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Research work has shown, however, that there is a variable proportion of nonspore-forming bacteria which are not destroyed at the Pasteurizing temperature. Fortunately, the disease-producing bacteria are not included in this class, for they are readily destroyed. It is the presence of these heat-resistant bacteria which must be considered in the interpretation of bacteria counts of Pasteurized milk.

There appear to be two general classes of these resistant nonsporeforming bacteria: First, those which have a low majority thermal death point and only a few cells survive; and second, those which have a high majority thermal death point. When milk is properly Pasteurized at 145° F., for 30 minutes, bacteria of the first class do not survive in large enough number to influence the count, but those of the second class often are responsible for high counts. It is at once apparent that milk might be properly Pasteurized and handled, and yet have a high count due to these heat-resistant organisms. Occasional high counts are due to these types of bacteria, but since these organisms are not pathogenic, it is difficult to see how they affect the safety of a Pasteurized milk, yet bacteria counts alone would indicate a defect in the Pasteurizing process. An occasional high count caused by these resistant organisms is merely another example of how the bacteria count of Pasteurized milk is useful as a means of indicating some conditions out of the ordinary.


Percentage reduction of bacteria by Pasteurization was formerly taken as an index of the efficiency of the process. Since the newer knowledge of the nonspore-forming bacteria with a majority thermal death point above the Pasteurizing temperature, the fallacy of such an index has been recognized. Generally speaking, about 99 per cent of the total bacteria are destroyed by Pasteurization at 145° F. for 30 minutes, yet samples of milk have been Pasteurized in this manner under controlled conditions in our laboratories, with a percentage reduction ranging as low as 82 per cent. This is due to the variable proportion of resistant nonspore-forming bacteria.


The colon test is sometimes used as an index of efficiency of the process and is of value provided it is properly used and its limitations understood.

Colon bacilli have a low majority thermal death point, but on account of the resistance of a few cells they may survive the Pasteurizing process. The colon test, therefore, as an index of the efficiency of the process of Pasteurization, is complicated by the ability of certain strains to survive a temperature of 145° F. for 30 minutes, and to develop rapidly when the Pasteurized milk is held under certain temperature conditions met during storage and delivery; Consequently, the presence of a few colon bacilli in Pasteurized milk under ordinary conditions, does not indicate that the milk was not properly heated.

The presence of a large number of colon bacilli, immediately after the heating process, indicates that the milk has not been heated to 145° F. for 30 minutes, and the test properly applied should be valuable in control work. Fermentation tubes can be used for making the test, but when gas formation is noted the presence of colon bacilli should be demonstrated by further tests. Often anaerobic spore-formers are encountered which survive Pasteurization and give the typical fermentation tube test. If the colon test is used, it should be applied to samples of the Pasteurized milk taken immediately after the process is completed.


There is a tendency on the part of some authorities to look upon the presence of streptococci in Pasteurized milk with suspicion. This situation has arisen because of the fact that streptococci are so often the cause of infections. But it must be borne in mind that there are more harmless than dangerous streptococci.

There have been extensive studies of pathogenic streptococci, and it has been found that they are destroyed by heating to 145° F. for 30 minutes. What has led to confusion is the lack of appreciation of the fact that there are heat-resistant species among the harmless streptococci.

There are two classes of streptococci which survive Pasteurization:

(1) Streptococci which have a low majority thermal death point, but among which a few cells are able to survive the Pasteurizing temperature. This ability of a few bacteria to survive may be due to certain resistant characteristics peculiar to them, or it may be caused by some protective influence in the milk.

(2) Streptococci which have a high thermal death point, and which, when such is the case, survive because this point is above the temperature of Pasteurization. This ability to resist destruction by heating is a permanent characteristic of certain strains.

These streptococci which have a thermal death point above the Pasteurizing temperature play an important part in the occasional high counts found in Pasteurized milk.

Among the streptococci of class 1 is the milk-souring organism, Streptococcus lactis, while those of class 2 seem to be closely related. These resistant streptococci are not only harmless but valuable, in so far as they cause the souring of Pasteurized milk and restrict undesirable decomposition by peptonizing bacteria.

Without a clear understanding of these facts, false interpretations may be given to the presence of streptococci in Pasteurized milk; and in view of the facts, it is not clear that a test for streptococci has any significance as a measure of the efficiency of Pasteurization unless it be to explain occasional high counts.


When bacteria counts are properly made of Pasteurized milk from a given plant, with the bacteria count of the raw milk running fairly uniform, and the Pasteurizing process accurately and properly controlled, the counts will be quite uniform from day to day for any given period of the year in which climatic conditions are the same. It sometimes happens, however, that the count may rise greatly for a few days and then drop again to normal.

This presents a problem for the laboratory either of the health department or the milk dealer. A jump in count of this nature may be due to at least three things: First, a lowering of the Pasteurizing temperature; second, heavy contamination after the heating process; or third, the presence of an unusual number of heat-resistant nonsporeforming bacteria.

The count alone does not indicate which of these three conditions may be the cause. If due to a low temperature or to contamination after heating it is serious; if due to an unusually high proportion of resistant bacteria no harm results, unless there are bacteria standards to be met or ratings for the milk dealer to consider.

A situation of this nature calls for cooperation between health authorities and milk dealers to locate the cause of the trouble. Since the count alone does not give an explanation, it is necessary to go to the plant. If the Pasteurizing temperature, holding period, and sterilization of apparatus and bottles are found to be satisfactory, then the high counts are quite certain to be due to resistant bacteria.

In this case the laboratory can readily detect the farm supplying milk with an unusually high proportion of resistant nonspore-forming organisms. To do this it is only necessary to take composite samples from individual shippers, then make a bacteria count of the raw milk, and again after Pasteurization, at the temperature used in the plant with same holding period. The Pasteurizing process must be carried on in sterilized containers under controlled laboratory conditions. Usually one or more producers can be found whose milk will show only a very low percentage bacteria reduction. If the dealer so desires, milk from such producers can be refused until bacteria flora of the milk becomes normal. Cleaning up at the farm, with sterilization of utensils, will usually remedy the trouble.

In my opinion, increased count due to an unusually high proportion of these organisms in raw milk is not a health issue. It is quite plain, however, that they complicate bacteria standards for Pasteurized milk.

The statement that occasional high counts are not a health issue must be qualified slightly. It is not a health issue if it is known by plant inspection that all the milk was heated to the proper temperature and handled without reinfection.

Within the last year or two there has been considerable agitation over the appearance of so-called pin-point colonies on plates made from Pasteurized milk. These very small colonies occasionally appear in large numbers in proportion to other more regular-sized colonies. Recent work in our laboratories has shown that these pinpoint colonies are due to a nonspore-forming thermophile. It is found in small numbers in raw milk and grows rapidly at the Pasteurizing temperature. This organism apparently causes a sort of accumulative contamination in Pasteurizing rats unless they are thoroughly steamed after the daily run. The organism is killed by heating for three minutes at 180° F.

There appears to be no cause for alarm over the presence of this thermophilic organism, for it appears commonly in raw and Pasteurized milk, and therefore has been consumed without giving evi

dence of causing trouble. It grows slowly at normal temperatures and is probably of less importance than many other bacteria found in milk.

The presence of these thermophiles probably indicates insufficient steaming of apparatus in milk plants, and in this connection its presence is suggestive. Further information about pin-point colonies will be published elsewhere.


Of the two most important steps which contribute to the efficiency of Pasteurization the first is to be sure that the proper temperature and holding period are used, and the second, to see that milk so treated is not reinfected with pathogenic organisms. Bacteria or chemical tests will not check these points, so other means must be found. Direct observation of the process at the plant by a person familiar with the bacteriological and mechanical details of the Pasteurizing process provides the best means of maintaining the efficiency of Pasteurization. I wish particularly to emphasize the importance of proper heating and holding, and also the prevention of reinfection.

First, let us consider the temperature and holding period. It is well known that pathogenic bacteria are destroyed when milk is heated to 145° F. for a period of 30 minutes. Every effort should therefore be made to see that Pasteurized milk be heated to this temperature and that every drop of milk be held for 30 minutes. Recording thermometers are the common instrument for supplying this information, but it is necessary to understand their peculiarities. A recording thermometer must be set so that its readings agree with a standard thermometer. Furthermore, it should be checked each day. If the bulb is in a tank during steaming it may not return to its proper point upon cooling, and so may read high. As a result, in the next Pasteurizing run, the temperature will be low. In some cases the chart may record the holding period, in other cases it does not. Fake records can of course be easily made.

Temperature control and the holding period are so important that a failure to meet requirements may nullify the whole process. Automatic temperature control is necessary, but no instruments have been devised to accomplish this and record the results without intelligent care in operation. Such care must be exercised, day in and day out, and there must be a realization by those concerned, of the seriousness of failure at this point in the process.

Unless health authorities can have an inspector at each plant, they must rely, to a large extent, upon the dealers to properly contros temperatures and holding periods, and no dealer can afford not to meet this obligation. It is my belief that the constant control which the dealer gives to this part of the process plays a predominating part in the efficiency of Pasteurization. When a milk dealer is found who is lax in temperature control, it must be due to ignorance of its importance, for no sane man would endanger his plant investment by improperly Pasteurizing and running the risk of an epidemic.


When milk is known to have been heated to the proper temperature and held the proper length of time, there can be complete assurance that it is free, at this point in the process, from living pathogenic bacteria. All that remains is to handle it in such a way that it reaches the consumer in this same condition. It is during this period, when cooling, bottling, and delivering the milk, that means must be taken to prevent reinfection by pathogenic organisms. The danger of such infection from coolers, bottlers, and bottles can be taken care of by a proper system for sterilizing. The effectiveness of a sterilizing system is a question of daily plant inspection, and here again much responsibility rests upon the dealer. Bacteria counts of washings over apparatus and from bottles serve as a check on the process.

There are other potential sources of infection of Pasteurized or raw milk, and any food, in fact, which deserve much thought. These are diseased employees and carriers. Efficiency in Pasteurization can not be complete without the elimination of danger of infection from such persons, and this applies as well to raw milk. No health department can feel that its control is complete, and no dealer can feel that his products provide unquestionable safety, unless all employees who handle milk at any time after Pasieurization have been medically inspected. It is a simple process for a person suffering from disease or a carrier to infect milk or any other food material. I realize it is difficult to provide a suitable system of medical inspection, but it must be accomplished. This sort of inspection is carried on in some cities and as its importance is realized it is certain to become general.


Efficiency in Pasteurization depends on

First. Maintenance of proper Pasteurization temperature and holding period.

Second. Prevention of reinfection by pathogenic organisms after Pasteurization.

The maintenance of these conditions can be best provided by intelligent supervision at the plant, and should be supplemented by bacteria tests properly applied and interpreted. An effective system of medical inspection for milk handlers must also be provided.

Chairman Weld. Mr. Avers' paper is one of the most important contributions to our list of papers on Pasteurization.

Dr. H. W. REDFIELD (United States Bureau of Chemistry). I would like to say that favorable investigations have been carried out, and they show that if milk is heated to 142° and kept at that temperature or above it will be absolutely safe milk and does not need to be heated to 145o.

Doctor HASTINGS. I want to express my appreciation of this most valuable paper. Mr. Ayers makes mention of the self-registering thermometers. It is absolutely essential that these thermometers be accurate at all times. We have these installed in all of our Pasteurizing plants. We imported them from Europe, and a daily

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