Thermal Death Time Curve

220 230 240 250 260 270 280 290 Temperature (°F)

Figure 13. Thermal death time (TDT) curve showing temperature dependency of D value (decimal reduction time of microorganisms) given by temperature change (Z) required for tenfold change in D value. Source: Ref. 3, reprinted with permission, copyright 1992 by Marcel Dekker, Inc., New York.

is 1 min at 250°F (121°C). Thus, if a process is assigned an F-value of 6, it means that the integrated lethality achieved by whatever time-temperature history employed by the process must be equivalent to the lethality achieved by 6 min of exposure to 250°F.

To illustrate, the example process calculation using the TDT curve in Figure 13 will be repeated by specifying the F-value for the required process. Recall from that example that the process was required to accomplish a 6-log-cycle reduction in spore population. All that is required to specify the F-value is to determine how many minutes at 250°F will be required to achieve that level of log-cycle reduction. The £)25o_value is used for this purpose, since it represents the number of minutes at 250°F to accomplish 1 log-cycle reduction. Thus, the F-value is equal to D250 multiplied by the number of log cycles required in population reduction, or

where a is the initial number of viable spores and b is the final number of viable spores (or survivors).

In this example, D250 = 1.16 min as taken from the TDT curve in Figure 13, and (logo - logft) = 6. Thus, F = 1.16(6) = 7 min, and the sterilizing value for this process has been specified as F = 7 min. This is normally the way in which a thermal process is specified for subsequent calculation of a process time at some other temperature. In this way, information regarding specific microorganisms or numbers of log cycles reduction can be replaced by the F value as a process specification. Note also that this F-value serves as the reference point to specify the equivalent process curve discussed earlier. By plotting a point at 7 min on the vertical line passing through 250°F in Figure 13 and drawing a curve with a slope of l/z parallel to the TDT curve through this point, the line will pass through the two equivalent process points that were calculated earlier (60 min at 235°F and 0.6 min at 270°F). Alternatively, the equation of this straight line can be used to calculate the process time (t) at some other constant temperature (T) when F is specified:

Equation 3 becomes important in the general case when the product temperature varies with time during a process, and the F-value delivered by the process must be integrated mathematically, such as at the center of a container of solid food.

Specification of Process Lethality. Establishing the lethality (F-value to be specified for a low-acid canned food) is undoubtedly one of the most critical responsibilities taken on by a food scientist or engineer acting on behalf of a food company in the role of a competent thermal processing authority. The steps normally taken for this purpose are outlined here.

There are two types of bacterial populations of concern in canned food sterilization. First is the population of organisms of public health significance. In low-acid foods with pH above 4.5, the chief organism of concern is Clostridium botulinum. A safe level of survival probability that has been accepted for this organism is 10"12, or one survivor in 1012 cans processed. This is known as the 12-D concept for a botulinum cook. Since the highest Z3250 value known for this organism in foods is 0.21 min, the minimum process sterilizing value for a botulinum cook assuming an initial spore load of one organism per can is

Essentially, all low-acid foods are processed far beyond the minimum botulinum cook in order to deal with spoilage-causing bacteria of much greater heat resistance. For these organisms, acceptable levels of spoilage probability are usually dictated by economic considerations. Most food companies accept a spoilage probability of 10"5 from me-sophilic spore-formers (organisms that grow and spoil food at room temperature). The organism most frequently used to characterize this classification of food spoilage is a strain of Clostridium sporogenese known as PA 3679 with a maximum £>250 value of 1.00. Thus, a minimum process sterilizing value for a mesophilic spoilage cook assuming an initial spore load of one spore per can is

Where thermophilic spoilage is a problem, more severe processes may be necessary because of the high heat resistance of thermophilic spores. Fortunately, most thermo-philes do not grow readily at room temperature; they require incubation at unusually high storage temperatures (110-130°F) in order to cause food spoilage. Generally, foods that show no more than 1% spoilage (spoilage probability of 10 ~2) upon incubation after processing will show less than the minimum 10"5 spoilage probability in normal commerce. Therefore, when thermophilic spoilage is a concern, the target value for the final number of survivors is usually taken as 10 ~2, and the initial spore load needs to be determined through microbiological analysis since contamination from these organisms varies greatly. For a situation with an initial thermophilic spore load of 100 spores per can and an average D250 value of 4.00, the process sterilizing value required would be

These procedural steps are guidelines for average conditions; they often need to be adjusted up or down in view of the types of contaminating bacteria that may be present, the initial level of contamination or bioburden of the most resistant types, the spoilage risk accepted, and the nature of the food product.

Heat Transfer Considerations

In traditional thermal processing of canned foods, containers are placed in steam retorts that apply heat to the out side wall. The product temperature cannot respond instantaneously, but will gradually rise in an effort to approach the temperature at the wall followed by a gradual fall in response to cooling at the wall. In this situation, the sterilizing value delivered by the process will be the integrated result of the time-temperature profile experienced at the slowest heating point of the container. This profile shape will depend in large part upon the mode of heat transfer experienced by the product.

Modes of Heat Transfer

Conduction-Heating. Solid-packed foods in which there is essentially no product movement within the container, even when agitated, heat largely by conduction heat transfer. Because of the lack of product movement and the low thermal diffusivity of most foods, these products heat very slowly and exhibit a nonuniform temperature distribution during heating and cooling, which is caused by the temperature gradient set up between the can wall and geometric center. For conduction-heating products, the geometric center is the slowest heating point in the container. Therefore, process calculations are based on the temperature history experienced by the product at the can center. Solid-packed foods such as canned fish and meats, baby foods, pet foods, pumpkin, and squash fall into this category. These foods are usually processed in still cook or continuous hydrostatic retorts that provide no mechanical agitation.

Convection-Heating. Thin-bodied liquid products packed in cans, such as soups, sauces, and gravies, will heat by either natural or forced convection heat transfer, depending upon use of mechanical agitation during processing. In a still cook retort that provides no agitation, product movement will still occur within the container because of natural convective currents induced by density differences between the warmer liquid near the hot can wall and the cooler liquid near the can center. The rate of heat transfer in nearly all convection heating products can be increased substantially by inducing forced convection through mechanical agitation. For this reason, most convection-heating foods are processed in agitating retorts designed to provide either axial or end-over-end can rotation. Normally, end-over-end rotation is preferred and can be provided in batch retorts; continuous rotary retorts can provide only limited axial rotation. Unlike conduction heating products, because of product movement in forced convection-heating products, the temperature distribution throughout the product is reasonably uniform under mechanical agitation. In natural convection, the slowest heating point is somewhat below the geometric center and should be located experimentally in each new case.

Broken-Heating. Broken-heating canned food products exhibit a break between the two modes of heat transfer; they will heat part of the time by convection and part of the time by conduction. The more common of these foods are those that initially heat by convection, then, because of starch gelatinization or other thickening agent activity, they set-up or thicken and proceed to heat by conduction. Less common are products that begin heating first by conduction, then for the remainder of the period heat by convection. Generally, these are products with solid pieces in a liquid brine that settle and pack into the lower two-thirds or so of the container when placed in the retort. After some time of heating, when convective currents become sufficiently strong, the solid pieces are lifted and disperse to begin moving with the liquid phase.

Heat Penetration Measurement. The primary objective of heat penetration measurements is to obtain an accurate recording of the product temperature at the can cold spot over time while the container is being heated under a controlled set of retort processing conditions. This is normally accomplished through the use of copper constantan thermocouples inserted through the can wall so as to have the junction located at the can geometric center. Thermocouple lead wires pass through a packing gland in the wall of the retort for connection to an appropriate data acquisition system in the case of a still cook retort. For agitating retorts, the thermocouple lead wires are connected to a rotating shaft for electrical signal pick up from the rotating armature outside the retort.

The precise temperature-time profile experienced by the product at the can center will depend on the physical and thermal properties of the product, size and shape of the container, and retort operating conditions. Therefore, it is imperative that test cans of product used in heat penetration tests be truly representative of the commercial product with respect to ingredient formulation, fill weight, head space, and can size and that the laboratory or pilot plant retort being used is capable of accurately simulating the operating conditions that will be experienced during commercial processing on the production-scale retort systems intended for the product. If this is not possible, then heat penetration tests should be carried out using the actual production retort during scheduled breaks in production operations.

Heat Penetration Curves. During a heat penetration test, both the retort temperature history and product temperature history at the can center are measured and recorded over time. A typical test process will include venting of the retort with live steam to remove all atmospheric air and then closing of vents to bring the retort up to operating pressure and temperature. This is the point at which process time begins, and the retort temperature is held constant over this period of time. At the end of the prescribed process time, steam is shut off, and cooling water is introduced under overriding air pressure to prevent sudden pressure drop in the retort. This begins the cooling phase of the process, which ends when the retort pressure returns to atmosphere and the product temperature in the can has reached a safe low level for removal from the retort. A typical temperature—time plot of these data is shown in Figure 14, which illustrates the degree to which the product center temperature can lag behind the retort temperature during both heating and cooling. The product center temperature history can be taken directly from this plot to perform a process calculation by numerical integration of equation 3; this will be discussed in further detail later.

A heat balance between the heat absorbed by the product and the heat transferred across the can wall from the

20 40

Process time (min)

Figure 14. Center temperature profile in cylindrical container of conduction-heating food product in response to constant retort temperature process during heating and cooling.

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