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BIOL 345 Set 5 Preservation 2.pdf

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BIOL 345
Barbara Butler

reminder: Biol 345 midterm exam Thursday October 24th, in lecture period in RCH 110: Adeli → Potts (inclusive) RCH 309: Rai → Zhou (incl) • exam is 50 minutes in length • written answer exam • bring your WATCard Preservation of food by drying: PR21 • direct dehydration &/or reduction in available water • some cells may be destroyed, but process generally not lethal to culture → substantial part of population may recover with re-hydration • low moisture foods (LMFs): 25% moisture or less, aw 0.0-0.60 • freeze-dried foods: lyophilization, reduce moisture to about 2% • intermediate moisture foods (IMFs): 10-40% moisture, aw 0.70-0.90 “alarm water content” - not to be exceeded if mold growth is to be avoided e.g., dehydrated vegetables 14-20% dehydrated fruits 18-25% wheat flour 13-15% example goal for LMF: • total count not to exceed 100,000/g, coliforms near 0/g, no food poisoning microorganisms Intermediate moisture foods (IMFs): PR22 • reduced aw products that are shelf-stable, microbiologically safe at ambient temperatures for varying periods of time e.g., traditional IMF: jam; newer IMFs: pop tarts, pouched moist pet food, chewy granola bars • drying in combination with other method(s) e.g., drying (+ pasteurization) + additives possible additives: • humectants (compounds that tie up water) e.g., glycerol, glycLoading...tol, sucrose • fungistats e.g., sorbate, benzoate, SO2 storage stability: • gram negatives, most gram positives will not grow (except some cocci, sporeformers, lactobacilli) • shelf life typically limited by xerophilic mold growth or chemical effects (lipid oxidation, Maillard browning) Preservation with low temperature methods PR23 (i) chill storage: ~16º to -2ºC (ii) frozen storage: at or below -18ºC objective: reduction of enzymatic rates to reduce or prevent growth • cells on/in foods not necessarily kil(preserve microbial cultures by freezing!) chill storage : to extend shelf life of fresh foods • psychrophilic, psychrotolerant microbes grow under refrigerated conditions (Table 27.1 MMK 3rd ed; Table 25.1 2nd ed) frozen storage : (at or below -18°C) PR24 • highly energy-intensive process slow freeze (home freezer) quick freeze (commercial) • immersion in refrigerant; indirect contact, blast freezer • smaller ice crystals, shorter solidification time liquid nitrogen • foods that support microbial growth at sub-zero temperatures: fruit juice concentrates, bacon, ice cLamo, certin rnits( cryoprotectants) processing foods for freezing: • select good quality material • blanching (brief immersion in hot water, steam) of fruits, vegetables to inactivate plant enzymes • usually package before freezing effects resulting from freezing/thawing of food: PR25 changes during freezing: • expansion of food volume • increased solute concentration in unfrozen liquor changes during storage: • slow irreversible protein dehydration; possible oxidation, hydrolysis of fat, myoglobin; other chemical, enzymatic reactions • oozing of unfrozen concentrated solutions; freezer burn changes during thawing: • thawing inherently slower process than freezing • changes reflect damage during freezing, storage • “drip” from meat, “leakage” from fruit, vegetables, wilting, mushiness microbial status: • lethal, sub-lethal damage; can reach ~90% lethality (unpredictable); any toxins present, endospores unaffected • must prevent post-processing contamination effect of freezing on microorganisms: PR26 • osmotic shock, mechanical damage, concentration of cellular liquids lethal effects = effects which kill • denaturation/flocculation of essential proteins (enzymes) • effects to membrane lipids → altered permeability, cell leakage • DNA strand breaks, rRNA degradation, enzyme effects sublethal effects = effects from which a microbe may potentially recover (without recovery → eventual lethality) • if recovery occurs a greater proportion of sublethally-damaged (injured) population ultimately will survive → possible recovery of spoilage microbes, foodborne pathogens factors important to recovery of injured population: • time for repair • availability of additional nutrients to damaged cells • e.g., medium rich in metabolizable C, N sources, vitamins; pyruvate, catalase supplements • repair requires RNA and protein synthesis PR27 effect of freezing and thawing on viability loss and injury of E. coli medium CFUs/mL CFUs/mL subpopulations before after freezing freezing tryptic soy 276 x 106 17.5 x proportion of 276 x 106 - 17.5 x agar (TSA) 106 initial population 106 (nonselective now dead: (93.7%) medium) TSA + 276 x 106 3.5 x 106 among survivors: deoxycholate uninjured 3.5 x 106 (19.9%) (selective medium) injured 17.5 x 106 - 3.5 x 106 (80.1%) E. coli cells suspended in water enumerated before and after freezing (-20°C for 16 h) and thawing simultaneously in tryptic soy agar (TSA) and TSA + deoxycholate by pour plating followed by incubation at 37°C for 24 h (modified from Ray& Bhunia. 2008. Fundamental Food Microbiology, 4th ed) hypothetical curves for injury & repaired in bacteria: PR28 unstressed controls plated on selective medium plated on nonselective medium (M&M 2nd ed; Fig 2.2a in MMK 3rd ed is similar) PR29 importance of sublethally-injured microbes in foods: • many physical, chemical treatments can result in sublethal injury → injured cells likely to be present • viable but nonculturable (VBNC, VNC) state (partial?) solutions to the problem: with respect to detecting undesirable microbes: • incorporate repair time before use of selective media to detect important microbes • design processing parameters to avoid possibility of survivors with respect to enhancing shelf life of foods: • use severe processing conditions to avoid survivors (may excessively damage food!) • injured cells susceptible to non-optimal physical & chemical environments (e.g., low temperature, low pH) • continued presence of these ensures sublethally-damaged cells cannot repair, then grow → product spoilage can be reduced physical & chemical treatments known to cause sublethal injury in PR29a microorganisms: physical stresses: • low temperature: refrigeration, freezing • heat: temperature and time below lethal treatment • drying: air drying, freeze-drying • high solids: sugars, salts • radiation: UV, X-ray chemical stresses: • acids: organic and inorganic • sanitizers: chlorine, quaternary ammonium compounds • preservatives: sorbate, benzoate • toxic chemicals: mercuric chloride microorganisms known to suffer reversible injury from sublethal stress: indicators: E. coli, Ent. aerogenes, Klebsiella, Strep. facaelis pathogens: Salmonella, Shigella, V. parahaemoliticus, Y. enterocolitica, C. jejuni, Staph. aureus, Cl. perfringens, B. cereus spoilage: Pseudomonas, Bacillus, Candida, Aspergillus bioprocessing: L. lactis, L. bulgaricus, S. cerevisiae (Ray, B. (ed.) 1989. Injured Index and Pathogenic Bacteria: Occurrence and Detection in Foods, Water and Feeds. CRC Press) Manipulation of atmospheric conditions: PR30 (i) controlled atmosphere storage (CAS) • mainly bulk storage and transport applications fruits and vegetables • increase CO2 and decrease O2 • slows respiration → slows ripening • 2-3 ppm O3 (ozone) • increases storage time e.g., strawberries, raspberries, grapes • increase ethylene • faster ripening, desirable colour change hypobaric (low pressure) storage (reduced O2) • controlled low temperature, high humidity also needed • reduced fat oxidation in meats, seafood • extended shelf life for cut flowers, fruits and vegetables PR31 (ii) vacuum packaging • evacuation of air from gas-impermeable pouch + sealing of pouch • some O2 removed, CO2 increases due to respiration • inhibits aerobic spoilage microorganisms • retards fat oxidation and discolouration percentage of CO2 and O2 in gas-impermeable packages of fresh pork stored at 2ºC or 16ºC 2ºC 16ºC CO2 O2 CO2 O2 storage time 3 hr 3-5 20 3-5 - 4 d 13 20 30 1 5 d - - 30 1 10 d 15 1 - - 14 d 15 1 - - (after Table 14-2, Jay et al, 2005) PR32 (iii) modified atmosphere packaging (MAP) • alteration of package atmosphere by flushing with various mixtures of CO2, N2 and/or O2 • high-O2 MAP – up to 70% O2, balance of CO2 or N2 • low-O2 MAP – O2 levels as high as 10%; CO2 maintained in 20-30% range, balance N2 examples of MAP gas mixtures used with foods: food CO2 (%) O2 (%) N2 (%) fresh meat 30 30 40 fresh meat 15-40 60-85 0 cured meat 20-50 0 50-80 eggs 0 0 100 poultry 25-30 0 70-75 processed meats 0 0 100 hard cheese 0-70 30-100 (after Table 4.13, Moss & Adams, 2008) PR32a effect of storage conditions on the microbiota of smoked pork loins held for 48 d at 4ºC vacuum CO2 N2 day 0 (day 48) (day 48) (day 48) log APC/g 2.5 7.6 6.9 7.2 pH 5.8 5.8 5.9 5.9 dominant Flavobacterium (20) lactics (52) lactics (74) lactics biota (%) Arthrobacter (20) (67) yeasts (20) Pseudomonas (11) Corynebacterium (10) (modified from Table 14-5, Jay et al, 2005) Tectrol process for long-distance shipping: PR32b Tectrol: refrigeration only: Protecting Retail Profits TECTROL© Atmosphere helps control retail shrink. Strawberries shipped with TECTROL© simply do not deteriorate or decay as fast as strawberries shipped without TECTROL©. This translates into direct profits delivered to the retail category Once strawberries arrive and the TECTROL© bag is removed, TECTROL©-treated- fruit stays fresher longer. Decay and wet and leaky conditions are controlled. htt
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