Most of us, quite reasonably, associate the pasteurization process with the problem of making milk safe to drink. Fewer of us are aware of how high a microbial load raw milk actually has when it comes from the dairy cow. Fewer still appreciate its pathogen content. Milk is a rather special food, with unique characteristics. The pasteurization cycles devised for milk often involve exposing milk to relatively high temperatures for very short lengths of time, the so-called high temperature, short time (HTST) pasteurization we are most familiar with. There are problems associated with this procedure. One of them is that it dramatically changes the flavor of milk, and not for the better. This is, presumably, one reason many consumers prefer to run the health risks of drinking raw milk.
In reality, there are many other foods, most of which are quite different from milk that are also pasteurized. For example, in Wisconsin the recommendation is that apple juice (or cider) having a pH below 3.9 be pasteurized at 155 degrees Fahrenheit for 14 seconds. In New York, the recommendation is that pasteurization of apple juice proceed at 160 degrees for 6 seconds. Is this an example of regulators in disagreement? No. Pasteurization efficacy is a function of both time and temperature. One is free to vary both, so long as a so-called "Five Log" reduction in bacterial numbers is met. One "Log" refers to a base 10 logarithm, or a factor of ten. A one log reduction means that 90 per cent of a population of microorganisms have been killed. After a one log heat treatment, the product of interest retains only ten percent of its original microbial numbers. Similarly, a two log heating cycle would leave only one percent of the original numbers. A five log heat treatment would therefore kill 99.999 percent of the microbial population originally present. If, for example, one can just barely see a slight bacterial turbidity in a clear medium containing bacteria, it means that the bacterial content is roughly one million bacterial cells per milliliter. A five log pasteurization cycle performed on a suspension containing a million bacterial cells per milliliter would be expected to leave only ten bacteria per milliliter [(1,000,000 - (99.999/100)(1,000,000)) = 10]. A company that has carefully analyzed the environmental microbial load of its manufacturing plant may feel that since the plant could only, at worst, introduce fewer than a thousand microbes per milliliter into the products it makes that it should be content to perform only a three log heat treatment to get a pasteurized product. Not so, because most regulatory language tends to define "pasteurization" as a five log reduction, whether the microbes are there to be destroyed or not. Of course, a three-log reduction from a microbial load of only 1,000 bacteria/milliliter would theoretically leave only one bacterial cell per milliliter, one tenth as many as a five-log reduction from a load of one million bacteria per milliliter. Nevertheless, the less safe product (in this example) could be labeled as "pasteurized" while the safer product could not be so labeled. Despite these apparent labeling irregularities, results like this could be of considerable importance to the HACCP plan put in place for this product.
To see why, imagine that the serving size of this hypothetical product is 250 ml and that the most active of the pathogens it might contain require, on average, one thousand cells to begin an infectious process. In the worst case, one may construe that the ten bacterial cells remaining surviving pasteurization will be the pathogen in question. Therefore, considering the worst case, a consumer ingesting one serving of this product might also ingest up to 2,500 pathogens per milliliter, more than enough to begin an infectious process. In the other case, even if the serving size is the same, the fact that only one bacterial cell remains after treatment will mean that a consumer ingesting one serving of the product will only have consumed one quarter of an infective dose and so is unlikely to get sick. Given how expensive recalls are to a company and the public relations damage they cause, it pays to be on the safe side, regardless of whether or not there is a marketing-based desire to make use of the word "Pasteurized" on the label.
It is also worth mentioning that most experiments showing the rates of kill consequent upon any particular sanitizing treatment seem to suffer from the same defect. They show an exponential decay in the numbers of the test bacterial culture. After one or two logs worth of bacterial killing, the experiment is usually terminated because the experimenter has enough points to establish a good curve fit and so is able to calculate a statistically significant decimal reduction value (D-value). Unfortunately, life is messier than that. There may often be considerable variation in the killing sensitivity of whatever lethal agent is being used against a given population of challenge microorganism. The presence of such resistant cells is often seen as a change in the slope (usually a reduction) of the killing kinetics towards the end of the treatment time and so is called "tailing." Over-reliance upon only the initial slopes will then tend to overestimate the efficacy of whatever treatment is being used leading to a HACCP plan that provides the manufacturer with a false sense of security.
Elsewhere on this website, the point was made that it is useful to keep a collection of the common organisms found in the plant, whether these come from plant surfaces, drains, water, ingredients or anywhere else. These are the microorganisms most likely to end up in the food products being manufactured. For that reason, it is prudent to use the most heat-resistant examples of known contaminants in a plant as the challenge organisms with which to validate the pasteurization cycles of your products.
Copyright © 2012 by M. Mychajlonka, Ph. D.