steaming is an effective processing technique used to inactivate ricin in castor oil. - However, dry heat is largely ineffective to deactivate lectins. ### Antiminerals - Antiminerals are substances that interfere with absorption or metabolic utilization of minerals. - Examples: phytic acid, oxalic acid, glucosinolates, dietary fiber, and gossypol. - Phytic acid's negative effect on iron absorption has been known for decades. - Phytic acid is found in bran and germ of many plant seeds and in grains, legumes, nuts and species. - In addition, phytic acid can compromise the absorption of magnesium, zinc, copper, and manganese, usually forming precipitates. - Formation of soybean protein-phytate complexes during processing has been associated with a reduction in bioavailability of minerals such as Ca, Zn, Fe and Mg. - On the other hand, fermentation and other processing techniques are useful in reducing phytate levels. - Oxalic acid, like phytic acid, reduces the availability of bivalent cations. - Sources of oxalate include rhubarb, spinach, beets, potatoes, tea, coffee, and cocoa. - Tea drinking was associated with concerns for Ca deficits via complexes, which apparently can be counterbalanced by using milk with tea drinking. - Glucosinolates are goitrogenic and inhibit iodine uptake into the thyroid. - Rutabaga, turnips, cabbage, peaches and strawberries are good sources for glucosinolates. - Decreases in plasma thyroxin for glucosinolate intakes are found at the 15% dietary level in rodent studies. - Dietary fiber, even without phytate, can affect calcium, magnesium, zinc and phosphorus absorption. - Collectively, dietary fiber refers to food components in plants cell walls that are not digested. - Dietary fiber can act like ion exchangers and bind minerals. - Gossypol is a phenolic compound isolated from the cotton plant and is capable of chelating iron and binding up amino acids. - This major toxic compound makes up 1% of the seed's dry weight. - In the 1930s and 40s, outbreaks of infertility in some areas of China were traced to gossypol in cottonseed oil used as a cooking oil. - Various processing techniques to isolate the protein can remove 90% of the gossypol. - Gossypol also exerts genotoxic effects in mammalian cell cultures, possibly through production of superoxide anions and singlet oxygen induced strand breakage in DNA. - It is not known whether any of these adverse effects of gossypol are related to effects on iron metabolism. ### Antivitamins - Antivitamins are substances that inactivate or destroy vitamins or inhibit the activity of a vitamin in a metabolic reaction and increase an individual's need for vitamins. Again, the collection of compounds involved as antinutrients is diverse and does not lend itself to simple characterization. - For example, ascorbic acid oxidase, an enzyme found in fruits and vegetables, can oxidize ascorbic acid. - Hence, in addition to aerobic conditions, fresh juices lose 50% of the vitamin in less than 1 h. - Raw fish contains thiaminase, which has antivitamin properties and splits the thiamin at the methylene linkage into two rings. - Also, it is known that tannins found in plants destroy thiamin. - Mushrooms contain vitamin B6 antagonists, and linatine found inlinseed oil is an antipyridoxine factor. - Finally, avidin, a heat-sensitive compound found in egg white, forms a complex with biotin. ## 1.2 Water - Water can universally be found in the solid, gaseous, and liquid states. - Saltwater oceans contain about 96.5% of our global water supply. - Ice, the solid form of water, is the most abundant form of freshwater and most of it, nearly 68.7%, is currently trapped in the polar ice caps and glaciers. - About 30% of the freshwater sources are present in aquifers as groundwater. - The remaining freshwater is surface water in lakes and rivers, soils, wetlands, biota, and atmospheric water vapor. - Water is the most abundant and surely the most frequently overlooked component in foods. - It is estimated that over 35% of our total water intake comes from the moisture in the foods we consume. - The other contributors to our water intake are beverages and metabolic water, which is produced through chemical reactions in the body. - The water content of foods is very variable. - It may be as low as 0% in vegetable oils and as high as 99% in some vegetables and fruit. - Water by itself is free of calories and plain water does not contain nutritive substances, but it may be an ingredient itself in foods. - Foods are described as dry or low-moisture foods if they have very low water content. These are most often solid food systems. - Liquid food systems and tissue foods where water is the dominating constituent of the solution are high-moisture foods. - Foods that contain moderate levels of water are intermediate-moisture foods (IMFs). ### Water and the Food Industry - For the food industry, water is essential for processing, as a heating or a cooling medium. - It may be employed in processes in the form of liquid water or in the other states of water such as ice or steam. - Almost all food-processing techniques involve the use or the modification of water in food. - Freezing, drying, concentration, and emulsification processes all involve changes in the water fraction of the food. - Without the presence of water, it would not be possible to achieve the physicochemical changes that occur during cooking such as the gelatinization of starch. - Water is important as a solvent for dissolving small molecules to form solutions and as a dispersing medium for dispersing larger molecules to form colloidal solutions. - The control of water activity in foods is an important tool for extending shelf life. - It is responsible for the quality of foods affected by microbiological, chemical, and physical changes. - The physical properties, quantity, and quality of water within the food have a strong impact on food effectiveness, quality attributes, shelf life, textural properties, and processing. ### Food-preservation Processes - Food-preservation processes have a common goal of extending the shelf life of foods to allow for storage and convenient distribution. - The activity of microorganisms is the first and most dangerous limitation of shelf life. - Water is essential for microorganisms that may cause food spoilage if they are present in a food that offers them favorable conditions for growth. - Hence, many food preservation techniques were developed to reduce the availability or activity of water in order to eliminate the danger of microbial spoilage. - The presence or activity of water in foods may also enhance the rate at which deteriorative chemical reactions occur. - Some products may become rancid through free radical oxidation even at low humidities and thus become unacceptable. - Labile nutrients such as vitamins and natural color compounds are oxidized more rapidly when stored at low moisture levels. - Enzyme-mediated hydrolytic reactions may reduce the quality of the food product. Other reactions such as the Maillard type of non-enzymatic browning may be enhanced by the presence of higher levels of water. - On the other hand, water content is crucial for the textural characteristics and the sensory perception of foods. - A food may be found unacceptable by consumers simply because it does not satisfy their textural (sensory) anticipation. ## 1.3 Carbohydrates - Carbohydrates are ubiquitous. - Every organism contains some carbohydrate. - Carbohydrates can range from a simple monosaccharide to a large complex polysaccharide. - Polysaccharides in combination with proteins, lipids, and nucleic acids play an important role in many plant and animal metabolic systems. - Carbohydrates have many roles in food systems, where they function to provide flavor, structure, and texture to food and nutritional benefits to the consumer. - Carbohydrates are primarily composed of the elements carbon, hydrogen and oxygen. - Some of the carbohydrates possess the empirical formula (C.H₂O)n, where n-3, satisfying that these carbohydrates are in fact carbon hydrates. - Carbohydrates may be defined as polyhydroxy-aldehydes or ketones or compounds which produce them on hydrolysis. - They are the most abundant dietary source of energy (4 Calorie per gram) for all organisms. - Carbohydrates are often referred to as saccharides. - They are broadly classified into three groups - (i) monosaccharides, (ii) oligosaccharides, and (iii) polysaccharides. - This categorization is based on the number of sugar units. - Mono and oligosaccharides are sweet to taste, crystalline and soluble in water, hence they are commonly known as sugar. ### Monosaccharides - Monosaccharides, which represent the most basic carbohydrate elements, are polyhydroxy aldehydes and ketones commonly referred to as aldoses and ketoses, respectively. - The most common pentose sugars are D-xylose and L-arabinose, which are both aldoses. - The most common hexose sugars are D-glucose, D-galactose, D-mannose, and D-fructose. - D-Glucose, D-galactose, and D-mannose are aldoses. - From a food ingredient use point of view, D-fructose is the only common ketose sugar in plants. - Glucose can be commercially produced by degradation of starch to glucose. - Starch can be liquefied by hot acids. - Commercially, hydrochloric acid is sprayed onto well-mixed starch, and the mixture is heated until the desired degree of hydrolysis is obtained. - The acid is neutralized and the product is recovered by filtration or centrifugation, and washed and dried. - Glucose can be converted to fructose by passing glucose syrup through a column containing glucose isomerase. This step typically converts about 40% of the glucose to fructose. - The fructose is concentrated by passing the mixture through a chromatography column. - High-fructose corn syrup contains "55% lactose and is produced by blending fructose back into the glucose syrup. - Liquefied starch can be used to produce syrups with dextrose equivalence values from 50 to 98. - (Dextrose equivalence 100/average number of monosaccharide units in the oligosaccharides found in the syrup). - These are usually produced by using an exoamylase (glucan 1, 4-a-glucosidase) also known as amyloglucosidase. - This enzyme can convert liquefied starch into 95%-98% glucose. - The syrup is filtered and passed over activated charcoal and an ion-exchange resin. - Glucose and fructose are generally used as syrups in food ingredients. - They have film-forming and adhesive properties and are used as coatings for roasted nuts, candy, fillers, and spray-dried flavors. - High-fructose corn syrups are used as sweeteners and seem almost ubiquitous where sweetening is desired. - Rare sugars such as D-allose, D-psicose, D-tagatose, and D-talose have been incorporated into health foods and drinks, where they are used as bulking, browning, and low-calorie sweetening agents in food preparation. - These rare sugars have a taste similar to sucrose. ### Oligosaccharides - Oligosaccharides are comprised of 2 to 20 glycosidically-linked monosaccharide units. - Based on the number of monosaccharides unit present, the oligosaccharides are further subdivided. #### Disaccharides - Disaccharides are composed of two monosaccharide units joined by a glycosidic bond. - Disaccharides can either be reducing (e.g., maltose and lactose) or non-reducing (e.g., sucrose), depending on whether one or both anomeric carbon atoms are involved in the disaccharide glycosidic bond. - Disaccharides on hydrolysis produce two molecules of the same or different monosccarides. * Maltose → glucose + glucose * Sucrose → glucose + fructose * Lactose → glucose + galactose - Maltose, a disaccharide formed by enzymatic hydrolysis of starch, is produced commercially from the malting of barley, and is the primary fermentable sugar used in the production of beer. - Lactose is found in milk at concentrations between 4 and 9%, and is the primary carbohydrate source for developing mammals. - For energy utilization, it is necessary that lactose be hydrolyzed by the enzyme lactase (B-galactosidase) to D-galactose and D-glucose in the small intestine to facilitate absorption into the bloodstream. - In some individuals, lactose is not (or is only partially) hydrolyzed (lactase deficiency), which condition is clinically termed lactose intolerance, and results in the bacterial, anaerobic fermentation of lactose in the large intestine to lactic acid and gaseous products. - Sucrose, a disaccharide, is composed of a-D-glucose and ß-D-fructose units that are linked (1→2) through their reducing ends. - Thus, sucrose is a nonreducing sugar. - Sucrose is made in the cytoplasm of cells in photosynthetically active tissue and is translocated via phloem to metabolically active sites, where it is usually cleaved into glucose and fructose and used as intermediates in various metabolic pathways. - In plants such as sugar beet (Beta vulgaris) and sugarcane (Saccharum spp.), sucrose is a storage carbohydrate. - In sugarcane, sucrose accumulates in the vacuoles of cells located in the internodal region of the stem, while in sugar beet; sucrose accumulates in the vacuoles of cells located in root tissue. - Sucrose affects colligative properties of water in food systems by depressing freezing point and elevating boiling point and can delay starch gelatinization. - Sucrose can interact with ingredients to enhance the retention of aroma and flavor of foods. - Its antioxidant properties have been used to prevent the deterioration of flavor in canned fruit. - Its humectants properties have been used to prevent moisture loss from baked products. - Sucrose can provide a yellow-brown color to food products, through thermal degradation, alkaline degradation, and Maillard products. - Sucrose is rapidly degraded into glucose and fructose, which are readily absorbed into the bloodstream in the small intestine. - Sucrose provides a quick source of energy for the human body. - However, overconsumption of sucrose can cause adverse health problems. - It can contribute to obesity, dental caries, and can be problematic to people suffering from defects in glucose metabolism, e.g., hypoglycemia. #### Other Important Oligosaccharides - The other important oligosaccharides include tri-, tetra-, or penta-saccharides, respectively, containing 3, 4, or 5 monosaccharides of plant origin. - Raffinose (trisaccharide) → Fructose + Galactose + Glucose - Stachyose (tetrasaccharides) → 2 Galactose + Glucose + Fructose - Verbascose (Pentasaccharides) → 3 Galactose + Glucose + Fructose ### Polysaccharides - Polysaccharides are composed of more than 20 monosaccharide units. - Polysaccharides often are classified as being starch or non-starch. - Starch polysaccharides represent a source of energy in human beings while non-starch polysaccharides generally are non-digestible and are important in maintaining intestinal health. - Polysaccharides are a very diverse set of compounds. - They can consist of one type of sugar (homoglycans) or several types of sugars (heteroglycans). - They can be branched or non-branched and can vary in type of linkage. The three dimensional structure or conformation of polysaccharide chains is determined by the monosaccharide units and position and type of glycosidic linkages. - For example, polysaccharides with a-(1-4)-D-glucosyl linkages or B-(1-3)-D-glucosyl linkages form hollow helixes while those with B-(1-4)-D-glucosyl linkages form ribbon-like conformations. - The ribbon-like and hollow helix conformations are the two basic chain conformations. #### Homoglycans (homopolysaccharides) - They yield only single types of monosaccharides when hydrolysis takes place. - They are based on the nature of the monosaccharides units. - Thus, glucans are polymers of glucose whereas fructosans are polymers of fructose. - **Starch** or amylum is a polymeric carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. - This polysaccharide is produced by most green plants as energy storage, It is the most common carbohydrate in human diets and is contained in large amounts in staple foods like potatoes, wheat, maize (corn), rice, and cassava. - Starch is composed of two polysaccharides: amylose (15-20%) and amylopectin (80-85%). - Amylose and amylopectin are both polymolecular [contain a-(1-4) and a-(1-6)-D-glucosyl linkages] and polydisperse (vary in degree of polymerization). - Amylopectin is a large, highly branched molecule with a degree of polymerization of 104-107 with 4%-5% of the linkages involved with branch points. - Amylopectin branches are categorized as A-chains, B-chains, and C-chains. - Starch isolation from cereal grains involves steeping, coarse milling, degerming, fine milling, screening, centrifugal separation, and drying. - During steeping, the grains are soaked in an aqueous solution of sulfur dioxide and lactic acid to loosen the granules in the packed endosperm cell structure. - The softened grain is coarsely ground and a hydrocyclone separator separates the germ which is less dense because it contains a high level of lipid. - The remaining material is finely ground to release the starch. - The starch is isolated from other cellular components by centrifugation. - Starch isolation from tubers/roots involves washing and peeling the tubers. - The tubers are soaked in an aqueous solution of sodium bisulfite to prevent discoloration. - The tubers are ground by a cylindrical drum containing rotary blades. - Starch is isolated from other cellular components by centrifugation. - Starch functionality in food systems is primarily related to its gelatinization, retrogradation, and pasting properties. - Starch and/or starch derivatives are nearly ubiquitous in food systems. - Starch affects the sensory and textural properties of food. - Starch is used to stabilize structure by acting as a bulking agent. - Starch can function as a thickening agent for sauces and pie fillings and as a colloidal stabilizer for salad dressings. - Starch is used as coating, glazing, and gel-forming agents for gum confections. - High temperature and shear makes native starch granules fragile and susceptible to rupture during processing. - Thus, native starches have limited use in the food industry. - Starches are often modified to improve the desired properties. - Starch suitability depends on the processing temperature, length of time at temperature, shear forces, and pH. - Cross-linked starches are more tolerant to harsh food processing conditions. - In general, increased cross-linking results in more tolerance to acidic condition and less breakdown of starch. - **Cellulose** is the key structural constituent of plant primary cell walls. - It consists of long, linear chains composed solely of (14)-linked B-D-glucopyranosyl units. - The nature of the cellulose glycosidic linkage, its regular monosaccharide sequence, and its linear backbone causes cellulose molecules to adopt flat, rigid, ribbon-like secondary structures that readily aggregate to form crystalline, water insoluble superstructures. - Thus, in the native state, while cellulose represents a good source of dietary fiber in indigenous whole foods or in isolated form (referred to as powdered cellulose), it generally requires further processing or derivatization to enhance functionality for broader food use. - **Dextrins** are the breakdown products of starch by enzyme amylase or dilute acids. - Starch is sequentially hydrolyzed through different dextrins and, finally to maltose and glucose. - The various intermediates (identified by iodine colouration) are soluble starch (blue), amylopectin (violet) erythrodextrin (red) and achrodextrin (no colour). - **Pectin** is a complex class of plant cell wall polysaccharides, composed of galacturonans and rhamnogalacturonans. - Pectin has a backbone of B-(1-4)-d-galacturonic acid with a-(1-2)-L-rhamnose units interspersed. - The galacturonic acid units are partially acetylated and methylated, and the L-rhamnose units contain side chains containing arabinose and/or galactose. - Pectins are water soluble and adopt a random coil conformation in solution. - There are two major types of pectin. - High methoxy pectins (HM-pectin) have >50% methvlation are soluble in water, and form gels in low pH and in the presence of a high concentration of sugar. - Low methoxy pectins (LM-pectin) swell and hydrate in cold water and form gels in the presence of calcium. - LM-pectin does not need sugar to form a gel. - Pectins are used as gelling agents either through the use of calcium cross-linking in the case of LM-pectins or sugar-induced dehydration with HM-pectin. - Pectins are used as thickeners and stabilizers in jams, jelly, and fruit products. - Pectin can act as a protein dispersion stabilizer in acidified dairy products like yogurt and milk-based fruit drinks. - Pectin is a soluble dietary fiber and exerts physiological effects on the gastrointestinal tract such as delayed gastric emptying, reduced glucose absorption, and reduced serum cholesterol, all of which probably relate to its gel-forming and water-holding capacity. - **Glycogen** is the carbohydrate reserve in animal, hence often referred to as animal starch. - It is present in high concentration in lever, followed by muscle, brain etc. - Glycogen is also found in plants that do not possess chlorophyll (e.g. yeast, fungi). - The structure of glycogen is similar to that of amylopectin with more number of branches. - Glucose is the repeating unit in glycogen joined together by a (14) glycosidic bonds at branching points. - The molecular weight (upto 1 108) and the number of glucose units (upto 25,000) vary in glycogen depending on the source of glycogen is obtained (Formula C6H10O5)n. - Glycogen forms an energy source that can be quickly mobilized to meet sudden demand. ## 1.4 Proteins and Amino Acids - Proteins are among the fundamental molecules of biology. - They are common to all life, are present in every cell on Earth today, and are responsible for most of the complex functions that make life possible. - There are an estimated 100,000 different proteins in the human body alone. - In plants, an inventory of more than 13,000 plant proteins has been cataloged. - Proteins consist of polymers of amino acids (AA) that are characterized by the following general chemical structure: * R-CH (NH2) COOH - Two hydrogens and a nitrogen comprise the amino group, -NH2, and the acid entity is the carboxyl group, -COOH. - Amino acids link to each other when the carboxyl group of one molecule reacts with the amino group of another molecule, creating a peptide bond -C(=O)NH- and releasing a molecule of water (H₂O). - Amino acids (AA) are the basic building blocks of enzymes, hormones, proteins, and body tissues. - Peptides consist of two or more amino acids, oligopeptides (2-10 amino acids), polypeptides (10-50 amino acids or more), and proteins both contain 10 or more amino acids. - The polypeptides consisting of more than 50 amino acids are classified as proteins. - A protein is a complex, high molecular weight organic compound that consists of amino acids joined by peptide bonds. - Proteins perform various functions in processed foods. - The functional properties of proteins, such as foaming, emulsifying, gelling, thickening, texturizing, dough forming, whipping, curdling, water binding, flavor binding, and fat binding properties, are important for imparting desirable sensory attributes in a variety of food products. - These various functional properties of proteins emanate from two molecular attributes of proteins, namely hydrodynamic properties and physicochemical attributes of the protein's surface. - While the hydrodynamic properties relate to the size and shape of the molecule, the properties of a protein's surface relate to its topology and to the pattern of distribution of polar and non-polar patches. - Although these molecular attributes of proteins in their native state can be determined fairly precisely from their crystallographic structure, it has been difficult to predict functionality in a complex food milieu. - This is principally due to denaturation of proteins that inevitably occurs during food processing, which might alter both their hydrodynamic attributes (shape and size) and surface characteristics. - In the denatured state, the intensity of intermolecular interactions between proteins, or between proteins and other constituents in the food milieu would dramatically alter the proteins' functionality. - 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 also are constituents of some proteins. ### Protein Classification Table: | Protein Type | Category | Examples | |---|---|---| | Simple | Globular protein | Albumins, Globulins, Glutelins, Prolamines, Histones, Globins, Protamines | | Simple | Scleroproteins | Collagen, Elastins, Keratins | | Conjugated | Nucleoproteins | | | Conjugated | Glycoproteins | | | Conjugated | Mucoproteins | | | Conjugated | Lipoproteins | | | Conjugated | Phosphoproteins | | | Conjugated | Chromoproteins | | | Conjugated | Metalloproteins | | | Derived | Primary | Coagulated proteins, Metaproteins | | Derived | Secondary | Proteoses, Peptones, Polypeptides, Peptides | ### Table 2: Important Functional Properties of Proteins in Several Food Systems | Functional Property | Food System(s) | |---|---| | Solubility | Infant formulas, protein beverages, beer, yogurt drinks | | Water-holding ability (capacity) | Tumbled hams, deli meats, frankfurters, poultry products, yogurt | | Gelation | Frankfurters, custards, gelatin, comminuted meat and poultry products | | Emulsification | Ice cream, liquid coffee creamers, salad dressings, milk, mayonnaise, gravies | | Foaming | Angel and sponge cakes, meringues, soufflés, marshmallows, whipped cream and toppings | - **Denaturation** is the phenomenon of breakdown of native structure of protein with the help of heat, acid, alkali, organic acids, UV rays, X-rays etc. - The extent of denaturation of proteins in a food milieu depends on the susceptibility of intra-molecular interactions that hold the compact native protein structure to temperature, pressure (shear), pH, ionic strength, types of ions, and specific and non-specific interactions with other food components such as sugars, polysaccharides, lipids, and other additives. - It should be recognized, however, that from a food application standpoint, protein denaturation during processing is not always undesirable. - In fact, in some cases it is highly desirable. - For instance, partial denaturation of proteins at the air-water and oil-water interfaces improve their foaming and emulsifying properties, whereas excessive thermal denaturation of soy proteins diminishes their foaming and emulsifying properties. - In protein beverages, where high solubility and dispersibility of proteins is a necessity, even partial denaturation of protein during processing may cause flocculation and precipitation during storage and thus may adversely affect the sensory attributes of the product. - Thus, to develop appropriate processing strategies, a basic understanding of the environmental and other factors that affect structural stability of proteins in food systems is imperative. - **Coagulation** refers to a semi-solid viscous precipitate of protein, irreversible denaturation results in coagulation. - Coagulation is optimum and requires lowest temperature at isoelectric pH. - Albumins and globulins (to a lesser extent) are coagulable protein. - Heat coagulation test is commonly used to detect the presence of albumin in urine. - **Flocculation** is the process of protein precipitation at isoelectric pH. The precipitate is referred to as flocculum. - Casein (milk protein) can be easily precipitated when adjusted to isoelectric pH (4.6) by dilute acetic acid. - Flocculation is reversible. - On application of heat, flocculation can be converted into an irreversible mass, coagulum. - The **isoelectric point** of a protein is the pH at which the protein is electrically neutral (it is denoted by pl). - At this pH, the global or overall charge on the protein is zero. - This does not mean that the protein contains no charged groups. - It means that the number of positive charges on the protein is equal to the number of negative charges. - At the isoelectric point, the protein molecules usually precipitate because they do not carry a net charge. - (Molecules that carry a like charge repel each other, and thus form a stable dispersion in water. Removal of the charge removes the repulsive force and allows the molecules to interact with each other and precipitate, in most cases.) - The isoelectric point is important in food processing. - For example, cottage cheese is made by adding lactic acid to milk to bring the pH to the isoelectric point of the major milk proteins (the caseins). - The proteins precipitate at this pH, forming curds. These are separated from the rest of the milk and may be pressed and/or mildly salted before being packaged as cottage cheese. - Good sources of protein include high-quality protein foods, such as meat, poultry, fish, milk, egg, and cheese, as well as prevalent low-quality protein foods, such as fresh vegetables and fruits, except legumes (e.g., navy beans, pinto beans, chick peas, soybeans, and split peas), which are high in protein. - Cereals are also a good source of proteins. - **Maillard browning** (Non-enzymatic) is the reaction that is responsible for the brown color of baked products. - A free carbonyl group of a reducing sugar reacts with a free amino group on a protein when heated and the result is a brown color. - The reaction is highly complex and has a significant effect on the flavor of foods as well as the color. - It is known as non-enzymatic browning, because the reaction is not catalyzed by an enzyme (Maillard browning must be distinguished from enzymatic browning, which is the discoloration of damaged fruits or vegetables and is catalyzed by an enzyme such as phenol oxidase. - The Maillard reaction is favored by the following: - High sugar content - High protein concentration - High temperatures - High pH - Low water content - Maillard browning is responsible for the discoloration of food products such as powdered milk and powdered egg. - Before drying, eggs usually are "desugared" enzymatically to remove glucose and prevent Maillard browning. - The reaction causes loss of the amino acids lysine, arginine, tryptophan, and histidine, as these are the amino acids with free amino groups that are able to react with reducing sugars. - With the exception of arginine, these are essential amino acids. (The body cannot make them, and so they must be included in the diet.) - Therefore, it is important to retard the Ma