Food components in food sciences (basic food chemistry) What “things” can we find in food? What are their chemical basis? How do these change during processing etc. What are food science implications? What happens during egg cooking? Why color changes during heating of meat? Why oil can go rancid? Where do flavor of toast bread come from?

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You can guess water content in food Beef (? ) Chicken meat ( ? ) Apples (? ) Jam (? ) Honey (? ) Milk powder (? )

Water: basic chemical and functional properties Chemical and physical properties that affect behavior (of food) during formulation, processing and storage. Determined by the functional groups in the food (i.e. interactions). Water functional properties: Diluent and carrier of hydrophilic ingredients Medium for chemical and enzymatic reactions Medium for dispersing and solvent actions Others: components of gels and emulsions, medium for heat transfer, as plasticizer, etc.

Food molecules can form hydrogen bonds with water (thus they can be dissolved or dispersed in water): a concept of solubility. Hydrophilic compounds: compounds that hydrogen bond easily to water to form solutions or colloidal dispersion. These are charged or polar molecules themselves (e,g. many biomolecules such as minerals, salts, vitamins, sugars, complex carbohydrates, amino acids, proteins). Water dissolves these substances by hydrating them. Hydration?: a process by which water molecules surround, and interact with solutes by acting as a solvent (see fig 4.11). Thus, water acts as a carrier for hydrophilic substances., as well as diluents of food ingredients.

The meaning of hydration

Amphiphilic molecules (e.g. proteins, certain vitamins, phospolipids, Sterols) contain both hydrophilic and hydrophobic regions in their structures. In water, these form micelles, i.e. Clusters of molecules in which the hydrophobic groups are directed away from the water, while the polar (charged) groups are exposed on the external surface (see fig). The non-polar hydrophobic groups form stable inner core due to Hydrophobic interactions. Thus micelles are stabilized structures of amphiphilic molecules.

oil

Water activity and moisture? Water is chemically the same but can exist in different forms. Moisture: absolute amount of water present, as a component relative to all solid constituents (such as protein, carbohydrate etc), versus Water activity has to do with the forms: (water exists in different forms, free or chemically bound). Free: lightly entrapped, and thus can be pressed/squeezed from food, the Expressed water can be seen and felt. This water acts as dispersing agent and solvent and can be removed by drying. e.g. the juice in an orange, or the water that sometimes separates in sour cream or yogurt.

(b) Adsorbed water or structural water: Associates in layers via intermolecular hydrogen bonds around hydrophilic (?) Food molecules. Water that is attached to the surface of molecules like polysaccharides and proteins. It is not readily squeezed out of the food. When food scientists refer to the hydration of proteins, such as gluten, and carbohydrates, such as starch, they are referring to adsorbed water. (c) Bound water: Tight chemical bound, within crystalline structure, via water-ion and water- Dipole interactions. It does not exhibit typical properties of water; fail to Freeze at 0 °C, or act as solvent. Water is physically trapped within crystals, such as crystalline starch, or other substances in food (some food scientists prefer to define just the free and bound forms of water). The important point is that free and adsorbed water can promote the growth of microorganisms, while bound water cannot.

Water activity is a measure of the amount of free and adsorbed water in food, and is measured on a dimensionless scale of zero to one. Water activity is the ratio of the vapor pressure (P) of water in food divided by the vapor pressure of pure water (P0) at the same temperature. The water activity of pure water is equal to 1.0

Water activity or aw definition: A measure of availability of water molecules to enter into microbial, Enzymatic or chemical reactions. This availability determines shelf life of a Food. Bound water is inversely related to aw . More bound water, lower aw.

Which food is more stable? Why? HOT 1 Water activity for jam is about 1.20! 0.25 0.77 0.95 HOT 2 Water activity for meat is about 1.20! 0.25 0.77 0.95 Which food is more stable? Why?

Food stability map. The rate of most reaction decreases rapidly * Rate of lipid oxidation may increase at very low water activities. Lipid oxidation is obviously not a conventionally aqueous chemical reaction so it is surprising that it is as affected as it is. The rate of reaction is expressed logarithmically on the y-axis so a small change on the graph means a very large change in reality. Food stability map. The rate of most reaction decreases rapidly With decreasing water activity. aW * Less water able to act as a solute for the reaction

Changes in physical properties often follow changes in aw. e.g. Water activity is related to the texture of food. The amount of moisture in food determines the mobility of molecules in the food. Proteins and polysaccharides provide the structure of food. If they are rigid the food will be hard, but if they are flexible the food will be soft. In this case water acts as a plasticizer (a softening agent). For example baked goods can undergo a crisp to soft transition as water activity and powders can begin to clump.