A common example is the retorting of canned foods to effect sterilization. The object of sterilization is to destroy all microorganisms, that is, bacteria, yeasts and moulds, in the food material to prevent decomposition of the food, which makes it unattractive or inedible. Also, sterilization prevents any pathogenic (disease-producing) organisms from surviving and being eaten with the food. Pathogenic toxins may be produced during storage of the food if certain organisms are still viable. Microorganisms are destroyed by heat, but the amount of heating required for the killing of different organisms varies. Also, many bacteria can exist in two forms, the vegetative or growing form and the spore or dormant form. The spores are much harder to destroy by heat treatment than are the vegetative forms.
Studies of the microorganisms that occur in foods, have led to the selection of certain types of bacteria as indicator organisms. These are the most difficult to kill, in their spore forms, of the types of bacteria which are likely to be troublesome in foods.
A frequently used indicator organism is Clostridium botulinum. This particular organism is a very important food poisoning organism as it produces a deadly toxin and also its spores are amongst the most heat resistant. Processes for the heat treatment of foodstuffs are therefore examined with respect to the effect they would have on the spores of C. botulinum. If the heat process would not destroy this organism then it is not adequate. As C. botulinum is a very dangerous organism, a selected strain of a non-pathogenic organism of similar heat resistance is often used for testing purposes.
has been found that microorganisms, including C. botulinum, are
destroyed by heat at rates which depend on the temperature, higher temperatures
killing spores more quickly. At any given temperature, the spores are
killed at different times, some spores being apparently more resistant
to heat than other spores. If a graph is drawn, the number of surviving
spores against time of holding at any chosen temperature, it is found
experimentally that the number of surviving spores fall asymptotically
to zero. Methods of handling process kinetics are well developed and if
the standard methods are applied to such results, it is found that thermal
death of microorganisms follows, for practical purposes, what is called
a first-order process at a constant temperature (see for example Earle
and Earle, 2003).
rates of destruction can in this way be related to:
Of course the surviving number must be small indeed, very much less than one, to ensure adequate safety. However, this concept, which includes the admissibility of survival numbers of much less than one per container, has been found to be very useful. From such considerations, the ratio of the initial to the final number of surviving organisms becomes the criterion that determines adequate treatment. A combination of historical reasons and extensive practical experience has led to this number being set, for C. botulinum, at 1012:1. For other organisms, and under other circumstances, it may well be different.
In this graph, these
times are plotted against the different temperatures and it shows that
when the logarithms of these times are plotted against temperatures, the
resulting graph is a straight line. The mean times on this graph are called
thermal death times for the corresponding temperatures. Note that these
thermal death times do not represent complete sterilization, but a mathematical
concept which can be considered as effective sterilization, which is in
fact a survival ratio of 1:1012, and which has been found adequate
Considering Fig. 6.4, the standard reference temperature is generally selected as 121.1°C (250 °F), and the relative time (in minutes) required to sterilize, effectively, any selected organism at 121°C is spoken of as the F value of that organism. In our example, reading from Fig. 6.4, the F value is about 2.8 min. For any process that is different from a steady holding at 121°C, our standard process, the actual attained F values can be worked out by stepwise integration. If the total F value so found is below 2.8 min, then sterilization is not sufficient; if above 2.8 min, the heat treatment is more drastic than it needs to be.
The other factor that must be determined, so that the equivalent killing powers at temperatures different from 121°C can be evaluated, is the dependence of thermal death time on temperature. Experimentally, it has been found that if the logarithm of t, the thermal death time, is plotted against the temperature, a straight-line relationship is obtained. This is shown in Fig. 6.4 and more explicitly in Fig. 6.5.
Also, if we define the z value as the number of degrees below 121°C at which t increases by a factor of 10, that is by one cycle on a logarithmic graph,
log 10F - logF = log (10F/F) = 1 = m[121 -
(121 - z)]
Therefore log (t/F) = (121 - T)/z
Now, the fraction of the process towards reaching thermal death, dS, accomplished in time dt is given by (1/t1)dt, where t1 is the thermal death time at temperature T1, assuming that the destruction is additive.
When the thermal
death time has been reached, that is when effective sterilization has
This implies that the sterilization process is complete, that the necessary fraction of the bacteria/spores have been destroyed, when the integral is equal to F. In this way, the factors F and z can be combined with the time-temperature curve and integrated to evaluate a sterilizing process.The integral can be evaluated graphically or by stepwise numerical integration. In this latter case the contribution towards F of a period of t min at a temperature T is given by t x 10-(121-T)/z Breaking up the temperature-time curve into t1 min at T1, t2 mm at T2, etc., the total F is given by
F = t1 x 10-(121-T1)/z + t2 x 10-(121-T2)/z + …………..
This value of F is then compared with the standard value of F for the organism, for example 2.8 min for C. botulinum in our example, to decide whether the sterilizing procedure is adequate.
Approximate stepped temperature increments are drawn on the curve giving the equivalent holding times and temperatures as shown in Table 6.2. The corresponding F values are calculated for each temperature step.
From the example, it may be seen that the very sharp decrease of thermal death times with higher temperatures means that holding times at the lower temperatures contribute little to the sterilization. Very long times at temperatures below 90°C would be needed to make any appreciable difference to F, and in fact it can often be the holding time at the highest temperature which virtually determines the F value of the whole process. Calculations can be shortened by neglecting those temperatures that make no significant contribution, although, in each case, both the number of steps taken and also their relative contributions should be checked to ensure accuracy in the overall integration.
It is possible to choose values of F and of z to suit specific requirements and organisms that may be suspected of giving trouble. The choice and specification of these is a whole subject in itself and will not be further discussed. From an engineering viewpoint a specification is set, as indicated above, with an F value and a z value, and then the process conditions are designed to accomplish this.
The discussion on sterilization is designed to show, in an elementary way, how heat-transfer calculations can be applied and not as a detailed treatment of the topic. This can be found in appropriate books such as Stumbo (1973), Earle and Earle (2003).
Pasteurization is a heat treatment applied to foods, which is less drastic than sterilization, but which is sufficient to inactivate particular disease-producing organisms of importance in a specific foodstuff. Pasteurization inactivates most viable vegetative forms of microorganisms but not heat-resistant spores. Originally, pasteurization was evolved to inactivate bovine tuberculosis in milk. Numbers of viable organisms are reduced by ratios of the order of 1015:1. As well as the application to inactivate bacteria, pasteurization may be considered in relation to enzymes present in the food, which can be inactivated by heat. The same general relationships as were discussed under sterilization apply to pasteurization. A combination of temperature and time must be used that is sufficient to inactivate the particular species of bacteria or enzyme under consideration. Fortunately, most of the pathogenic organisms, which can be transmitted from food to the person who eats it, are not very resistant to heat.
The most common application is pasteurization of liquid milk. In the case of milk, the pathogenic organism that is of classical importance is Mycobacterium tuberculosis, and the time/temperature curve for the inactivation of this bacillus is shown in Fig. 6.7.
An enzyme present in milk, phosphatase, is destroyed under somewhat the same time-temperature conditions as the M. tuberculosis and, since chemical tests for the enzyme can be carried out simply, its presence is used as an indicator of inadequate heat treatment. In this case, the presence or absence of phosphatase is of no significance so far as the storage properties or suitability for human consumption are concerned.
Enzymes are of importance in deterioration processes of fruit juices, fruits and vegetables. If time-temperature relationships, such as those that are shown in Fig. 6.7 for phosphatase, can be determined for these enzymes, heat processes to destroy them can be designed. Most often this is done by steam heating, indirectly for fruit juices and directly for vegetables when the process is known as blanching.
The processes for sterilization and pasteurization illustrate very well the application of heat transfer as a unit operation in food processing. The temperatures and times required are determined and then the heat transfer equipment is designed using the equations developed for heat-transfer operations.
From Fig. 6.7, pasteurization times tT can be read off the UK pasteurisation standard, and from and these and the given times, rates and fractional extents of pasteurization can be calculated:
Total pasteurization extent = (0.13 + 0.33 + 0.37) = 0.83.Pasteurization remaining to be accomplished = (1 - 0.83) = 0.17.
At 66°C this would be obtained from (0.17 x 5.4) min holding = 0.92 min.
So an additional 0.92 min (or approximately 1 min) at 66°C would be needed to meet the specification.
Heat-Transfer Applications > REFRIGERATION, CHILLING AND FREEZING
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