| High temperature processes | pasteurization, sterilization, ultra high temperature |
| Pasteurization | - destroys pathogens (vegetative cells)
- destroys or reduces spoilage microbes
- 63C 30min for batch process, 72C 15s for HTST process |
| HTST Pasteurization | high temperature short time |
| Pasteurization | - destroys the most heat resistant non spore forming pathogens (Coxielle brunette, Myobaterium tuberculosis)
- thermoduric and thermophilic microbes survive (Lactobacillus spp, Streptococcus spp, Bacillus & Clostridium)
- beer is pasteurized to destroy spoilage microflora @ 60C for 8-15min |
| Sterilization | commercial sterility
- no viable organisms detected by usual cultural methods
- low # survivors is insignificant under conditions of storage
- survivors cannot grow in product due to undesirable conditions (low pH, storage temp) |
| Ultra-high temperature (UHT) | - 140-150C for a few seconds can result in commercial sterility
- continuous heating plus aseptic packaging |
| Factors affecting microbial heat resistance in foods | - Moisture: moist heat is more lethal than dry heat, greater protein denaturation
- Fat content: low aw, water excluded from fat, microbes more heat resistant
- Salt: lower aw in foods, microbial heat resistance increases
- Carbohydrates: lower aw, increased microbial heat resistance
- pH: extremes of pH + heat are more lethal to microbes
- Number of microbes: large # give protection, production of protective substances
- Growth temperature: greater resistance in microbes with high growth temperature
- Age of microbes: older cells more resistant, stationary phase cells more resistant than exponential, long term survivor cells the most resistant
- Inhibitory compounds
- Heating temp and time
- Prior environmental stress: certain stresses can increase heat resistance of microbes (sub lethal heating, starvation, desiccation), production of stress proteins |
| Antimicrobial action of heating on cells | - sublethal injury or death of vegetative cells and spores
- cellular lesions: damage to cell membrane/wall/DNA, degradation of rRNA, denaturation of enzymes
- cell death: irreparable damage in important functions/structural components
- death of bacterial spores: damage prevents spores from germinating or outgrowing (producing a vegetative cell) |
| Low temperature processes | chill, refrigerator, and freezing temperatures |
| Low temperatures | - reduce or prevent microbial enzyme activity and microbial growth
- reduce spore germination
- can be lethal to microbial cells during storage
all metabolic activities are enzyme catalyzed and rate of enzyme catalyzed rxns is dependent on temp |
| Temperature coefficient (Q10) | Q10 = (Rate, + 10^0)/Rate
Reaction rate for 10 deg C increase in temp
- Q10 for most biological systems is 1.5-2.5
- 10C rise in temp --> 2 fold increase in rxn rate
- reverse is true for every 10C decrease in temp ^
- important in predicting shelf-life of foods |
| Chill temperatures | usually 10-15C, storage of certain fruits/vegetables |
| Refrigerator temperatures | 0-7C, ideally not >4.4C
- home refrigerator: 4-5C
- commercial refrigerator: as low as 1C |
| Freezer temperatures | at or below -18 C, generally prevents microbial growth
- some can grow at a very slow rate, depending on aw of food product
- home freezer generally -20 C |
| Slow freezing: effect on foods | - freezing temp achieved within 3-72h
- large extracellular ice crystals formed
- large crystals cause disruption of cell walls, membranes, and internal structures
- large amount of exudate (drip or leakage) upon thawing |
| Quick/fast freezing: effect on foods | - food temp lowered to -20C within 30min
- favors formation of small intracellular ice crystals
- less cellular damage
- less drip loss (exudate) |
| Effects of freezing on microbes | - sudden death of some cells
- portion of survivors die gradually during storage
- reduction in microbial population: rapid at temp just below freezing, slows down at lower temps, very slow below -20C
- dehydration of cells as water is converted to ice
- increased viscosity of cytoplasmic material
- loss of cytoplasmic gases (O2, CO2)
- pH changes in cellular matter
- protein denaturation (some enzymes damaged)
- metabolic injury
- temperature shock |
| Microbes resistant to freezing | Gram positive cocci are most resistant, endospores and toxins are unaffected |
