Heat processing

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Heat processing

• Applying heat to foods to decrease the
  concentration of the viable microorganisms to
  such a level that would only allow growth of
  microorganisms and spores in the food under
  defined storage conditions to an acceptable level
  (commercial sterility).

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• In heat processing, to achieve microbial
  stability and eating quality both
• The temperature of heating and
• The duration of the thermal process
• are important. An optimum balance needs
  to be found to avoid over- and
  underprocessing.
• To design a heat process it is necessary to
  determine
• The heat resistance of the spoilage
  microorganisms (target microorganism)
• The temperature history of the food at the
  slowest heating point. (thermal center)

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Thermal destruction of bacteria

• Bacteria have a logarithmic order of death when
  subjected to high temperatures.
• Log of viable bacteria concentration vs. time of
  exposure is a straight line relationship called a
  survivor or a thermal destruction curve.

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Survivor or thermal destruction curve
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• For the target microorganism, if the initial
  viable cell concentration is Ni, viable cell
  concentration at time t can be estimated by
• log (N/Ni) Slope (t-0)
• The slope of the survivor curve is defined as
  -1/D,
• log (N/Ni) -t/D
• D is called the decimal reduction time which is
  constant at a given temperature. D D(T)
• D is the time period needed to decrease
  viable cell concentration 10-fold at a given
  temperature.

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• The decimal reduction time, D is determined for
  each type of target microorganism in certain
  types of food (growth medium, aw, pH, composition
  etc.) for different temperatures. It is strongly
  dependent on temperature.
• From survivor curve equation
• N Ni x 10-(t/D) N?0 only if t ? ?
• An infinite time will be required for the
  destruction of all viable microorganisms.
  Basis for
  defining commercial sterility.

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• Product will be accepted as commercially sterile
  when the concentration of the viable cells of the
  target microorganism is reduced below a certain
  level N0 just low enough that the spoilage hazard
  it presents is commercially acceptable within the
  period of suggested shelf life.
• A reduction exponent is defined as
  
• m log(Ni/N0)

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Effect of varying temperatures

• During a thermal process temperature varies
  with time at the thermal center of the food.
• Since D D(T), an integration w.r.t. time is
  necessary
• T T(t), D D(t)
• Ni, Nf initial and final viable cell
  concentrations,
• tf duration of the thermal process needed
  to achieve commercial sterility.

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• Condition for commercial sterility Nf ? N0
• log (Nf/Ni) ? log (N0/Ni) , since m - log
  (N0/Ni)
• condition for commercial sterility becomes
• The processing time tf can be estimated by
  graphical integration of 1/D versus T
• Steps Generate T vs. t data ? find D versus T
  data from literature ? plot 1/D versus t.

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Tr
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Modeling temperature dependency of D

• The variation in the logarithm of the decimal
  reduction time D could be well correlated as a
  linear function of temperature.

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• If at temp. ? the decimal reduction time is
  D? , then at T, D will be
• z-value is the temperature difference
  required to change the decimal reduction time
  tenfold. From the equation above
• Plugging into condition for commercial
  sterility
• 
  
• letting
• For commercial sterility
• L is defined as the lethal rate

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• For each kind of microorganism z-values can be
  found in literature.
• ? is called the reference temperature. For
  sterilization operations it is taken as 2500F
  (121.10C), the max. temperature experienced by
  the food in retorts.
• The value of the integral is called
  the equivalent time of the heat process and it is
  denoted by F.
• The equivalent time values are estimated for
  certain target microorganisms with known z-values
  at a fixed reference temperature. Therefore,
  equivalent time needed for commercial sterility
  is denoted as
• Since most target microorganisms have z-values
  close to 10 and since the reference temperature ?
  is usually taken as 121.10C, for this specific
  case
• F F0 (F121.1)
• is used. For commercial sterility F ? mD?

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Example Problem Heat penetration data on a
vacuum packed corn are given in the Table. The
target organism for this food is B. Sporogenes
(D?0.8). What is the minimum processing time
necessary to achieve commercial sterility for
this food assuming instant cooling after the
process ?
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Formula method for thermal process evaluation

• This method aims to perform the integral
  analytically to estimate the equivalent
  time.
• Let Tr be the constant temperature of the medium
  where the food is heated. A dimensionless
  temperature V is defined as
• V ( Tr T) / ( Tr
  T0 )
• T0 initial temperature at the thermal
  center,
• T temperature at the thermal center at time
  t
• at t 0 V 1.0 , as t?? , T?Tr ,
  V?0.0
• A plot of logV vs. time can be approximated with
  a straight line.

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Tr
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• Thermal destruction of microorganisms occurs to
  the most part when the linear asymptote forms a
  good approximation to the heating curve.
• The linear asymptote is specified by defining two
  parameters the lag factor j ( j1.06-1.40 for
  conduction-, j?1.0 for convection heating) and
  the slope 1/f .

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• The equation for the asymptote is
• -1/f (logV-logj) / (t-0) ? t/f
  log(j/V)
• log j ? ( Tr T0 ) / ( Tr T ) ? (1/f)
  t
• dt f M ? dT/(Tr-T) ? , M loge 0.4343

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• Inserting dt f M ? dT/(Tr-T) ? into the
  integral for equivalent time ?o 10(T- ?)/z
  dt
• F ?o 10(T- ?)/z fM dT/(Tr-T) this
  integral is
• analytically solved in many
  steps to obtain
• F M f exp?(Tr-?)/Mz? ?-Ei(-g/Mz) Ei
  ?-(Tr-T0)/Mz ? ?
• g Tr-T at the end of the heating period
  (ttf)
• Ei(-x) is an exponential function, values of
  which are read from mathematical
  tables.
• Since (Tr-T0)/Mz has a high value, Ei
  ?-(Tr-T0)/Mz ? is very
  small, this term is usually neglected.

tf
tf
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• F M f exp?(Tr-?)/Mz? ?-Ei(-g/Mz) ?
• This equation relates the equivalent time to
  the processing temperature (Tr) and processing
  time (contained in g), for a given target
  microorganism of given z-value, for a certain
  food (heat transfer characteristics, contained in
  g and f)
• g Tr-T
• log j ? ( Tr T0 ) / ( Tr T ) ? (1/f)
  t

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Summary of heat process calculations

• Microbiological input Heat
  penetration input
• D, z-values for the target microorg. T
  vs. time data
• F, the equivalent time necessary
  f, j-values
• processing
  conditions
• initial
  temperature
• heating
  medium temp.
• cooling
  medium temp.
• established process (processing
  time to
• meet microbiological, heat
  penetration
• and processing requirements)
  

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Sterilization methods

• Mainly two methods
• Sterilization in containers,
• Sterilization before placing into the container
• Selection of sterilization method largely depends
  
• on the packaging material used
• - tin (metallic) cans
• - glass jars
• - film pouches

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Sterilization in containers

• Mostly carried out by heating the packaged foods
  in saturated steam
• Sterilization of low acid foods is carried out at
  temperatures above 1000C, therefore pressurized
  vessels (retorts) are used.

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• In retort operations it is important to
• a) have adequate venting of air from the
  retort
• and container surfaces to avoid air
  pockets,
• b) minimize thermal shock to the food,
• c) limit thermal and pressure strain on the
• containers by
• 1. control of heat-up, cool-down rates,
• 2. use of pressurized air during cooling
  to
• balance increased internal pressure
  in
• the container,
  
• 3. processing jars immersed in water

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• Internal pressure increase of containers
• Thermal expansion of food
• Thermal expansion of headspace gas
• Increased vapor pressure of water

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A vertical batch retort
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End-over-end agitation
Horizontal Orbitorts
Axial agitation
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Hydrostatic sterilizer
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Sterilization of food outside container

• High temperature processing (T?1500C) by means of
  high speed heat exchangers reduces
  processing time substantially (to few seconds)
  and improves product quality.
• Such processes are called high-short processes
  (HTST -applied to sterilization of milk).
• Improved product quality is due to the fact that
  destruction of nutrients and flavor components in
  foods (vitamins, colors, antioxidants, enzymes,
  amino acids) are similar to destruction
  of bacteria with considerably higher z-values.

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• Example For a certain food F10120 10 min is
  needed for commercial sterility. Two alternative
  procedures
• Heat food instantaneously to 1200C, hold at this
  temperature for 10 min and cool instantaneously.
  F10(T-?)/z tf 10(120-120)/10 x 10 10min.
• Heat food inst. To 1400C, hold at this T for
  0.1min and cool inst. F 10(140-120)/10 x
  0.1 10min.
• Suppose this food contains a valuable
  enzyme with a z-value of 50C0which requires 4 min
  at 1200C for inactivation. At 1400C time
  required for inactivation will be
• t 4 x 10(120-140)/50 1.6 min.
• Processing time needed Time needed
  for enzyme inactiv.
• Procedure 1 10 min
  4 min
• Procedure 2 0.1 min
  1.6 min

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Aseptic processing

• Sterilized food packed in sterile containers
  under aseptic conditions.
• Advantages
• Product with higher organoleptic and
  nutritional quality,
• Possibility to use large containers to pack
  the food,
• Extended possibilities for using packaging
  materials of many package sizes, shapes
  and materials,
• Handling of containers during subsequent
  sterilization is avoided, recontamination
  risk during cooling is minimized.

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• Limitations
• Large capital investment.
• Pumping at high pressures, product must be
  relatively homogeneous.
• Need for specific design of systems for a
  specific product.
• Complex operation requiring careful control and
  sophisticated instrumentation, need for highly
  trained personnel
• Relatively limited filling rate (200 packages
  per min. versus 600 tin cans per min