1. Thermal Preservation/Processing of Foods
Ch 19

2. Objectives To explore :
Scientific principles behind thermal processing in home and industry
Terminology used to describe thermal preservation
Conduction versus convection heating
Packaging used for thermally processed foods

4. Thermal Preservation/processing of Foods
Refers to controlled processes that are performed commercially
Blanching
Pasteurizing
Commercial Sterilization
Inactivate disease and spoilage causing microorganisms
Inactivate enzymes in food that can cause spoilage
Increasing
severity of
the process

5. BLANCHING Type of process: Mild intensity heat process
Exposes fruit / vegetables to boiling water OR steam for a short period of time
( 2 sec to 2-3 min)

6. BLANCHING Preservation effect: INACTIVATES endogenous enzymes
Enzymes that can brown, wilt, etc.
Drive off oxygen and other gases ( minimize pressure build up)
Short term shelf life extension
Combined with other forms of processing…. Canning and freezing

7. Pasteurization Louis Pasteur 1864
Type of Process: moderate intensity heat process
Temperature below boiling point of water 80-90o C ( o F)
Eg) Milk
15 sec at 72oC (162oF)- high temperature, short time HTST (flash pasteurizer)
Process performed PRIOR to packaging

8. Pasteurization Preservation effect: Depend on on the type of food
1) low acid foods (milk, eggs)
To destroy pathogenic ( disease causing) bacteria and viruses
To inactivate enzymes
2) acid foods (beer, wine, fruit juices)
Extend product shelf life
Destroy some spoilage-causing microorganisms and enzymes
Acid foods are not expected to be a source of pathogens…
Except E. coli 0157:H7
Pasteurized vs unpasteurized juices
* Primary reason- get rid of enzymes to extend shelf life.

9. Pasteurization Many spoilage-causing Microorganisms can survive
Especially in low acid foods
Combined with other forms of processing like refrigeration
Moderate shelf life extension
“Best if used before date” at less than or equal to 4oC
“Intermediate heat treatment”

10. Commercial Sterilization (CS)
Type of Process: high intensity heat process
Also known as “canning”
Cans, bottles, bags
Requires a minimum of 121oC (250oF) moist heat for 15 minnutes

11. Commercial Sterilization
Preservation Effect:
Destroys spoilage causing and disease causing microorganisms
Free from viable forms of microorganisms (including SPORES)
Ensures spores of Clostridium botulinum are destroyed’
Bot. destruction not done on other two methods
Aka: old term “ botulism cook”

12. Commercial Sterilization
What does survive is a small number of heat resistant spores
Do not cause disease
Ex) extreme thermophiles ( like basillus bacteria)
BUT are able to multiply (germinate) in the food product even if it is held at room temperature

13. Commercial Sterilization (CS)
Performed once the food is packaged in suitable containers
Containers must withstand
high temperatures
High pressures
(sterilization is done in the container it is stored in)
Must be hermetically sealed
Impermeable to gas and liquid and microorganisms
CS products have a long shelf life ( 2 years or more)
Eg) canned products

14. UHT processing and Aseptic Packaging= Commercial Sterilization
1. Ultra High Temperature (UHT)
Injection of hot steam under pressure ( oC; oF) for a short time (4-6 sec), followed by immediate cooling
2. Aseptic packaging-
UHT food is aseptically placed into
presterilized containers and sealed
in aseptic environment
Eg. Shelf stable milk

15. UHT processing and Aseptic Packaging
UHT treated & aseptically packaged
Shelf life > 6 months, w/o refrigeration
( more likely to breach seal of container)
Some UHT treated foods are not necessarily aseptically packaged…. Not commercially sterile
Longer shelf at refrigeration temperatures
Not shelf-stable ( at room temperature) ******
Post processing recontamination

16. Commercial Sterilization
Survivor Curve or Thermal Death Rate Curve (TDRC)
(speed at which microorganisms are killed)
If a given temperature kills 90% microbial population in the 1st minute of heating,
90% of remaining population will be killed in the 2nd minute,
90% remaining in the 3rd….
10,000 start
1, D D D D D
Logo rhythmic order of death
1 log=90% or 10% survive
If a food contains 10,000 mo,
How many survivors will there be if
A 3D process is applied?
10 4-3= 101= 10

17. Do the math….
How would you find the number of survivors after an application of a 3D thermal process to a food containing a starting microbial load of 1,000,000?
Would the answer be…..
A. 20,000
B. 5,000
C. 1,000
D. 100
E. 10
Death rate =90% per cycle/ log
Each log survival = 10% or x .10
Math:
log 1= 100,000 (90% survive)
log 2= 10,000 (90% survive)
log 3= ,000 (90% survive)

18. Commercial Sterilization (CS)
Survivor Curve or Thermal Death Rate Curves (TDRC)
D-Value (decimal/ chemical reduction time)
Time in minutes at a particular temperature (oC or oF) required to kill 90% of microbial population
The more microbial load (microbes present) then the more logs (time) will be needed to reduce the number
D-value = decimal reduction time

19. Thermal Death Curve
Used to determine cooking time and temperature needed to kill a specific microorganism
The lower the temperature the greater the exposure time needed to kill
* Not necessarily sterile, but kills 90% of the population of specific microbe

20. Why is this important?????

21. What do these charts tell you?

22. Survivor curve (TDRC) One log cycle Thermophilic spores Population
C. Botulism spores
Vegetative state

23. Determine the thermal process required for different microorganisms of concern based on:
Thermal Death Time Curves (TDTC)
Times required for destruction of microbe under specific conditions at different temperatures
Z-value (oC or oF) and F- value (minutes)

24. Z- value In the real world we don’t have instantaneous heat to 240oF
Product may already be “ cooked “ at 220o F
Z-value allows comparison of heating processes at different temperatures

25. Z-value= resistance of a microorganism to different temperatures
D-value= time needed at a constant temperature to kill 90% of microorganism population
Z-value= resistance of a microorganism to different temperatures
The temperature change causes the 1 log cycle change in the Dvalue
(it takes time for the change to happen)
F-value = minutes required to kill microorganisms at a certain temperature

26. Heating time in
“D-value”
minutes
F-value
temperature
If it takes an increase of 20oF for the D-value to change 1 log (90% death),
then D-value slope =20oF

27. Now what do these charts tell you?

28. How can we determine if C.botulinum spores have been destroyed?
1. inoculated pack studies
Clostidium sporogens PA3679
Spoilage causing microorganisms (anaerobic, spores)
More heat resistant than spores of C. botulinum
2. Margin of safety (MS)

29. Margin of Safety (MS)
MINIMIZE possibility of viable C. botulinum spores
1. Low acid foods – use a 12D thermal process (MS)
12D = 12 log cycle reduction= can kill 1012 (1 trillion) Microorganisms
No food exists that has 1012 spores
Large margin of safety ( usually large # of microbes< 1012 )
How much time required for the thermal process?
Depends on the D value decreases as temperature increases

30. Margin of safety 2. acid foods- use a 5D thermal process/ or 3D
C. botulinum will NOT grow in acid foods ( pH below 4.6)

31. Other factors that need to be controlled: (related to the FOOD constituents)
Mechanism of heat transfer
Consistency of the food ( solid, vicious liquid, a combination)
Cold point- last area to heat in container
Chemical composition of the food (eg. Fat, protein, …)
Can act as insulators

32. Type of heat transfer (related to the Food constituents)
Combination
Like corn, beans
and peas
Solid foods
Like canned
tuna a salmon
Non-vicous liquids
Like broth or milk
Conduction heating –
transfer from molecule
to molecule in straight lines
Convection heating–
transfer from fluid motion.
Fluid heats along walls and rises

33. ….. Effect of food constituents
Sugars, starches, fats and proteins can protect spores & vegetable cells from the killing effect of heat
D194 for salmonella in chocolate milk = 78 min
(90% of salmonella)
Chocolate milk has other ingredients
D194 for salmonella in regular milk = min
Also effect the heat transfer mechanism
Other constituents may decrease the resistance of microorganisms to thermal processes.
Eg.) acids, spices, some antimicrobial components

34. Types of packaging for thermally processed foods
Cans (2-3 piece)
Retort(able) pouches
Tetrapak
Glass jars or bottles
Plastic

35. Commercial processing concerns:
Safety- destroy harmful bacteria
Quality- destroy organisms and maintain quality
Cost- packaging,equipment and energy use, shelf life, shipping -

36. Heating food before or after pkg
Either enclosed or conveyor belts
Retort- huge pressure canners page 579
Pressure sealed to hold canned foods
Still retorts- Like home pressure canners, limited temp 250oF
Agitating retorts- gentle agitation

38. Retortable pouch 3 layers ( laminate) A. mylar or polypropylene
outside surface
B. aluminum –
barrier to gases and water vapor; Hermatic container
C. polypropylene-
in contact with the food;
inert (prevent contact of food with aluminum)
heat resistant surface