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Food Microbiology 1

Food Microbiology 1. Unit 5 Thermal and Non-Thermal Preservation. Thermal Pasteurization Commercial Sterilization. Non-thermal Low Temperature Irradiation Chemical Micro filtration High Pressure Pulsed electric field. Thermal (High Temperature) Processing.

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Food Microbiology 1

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  1. Food Microbiology 1 Unit 5 Thermal and Non-Thermal Preservation

  2. Thermal • Pasteurization • Commercial Sterilization • Non-thermal • Low Temperature • Irradiation • Chemical • Micro filtration • High Pressure • Pulsed electric field

  3. Thermal (High Temperature) Processing • Logarithmic Death: Microbial destruction by heat occurs in a logarithmic fashion allowing us to predict the death of a population of organisms. • The theory of logarithmic death is based on a single hit or one event equals death

  4. Pasteurization • Derives its name from the mild heat treatments developed by Louis Pasteur to prevent or delay spoilage of wine and beer • Today it refers to a heat process that results in destruction of all vegetative cells (non-spore formers) of pathogens expected in that food

  5. Pasteurization • The process of pasteurization is based on food safety and not on food preservation alone • It kills target pathogens • Extends shelf life ( shelf-life refers to the amount of time from packaging of the food product to the time of spoilage under appropriate storage conditions). • Does not inactivate all microbes present • Pasteurized food usually requires additional control measures (such as refrigeration, low aw, low pH) to prevent rapid spoilage

  6. Pasteurized Foods • The most common pasteurized food is milk • Originally designed to eliminate Mycobacterium tuberculosis and Coxiella burnetti • Fruit juice • Spoilage yeast and bacteria, E. coli O157:H7 • Beer • Spoilage bacteria and yeast

  7. Pasteurized Foods • Liquid egg • Salmonella and spoilage bacteria • Honey • Spoilage yeast • Meat surfaces (steam, hot water) • E. coli O157: H7, Salmonella, Campylobacter

  8. Milk Pasteurization Time/Temperature Combinations • High Temperature Short Time (HTST) 15 sec @ 72oC • Low Temperature Long Time (LTLT) 30 min at 63oC • Heat treatments are established on the basis of safety first (elimination of pathogens) and spoilage (extension of shelf life) second.

  9. Applying high temperatures over a short time preserves the sensory and nutritional quality of milk • Other combinations may result in a sensory quality not accepted by consumers • Can effect the quality of products derived from treated milk (e.g. cheese)

  10. Commercial Sterilization • Some milk is sold in cans (evaporated or sweetened condensed milk) or in boxes that remain at room temperature • The boxed milk is known as Ultra High Temperature milk (UHT) milk • UHT milk has undergone commercial sterilization and so can be stored at room temperature • UHT treatment is 2 sec @ 140-150oC

  11. Sterilization: Inactivation of all microorganisms • Essential in clinical settings (surgical instruments) • Commercial Sterilization: “ A product is not necessarily free of all microorganisms, but those that survive the sterilization process are unlikely to grow during storage and cause spoilage”

  12. Commercial Sterilization • A product that has undergone commercial sterilization is free of vegetative and spore-forming pathogens and spoilage microorganisms that are capable of growing in that food under typical non-refrigerated storage conditions • Most common commercially sterilized foods are canned products

  13. Commercial Sterilization • Primary Objective: • Destroy the most heat resistance pathogenic spore-forming organisms- Clostridium botulinum • Secondary Objective: • Destroy vegetative and spore-forming microorganisms that cause spoilage. Spoilage spore-formers are usually more heat resistant than pathogenic spore formers

  14. Thermal Destruction Curves • Thermal destruction curves provide an empirical model to calculate time/temperature relationships used in processing • D value • Z value • F value

  15. D -value • D-value- Decimal Reduction Time: Is the time needed to reduce a population of microorganisms by 90% (1 log cycle) at a specified temperature and in a specified medium • If the initial population was 100 CFU/ml • 10 CFU/ml would remain after a 1 log cycle reduction

  16. D -value 105 D-value 104 Time (s) @ 121oC

  17. D –value Formula DT Value = t2-t1/ (log N0-log N1) T= temperature t1= initial time t2= final time N0= initial population N1= final population From previous example: D121= 45-30/5-4 = 15/1= 15 sec

  18. Z- Value Z-value: is the change in temperature required to produce a 10-fold change (1 log) in D-value. Z-values are calculated from the slope of the curve of D-value vs temperature Z- value is the measurement of the sensitivity of an organism to changes in temperature

  19. Z- Value Z D-value

  20. Z- Value Formula Z = T2 – T1/ log a- log b T2= Final temperature T1= Initial temperature a = upper D-value b = lower D-value From previous figure: Z= 240-220/log 100- log 10 Z= 20/2-1 Z= 20oF

  21. D value determination for E. coli O157:H7 in beef at 60oC: Calculate the D value of the organism under these conditions Exercise

  22. Temperature (oC) Log10 D value (min) 55 60 0.75 -0.7 Calculate the Z value of the organism

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