. In heat processing, to achieve microbial stability and eating quality both:The temperature of heating andThe 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 determi
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1. 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).
2. 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)
3. 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.
5. 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.
6. 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.
7. 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)
9. 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.
10. 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.
13. 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.
14. 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
16. 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?
20. 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.
23. 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 ( j=1.06-1.40 for conduction-, j?1.0 for convection heating) and the slope 1/f .
24. 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
25. Inserting dt = f M ? dT/(Tr-T) ? into the integral for equivalent time
F= ?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 (t=th)
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.
26. 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
28. 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)
29. 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
31. 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.
32. 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
33. Internal pressure increase of containers:
Thermal expansion of food
Thermal expansion of headspace gas
Increased vapor pressure of water
37. 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. But the z-values of nutient compounds are considerably larger than that of the microorganisms.
38. 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. F=10(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
39. 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.