FST613 - Water PDF

WATER OBJECTIVES: At the end of this chapter, students should be able to: 1. Differentiate between water content and water activity. 2. Explain several terms related water & water activity such as equilibrium relative humidity 3. Explain the effect of water activity on rate of many chemical...

WATER OBJECTIVES: At the end of this chapter, students should be able to: 1. Differentiate between water content and water activity. 2. Explain several terms related water & water activity such as equilibrium relative humidity 3. Explain the effect of water activity on rate of many chemical reactions in foods & microbial growth. 4. Draw and explain the water sorption isotherm, desorption and adsorption 5. Describe the types of water in foods 1  INTRODUCTION Important constituents of many foods. Acts as dispersing medium (in emulsified products such as margarine) & solvent Physical properties of water & ice 2  Understanding of its properties & behaviour is necessary as it influences chemical & microbiological deterioration of foods 3  Water Molecules Arrangement The three diagrams above illustrate the distinct arrangement patterns of water molecules as they change their physical state from ice to water to gas. Frozen water molecules arrange themselves in a particular highly organized rigid geometric pattern that causes the mass of water to expand and to decrease in density. The diagram above shows a slice through a mass of ice that is one molecule wide. In the liquid phase, water molecules arrange themselves into small groups of joined particles. The fact that these arrangements are small allows liquid water to move and flow. Water molecules in the form of a gas are highly charged with energy. This high energy state causes the molecules to be always moving reducing the likelihood of bonds between individual molecules from forming.  INTRODUCTION Perishability of foods is related to moisture content Concentration & dehydration processes decrease moisture content & decrease perishability Moisture content alone is not a reliable indicator of perishability because various foods with same moisture content differ significantly in perishability. Some foods with low moisture content are unstable (eg. peanut oils with about 1% moisture content); whereas some foods with relatively high moisture content are stable (eg. potato starch at 20% moisture content). 5  INTRODUCTION Deterioration of foods due to: i. microorganisms ii. chemical reactions iii. enzymic reactions Availability of water for microbial, enzymic & chemical activity that determine the shelf-life of foods & this is measured by water activity. Availability of water in foods can be reduce by: i. Physically remove water by dehydration ii. Immobilize water using humectants as in Immediate Moisture Foods (IMF) 6  WATER ACTIVITY is a measure of how efficiently the water present can take part in a chemical/physical/microbiogical reaction. If half the water was so tightly bound to a protein molecule that it could not take part in a hydrolysis reaction the overall water activity would be reduced. Water Activity (aw) : ratio of partial pressures of water above the food to vapour pressure of pure water at the same temperature. aw = p/po p = partial pressure of water above the food po = vapour pressure of pure water at the same temp. 7  WATER ACTIVITY The tightly bound water has no tendency to escape from a food as a vapor and therefore exerts no partial pressure and has an effective water activity of zero. The amount of water binding is therefore indicated by the water activity. At high mc, when amount of water exceeds amount of solids, the aw is close or equal to 1. When mc is < that of solids, aw is < 1. Pure water has an aw of 1.00. 8  EQUILIBRIUM RELATIVE HUMIDITY The water activity of a food in equilibrium with its surroundings is also defined as the equilibrium relative humidity of the surrounding atmosphere (a fraction). aw = ERH/100 The moisture content of foods change A food will gain or lose moisture to the sorroundings At the point where the food neither gains or lose moisture ----Equilibrium Relative Humidity (RH of storage atmosphere) 9 Table 2. Approximate water activities of some common foods 1-0.95 Fresh fruit, meat, milk 0.95-9 Cheese 0.9-0.85 Margarine, 0.85-0.8 Salted meats 0.8-0.75 Jam 0.75-0.65 Nuts 0.65-0.60 Honey 0.5 Pasta 0.3 Cookies 0.2 Dried veg., crackers 10  WATER ACTIVITY & FOOD STABILITY aw affect rate of many chemical reactions in foods and rate of microbial growth 11  MICROBIAL GROWTH Minimal levels of aw require differ according to microorganisms: bacteria - 0.91 yeasts - 0.88 moulds - 0.80 However, certain species exhibit lower aw for growth: Halophilic bacteria – develop at 0.75 Xerophilic moulds – develop at 0.65 Osmophilic yeasts – develop at 0.60 Below 0.6 – almost all microbial activity inhibited Below 0.7 – most fungi inhibited Below 0.8 – most yeasts inhibited Below 0.9 – most bacteria inhibited 12  MICROBIAL GROWTH majority m/os :optimum growth: 0.92< aw0.5-0.7 speed decrease – at higher aw water is a product of condensation reaction in browning & at higher moisture content levels, browning is inhibited by “end- product dilution”. At high mc, water dilutes reactants & rate of browning decreases. Low aw restricts mobility of reactants & browning is reduced 17  OXIDATION REACTIONS Oxidation of lipids occurs at low aw due to action of free radical Above BET monolayer, antioxidants and chelating agents become soluble & reduce rate of reaction. Classify into 4 phases: aw95% of total water in high moisture food Water that existing in many foods that governs stability 28  Reaction Rates in Foods as determined by water activity Reaction Monolayer water Capillary water Loosely Bound water Enzymic Zero Low High activity Mold growth Zero Low* High Yeast growth Zero Low* High Bacterial Zero Zero High growth Hydrolysis Zero Rapid Increase High Non-enzymic Zero Rapid increase High browning Lipid High Rapid increase High oxidation *Growth starts at aw 0.7 to 0.8 29  USES OF WATER SORPTION ISOTHERM To determine rate and extent of drying To determine optimum frozen storage temperatures To determine moisture barrier properties required in packaging materials Indicates the aw at which a food is stable Allows predictions of effect of changes in mc on a w & storage stability 30