Thermal processing

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Thermal processing

• Sterilization/pasteurization
• Appertization
• Canning

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History

• 1810 Nicolas Appert
• about 1860 Luis Pasteur provides scientific
  explanation
• 1920 Bigelow and coworkers described method of
  calculation
• 1945 Otto Rahn applies the principle that
  microorganisms die according to logarithmic order.

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HEAT is the energy in transit due to a
temperature difference between two regions and is
always transferred from the region of higher
temperature to the region of lower temperature.
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HEAT TRANSFER MODES

• Conduction
• Convection
• Radiation

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CONDUCTION
Transport of energy due to direct molecular
interaction without appreciable displacement of
molecules.
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CONVECTION

• Transport of energy from one point of fluid to
  another point by actual movement of fluid itself.
  
• MODES OF CONVECTION
• Natural convection
• Forced convection

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RADIATION
Transport of energy in the form of
electromagnetic waves between materials across
the space.
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HEAT PENETRATION TEST
Process of putting a temperature sensitive
element in food container and gathering
temperature data over a time course during a
thermal process. Reports on penetration tests
need to state the type of thermocouple used and
where the thermocouple was placed.
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For penetration test the thermocouple should be
placed in the coldest place of the container

• for conduction heating product it is
• located at a geometric center of container
• for convection heating products it is located
• 1/10 to 1/5 of the container height from the
• bottom of the container

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Primary Objectives of Canning are

• to kill microorganisms
• to keep away microorganisms

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Microorganisms

• public health hazard
• economic spoilage

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Public health hazard
Clostridium botulinum is the main public hazard
b/c spores are heat resistant. Spores may survive
when heat processing is insufficient. The health
hazard is due to ability of Cl. botulinum to grow
under anaerobic conditions and to produce toxin.
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Economic spoilage is due to un-processing

• sporeformers from the genera Bacillus
• sporeformers from the genera Desulfotomaculum
• sporeformers from the genera Clostridium

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FOOD CONTAINERS

• Metal containers
• Glass containers
• Retort pouches

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Double seam
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HEADSPACE
GROSS - vertical distance from the top of double
seam or the top edge of the glass jar to the
level of product in the container. NET -
vertical distance from the level of food to the
inside surface of the lid (only metal
containers). A retort pouch does not have a true
headspace, what is of importance is the volume of
non con- densible gases.
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Classification of thermal processes based on
processing temperature

• Sterilization gt100 oC
• Pasteurization lt 100 oC

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Classification of thermal processing based on
method of product packaging

• Terminal processes
• Aseptic processes
• Hot pack/hot fill processes

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Food classification

• Low acid food pH gt 4.6
• Acid food pH ? 4.6
  

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Acid Foods

• pH 4.6
• Generally all fruits
• Tomatoes, with added acid
• Sauerkraut and fermented pickles
• Foods to which large amounts of acid are added

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Low Acid Foods

• pH gt 4.6
• Generally all vegetables
• Meats
• Poultry
• Seafood
• Soups
• Mixed canned foods

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Retorting consist of

• heating phase
  removal of air, come-up time
  (CUT) , holding time at processing temperature
• cooling phase

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Types of retorts

• Still retorts (nonagitating retorts) vertical,
  horizontal, malo, hydrostatic.
• Agitating retorts sterilmatic, orbitort,
  rotomat,
• Flame sterilizers
• Aseptic systems

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Still retort - vertical retort
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Still retort -hydrostatic retort
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Agitating retort - Sterilmatic
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Agitating Retort - ORBITORT
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Effect of heat on microorganisms
Yeast are the least resistant, followed by mold
and then bacteria. All vegetative cells are
destroyed instantly at 100 0C . Spores of C.
botulinum, C. sporogenes, C. bifermentans, C.
butiricum, C. pasteurianum, C. perfringens, C.
thermosaccharolyticum, D. nigrificans, and B.
stearothermophilus are very heat resistance.
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Equivalent processing times

• 330 min at 100 oC
• 150 min at 104 oC
• 36 min at 110 oC
• 10 min at 116 oC
• 5.27 min at 118 oC
• 2.78 min at 121 oC
• 1.45 min at 124 oC
• 0.78 min at 127 oC

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The destruction of microorganisms is affected by

• their inherent resistance
• by environmental influences during the growth
  and formation
• the heating time temperature
• pH
• humidity
• protective effect of food components fat,
  proteins, salt

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Order of destruction of microorganisms

• The death of bacteria exposed to wet heat is of
  logarithmic order.
• The logarithmic order means that theoreti- cally
  the survivors can be reduced to less than one.
  Thus the number of survivors may become very
  small such as one survivor in million units etc.

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The survivor curve
The number of viable bacteria plotted on the
logarithmic scale against the corresponding
heating time (processing time) on the linear
scale provides the graph known as
the
survivor curve.
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DECIMAL REDUCTION TIME, DT
The time in minutes required to reduce the viable
cells in suspension of bacteria to one tenth of
their original value. The slope of
semilogarithmic survivor curve determi -nes the
decimal reduction time.
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The logarithmic model for microbial destruction
is described by the equation
U DT (log N0 - log Nu) U - the equivalent
heating time at proces- sing temperature
N0 - the initial numbers of microorga-
nisms Nu -the number of microrganisms after
heating
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Survivor Curves at Different Temperatures.
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Comparison of D for Microbial Population
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THERMAL DEATH TIME
The longest time when the unit test are positive
for growth and the shortest time when the units
are negative
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The thermal death (TDT) curve
The value of F plotted on the logarithmic scale
against the corresponding tempe- ra ture on the
linear scale provides graph known as TDT curve.
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Thermal Death Time Curve
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The thermal resistance curve
Values of DT plotted on the logarithmic scale
against the corresponding temperature on the
linear scale provide a graph called phantom
thermal death time curve or thermal resistance
curve.
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Parameter Z
The parameter Z represents the number of
degrees of Fahrenheit, centigrade, or Kelvin
necessary to cause the F-value or D value to
change by a factor ten.
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TDT curve is described by equation
Log FT ( Tref - T)/Z Log Fref Z
10/log Q10 Q10 - 2.2- 4.6 dry heat Q10 - 8 -
20 wet heat
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Lethal rate
It is described as minutes at T ref per minute
at T. Can be calculated using the following
equation L Fref/FT 10 (T -Tret)/ Z Z 10
C or 18 F, Tref 121.1 C or 250 F
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Sterilization value of heat process, F
F Dref (log N0 - log NF )
F ? t ? Li F ?
L dt