Outlines of dairy bacteriology, 10th edition / A concise manual for the use of students in dairying

by H. L. Russell; E. G. Hastings

Filtration of milk. It is possible to remove all bacteria from water and other fluids and thus render them sterile by passing through filters of unglazed porcelain. This process can not be used with milk for the fat globules are larger than the bacteria (see Fig. 6) and any process that would remove the latter would also remove the former. The term "filtration" is applied to a process used in some European cities for the removal of the insoluble dirt that has been introduced into the milk. Suitable containers are filled with layers of coarse sand at the bottom and with finer sand at the top. The milk is introduced at the bottom and is forced upward through the sand. Such a filtering process is a very efficient means of removing the dirt; but unless the filters are kept scrupulously clean, the bacteria are likely to grow in the filtering material, so that the number of organisms in the milk may actually be increased by the filtering process. It is necessary to remove the sand daily and thoroughly wash and sterilize the same. The extra care required in keeping these sand filters in sanitary condition has been the great objection to their employment in this country. Filters of other material such as cellulose have been employed but with no marked success.

Clarifying milk. A much more efficient and less troublesome means of removing the insoluble foreign particles from milk is to pass it through a cream separator, allowing the cream and skim milk to mix in the same container. The slime that collects on the wall of the separator bowl is made up of dirt, casein, bacteria, and the cellular debris from the interior of the udder. The bacteria are heavier than the milk serum, and would, therefore, be deposited on the wall of the bowl were it not for other factors that in a measure prevent this. The movement of the fat toward the center of the bowl carries into the cream a considerable proportion of the bacteria in the milk. The slime will always contain many more bacteria than the milk, but the per cent of bacteria thus removed is relatively low, due to the small amount of slime obtained from the milk, so that the actual effect of clarification on the keeping quality of milk is insignificant. The complete removal of all insoluble and therefore visible dirt is, however, regarded of sufficient value to warrant the use.

Machines designed especially for the clarification of milk are now widely used. They differ from the cream separator in that the milk is introduced at the outside of the bowl and hence there is no separation of the fat from the serum. It is claimed that the removal of the dirt, cells from the interior of the udder and bacteria is as efficiently done as when the separator is used. The advantages claimed for the machine are that it has no effect on the subsequent gravity creaming of the milk and that less power is demanded than for the separator.

From the standpoint of the consumer, all processes by which dirt is removed from milk are objectionable, since they make the milk appear cleaner and better than it really is, the harm having been done when the dirt with the adherent bacteria found its way into the milk. The removal of the foreign matter that has been introduced into the milk will have but little effect in reducing the number of bacteria, since a large part of the organisms will have been washed off the insoluble material. All of these processes improve the appearance of the milk but have little or no influence in increasing its keeping quality or its healthfulness.

Preservation by cold. The only legitimate way of preventing the growth of bacteria in milk is by holding it at temperatures at which the ordinary forms of bacteria cannot thrive. Bacterial growth is greatly checked at temperatures approximating 50° F., or below, although certain types multiply at the freezing point or slightly above. If food products are actually congealed, no germ growth occurs, and they may be kept quite indefinitely, but this process cannot be successfully applied to milk, as the fat and casein are physically changed, so that a normal emulsion can not again be made when the frozen milk is melted. The fat separates in visible masses as though the milk had been partially churned. On account of this fact milk must be stored at temperatures above the freezing point. In Denmark efforts have been made to preserve milk, that is to be shipped long distances, by freezing a portion of the milk, and placing a block of the frozen milk in each can after cooling the main mass of milk nearly to the freezing point. Even this method has not proven practical, and at present reliance is placed on thorough chilling of the milk. At 32° F., the lactic bacteria cannot grow, but other types, such as certain of the putrefactive forms grow slowly; the milk may, therefore, have no objectionable odor or taste and yet be swarming with bacteria. In cities the practice is followed of placing cream in cold-storage during the cooler periods of summer in preparation for an increased demand, during hot weather or on holidays. It seems probable that poisoning from ice cream may, at times, be due to the use of such cream.

Preservation by the use of antiseptics. Many chemical substances prevent the growth of bacteria when added to food supplies; such substances thus used are called preservatives. In the past some of these have been used in milk to a great extent, but at present, on account of stringent pure food laws, they are employed only to a slight extent. There is a great temptation for the small milk dealer in the city to employ them to preserve the excess of milk from day to day, as through the use of a few cents worth of some preparation, many dollars worth of milk may be kept from spoiling until it can be sold to the unsuspecting consumer.

Formalin has been most widely used in milk because it is a most efficient preservative; it is cheap and cannot be detected by the consumer, although it injures the digestibility of the casein. One ounce will keep one thousand pounds of milk sweet for twenty-four to forty-eight hours. Borax, boric acid, and salicylic acid have also been used, but these substances must be employed in much larger quantities than formalin. Bicarbonate of soda has sometimes been used although it is not a true preservative. Its effect is based upon the neutralization of the acid produced by bacterial growth. The treated milk does not taste sour so quickly, and the curdling of the milk is also delayed.

Many proprietary compounds for milk preservation have been placed on the market in the past, but the use of all of these is illegal in most states. The federal law also prohibits their use in all dairy products that pass into interstate commerce.