2/25/14 Density mass per volume H201iq)=0.9998(0rC) g/mL FreezeDried H,0(icc)=0.9168(0rC) Lower density expansion ice hquid Thermal Conductivity of Ice is about four times faster Thermal Conductivity of Ice is about four times faster than Liquid Water than Liquid Water Freezing Thawing +100 20 20C -20C +10C 6
2/25/14 6 H2O(liq) = 0.9998 (0ºC) H2O(ice) = 0.9168 (0ºC) For a constant weight of water: Lower density results in volume expansion Density = mass per volume = g/mL liquid ice +10C -20C +10C -20C Thermal Conductivity of Ice is about four times faster than Liquid Water Freezing -20C +10C -20C +10C Thermal Conductivity of Ice is about four times faster than Liquid Water Thawing
2/25/14 Describing Water in Foods A major Problem In the development of stable food Several approaches: 1)Water content,%H,0 (traditional) products,a major problem is getting the -simple in concept and measurement water in foods to stay in place.That means to prevent it from migrating from place to place. recend ences b -treats food as polymer matrix,water as key plasticizer -glass transition explains some imregularities in a data Eacag2oib5ntnaynpete.ogeterheyae Water Availability Relative Humidity of Food! Water content of food -Useful knowledge,but. Po Po=water vapor pressure -Does not account for hindered water above pure water food Relative Humidity=p/p.1 Need measure of water availability of water Wait for e uilibrium ther RH=ERH=p/p,x 100 Water activity (a)=p/p.0<a.<1) >
2/25/14 7 A major Problem • In the development of stable food products, a major problem is getting the water in foods to stay in place. That means to prevent it from migrating from place to place. Describing Water in Foods To manage water we must first be able to describe and quantify water that is in a food Several approaches: 1) Water content, %H20 (traditional) – simple in concept and measurement 2) Water activity, aw (1950s →) – accounts for effects of solutes and differences between foods 3) Molecular mobility, Tg (Slade & Levine, 1988) – treats food as polymer matrix, water as key plasticizer – glass transition explains some irregularities in aw data Each approach is individually imperfect, together they are complimentary. Water Availability • Water content of food – Useful knowledge, but. – Does not account for hindered water • Need measure of water availability of water Relative Humidity of Food! p p = water vapor pressure above food po water po = water vapor pressure above pure water Relative Humidity = p/ po x100 Wait for equilibrium then RH = ERH = p/po x 100 Water activity (aw) = p/po (0 < aw<1) food