BWD22303 Food Chemistry and Analysis PDF

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Universiti Tun Hussein Onn Malaysia

Norhayati Binti Muhammad

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food chemistry food science water activity food stability

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This document is a set of lecture notes on Food Chemistry and Analysis, specifically focusing on the fundamental properties of water, moisture content and water activity, and how these relate to food stability. The document contains detailed information on topics such as physical properties of water, and water activity in foods.

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BWD22303 Food Chemistry and Analysis By Name / Title / Address BWD22303 Food Chemistry and Analysis ASSOC. PROF. ChM Dr. NORHAYATI BINTI MUHAMMAD Department of Technology and Natural Resources Faculty of Applied Sciences and Technolo...

BWD22303 Food Chemistry and Analysis By Name / Title / Address BWD22303 Food Chemistry and Analysis ASSOC. PROF. ChM Dr. NORHAYATI BINTI MUHAMMAD Department of Technology and Natural Resources Faculty of Applied Sciences and Technology Universiti Tun Hussein Onn Malaysia Pagoh Educational HUB KM 1, Jalan Panchor 84600 Pagoh Johor, MALAYSIA 069742088 0127237295 [email protected] 4.1 Fundamental properties and structure 4.3 Moisture determination by loss on drying, distillation, and chemical reaction 4.4 Direct and indirect moisture method 4. WATER 4.2 Water activity and food stability 4.1 Fundamental Properties and Structure Introduction Water (moisture) is the predominant constituent in many foods. Because of the importance of water as a food constituent, an understanding of its properties and behavior is necessary. The presence of water influences the chemical and microbiological deterioration of foods. 4.1 Fundamental Properties and Structure Physical Properties of Water 4.1 Fundamental Properties and Structure Physical Properties of Ice 4.1 Fundamental Properties and Structure Water Molecule A water molecule consists of 1 atom of oxygen and 2 atoms of hydrogens. Oxygen in water molecule has six (6) valence electrons. 4.1 Fundamental Properties and Structure Water Molecule The six valence electrons of oxygen in a water molecule are hybridized to sp3 orbitals. 4.1 Fundamental Properties and Structure Water Molecule Non-bonding electrons pairs The six valence electrons of oxygen in a water molecule are hybridized to four sp3 orbitals that are elongated to the corners of imaginary tetrahedron. Two hybrid orbitals form O-H covalent bonds with a bond angle of 104.5o for H-O-H, whereas the other 2 orbitals hold the non-bonding electron pairs. The O-H covalent bonds, due to the highly electronegative oxygen, have a partial (40%) ionic character. 4.1 Fundamental Properties and Structure Water Molecule Each water molecule is tetrahedrally coordinated with four other water molecules through hydrogen bonds. The two unshared electron pairs (sp3 orbitals) of oxygen act as H-bond acceptor sites, and the H-O bonding orbitals act as hydrogen bond donor sites. 3.1 Fundamental Properties and Structure Water Molecule The simultaneous presence of two acceptor sites and two donor sites in water molecules permits association in a three-dimensional network stabilized by H-bridges, which explains the physical properties of water. The polarization of H-O bonds is transferred via hydrogen bonds and extends over several bonds. Therefore, the dipole moment of a complex consisting of increasing numbers of water molecules (multi-molecular dipole) is higher as more molecules become associated and is certainly much higher than the dipole moment of a single molecule. Thus, the dielectric constant of water is high 4.1 Fundamental Properties and Structure Water Molecule Proton transport takes place along the H-bridges. It seems the jump of a proton from one water molecule to a neighboring water molecule. In this way a hydrated H3O+ ion is The transition of a proton from one formed with an exceptionally strong oxygen to the next occurs extremely hydrogen bond (dissociation energy about 100 kJ/mol. rapid (v>1012/s). 4.1 Fundamental Properties and Structure Liquid Water and Ice Due to the pronounced tendency of water molecules to associate through H-bridges, liquid water and ice are very different in terms of the distance between molecules, coordination number and duration of stability. 4.1 Fundamental Properties and Structure Liquid Water and Ice Stable ice is formed at 0oC and 1 atm pressure, with the coordination number of 4, and the O-H~~~O (nearest neighbor) distance is 0.276 nm, and the H-atom between neighboring oxygen is 0.101 nm from the oxygen to which it is bound covalently and 0.175 nm from the oxygen to which it is bound by a hydrogen bridge. The H-O-H angle is 109.5o. 4.1 Fundamental Properties and Structure Liquid Water and Ice Why coordination number increase when ice change to water? liquid water has a smaller bond angle than ice, the molecules can be packed together more tightly, and so the coordination number or, in other words, the average number of nearest neighbors is higher for water than for ice. 4.1 Fundamental Properties and Structure Liquid Water and Ice Why ice is less dense than water? (density of ice= 0.9168 g/cm3; density of water = 0.9998 g/cm3) When the coordination number decreases, the density will be decreased. 4.1 Fundamental Properties and Structure Liquid Water and Ice Why ice is less dense than water? (density of ice= 0.9168 g/cm3; density of water = 0.9998 g/cm3) The reason is “hydrogen bonds”. As water cools, so the hydrogen bonds align to cause the water molecules to become aligned and each molecule takes up more space, so the solid is less dense. 4.1 Fundamental Properties and Structure Summary Water is a structured liquid with a short-range order. The water molecules, through H-bridges, form short-lived polygonal structures which are rapidly cleaved and the reestablished giving a dynamic equilibrium. These fluctuation explain the lower viscosity of water (which could not be explained if H-bridges were rigid. The hydrogen-bound water structure is changed by solubilization of salts or molecules with polar and/or hydrophobic groups. 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Content of Food Varies greatly as shown in the table below: 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Content of Food Water is a major constituent of most food products. The approximate, expected moisture content of a food can affect the choice of the method measurements. It can also guide the food analyst in determining the practical level of accuracy required when measuring moisture content, relative to other food constituents. 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Forms of Water in Foods The ease of water removal from foods depends on how it exists in the food products. The 3 states of water in food products are: 1. Free water 2. Adsorbed water 3. Water of hydration 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Forms of Water in Foods The 3 states of water in food products are: 1. Free water: This water retains its physical properties and thus acts as the dispersing agent for colloids and the solvent for salts. 2. Adsorbed water: This water is held tightly or is occluded in cell walls or protoplasm and is held tightly to proteins. 3. Water of hydration: This water is bound chemically, for example, lactose monohydrate, also some salts such as Na2SO4.10H2O 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Loss On Drying This is the primary method used for moisture determination. The sample is heated under specified conditions, and the loss of weight is used to calculate the moisture content of the sample. The amount of moisture determined is highly dependent on the type of instruments used, condition within the oven and the time and temperature of drying. 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Loss On Drying This is the primary method used for moisture determination Calculation % Moisture (wt/wt) = 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Loss On Drying 1. Oven Drying Method Forced draft oven Vacuum oven Microvave Analyzer 2. Infrared Drying 3. Rapid Moisture Analyzer Technology Moisture Analyzer 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Distillation Distillation techniques involve co-distilling the moisture in a food sample with a high boiling point solvent that is immiscible in water (eg: toluene, xylene, benzene), collecting the mixture that distills off, and then measuring the volume of water Two distilling procedures are in the use today; i. Direct distillation ii. Reflux distillation 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Distillation Two distilling procedures are in the use today; i. Direct distillation – eg: sample heated in mineral oil ii. Reflux distillation – eg: sample reflux with toluene (Bp 110oC) Bidwell-Sterling moisture trap Direct distillation Reflux distillation 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Chemical Reaction The well-known chemical reaction method used is Karl Fisher titration method. This method is particularly adaptable to food products that show erratic results when heated or submitted to vacuum. This is the method of choice for determination of water in many low-moisture foods such as dried fruits and vegetables, candies, chocolate, roasted coffee, oil & fats or any low-moisture food high in sugar or protein. Direct distillation Reflux distillation 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Chemical Reaction This method involves the reduction of iodine by SO2 in the presence of water, 2H2O + SO2 + I2  C5H2SO4 + 2HI Reflux distillation 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Chemical Reaction Before the amount of water found in a food sample can be determined, a KFR water (moisture) equivalence (KFReq) must be determined, 36𝑔𝑔𝑔𝑔2𝑂𝑂 𝐻𝐻2𝑂𝑂. 2𝐻𝐻2𝑂𝑂 𝑋𝑋 𝑆𝑆 𝑋𝑋 1000 𝐾𝐾𝐾𝐾𝐾𝐾𝐾𝐾𝐾𝐾 𝑚𝑚𝑚𝑚 = 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 𝐶𝐶 𝐻𝐻 2 4 4 6𝑂𝑂 𝑚𝑚𝑚𝑚 𝑔𝑔 230.08 𝑋𝑋 𝐴𝐴 𝑚𝑚𝑚𝑚𝑚𝑚 Where: KFReq = Karl Fisher reagent moisture equivalence S = weight of sodium tartrate dihydrate (g) A = ml of KFR required for titration of sodium tartrate dihydrate 4.3 Moisture determination by loss on drying, distillation, and chemical reaction Moisture Determination By Chemical Reaction Once the KFReq is known, the moisture content of the sample is determineds as follows: 𝐾𝐾𝐾𝐾𝐾𝐾𝐾𝐾𝐾𝐾 𝑋𝑋 𝐾𝐾𝐾𝐾 %𝐻𝐻2𝑂𝑂 = 𝑋𝑋𝑋𝑋𝑋 𝑆𝑆 Where: KFReq = Karl Fisher reagent moisture equivalence Ks = ml of KFR used to titrate sample S = weight of sample (mg) 4.4 Direct and indirect moisture method Dielectric Infrared Freezing Method Hydrometer Pycnometer Refractometer Analysis Point 4.2 Water Activity and Food Stability Definition What is water activity? Water activity (aw) is the measure of the energy status of water in a system Higher activity = More energy = Water can do more (microbial growth, moisture migration, chemical & physical reactions) Differences in aw will dictate how moisture will move Higher aw  Lower aw 4.2 Water Activity and Food Stability Definition What is water activity? Water activity (aw) = p/po 4.2 Water Activity and Food Stability Definition What is water activity? aw = 4.2 Water Activity and Food Stability 4.2 Water Activity and Food Stability Moisture Content vs Water Activity Bound water 4.2 Water Activity and Food Stability Moisture Content vs Water Activity 4.2 Water Activity and Food Stability Shelf stability means the product ’won’t get moldy, but it also affects the food’s texture, moisture migration and caking and clumping. With aw = 0.3, the product is most stable with respect to lipid oxidation, non-enzymatic browning, enzyme activity, and of course, the various microbial parameters. As aw increases toward the right, the probability of the food product deteriorating increases. 4.2 Water Activity and Food Stability Microbial Growth Water activity (aw), unlike water content, can determine a food’s shelf stability. It can predict which microorganisms will be potential sources of spoilage and infection. 4.2 Water Activity and Food Stability Microbial Growth 4.2 Water Activity and Food Stability Osmotic dehydration It happens when the addition of additional solute to the food products. Their penetration of solute can be performed by moist infusion or by dry infusion. Moist infusion consists in soiling the food pieces in a water/solute solution of lower aw while dry infusion involves direct mixing of food pieces and solute in required proportions. When water/rich solid products, such as fruit and vegetables, are subjected to moist or dry infusion, three flows arise: a water outflow, from product to the environment; a solute flow, from the environment to product; an outflow of the product’s own solutes. By controlling these above complex exchanges it is possible to conceive different combinations of water loss and solid gain, from a simple dewatering process (with substantial water removal and only marginal sugar pickup) to a candying or salting process (in which solute penetration in favored and water removal limited) 4.2 Water Activity and Food Stability Moisture Sorption Isoterm The food sorption isotherm describes the thermodynamic relationship between water activity and the equilibrium of the moisture content of a food product at constant temperature and pressure. 4.2 Water Activity and Food Stability 4.2 Water Activity and Food Stability 4.2 Water Activity and Food Stability Class Activity Question 1: The results obtained: Weight of dried pan and glass cover =1.0376 g Weight of pan, liquid sample & glass cover = 4.6274 g Weight of pan, dried sample & glass cover = 1.7321 g What is the moisture content of the sample? 80.65% Class Activity Question 2: A company is producing dried fruits and wants to ensure they have a long shelf life. Explain how controlling water activity can help achieve this goal. Identify two methods the company could use to lower the water activity in the dried fruits and explain how each method works. 80.65% Class Activity Question 3: Question 4: A food product has a high water activity (aw) of To reduce the water activity in a food 0.95. What preservation method would be most product like jerky, which process would effective for extending its shelf life? you apply? o A) Freezing o A) Add more water o B) Adding salt o B) Increase fat content o C) Heating briefly o C) Dehydrate the product o D) Storing at room temperature o D) Refrigerate it immediately 80.65%

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