Unformatted text preview: ure of water in sample Vapor pressure of pure water Aw = 0.95 1.0 All Microorganisms (MO) can live Aw = 0.91-0.95 Important bacteria cannot survive. Aw = 0.87-0.91 Dangerous bacteria and yeast can survive Aw= 0.80-0.87 molds can survive s s s s Water Activity
Food Beef Watermelon Lettuce Pound Cake White Bread Cheddar cheese Corn syrup Aw 0.95-0.99 0.95-0.99 0.95-0.99 0.90-0.95 0.90-0.95 0.80-0.90 0.60-0.70 % water 50 93 96 18 38 41 26 Functions of Water in Food Preparation.
s Medium for Transfer of Heat. s Absorbs heat and sets up convection currents. s Equalizes temperature throughout food. s Good conductor of heat. Readily gives up heat to food. Transfer of Heat
Conduction: Transfer of energy from one molecule to the adjacent molecule in a continuous and progressive fashion. Heat goes from the source to the pan to the food. s Convection: Transfer of heat by the circulation of currents of hot air or liquid.
s Functions of Water in Food Preparation
s Dispersing Medium s Dispersed particles could be crystals, cells, liquid droplets, gas bubbles. s Continuous phase: usually water but it could be oil. Types of Dispersions
1. 2. 3.
Solutions Colloidal Dispersion Coarse Suspensions 1. Solutions
Solutions: Smallest particle size of the three types of dispersion s Example sugar solution (< 1 mmicron) s Starch and protein too big to form solutions. s Most stable of the three types of dispersions.
s 2. Colloidal Dispersions
2. Colloidal Dispersions. Particles are larger than in a solution but smaller than in coarse dispersions. s Colloidal systems, two phases = Dispersed phase & continuous phase. s Continuous phase extends throughout system s Dispersed phase: Isolated or disconnected.
s 2. Colloidal Systems
s May be a combination of solid, liquid or gas. s A. Emulsion: liquid in liquid s B. Sol: Solid in a liquid. s C. Gel: Liquid in solid s D. Foam: Gas in liquid A. Emulsions
Types of Emulsions s Oil in water emulsion: Oil dispersed phase Water continuous phase Example: Mayonnaise s Water in oil emulsion Water dispersed phase Oil continuous phase Example: Butter
s B. Sol
Dispersed phase solid s Continuous phase: liquid s Example: gravy s Sols are pourable because the continuous phase is liquid. s Temperature and concentration of solid determine these properties.
s C. Gels
Dispersed phase: Liquid s Continuous phase: Solid s Example: gelatin, custard s Syneresis: Weeping or draining of liquid from a gel s Cranberry jelly is a good example
s D. Foams
Dispersed phase: Air s Continuous phase: Liquid s Examples: egg foams, milk foams s They contribute to volume and texture of various food products.
s 3. Coarse Suspensions
s Coarse Suspensions: Mixtures of substances, larger than in colloidal suspension. s Influenced by gravity s Tend to separate s EX. oatmeal flakes in water, uncooked starch mixture....
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- Fall '10
- Bound Water