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The Bacterial Content of Ice Cream¹

Kansas Agricultural Experiment Station, Manhattan, Kansas

ABSTRACT

In this work an effort has been made to determine the possibilities of producing ice cream with a low bacterial content under commercial plant conditions. The conditions under which the work was done and the products used in some of the mixes were far from ideal. Care was taken not to employ methods to lower the bacterial count that were impractical or that could not be followed by any manufacturer.

Since the purpose of the work was to determine the possibilities of producing ice cream with a low bacterial count, a slightly higher temperature of pasteurization was employed in most of the experiments than is required by the state law. The data presented show that it is possible to produce ice cream consistently having less than 100,000 bacteria per gram, even though in many instances the raw mix contained several million per gram. It is also quite evident from the data that the bacterial count of the finished product is more dependent upon the temperature and time used in pasteurization than upon the original number of bacteria in the raw mix. In this respect the bacterial count of ice cream may not reveal the use of raw products of poor quality. It is possible for a manufacturer to utilize a high temperature of pasteurization to conceal the low grade of ingredients used in the mix. It is quite probable that some manufacturers would resort to such practices if bacterial counts were being determined on the ice cream. It would at least have the desirable effect of stimulating careful pasteurization.

One point repeatedly observed throughout the progress of the experiments was that any carelessness on the part of the ice cream maker invariably resulted in a high bacterial count of the finished product. If the pasteurization time or temperature were reduced, utensils not thoroughly cleaned, the temperature during aging was not properly controlled, a high count in the finished ice cream was almost certain to result. This observation was so consistent that it was even possible to predict a high or low bacterial count of the finished product from the time and care used in its manufacture.

The results in this paper on aging of the mix may not be applicable to those plants using a longer period of aging than was in use with this work. These results show practically no change in the bacterial count before and after aging except in a few instances. If these mixes had been aged three to six days, as is occasionally practiced, there would probably have been a slight increase in the number of organisms after aging. The temperature used for aging is such that bacterial growth is very slow and is negligible in fifteen to eighteen hours. When long aging periods are used, a slight increase may be expected, but if the temperature is properly controlled, this increase should not be sufficient to cause an excessively high count in the finished product. However, if the temperature is not well controlled and is allowed to raise only a few degrees the bacterial content will be greatly increased. Increasing the temperature ten degrees may mean an increase of several hundred per cent in the bacterial count after twenty-four hours. This increase in the bacterial content may be constituted of perfectly harmless types of bacteria, and the ice cream made from the mix may taste just as good as though the bacterial growth had been checked. It is the contention of the writers, however, that the failure to control the temperature during the aging process is indicative of careless methods and careless methods in turn indicate an undesirable if not an unsafe product. Hammer and Sanders (12) state that, "while in most ice cream of high bacterial content Bacterium lactis acidi, a harmless type, predominates, the entrance and rapid multiplication of this organism occurs under conditions that make possible the entrance and multiplication of undesirable and possibly harmful types." Although it is true that ice cream having a high bacterial count due to a dirty plant, filthy utensils and poor raw products is much less desirable than a product having a high count due to growth of bacteria during the aging process, nevertheless it is difficult to determine to which factor the large number of bacteria is due.

This direct relation between the care of manufacture and the bacterial count indicates that a bacterial standard for ice cream would be particularly valuable. The increased use of ice cream emphasizes the necessity of controlling its manufacture. The consumer should be protected against the use of a product made under undesirable conditions. Since the bacterial count affords a fairly good check on the conditions surrounding the manufacture of ice cream, its use is recommended for such an index.

The problem of establishing standards is one that requires more data than is available at present. In this work ice cream has been consistently produced with less than 100,000 bacteria per gram by pasteurizing at 150°F. for thirty minutes. It remains to be shown what the bacterial count of ice cream should be when it is pasteurized at the minimum requirements of the present law, 145°F. for thirty minutes.

¹ Contribution No. 61 from the Department of Bacteriology and No. 48 from the Department of Dairy Husbandry of the Kansas Agricultural Experiment Station.