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Unit 3 Notes part 1.docx

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University of Guelph
Food Science
FOOD 2010
Massimo Marcone

Unit 3 Notes Chapter 4 Review: 4.1, 4.2 and 4.3 if you are not familiar with chemical bonds, reactions and functional group nomenclature (not responsible for specific material from these sections) 4.4 The Chemical and Functional Properties of Water Functional Properties: the physical and chemical properties of food molecules that affect their behaviour in foods during formulation, processing, and storage. E.g. flavour, texture, physical condition - they are determined by functional groups in foods and many depend on weak bonds (e.g. hydrogen) Water Molecular Structure: - a charge separation (dipole) occurs where the O atom has a partial - charge and the H atom has a partial + charge Waters functional properties in food are that it provides a medium for chemical/enzymatic reactions and the following: Solvation and Dispersing Action: - food molecules form H bonds with water and therefore dispersed or dissolved (Solubility) Hydrophilic Compounds: compounds that hydrogen bond easily to water to form solutions or colloidal dispersions Hydration: (considered a solvation) the process by which water molecules surround and interact with solutes by acting as a solvent. Water acts as a carrier for hydrophilic substances, and a diluent Amphiphilic molecules: e.g. proteins, vitamins, phospholipids contain both hydrophilic and hydrophilic regions Micelles: stable structures of amphiphilic molecules, which form clusters of molecules in which the hydrophobic groups are directed away from the water and form a stable core while the polar (charged) groups are exposed on the external surface non-covalent interactions: include hydrogen bonding, ionic interactions, and hydrophobic interactions (micelle structure) Water activity and moisture:  1. Moisture: the amount of water present in a food, relative to all the solid constituents (e.g. proteins) 2. Water activity: (aw orAw) measure of the availability of water molecules to enter into microbial, enzymatic, or chemical reactions, which determines the shelf life. (or measure of relative humidity) - temperature dependent – increases with increasing temperature - as the amount of bound water in a food increases, the water activity decreases (inverse relationship) - Aw is calculated as the ratio of water vapor pressure of the substance divided by the vapor pressure of pure water at the same temperature (Po) :Aw = P/Po - therefore water activity of pure water = 1.0 - relative humidity RH(%) = 100 xAw Water Sorption Isotherms: graphs of data that interrelate the water (moisture) content of a food with its water activity at a constant temperature - indicates the water activity at which the food is stable - allows predictions of the effect of changes in moisture content on water activity and therefore on storage stability - temperature dependent - used to determine the rate and extent of drying, the optimum frozen storage temperature, and the moisture barrier properties required in food packaging materials Forms of water in food: 1. Free water: most water in food, water that is lightly entrapped and therefore easily pressed from food matter.Acts as dispersing agent and solvent, can be removed by drying food 2. Absorbed water: associates in layers via intermolecular hydrogen bonds around hydrophilic molecules 3. Bound water: a.k.a water of hydration, tightly chemically bounded, such as with crystalline structure, via water-ion and water-dipole interactions. Does not exhibit the typical properties of water (does not freeze at 0 degrees C) Water as a component of Emulsions: Emulsion: type of colloidal dispersion, a system containing two liquids or phases that normally do not mix: a dispersed phase and a continuous phase - water can function as a dispersed OR continuous phase - water as an emulsion is hydrophilic in character and fat phase as an emulsion is lipophilic (hydrophobic), therefore presence of water creates emulsion Water and Heat Transfer: - acts as an important vehicle for heat transfer in food during food processing operations and in food preparation - water possesses kinetic energy when the temperature is above 0 K (-273C) & with the addition of heat the Ke increases - water can act as a conductor of thermal energy to food molecules, (Heat transfer) Water as an Ingredient: - water commonly incorporated into processed food/low fat foods - increasing water content of food can reduce quality as water can act as a solvent and change state with temperature (freeze/thaw) and exhibit motion in a food system - movement of water must be stabilized in frozen foods – freeze-thaw cycles can result in the production of concentrated and diluted portions of previously homogeneous product when freeze-thaw stability is poor Water as a Plasticizer: - water acts as a plasticizer: lowers the glass transition temperature when added to a polymer food system especially when there is low moisture or food is frozen - acts as a food system softener, increasing food polymer molecular volume as well as mobility e.g. increasing water content in starch Glass transition temperature (Tg): temperature at which a change in the physicochemical state and the mobility of the water and polymer molecule constituents of a food occurs - decrease of Tg and increase ofAw show a linear relationship 4.5 The Chemical and Functional Properties of Food Acids - first sour substance: fermentation of cider & honey created 10% acetic acid (contains COOH group) - proteins are composed of amino acids – responsible for shape, functionality & nutrition quality - fats are composed of fatty acids e.g. citric acid, phosphoric acid in cola drinks, tartaric acid in leavening systems of baked goods Food Acid Structure - typical food acid is a carboxylic acid (COOH) – acids lacking this group, like phosphoric acid, are inorganic acids - acids influence flavor and act as antimicrobial agents that extend the life of foods - some foods that must remain free-flowing have acids added to them because acids exhibit low hygroscopicity: a low attraction for moisture - functions of food acids are related to their molecular size and structure - acids that possess double bonds are called alkenes and are less polar molecules (e.g. fumaric acid) therefore less soluble in water Acid Strength - food acids donate (lose) protons and are all considered weak acids Weak acids: mainly in the form of –COOH but a small amount has H+ separated to form COO- and H+ Strong acids: have large amounts of dissociated ions Weak acids: have a small ionization constant, Ka - Ka is given for the reaction HA↔H+ + A-, where HAis the unionized acid, and H+ (proton) andA- (the conjugate base or anion of the original acid) are ionized species derived from the HA acid - pKa can also be measured, it is the inverse of Ka, therefore the lower the pKa, the stronger the acid Fumaric Acid and Dough Softening - when added to flour dough, fumaric acid softens it - under typical oxidizing conditions of flour mixing, baking and storage, the sulfhydryl groups of the amino acid cysteine (found in flour protein) become oxidized, losing hydrogen. - thus the remaining sulfur atoms on each systeine can join, forming disulfide bonds (-s-s-) between cysteine amino acids in flour dough proteins - these bonds make the dough structure
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