Principles of thermal processing

Canned foods subjected to thermal processing are not sterile and the processes are not designed to make them sterile. The success of thermal processing does not depend on the complete destruction of all microorganisms which would result in low product quality caused by the long heating required. Instead, all pathogens and most spoilage-causing microorganisms in a hermetically sealed container are destroyed, and an environment is created inside the package that does not support the growth of spoilage-type microorganisms and their spores. Indeed, together with the nature of the food (pH), environment (vacuum), hermetic packaging and storage temperature, the given heat process prevents the growth of microorganisms of spoilage and satisfies public health concerns. Hence, to determine the extent of heat treatment several factors must be known (Fellows, 2000): the type and the heat resistance of the target microorganism, spore or enzyme present in the food; the pH of the food; the storage conditions following the process; the heating conditions and the thermophysical properties of the food and the container shape and size.

10.3.1 Thermal resistance of microorganisms

Oxygen, pH and temperature sensitivity. In foods that are packaged under vacuum in hermetically sealed containers, low oxygen levels are intentionally achieved. Therefore, the prevailing conditions do not support the growth of microorganisms that require oxygen (obligate aerobes) and result in food spoilage or public health problems. Furthermore, the spores of obligate aerobes are less heat resistant than the microbial spores that grow under anaerobic conditions (facultative or obligate anaerobes). The growth and activity of these anaerobic microorganisms are largely pH dependent. From a thermal processing standpoint, foods are divided into three pH groups: (1) high-acid foods (pH < 3.7), (2) acid or medium-acid foods (3.7 < pH < 4.5), and (3) low-acid foods (pH > 4.5).

The most important distinction in the pH classification, especially with reference to thermal processing, is the dividing line between acid and low-acid foods. It has been generally recognized that C. botulinum does not grow and produce toxin below a pH of 4.6. Hence, the dividing pH between the low-acid and acid groups is set at 4.5. In the low-acid foods (pH > 4.5), destruction of C. botulinum spores is the primary concern in these processes. However, there may be other microorganisms, for example, Bacillus stearothermophilus, B. thermoacidurans and C. thermosaccolyaticum, that are more heat resistant than C. botulinum. These are generally thermophilic in nature (optimal growth temperature around 50-55°C) and hence are of little concern if the processed cans are stored at temperatures below 25°C.

Microbial destruction kinetics. To establish the thermal processing schedule, the thermal destruction rate of the test microorganism must be determined under the conditions that normally prevail in the container so that an appropriate heating time can be determined at a given temperature. Furthermore, because packaged foods cannot be heated to process temperatures instantaneously, data on the temperature dependence of the microbial destruction rate are also needed to integrate the destruction effect through the temperature profile under processing conditions.

Survivor curves and D-value. Evidence suggest that the thermal destruction of microorganisms follows a first-order reaction indicating a logarithmic order of death (Fig. 10.1). The microbial destruction rate is defined as a decimal reduction time (D-value), which is the heating time in minutes at a given temperature required to result in one decimal reduction (90% destruction) in the surviving microbial population. Graphically, this represents the time range between which the survival curve passes through one logarithmic cycle (Fig. 10.1). Mathematically:

where a and b represent the survivors following heating for t1 and t2 min, respectively.

Thermal death time (TDT) and D-value. In food microbiology another term is often employed, thermal death time (TDT), which is the heating time required to cause microbial death or destruction. TDT data are obtained by subjecting micro-bial population to a series of heat treatments at a given temperature and testing for survivors. TDT represents a time between the shortest destruction and the longest survival times. The death in this instance generally indicates the failure of a given microbial population, after the heat treatment, to show a positive growth in the subculture media. Comparing TDT approach with the decimal reduction approach, it can easily be recognized that TDT value depends on the initial microbial load (while the D-value does not).

Temperature dependence and z-value. The D-value depends strongly on the temperature employed. The temperature sensitivity of D-values at various tem-

10000

1000

Time at a constant temperature (min)

Time at a constant temperature (min)

Fig. 10.1 A typical survivor curve.

peratures is normally expressed as a thermal resistance curve with log D-values plotted against temperature (Fig. 10.2). The temperature sensitivity indicator is defined as a z-value, which represents a temperature range that results in a 10fold change in D-values, or graphically it represents the temperature range through which the D-value curve passes through one logarithmic cycle. Mathematically:

where D1 and D2 are D-values at T1 and T2, respectively. The D-value at any given temperature can be obtained from a modified form of equation [10.2] using a reference D-value (D0) at a reference temperature, Tr, usually 121°C for thermal sterilization):

Lethality concept. Lethality (F-value) is a measure of the heat treatment or sterilization processes. To compare the relative sterilizing capacities of heat processes, a unit of lethality needs to be established. For convenience, this is defined as an equivalent heating of 1 min at a reference temperature, which is usually taken to be 121°C for the sterilization processes. Thus the F-value would represent a certain multiple or fraction of the D-value depending on the type of the microorganism; therefore, a relationship like equation [10.3] also holds good with reference to the F-value:

The F0 in this case will be the F-value at the reference temperature (Tr). A

Thermal Resistance Microorganisms
Temperature (°C) Fig. 10.2 A typical thermal resistance curve.

reference (or phantom) TDT curve is defined as a curve parallel to the real TDT or thermal resistance curve (i.e. having the same z-value) and having a TDT (F-value) of 1min at 121°C. With a phantom TDT curve so defined, it will be possible to express the lethal effects of any time-temperature combination in terms of equivalent minutes at 121°C or lethality:

For real processes where the food passes through a time-temperature profile, it should be possible to use this concept to integrate the lethal effects through the various time-temperature combinations. The combined lethality so obtained for a process is called process lethality and is also represented by the symbol F0. Furthermore, with reference to the processing situation, the lethality can be expressed as related to a specific location (normally thermal center) or any other arbitrarily chosen location or integrated over the container. From a microbiological safety point of view, the assurance of minimal lethality at the thermal center is of utmost importance, while from a quality standpoint it is desirable to minimize the overall destruction.

10.3.2 Heat penetration curves

Simple time-temperature curves during heating and cooling by conduction and convection heating are shown in Fig. 10.3. The general and improved general methods of process calculation make use of this type of information. On the other hand, most formula methods make use of heat penetration data obtained from a

Time (min) Fig. 10.3 Typical heat penetration.

Time (min) Fig. 10.3 Typical heat penetration.

semilogarithmic plot of the temperature difference (TR - T, on log scale) between the retort (TR) and the product (T) against time (on linear abscissa) as shown in Fig. 10.4 (Jackson plot). This can be easily accomplished by rotating the semilog paper through 180° and labeling the top line 1°C below the retort temperature, then plotting temperatures directly (Fig. 10.4). The heating rate index (fh) can be obtained as the negative reciprocal slope of the straight line portion of the curve or time to cross one log cycle. The lag factor jh is obtained using the following relationship:

where TR is the retort temperature, Tih is the initial product temperature and Tpih is the pseudo-initial product temperature.

A similar plot of T - Tw, the temperature difference between the product and the cooling water temperature during cooling (Fig. 10.5) is used to obtain the cooling parameters. To plot the cooling curve, the semilog paper is kept in the normal position and the bottom line is marked 1°C above the cooling water temperature and the temperatures are plotted directly. From this plot, the cooling rate index fc and cooling lag factor jc can similarly be obtained.

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