1. Thermal and Non-Thermal Preservation
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

5. 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

6. 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

7. 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

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

9. Time/Temperature Combinations
Milk Pasteurization
Time/Temperature Combinations
High Temperature Short Time (HTST) 15 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.

10. 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)

11. 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 oC

12. 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”

13. 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

14. 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

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

16. 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

17. D -value
105
D-value
104
Time 121oC

18. 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

19. 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

20. Z- Value Z
D-value

21. 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= /log 100- log 10
Z= 20/2-1
Z= 20oF

22. 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

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