《食品化学》课程教学课件（国外讲稿）02 water

2/25/14 Fennema's Food Science Technology Food Chemistry Fourth Edition ·Food preservation: food safety food quality

2/25/14 1 Food Science & Technology • Food preservation: food safety food quality

2/25/14 2 Water and lce Purt III Food Systems. 多 Davd s Reid and Owen R.Fennema Less important CONTENTS r'5a 12 r加c作okan ra6som0og 21 Mo 213.1 The lle 2

2/25/14 2 Less important

2/25/14 Food Safety Food Quality ·Chemical Nutrition ·Biochemical ·Sensory flavor ·Microbiological appearance texture Water Food Water is the principle component of foods Water functions as a medium for chemical reactions. Water functions as a medium for microbiological growth

2/25/14 3 Food Safety • Chemical • Biochemical • Microbiological Food Quality • Nutrition • Sensory flavor appearance texture Water & Food • Water is the principle component of foods. • Water functions as a medium for chemical reactions. • Water functions as a medium for microbiological growth

2/25/14 Phases or States of Water Phases or States of Water ·Solid(ice) State of water depends on temperature and pressure. ·Liquid At constant pressure it requires heat to change temperature and state. ·Gas or vapor At constant temperature,it requires changes in pressure to change state. Temperature change Phase change ercee2gacwaeraequeuoasee ·Ice to liquid water requires80 calg at0℃ ·10 g ice at0℃requires: Toraise the temperature of 100g water from 10to20c 10x100=1000 calories 10 x 80=800 calories One calorie is not the same as one food Calorie ·“Ice Diet" .1 Calorie(food calorie)=1000 calorie one meter. 800 Calories will melt 8,000g ice 4

2/25/14 4 Phases or States of Water • Solid (ice) • Liquid • Gas or vapor Phases or States of Water • State of water depends on temperature and pressure. • At constant pressure it requires heat to change temperature and state. • At constant temperature, it requires changes in pressure to change state. Temperature change • 1 calorie of heat per gram of water is required to raise the temperature of water one ºC. • To raise the temperature of 100 g water from 10 to 20 ºC requires 10 x 100 = 1000 calories • One calorie is not the same as one food Calorie • One Joule is 0.2390 cal (1 J = 1N . M) 1 J is the energy required to lift a small apple one meter straight up. Phase change • Ice to liquid water requires 80 cal/g at 0 ºC • 10 g ice at 0 ºC requires: 10 x 80 = 800 calories • Ice Diet • 1 Calorie (food calorie) = 1000 calorie • 800 Calories will melt 8,000g ice

2/25/14 Phase diagram of Water Temperature control Liquid Ice and liquid water mixture I atm Temperature =0C Solid Vapor Temperature Liquid Pressure cooke 121 Liquid or 1 atm 0℃ 100C Retort 1 atm 0℃ 100rC Solid Solid Vapor Vapor or freeze dricd Temperature Temperature 5

2/25/14 5 Temperature control • Ice and liquid water mixture Temperature = 0 ºC • Liquid water and steam Temperature = 100 ºC • At normal pressure (one atm) the temperature cannot go above 100 ºC with liquid water present. Temperature Pressure 1 atm 0ºC 100ºC Phase diagram of Water Temperature Pressure 1 atm 0ºC 100ºC 121 ºC Temperature Pressure 1 atm 0ºC 100ºC Sublimination or freeze dried

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

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

2/25/14 Water Movement Water Sorption Curve (same food with different moisture contents) 05 0.9 ERH=50%or90%? 50%<ERH<90% 02 0.6 0.8 Aw Table 1.Water Content of Various Foods Dried to aw=0.70 Case Study #1:Raisin Bran Multi-component food in same package Food -want flakes crispy.raisins moist throughout shelf life Nuts Fundamental problem:water migration Whole milk powder 49 y vaetenere high aw low aw Dried lean meat and fish Raisin Bran Wheat flour,noodles 13-15 Dried soup mixes Raisins:high a Dried fruits Flakes:low aw 8

2/25/14 8 Water Movement (same food with different moisture contents) ERH = 50% or 90% ? 0.5 0.9 50% < ERH < 90% Water Sorption Curve Case Study #1: Raisin Bran • Multi-component food in same package – want flakes crispy, raisins moist throughout shelf life • Fundamental problem: water migration – Water activity values determine direction of water migration – high aw low aw Raisin Bran Raisins: high aw Flakes: low aw

2/25/14 P/B Texture and water 7 0.4 06

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2/25/14 Probable Spoilage Table 4.Water Activity of Confectionery Products Food Type of Product Water Activity 0100 Bacteria Cottage cheese,fresh meat Fondant 07508 .85-0.90 ia and molds rgar Fruit jellies ed condensed milk 0.63-0.7 ed bu 0.60-0.70 .80-0R5 053-0.66 0.75-0.80 ohilic molds Dried figs 0T10 Molds and yeasta Gu and pastilles 0.51-0.64 Confections colate 0.37-0.50 070075 0.65-0.70 Toffee <0.48 0.60-0.65 Dried fruit Boiled sweets ≤0.30 Osmophilic yeasts Ho Chemical reaction rates are affected by water activity General Technology Caramels (2%Water) 4.Fondant (10-12%Water) Low Molsture,Fine Sugar Crystals Set Foam uaat (7-12%Water) Fine ug 8.Hard Panned Coa ng (0%Waer)Sugar Crystallza ed (7%Vhter)Low Moisture,Wheat Guter 0 0.8 10.Cream Pastes (7-8%Water) Water Activity 10

2/25/14 10 General Technology Structure Body Derived From 1. Hard Candy (1.5% Water) High Viscosity Glass 2. Jellies – Various (25% Water) High molecular weight colloidal network 3. Caramels (2% Water) Milk Proteins 4. Fondant (10-12% Water) Very Fine Sugar Crystallization 5. Lozenges – Compressed & Extruded (0.5%Water) Low Moisture, Fine Sugar Crystals 6. Marshmallow (14-25% Water) Colloidally Set Foam 7. Nougat (7-12% Water) High Viscosity – Glass Foam or Fine Sugar Crystallization Foam 8. Hard Panned Coating (0.2% Water) Sugar Crystallization 9. Liquorice – Jelly Related (7% Water) Low Moisture, Wheat Gluten 10. Cream Pastes (7-8% Water) Sugar Crystallization and Colloidally Set Water Activity Rate of reaction or growth Chemical reaction rates are affected by water activity