Mod. 6.docx

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University of British Columbia
Food, Nutrition and Health
FNH 200
Nooshin Alizadeh- Pasdar

MODULE 6: Thermal Preservation of Foods Thermal Preservation - thermal preservation = extending shelf life by use of high temperatures to inactivate undesirable disease and spoilage-causing microorganisms and inactivate enzymes in food that cause spoilage  Blanching - mainly to veggies and fruits - expose to heat or boiling water or culinary steam for short period of time 1. Inactivate enzymes in plant tieeus to enzymatic degradation does not occur 2. Wilt veggie products to enable packaging and achieve proper weights 3. drive off inter-/intracellular oxygen and other gases so containers not deformed  Pasteurization - temperatures of at least 72°C for 15 sec - HTST – high temperature short time - basis: inactivate pathogenic bacteria and viruses in low acid foods (ex. milk) - in low-acid and acid foods, many spoilage-causing microorganisms can still survive pasteurization; therefore, not all psychrotrophic bacteria killed must refrigerate (milk) - durable life date = expiry date  Commercial sterilization - minimum treatment 121°C moist heat for 15 min for each food particle - canning (1800s, “botulinum cook”) before heating, or heating before aseptically packaged (UHT-Aseptic packaging) - basis = CS to destroy both spoilage and disease causing microorganisms in low-acid and acid foods => food “commercially sterile” - time to achieve sterility depends on size of can since heat transfer to food happens at slower rate - shelf life 2+ years  Ultra-high temperature (UHT) and aseptic packaging - application of ultra-high heat to food before packaging food into pre-sterilized containers (cartons of laminated plastic, aluminum and paper, plastic cans, flexible pouches, etc.) in a sterile atmosphere - no need for refrigeration - decrease in processing time – 140-150°C for 4-6 sec of direct injection of steam and then cooled in vacuum chamber water from steam flushed out - higher quality product - shelf life 6 months + without fridge - liquid products: milk, juices, cream yogurt, etc. - no added agents; solely application of heat and packaging under aseptic conditions - not all UHT products aseptically packaged => advantage of longer storage life at refrigeration but not shelf-stable at ambient temperatures due to post- processing recontamination Selection of Heat Treatments - main considerations in selecting required temperature-time conditions for thermal processing: 1. Objective /purpose? (blanching, pasteurization, or commercial sterilization) 2. Any additional preservation steps (combined with other methods?) 3. Physical, chemical properties of food (type of food) 4. Heat resistance of microorganisms in food? - examples:  Foods consumed shortly after processing can use pasteurization and refrigerated storage  Longer storage times at ambient temperatures in evacuated sealed containers requires commercial sterilization  Time-temperature combination for pasteurization and CS determined be most heat- resistant disease and spoilage-causing microorganisms in particular food  Type of thermal processing and rate of heat penetration into slowest heat portion of food in container governed by food’s physical and chemical properties; slowest heating portion of food must receive specified time-temperature thermal treatment - low acid foods biggest concern is Clostridium botulinum (soil, water, mud => agricultural foods)  Destruction of C. botulinum spores determined via “inoculated pack studies” using a non-pathogenic spore-forming bacterium, C. sporogenes PA3679 – more resistant to heat than C. botulinum so process killing PA3679 will guarantee death of C. botulinum spores with wide margin of safety Thermal Death Curves - microbial death during thermal processing follows logarithmic order – killed at a rate nearly proportional to number present in system being heated - survivor curve or thermal death rate curve: logarithmic order of death; - decimal reduction time (D-value) – time taken (one logarithmic cycle) at specific, constant temperature to kill 90% of microbial pop. - D-value useful index of heat resistance of particular microorganism; apply to specific microorganism under specific conditions - temperature increase = D-value decrease (take less time to kill 90% because rate of microbial death increases) - magnitude D-value depends on how constituents of food affects sensitivity of microorganisms
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