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University of British Columbia
Food, Nutrition and Health
FNH 340

Heat Transfer Heat-energy from rapid vibration of molecules (a transfer of thermal energy) Temperature-measures average intensity of heat (i.e. thermal energy) Kinetic Energy-molecules that are constantly in motion Change of state-eg. ice (molecules vibrate, tightly bound) -> liquid -> gas Heat Capacity/Specific Heat of H20=1.0 (standard b/c good for absorbing heat) =amount of heat needed to raise 1g of water 1 C (1 calorie) eg. to raise 100g from 0C to 100C, need 10 000 C of heat/energy -air has higher specific heat -> takes longer to bake vs. boil food -fat + sugar SH lower -> heat faster than foods high in water -steam + oil have low specific heats -> can cause severe burns Calorie-energy value of foods (1000 cal = 1 kcal = 4.18 kj) British Thermal Unit (BTU)-heat needed to raise 1 lb of water 1 F Specific Gravity-weight of material in reference to an equal volume of water, at given T (water SG=1); calculated by ratio of weight to volume -varies with composition (eg. higher fat = lower SG b/c less non-fat solids) Latent Heat-heat absorbed to change physical state, but not change in T eg. latent heat of fusion/solidification -> 80 cal to melt 1g of water (heat gets absorbed by water, so is hidden) Ice Cream-salt/H20 mixture -> salt lowers freezing point -> ice melts at lower T by absorbing latent heat -> draws heat from mix -> lowers T -> freeze liquid Ice-water baths (to cool food rapidly)-when ice melts (at 0C), it absorbs heat + cools another substance (more effective than water) Heat of Vaporization-H20 to steam (opposite=latent heat of condensation) Steaming-condenses on surface of cooler food -> releases heat -> food absorbs Boiling-H20 vaporizes + gas escapes as steam -> pushes air at surface aside -> more gas molecules form at surface -> vapour P increases till past atm P -> bubbles break the surface -> boiling -at sea level, water boils at 100C -high elevation or in vacuum (air pumped out), less air=less P at surface=less resistance for gas -> lower b.p. -> food takes longer to cook (b/c at lower T) -used to make sugar, evaporated, condensed milk w/o changing taste -pressure cooker=opposite (vapour P >> atm P -> raises b.p. -> food cooks faster 4 types of Heat Transfer Conduction-slow, direct transfer btwn molecules (eg. element -> pan -> water/fat on bottom/sides of pan -> through food) Good pan? flat bottom, metal (Cu, Al, Fe, stainless steel-v. durable alloy but don't heat uniformly, so combine w/ Cu, Al), durable (depends on gauge of metal-# of sheets needed to equal 1 inch eg. 10 gauge is sturdy pan) Convection-heat transfer thru air/liquid from more to less dense areas eg. canning, boiling, stews, baking, deep fat frying -as gets heated, gets less dense -> rises -> replaced by more dense eg. cook in water -> convection currents move heated water molecules around food, heat transferred to food via conduction -currents do not occur in solids (food would burn) -bake food in centre of oven b/c convection currents create uniform T in centre -currents remove moist air from surface of cake in oven -> lets cake brown -if use convection oven, reduce cooking T/time to prevent over-cooking Radiation-E waves vibrate, travel at speed of light -> rays fan out over distance but less heat in contact with food -E absorbed at surface -> inside heated via conduction (rays don’t penetrate) -dull, black, rough surfaces absorb heat while shiny, white, smooth surfaces reflect (latter = slower cooking) -glass transmits radiant waves v. efficiently -> if used, lower T of oven -toasting, broiling, BBQ, baking (in oven, air absorbs some E + rises -> convection currents -> help distribute heat) -infrared radiation used to dry fruits + veggies, heat blanching, keep food warm on serving line Microwaves-cook differently from radiation Induction Heating-2 coupled circuits create a magnetic current that heats up ferrous cooking pans -> food heated via conduction -pans must be flat, cast iron, magnetic stainless steel, or enamel over steel -v. fast, easy cleanup Heat Transfer Mediums Air-roasting, baking, broiling, BBQ, grilling (dry heat methods) -baked + roasted food partly cooked by moist heat, if food high in water (surface cooked by dry convection currents + conduction from container; interior by moist heat + conduction from surface) -convection ovens use fans to circulate air -> faster -> decrease T so heat has time to reach center of food Water-boiling, parboiling, simmering, poaching, braising, stewing (simmering + poaching use T lower than 100C) -transfer via conduction (heat to pan) + convection (currents in water) -> faster b/c water better conductor than air Steam-waterless cooker (bit water for steam), pressure cooker, wrapping in foil, on rack above water in pan w/ lid -> all trap steam -steam touches cold food -> condenses -> gives off heat Fat-sauteing, stir-frying (use little fat, so conduction), pan-frying (more fat, both conduction + convection), deep fat frying (heated fat transferred to food by conduction, distributed by convection?) -effective b/c can be heated to higher T than water, also more readily absorbed by food -> changes flavor Microwave Cooking Microwaves-short, high frequency waves -non-ionizing (don’t cause dangerous chemical changes); cause thermal change -either absorbed (high moisture/fat foods), transferred (paper, glass, plastic), or reflected (metals) -commercial use: temper/thaw meat, poultry, fish; dry pasta; process chips -ovens lined w/ metal, magnetron makes microwaves-> walls reflect, food absorb -> metal stirrer breaks up + distributes waves Vapor Pressure-pressure exerted by vapour when in equilibrium w/ liquid/solid form; measures tendency to change into gas state; increases w/ T -high moisture
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