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Food, Nutrition and Health
FNH 200
Nooshin Alizadeh- Pasdar

MODULE 8: Dehydration as Food Preservation Method Terms  water activity (aw) = defines proportion of water in food in free, unbound form - range 0-1 - dehydrated foods: 0.2 – 0.6 - microorganisms cannot grow below 0.6 - chemical reactions begin at 0.3  dehydration vs. concentration - dehydration = removal of as much water from food as possible to increase storage life - concentration = some water removed to concentrate food constituents; no shelf-stable, require other forms of preseravtion  case hardening = sugars from hard case around food during rapid dehydration  mass transfer  water soluble components  sublimation = from frozen state directly to gas phase, without transition through liquid  hygroscopic = readily pick up moisture  atmospheric pressure & vacuum  different dehydration methods Reasons for dehydrating  food preservation  retention of size and shape while preserving  reduce weight and bulk for easy storage, transportation  convenience items (instant coffee, instant soup preps) * dehydration does not kill microorganisms; once rehydrated => resume growth under favorable conditions Changes in food during dehydration  cell/tissue shrinkage - as water removed cell tissue shrinks and loses elasticity - water migrates from interior of food to surface and evaporates, carried away by dehydrating medium - loss of water soluble substances => poor rehydration properties  case hardening - rapid drying causes sugars to form hard, impermeable cases around food - decrease rate of dehydration - minimized as much as possible during dehydration  chemical changes - Maillard browning: rapid in foods with water content 15-20% because reactants in close proximity - eggs treated with enzyme, glucose oxidase, which “desugars” egg and minimizes color and flavor changes caused by maillard reactions involving glucose - heat denaturation of proteins, starches, gums decrease rehydration ability of foods - dehydrated fruits sweeter than fresh because sugars concentrated on outside of fruit - higher the temperature, more loss of volatile substances => dehydrated food less flavorful Factors affecting dehydration * goal: dry as fast as possible, at least cost, fewest changes in food quality  surface area - maximize surface-to-volume ratio => minimize resistance to heat and mass transfer  temperature - maximize temperature difference between product and drying air - hotter air the better - upper limit dictated by chemical reactions at high temperatu
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