Unit 2 Weekly notes 1
Unit 2 Weekly notes 1 Micr 407
Popular in Food and Dairy Microbiology
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This 5 page Class Notes was uploaded by Molly Gersbach on Monday February 15, 2016. The Class Notes belongs to Micr 407 at Clemson University taught by Xiuping Jiang in Spring 2016. Since its upload, it has received 32 views. For similar materials see Food and Dairy Microbiology in Microbiology at Clemson University.
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Date Created: 02/15/16
2.10.16 -2.12.16 Physical Methods of Food Preservation I. Definition a. Utilize physical treatments to inhibit, destroy, or remove undesirable microorganisms without involving antimicrobial additives or products of microbial metabolism as preservative factors II. Low temperature preservation: reducing temperature to slow down the growth rate; not a sterilization method. a. Q10: reduce the temperature by 10 degrees C, the growth rate will slow by 1-1.5x b. History: i. Romans using ice ii. Natural ice from frozen lakes used (early 1800s) iii. Linde (1874) developed first domestic refrigerator – commercialized in 1890 iv. Frozen foods popular in 1950’s-60’s v. 65% of foods eaten are frozen or refrigerated today c. Methods i. Chilling: foods held on ice 1. Pro: Lasts a few days; allows consumers to see foods displayed; more convenient for local vendors 2. Con: Too many temperature fluctuations within foods, cross-contamination ii. Refrigeration: 1. Domestic fridge: 4-5 degrees C 2. Commercial fridge: ~1 degree C or lower 3. Lasts 60 days iii. Freezing: 1. Domestic freezer: -20 degrees C 2. Storage life of months to a year or more 3. Preparation for freezing: a. Selecting, sorting, washing b. Blanching: put in boiling water or steam for a short period of time i. Purpose 1. To kill outside microbes 2. Keeps color of some vegetables 3. Removes extra air 4. Reduce enzymatic activity c. 2 processes: i. Quick freezing: within 30 min ii. Slow freezing: within 3-72 h d. Shelf-life extension of raw foods i. Helps with importing and transportation; important to know how much time you have before it goes bad e. Microorganisms of concern: These can grow at refrigerated temperatures i. Psychrophiles (max 15 degrees C) ii. Psychrotrophs (optimimum is 20-30 degrees C): In general, grow slower than mesophiles 1. Stenopsychorotroph: forming colonies in about 5 days 2. Eurypsychrotroph: forming colonies between 6-10 days 3. Organisms: a. Most are gram negative b. Major sources include: water, soil, and improperly sanitized equipment c. Most are heat-sensitive d. Some pathogenic psychrotrophic: Listeria, Yersinia e. Important food spoilage genera: Alcaligenes, Chromobacterium, Flavobacterium, Pseudomonas i. All aerobic/high Oxidation-reduction potential ii. High growth rate iii. Prevention: remove oxygen (modified atmosphere packaging) iii. Characteristics of Psychrotrophs/Psychrophiles 1. Cold-resistant transport system: a. An increase in unsaturated fatty acid residues to keep membrane lipid in a liquid and mobile state (membrane fluidity) b. Active transport permeases i. Helps move nutrients across membrane 2. Synthesis of high levels of polysaccharides 3. Pigment production (phenazine and carotenoid pigments) iv. Significance of psychrotrophs and psychrophiles 1. Growth and spoilage at refrigerated temps 2. Some are heat resistant (producing heat-resistant enzymes – lipases/proteases) and survive pasteurization 3. Pathogenic: Yersinia enterocolitica, Vibrio, Listeria, Aeromonas f. Preservation effect: i. Growth rates decline as temperature shifts from optimum to minimum for growth ii. Increased generation time iii. Microbiota will be changed during cold storage depending upon types of m/o g. Mechanisms of freezing on microbes i. Extracellular or intracellular formation of ice crystals 1. Intracellular crystals much smaller 2. Create loss of available water ii. Increase in viscosity of cellular matter & loss of cytoplasmic gasses (O2 & CO2) 1. Due to loss in water iii. Change in pH value and cellular electrolytes iv. General alteration of colloidal state of cellular protoplasm v. Denaturation of cellular proteins vi. Metabolic injury to some cells of microbes h. Factors affecting microbes in foods at low temps i. Effect of freezing on microorganisms – NOT STERILE i. Sudden mortality immediately on freezing ii. Storage at freezing temperature iii. Slow freezing causes more death and injury compared to fast freezing iv. The cocci being generally more resistant than gram- negative rod v. Bacterial spores or toxins – almost no effect on these vi. Repeated cycles of freezing/thawing results in increased killing/injury vii. Cell death in frozen foods can sometimes release enzymes that lower quality of food viii. Microbial survivors in frozen foods even after long term storage j. Food environment i. Composition of food such as proteins, CHO, lipids, etc. ii. pH iii. aw – if you have high water activity, microbes can be killed quicker via ice crystal formation iv. preservatives k. Inherent properties of microbes i. T-mperature class of microbes ii. G bacteria are more susceptible to freezing damage iii. Rapidly growing cells vs non-growing cells iv. Some endospores can outgrow at ca. 2 C o v. Spores remain viable in frozen foods l. Factors unique to low temperature storage i. Psychtrotrophic & psychrophilic bacteria, yeasts, and molds can grow at ~2* C and lower. (to around -7 to -10*) ii. Endospores of some species of Clostridium and Bacillus can germinate and outgrow at 3-5*C iii. Pathogens of concern for refrigerated foods: 1. L. monocytogenes 2. V. parahaemolyticus 3. Y. enterocolitica 4. A. hydrophila 5. C. botulinium type E 6. Bacillus III. Heat Preservation a. Mechanism of thermal death i. Heat can result in death or injury 1. Wet heat: denaturing DNA, RNA, structural proteins, and enzymes a. Heat transfer is much faster than dry heat b. Temp: 121* C c. Pressure: 15 psi d. Time: 20-30 minutes 2. Dry heat: dehydration and oxidation a. Temp: 170* C b. 2 hours b. Factors affecting thermal death i. Nature of microorganism 1. Thermophiles are more heat resistant 2. Spores of all microbes are more resistant than vegetative cell counterparts 3. Initial population (o ) a. Large population, there will be some extremely sensitive/resistant/etc 4. Physiological stage of cells a. Stationary cells vs. exponentially growing cells b. Heat shock i. Expose to sublethal temperature (48*C for mesophiles), ii. A lot of “heat shock proteins” are formed allowing for heat resistance 5. Heat resistance a. Thermophiles > mesophiles > psychrophiles b. Sporeformers > non-sporeformers; thermophilic sporeformers > Mesophilic sporeformers i. Yeast & Mold spore: 65*-70*C for a few min ii. Some mold spore: 90*C for 4-5 h c. Gram+ > Gram – ii. Nature of the food 1. Chemical composition: proteins, CHO, fats, etc. 2. Aw – available water for bacteria to use a. the lower the aw, the fewer the deaths 3. pH – microbes are more sensitive at low pH, especially in presence of acetate, lactate, or propionate a. hurdle concept c. Important terms i. Thermal Death Time (TDT): time necessary to kill given number of organisms at a specific temperature 1. Higher temperature, faster time 2. Higher population, more time 3. TDT Curve a. Y-axis: TDT in minutes b. X-axis: Temperature c. Open circle – at least one tube is positive (at least one cell is still alive) d. Closed circle – all tubes are negative (all population is killed) ii. D-value: the time required to destroy 90% of the organisms at a spec. temperature 1. Important: Not ENTIRE population 2. If D-value is high, more heat resistant 3. D-value curve: plot log CFU (Y) vs. time of heating (min) (X) 4. Regardless of population, you can get the same D- value o iii. Z value: it refers to the degree Fahrenheit (or C) required for the thermal destruction curve to traverse one log cycle iv. F value: the time at a specific temperature required to kill a specific number of cells having specific z value v. F 0alue: a universal reference F value; the temperature is set at 250 F (121 C), and the z value is 10 C (18 F)o vi. 12-D concept: (botulinum cook) 1. A heat process that reduces the population of C. botulinum spores in low acid (pH > 4.6) canned foods by an arbitrarily established factor of 12 decimal (log10 values 2. The 12-D concept provides a large safety margin in low acid canned foods 3. 12-D botulinum cook: F = 10D =12 x 0.20 (D 121C) = 2.4 min vii.