Ch' 8 Heat Preservation of Foods

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Ch. 8 Heat Preservation of Foods
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• Cooking/heating foods
• kills some microorganisms
• destroys most enzymes
• improves shelf life
• does not indefinitely preserve

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Degrees of Preservation

• Sterilization
• Complete destruction of microorganisms
• 121oC (250oF) for 15 minutes (internal temp)
• Commercially Sterile
• All pathogenic toxin-forming organisms are dead
  (applies to most preserved foods)

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• Pasteurization - Temps lt 100oC (212oF)
• Destroys pathogenic microorganisms
• Extends product shelf life (but not extensively)
• Ex milk, beer, fruit juices, liquid eggs
• Blanching
• Primarily used for fruits vegetables
• Deactivates enzymes
• Kills some bacteria

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Selecting Heat Treatments

• Heat that is sufficient to destroy all
  microorganisms and enzymes is detrimental to
  other food quality factors such as
• Color
• Flavor
• Texture
• Nutrition
• Consistency

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To pick the right heat treatment severity for a
specific food you must determine...

• Time/temperature combination required to
  inactivate the most resistant microbe
• Heat penetration characteristics of
• the food (varies with consistency, particle size)
• the container (varies with size, shape, material)

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Time/temperature combination required to
inactivate the most resistant microbe

• The most heat resistant pathogen in canned foods
  is Clostridium botulinum (botulism)
• Therefore, must use time/temperature combination
  adequate to kill this species

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Thermal Death Curves

• Heat kills bacteria logarithmically
• i.e. If 90 are killed in the first minute at a
  certain temperature, then
• 90 of those remaining alive will die during the
  second minute, and
• 90 of those remaining alive will die during the
  third minute, etc.
• Spores are more heat resistant than vegetative
  cells

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Margin of Safety

• Unknowns in raw foods, especially those which are
  low acid
• types of microbes present
• number of microbes present
• Therefore assume
• C. botulinum to be present in large numbers

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1,000
100,000/ml
100
10,000/ml
Killing time (min)
1,000/ml
10
1
210
230
250
TEMPERATURE (oF)
Thermal death curves for bacterial spores at
different initial concentrations.
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Food Processing Requirements Based Upon Acidity
(pH)
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Food Processing Requirements Based Upon Acidity
(pH)
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Food Processing Requirements Based Upon Acidity
(pH)
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HEAT TRANSFER

• Every food particle inside a can must reach the
  critical temperature for the required time
• Factors affecting heat penetration include
• size of can
• shape of can
• consistency of the food item (thick or thin)
• nature of the food (particulate vs liquid)

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Principals of Heat Transfer

• Conduction
• from one particle to another by contact
• food particles in can do not move
• Convection
• movement inside can distributes heat
• Radiation
• energy transfer through a medium which itself is
  not heated

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Thermocouple Placement atCold Point in Can
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Protective Effects ofFood Constituents

• Sugar protects bacterial spores in canned fruit
• Starch protein protect spores
• Fats Oils protect bacterial spores

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Examples of Process Times
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Time-Temperature Combinations

• From thermal death curves, the following
  time/temperature treatments yield the same
  microbe killing effect
• 0.78 min _at_ 127oC 10 min _at_ 116oC
• 1.45 min _at_ 124oC 36 min _at_ 110oC
• 2.78 min _at_ 121oC 150 min _at_ 104oC
• 5.27 min _at_ 118oC 330 min _at_ 100oC

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Heating Before orAfter Packaging

• After
• simplest
• oldest form of preservation using heat
• Before
• less damaging to food
• requires aseptic packaging

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Temperature/Pressure Relationship

• 10 psi 116oC
• 15 psi 121oC
• 20 psi 127oC

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Heating Food in Containers

• Still Retort
• Max temp of 121oC to prevent food damage near can
  wall
• Long cook time
• Agitating Retort
• Shorter cook time
• Less food damage

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• Hydrostatic Retort
• Continuous flow of cans
• Uses hydrostatic head
• to control pressure
• Is an agitating system

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Heating Food Prior to Packaging

• Pasteurization (liquid products)
• Batch
• Product in steam-jacketed kettle heated to
  specified temperature and rapidly cooled
• (ex. milk heated to 145oF for 30 min)
• Continuous
• High Temp Short Time (HTST)
• Ex. Milk heated to 161oF for 15 sec.

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Aseptic Packaging

• Food is sterilized outside the can
• Placed into a sterile container and sealed under
  aseptic conditions
• Paper and plastic packaging materials most
  commonly used
• Most suitable for liquid-based food products

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Hot Pack/Hot Fill

• Filling unsterilized containers with sterilized
  food that is still hot enough to render the
  package commercially sterile.

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Microwave Heating

• Eliminates temperature gradients
• Rapid heating
• Limited packaging (no metal containers)