Thermal and Non-Thermal Preservation 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 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

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

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

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

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

Time/Temperature Combinations 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.

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)

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

Essential in clinical settings (surgical instruments) 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”

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

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

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

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

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

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

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

Z- Value Z D-value

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

Log10viable count (cfu/g) Exercise D value determination for E. coli O157:H7 in beef at 60oC: Calculate the D value of the organism under these conditions Time (min) Log10viable count (cfu/g) 0.2 7.1 0.5 6.5 1.0 6

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