2/25/14Density=masspervolumeH,O(liq) = 0.9998 (0°C)= g/mLH,O(ice)= 0.9168 (0°C)Freeze DriedFor a constant weightof water.FreezDriedLower densityresults in volumeexpansionIceliquidThermal Conductivity of Ice is about four times fasterThermal Conductivity of Ice is about four times fasterthan Liquid Waterthan Liquid WaterThawingFreezing+10C-20C-20C+10C-20C+10C
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
2/25/14DescribingWaterinFoodsAmajorProblemTo manage water we must first be able to describe andquantify water that is in a foodSeveral approaches:.Inthedevelopmentofstablefood1)Watercontent.%H0(traditionalproducts,amajorproblemisgettingthe simple in concept and measurement2) Water activity,aw (1950's →)water in foods to stay inplace.Thataccounts for effects of solutes anddifferences betweenmeanstoprevent itfrommigratingfromfoodsplacetoplace.3) Molecular mobility, T, (Slade & Levine, 1988)-treatsfoodaspolymermatrix,wateraskeyplasticizer- glass transition explains some irregularities in adataEach approach is individually imperfect, together they arecomplimentaryWaterAvailabilityRelativeHumidityof Food!:Water content offoodp=watervaporpressureabove foodPo--Useful knowledge, but...DP, = water vapor pressureabove pure water=DoesnotaccountforhinderedwaterwaterRelative Humidity = p/ pax100food.NeedmeasureofwateravailabilityofwaterWait for equilibrium thenRH= ERH= p/p, x 100Water 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