1. Heat Preservation

2. Introduction
Cooking is the transfer of heat energy from some source to the food
Top 3 ways to heat in kitchen
stovetop
conventional oven
microwave oven
Different methods of heat transfer

3. Conduction Heat transferred through direct contact
Only the flat surface of the pan is hot enough to cook anything
Pans are made of metals - conduct heat efficiently & do not melt on the stovetop
Requires less time

4. Convection Heat transfer through a fluid
Liquid or gas
Oven is a confined area that gets hot by flames or electric coils
Cooks the food from all directions
Ovens heat foods from the outside in
Requires more time

5. Radiation Transfer of heat using electromagnetic radiation
Microwave oven

6. Microwave Oven – History

7. Microwave Oven
Microwave ovens work by spinning water, fats, sugars & oils inside food
Causes friction, which then heats the food & cooks it from the inside
Fastest method

8. Microwave Acceptable dishes for the microwave:
Paper, plastic, glass, microwaveable dishes
Unacceptable dishes for the microwave:
Metal, dishes with gold or silver on them
Cooking Tips – see handout

9. In Review

10. Degrees of Preservation & Thermal Death Curves

11. Heating Foods
Kills some microorganisms, destroys most enzymes & improves shelf life
Does not preserve a food indefinitely
Creates 4 degrees of preservation – depending on product
Sterilization
Commercial Sterility
Pasteurization
Blanching

12. Overview of Heat Treatments
Comparisons
Mild
Severe
Aim
Kill pathogens; reduce bacterial count
Kill all bacteria; food will be commercially sterile
Advantages
Minimal damage to flavor, texture, nutritional quality
Long shelf; no other preservation method is necessary
Disadvantages
Short shelf life; another preservation method must be used, such as refrigeration or freezing
Food is overcooked; major changes in texture, flavor, nutritional quality
Examples
Pasteurization, blanching
Canning

13. Sterilization Complete destruction of microorganisms
At least temps of 250 F (121 C) for 15 mins

14. Commercial Sterility
All pathogenic & toxin-forming organisms have been destroyed
End products may contain viable spores, but they will not grow under normal conditions

15. Pasteurization Low-energy thermal process with two main goals
1. destroy all pathogenic microorganisms that might grow in a specific product
2. extension of shelf life by decreasing number of spoilage organisms present
Product is not sterile & will spoil

16. Blanching Mild heat treatment Usually used on fruits & vegetables
Primary objective – enzyme inactivation
Lead to increase product shelf-life

17. Picking the Right Heat Treatment
Think about these things:
Time-temperature combination required to inactivate the most resistant microbe
Heat penetration characteristics of the food & container

18. Clostridium botulinum (botulism)
Most resistant microbe in canned foods
Must kill this with right time-temp combination
If food contamination is unknown processors assume C. botulinum to be present

19. In order to determine the time-temp combinations required to kill the most heat resistant pathogen and or spoilage organism in products you must understand thermal death curves!!!

20. Thermal Death Curves Plot the lethality of heat to microorganisms
Two types
thermal death time curves
plot combinations of heats & times required to reduce a microbial population by ninety %
thermal death rate curves
plot the lethality of a given temperature to a microbial population over time
Both specific to each microorganisms in given environmental conditions

21. Thermal Death Curves
Heat kills bacteria logarithmically – example: if 90% are killed in 1st min at temp., then 90% of those remaining alive will die during 2nd min, & 90% of those remaining alive will die during 3rd min and so on

22. Thermal Death Curves
Microbes are killed at a rate approximately proportional to their population
the more of any given microbe one hopes to kill, the greater must be the application of heat
Understanding Thermal Death Curves – see handout

23. More acidic (lower pH) foods generally take longer to kill all microorganisms