Transcript Heat Storage Notes

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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
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Different methods of heat transfer
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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
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Heat transfer through a fluid
 Liquid or gas
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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
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Requires more time
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Transfer of heat using electromagnetic
radiation
 Microwave oven
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http://www.ideafinder.com/history/invention
s/microwave.htm
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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
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Acceptable dishes for the microwave:
 Paper, plastic, glass, microwaveable dishes
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Unacceptable dishes for the microwave:
 Metal, dishes with gold or silver on them
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Cooking Tips – see handout
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http://www.wisconline.com/Objects/ViewObject.aspx?ID=SC
E304
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Kills some microorganisms, destroys most
enzymes & improves shelf life
 Does not preserve a food indefinitely
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Creates 4 degrees of preservation –
depending on product
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Sterilization
Commercial Sterility
Pasteurization
Blanching
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
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Complete destruction of microorganisms
At least temps of 250 F (121 C) for 15 mins
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All pathogenic & toxin-forming organisms
have been destroyed
End products may contain viable spores, but
they will not grow under normal conditions
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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
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Product is not sterile & will spoil
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Mild heat treatment
Usually used on fruits & vegetables
Primary objective – enzyme inactivation
Lead to increase product shelf-life
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Think about these things:
 Time-temperature combination required to
inactivate the most resistant microbe
 Heat penetration characteristics of the food &
container
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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
http://www.youtube.com/watch?v=RI7FVOBKTXM
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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
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Both specific to each microorganisms in given
environmental conditions
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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
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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
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Understanding Thermal Death Curves – see
handout
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More acidic (lower pH) foods generally take
longer to kill all microorganisms