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Food, Nutrition and Health
FNH 200
Nooshin Alizadeh- Pasdar

MODULE 7: Preservation of Food by Low Temperatures Use of Low Temperatures in Food Preservation -refrigeration or cool storage = storage at temperatures above freezing -freezing and frozen storage = storage at temperatures below freezing -pure water freezes at 0°C; most foods freeze at -2°C or lower -refrigerated/cool storage generally preserve perishable foods for days or weeks -good frozen storage conditions with proper packaging can preserve foods for months or years -some psychrotrophic pathogens can grow (slowly) at refrigerator temperatures and some spoilage microorganisms can grow below 0°C provided there is unfrozen water available -below -9.5°C, no significant growth of spoilage or pathogenic organisms, and gradual decrease in living organisms; however, freezing does not lead to complete destruction and rapid growth/multiplication can occur during thawing Refrigeration/Cool storage of Foods -refrigeration one of mildest approaches to food preservation -refrigeration temperatures fall between 16°C to -2°C; food industry refrigeration temperatures should be 4°C or lower (Food and Drug Regulations of Canada) -drop of 10°C slows down rate of senescence by 2-3 times and microbial growth by 3-6 times -refrigeration temperatures inhibit growth of most disease-causing microorganisms but can favor psychrotrophic organisms -some spoilage-causing microorganisms, moulds, can grow as low as -8°C -refrigeration storage temperatures provide only short term extensions of storage life of foods -refrigeration cannot improve quality of food -refrigeration sometimes applied to achieve desirable attributes of fermented food products Optimal Conditions for Refrigerated Storage of Foods -each food commodity has own optimum refrigeration conditions for max storage life and retention of quality and nutritional value -storage life of meats, fish, and dairy products maximized by temperatures approaching 0°C -maintenance of controlled temperatures important, prevent chill injury to certain foods Ex) –banana peels yellow to black, sweet potatoes decay/pitting and internal Discolorations when stored below 13°C - apples: soggy and internal browning if stored below 1-2°C - potatoes and avocados: browning if stored below 5 and 7°C, respectively -humidity and gas atmosphere composition should also be controlled to prevent dehydration and avoiding conditions that are so humid that mould growth and food spoilage are favored -controlled and modified atmospheres in conjunction with refrigerated storage extend storage life -controlled atmosphere = condition in which atmosphere surrounding food is different from that of normal atmosphere, and composition of atmosphere constantly monitored and maintained as preset level -modified atmosphere = creation of atmospheric conditions around food that are different from normal atmosphere. Food placed in package, air removed from package by drawing a vacuum, then backflushing package with desired gas mixture (usually CO2 and N2) before sealing, or directly flush package with desired gas mixture => MAP (MA- packaging) -composition of atmosphere in modified packaged foods changes over time due to Metabolic activities of food, microorganisms, gas permeability of packaging materials -vacuum packaged processed or cured meats another example of MAP, except no backflushing with gas mixture prior to sealing after vacuum drawn => much longer storage life because removal of oxygen suppresses growth of aerobic spoilage bacteria (retail cuts of cheese) -although foods packaged via modified temperatures have extended storage life, still must be kept in refrigerated storage to maximize benefits of inhibition of spoilage and disease-causing bacteria Changes in Food During Refrigerated Storage -over time, numerous undesirable changes cause by microbial spoilage: Bread staling, loss of crispiness in fruits and veggies, change in color of fresh meat, loss of flavor and nutrient value, oxidative changes, drip or syneresis from fish Freezing and Frozen Storage of Foods -extends shelf life -temperatures well below 0°C: commercial freezing requires min -18°C, household -12 to -14°C -basis: freezing permits longer term storage than refrigerated storage because 1) lower temperatures 2) lower water activity -both factors slow down chemical and enzymatic reactions and microbial growth -may still take place, especially if freezing conditions not optimal -freezing slows microbial growth but does not kill all microrganisms; surviving organisms may resume activities once food thawed -frozen foods generally higher in aesthetic quality and nutrition; faster the rate of freezing, better the retention of nutrition and quality -freezing point = temperature at which ice crystals in equilibrium with air-saturated water at 1 atmosphere pressure; solutes in water lower freezing point  freezing point of pure water = 0°C; although does not begin to freeze until several degrees below 0°C  freezing point of foods somewhat below 0°C because foods contain solutes dissolved in aqueous (water) phase  once water begins to crystallize (freeze), abrupt rise in temperature due to evolution of latent heat of fusion (crystallization); temperatures approaches freezing environment temperatures after water frozen  considerable portion of unfrozen water below 0°C – amount depending on temperature and food product  ex) beef – 70% unfrozen water at -4°C, only 3% at -9, trace amounts at -18°C - freezing and thawing curves = describes transitions occurring n hypothetical food material during freezing, thawing, including:  removable of heat from product (sensible heat)  freezing of water (liquid) into ice crystals (solid) (latent heat of fusion)  further cooling to surrounding temperature Changes in Food During Freezing, Frozen Storage, and Thawing - rupture of cell walls and membranes during freezing and thawing can lead to drip formation with product is thawed - process of freezing great influence on quality of food once thawed - chemical, enzymatic, physical changes result of concentration of components (concentration effects) in unfrozen water phase within frozen foods:  chemical – oxidative rancidity/oxidation of flavor components, pigments, vitamins  enzymatic – enzymatic browning, lipolytic rancidity  meats become tougher due to protein denature by chemical effects and cell breakage by ice crystals - aim to promote formation of numerous tiny ice crystals rather than fewer but larger ones that cause cellular damage - ice crystal damage => loss of water from food once thawed:  drip (loss of water) found in thawed strawberries, beef due to leakage of cellular fluids and loss of water-holding capacity of food components  emulsions and dispersions destabilized by growth of numerous small ice crystals to fewer, larger, more damaging ones; such growth caused by temperature fluctuations - other undesirable changes: formation of package ice and freeze dehydration/freezer burn = produce unsightly ‘burnt’ food surfaces and loss of nutrients due to water loss - ways to minimize changes:  blanching  proper temperature control for freezing and frozen storage  appropriate packaging Factors affecting quality of frozen foods - Rate of freezing (faster the better)  small ice crystals favored 1) Temperature difference: greater the temperature between food and refrigerant, faster the freezing rate 2) Product thicknes
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