Chapter III.

=Pepsin= is another enzyme the office of which in the process of digestion is to dissolve albuminoids. It is not considered identical with rennet though in an acid solution it will curdle milk. It occurs in the stomachs of grown animals fed on solid food and is usually produced from hogs’ stomachs.

Bacteria

Everywhere, in the air, in water, in the soil, and clinging to every object in the world, are minute organisms known under the common names of _bacteria_ or _microbes_. In contrast to rennet and the other “unorganized” ferments, bacteria belong to the “organized” class. Some are harmful, producing putrefaction, dissolution, poisons or disease; others are beneficial, leading to desirable fermentations and changes; others again are indifferent, neither good nor bad, but harmless.

As the milk comes from the cow it is almost free from bacteria, but milk makes an excellent soil for many of these organisms to grow in, and they soon get in, to multiply with enormous rapidity at any temperature from 60° to 100° F.

=Lactic Acid Bacilli= are bacteria of special importance to the dairyman, for they convert sugar of milk into lactic acid and produce various more or less agreeable flavors. They are also powerful germicides and scavengers, destroying or neutralizing the products of other bacteria which in the absence of these bacteria and the lactic acid produced by them would play havoc with the food and produce putrefaction or disease. Everybody who handles milk knows that pure sour milk or buttermilk in which lactic acid bacteria abound keeps well for a long time, free from other fermentations which have no chance to develop in their presence. It is due to this purifying property that Metchnikoff recommended Bulgarian sour milk as a health food, asserting that it prevents harmful fermentations in the digestive channel.

The Control of Bacteria

The principal means at our disposal to prevent or control and regulate bacterial growth are:

_Cleanliness_, _Heat_, _Cold_ and _Disinfectants_.

=Cleanliness.=—Only the most scrupulous cleanliness will prevent contamination. Hence the necessity of thorough scouring and sterilizing of all utensils, and the need for fresh air and pure water.

=Heat.=—The prevention of fermentation in milk by heating is called “sterilization” or “pasteurization” according to the intensity of the heat and the length of time the milk is subjected to it. Boiling destroys almost all bacterial life. Some germs require, however, exposure to a much higher temperature, up to 250° F. (boiling under pressure), to be entirely eradicated, but for all practical purposes a thorough boiling is considered sufficient to eliminate all danger of contamination. Such boiling is usually termed _Sterilization_.

=Cooling= checks bacterial growth and but few germs thrive at a temperature below 50° F. The following table and diagram[4] show how quickly bacteria multiply in milk at ordinary room temperature, 68° to 70°, which emphasizes the importance of keeping milk cool.

_Relative growth of bacteria when held at different temperatures_

──────────┬──────────┬──────────┬──────────┬──────────┬────────── │ No. of │ │ │ │ Tem. of │ bacteria │ At end │At end of │At end of │At end of Milk │ per c.c. │of 6 hrs. │ 12 hrs. │ 24 hrs. │ 40 hrs. │ at │ │ │ │ │beginning │ │ │ │ ──────────┼──────────┼──────────┼──────────┼──────────┼────────── °F. │ │ │ │ │ 50 │ 10 │ 12 │ 15 │ 41 │ 62 68 │ 10 │ 17 │ 242 │ 61,280 │3,574,990 ──────────┴──────────┴──────────┴──────────┴──────────┴──────────

If the milk had contained 1,000 bacteria per cubic centimeter at the beginning, the part held at 50° F. would have contained 4,100 bacteria at the end of 24 hours, while that held at 68° F. would have contained 6,128,000. The effect of temperature upon the growth of bacteria is shown graphically in the cut.

But even _frost does not kill_ the bacteria. If milk which has been kept sweet or at the desired degree of sourness by cooling is allowed to get warm again, the bacteria which have been kept dormant will get a fresh start. For this reason milk and cream for city supply should not only be cooled strongly, but must be kept thoroughly chilled up to the time they are used, which means that they should be delivered cool to the consumer, and kept on ice in the house, never being allowed to warm up until used.

=Disinfectants.=—Antiseptics, such as benzoate of soda, formalin, boracic acid, etc., are not permissible _in_ milk, but _disinfectants_ such as soda, lime, washing powders, etc., should be freely used in cleaning utensils, bottles, floors and walls where milk is handled, so as to prevent as far as possible any infection.

PASTEURIZATION

Pasteurization depends upon the fact that almost all bacteria, and especially the disease-producing species, are checked in their growth and made harmless, if not totally destroyed, by instantaneous exposure to a temperature of 175° F., or a more or less prolonged exposure to lower temperature, for instance for 20 minutes to 157° or 30 minutes to 145°. These lower temperatures are recommended in the case of new milk to be consumed as such, in order not to change its digestibility. Pasteurizers with “holding devices” are therefore largely taking the place of the “continuous” sterilizing machines, which allow only momentary exposure to the highest temperature.

Whatever method is used it is essential that after the heating is finished the milk should be cooled as quickly as possible to a temperature sufficiently low to prevent development of the germs that have not been entirely destroyed by the heat and which, if the milk is left for any length of time at a temperature favorable for bacterial life—anywhere between 65° and 110°,—will begin to grow again. Cool the milk to below 60° and if possible to 50° or 40°.

Sometimes a second pasteurization is practiced, the milk being purposely allowed to stand at 70° to 80° for 12 to 24 hours after the first pasteurization for the germs which escaped destruction to develop into full-grown bacteria. They are then killed by the second heating before the milk is finally chilled to stop any further growth.

Pasteurization can be done by placing the milk in any tin or enamelled or glass vessel, set in another vessel containing water (a double boiler). Heat until the milk has reached the desired temperature, hold it at that temperature the necessary time, and then place the vessel in cold running water or in ice water until the milk is thoroughly chilled. It is not advisable to place the hot milk in the ice box as cooling in air is too slow. Not until it is thoroughly cooled in water is it safe to put it in the ice box to _keep_ it cool.

In the seventies Prof. N. J. Fjord, in Denmark, applied to milk the process which had been developed by Louis Pasteur in France to give keeping quality to wine and beer. A Danish dairy expert, J. Moldenhauer, now connected with the New York State Department of Agriculture, first brought the process to this country and used pasteurization in a city creamery in Kentucky. American experiment stations established the temperatures and the time of exposure necessary for best results, and no one has done more for the introduction of pasteurization than the New York philanthropist, Nathan Straus, who has established pasteurizing plants and milk distributing stations in many localities, thereby contributing so largely to the lowering of the death rate among the children of the poor.

The following directions are given by the Straus Pasteurized Milk Laboratories of New York for the pasteurization of milk for babies:

1. Only use fresh, filtered milk, which has been kept cold, and proceed as follows:

2. Set the bottles, after they have been thoroughly cleaned, into the tray (a), fill them to the neck, and put on the corks or patent stoppers.

3. The pot (b) is then placed on a wooden surface (table or floor) and filled to the three supports (in the pot) with boiling water.

4. Place tray (a), with the filled bottles, into the pot (b), so that the bottom of the tray rests on the three supports, and put cover (c) on quickly.

5. After the bottles have been warmed up by the steam for five minutes, remove the cover quickly, turn the tray so that it drops into the water. The cover is to be put on again immediately. This manipulation is to be made very quickly, so that as little steam as possible can escape. Thus it remains for twenty-five minutes.

6. Now take the tray out of the water and cool the bottles with cold water and ice as quickly as possible, and keep them at this low temperature till used.

7. Before use, warm the milk—in the bottles—to blood heat. Never pour it into another vessel.

8. The milk must not be used for children later than twenty-four hours after pasteurization. Never use remnants.

In a _Continuous Pasteurizer_ a constant stream of milk is fed into the machine, heated by flowing over a metal surface with steam or hot water on the opposite side, and cooled by running over a cooler furnished with a stream of cold water or ice water.

PURE CULTURES

Before 1890 it was supposed that the flavor of fine butter depended upon certain volatile oils and acids peculiar to butter-fat. In the early nineties Professor V. Storch of the Danish Experiment Station showed, however, that it was due rather to the products of bacteria and he isolated the lactic acid bacilli which would produce such exquisite flavor even when perfectly neutral and tasteless butter-fat was churned with milk acidified or ripened with a pure culture of these bacilli. In this country Dr. H. W. Conn of Wesleyan University, Storrs, Conn., did much to advance the theory and practice of ripening cream with a pure culture starter.

“Pure cultures” are produced in the bacteriological laboratory by picking out under the microscope colonies of the desired species of bacteria, planting them in a sterilized medium and allowing them to grow under the most favorable conditions and with the exclusion of all other germs.

When such a culture has reached its maximum growth it is transplanted into a larger quantity of a sterilized medium containing proper nourishment for the particular organism. In the bacteriological laboratory, where alone absolute sterility of utensils and medium, and entire exclusion of foreign infection are possible, the culture may remain pure while this inoculation and propagation are repeated over and over again. But when the propagation is carried on in the house or the dairy, for instance in preparing starters or buttermilk, such absolute cleanliness is impossible and in the long run infections will creep in from the air or from the utensils and after a while it becomes necessary to start with a new “pure culture.” How often such renewal must be resorted to depends largely upon the surroundings and the care of the operator. Usually it must be done after a week’s time, although it is surprising to find milk preparations made by the simplest processes equal in purity to those prepared with the assistance of bacteriological science and technique. This is, for instance, the case in Bulgaria, where the famous Yoghourt sour milk is prepared pure without special care and in Denmark where the country is fairly permeated with the lactic acid bacilli used in ripening the cream for the celebrated Danish butter and where careful buttermakers often maintain their starters for months or even for years without “renewal.”

There are many different varieties of bacteria which convert sugar of milk into lactic acid, at the same time developing flavors more or less agreeable and characteristic for the various products. In the bacteriological laboratory certain species are selected which will produce the results desired for the particular purpose in view.

=Starters.=—Beginning with a commercial dry culture in the form of a powder as generally used in the creamery or the cheese factory as well as for the preparation of commercial buttermilk, or with buttermilk tablets as used in the ordinary household or the hospital, such culture is added to a small quantity of thoroughly pasteurized milk. If fresh, sweet skim milk is available it is preferable to whole milk as the butter-fat in the latter only interferes with the process; but either can be used.

Milk for starters should be strongly pasteurized by being kept at a temperature near the boiling point—at least 180°—for 40 to 60 minutes, then cooled to the degree at which it is to be set, usually between 65° and 75°, somewhat higher for the first propagation with the pure culture than for the subsequent transplantings when the bacteria, more or less dormant in the dry powder or tablets, have attained full vitality. Some species of bacteria, as the Bacillus Bulgaricus, require higher temperatures—90° to 100° or even 110°—than others. The culture having been thoroughly incorporated in the milk by vigorous and repeated stirring or shaking, the milk is left at rest in an incubator or a waterbath or wrapped in paper or cloth in a warm room where an even temperature can be maintained, until it is curdled, which may take 18 to 24 hours or even longer for the first propagation.

One part of this curdled milk is now added to 5 or 10 parts of fresh pasteurized milk and set to ripen in the same way as described above, possibly at a little lower temperature, and this is repeated every day, thus maintaining the “Mother Starter.” After the second or third propagation the bulk of each batch is used as a starter in the larger lot of material to be ripened, be it cream for butter or milk for cheese or for commercial buttermilk, while a little is taken for maintenance of the mother starter as described above.

The _amount of starter_ to prepare every day depends upon the amount of milk or cream to be ripened and the per cent of starter used in same. For instance, if you have ten gallons of cream to ripen every day in which you wish to use about 10% or 12% starter, or one gallon, take a little less than one pint of the first or second propagation for one gallon of milk; the next day use one pint of this to add to a gallon of fresh starter milk, and the remaining gallon to add to the ten gallons of cream, and so on every day.

If you have 4,000 lbs. of milk in the cheese vat to ripen with 2% or 80 lbs. starter, prepare 88 lbs. of mother starter. If, on the other hand, you wish to make only a quart of buttermilk every day, take, say, two buttermilk tablets, crush them thoroughly in a spoonful of pasteurized milk and stir this into a tumblerful of the same milk; let stand till it is thickened the next day and use a tablespoonful of this thickened milk in a quart of fresh pasteurized milk which when ripened is your buttermilk, from which you take out a tablespoonful for starter in the next batch, and so on. In this case there is no “mother starter” except that perhaps the first tumblerful prepared with the tablets may be called so, but afterwards the starter is taken right out of the finished product every day.

The process may be modified to suit special purposes and local conditions, but the following precautions should be strictly observed: (1) to interrupt the ripening immediately by quick and intense cooling as soon as it has reached the proper point in case the ripened product is not used at once, and (2) to _keep_ it ice-cold until it is used. If this is done, it may be kept for two or three days without deterioration if it is not convenient to make it fresh every day which, however, should be the rule.