食品化学英文.pptVIP

* Thermodynamics of Binding DG=DH-TDS Entropy cost to solvent (DS0, -TDS0) STRONG Enthalpy gain to solvent (DH0) Net DG0; reaction proceeds D:\lecture notes\Lectures- UCLA BC 205 - Biol_ Chem_ and Nut_ Lecture.htm Non Polar Solutes Weak transient dipole (Van der Waals attraction) Strong structuring of water into a hydrogen bonded clathrate cage – entropy cost D:\lecture notes\Lectures- UCLA BC 205 - Biol_ Chem_ and Nut_ Lecture.htm Non-polar Solutes OIL H O H H O H H O H H O H H O H H O H Effect on Storage Life Drying and/or storage at low temperatures are among the oldest methods for the preservation of food with high water contents. Water Activity 1952,W.J.Scott The storage quality of food does not depend on the water content, but on water activity( Aw ). Aw=P/P0=ERH/100 P=partial vapor pressure of food moisture at temperature T P0=saturation vapor pressure of pure water at T ERH=equilibrium relative humidity at T The typical water activity of some foodstuffs Type of product Water Activity (aw) Fresh meat and fish .99 Bread .95 Aged cheddar .85 Jams and jellies .8 Plum pudding .8 Dried fruit .6 Cookies .3 Milk powder .2 Instant coffee .2 The relationship between water content and water activity is indicated by the sorption isotherm of a food. At a low water content(50%), even minor changes in this parameter lead to major changes in water activity. The course of a drying process lies slightly above the adsorption isotherm pertaining to the storage of moisture-sensitive food. Moisture Sorption Isotherm aw Moisture content (d.w.b.) http://www.pentech.ac.za Moisture Sorption Isotherm aw Moisture content (d.w.b.) Moisture Sorption Isotherm aw Moisture content (d.w.b.) Zone 3 Zone 2 Zone 1 Zones in Isotherms Zone 3: Bulk water Zone 2: Loosely bound water Zone 1: Tightly bound water. Temperature Dependency aw Moisture content (d.w.b.) cold hot Moisture content (d.w.b.) Sorption and Desorption desorption sorption Hysteresis: Typi