The Wisconsin curd test is made as follows: Samples of the milk to be tested are placed in sterile pint fruit jars. The milk is warmed to 90° F., ten drops of rennet are added to each sample, and as soon as the curd is solid, it is cut into small pieces with a case knife so as to facilitate the expulsion of the whey. As the curd settles to the bottom of the vessel, the whey is poured off at intervals so that a pat of firm curd is left. As the milk curdles the bacteria are enmeshed and are carried with the curd. The jars are kept at a temperature of 100° to 105° F., since this temperature is favorable to growth of the bacteria that are sought, the gas-forming organisms. At the end of ten to twelve hours, the jars are examined; if the curd is solid, the texture firm, not mushy or slimy on the surface, if the odor is agreeable, it indicates that the milk contains few or none of the undesirable forms of bacteria. If the curd is full of gas holes, it is apparent that undesirable bacteria are present and under such circumstances the curd will not have an agreeable odor. If the gas-forming bacteria are numerous, the curd may even be spongy from the abundance of gas holes, and the undesirable odor more pronounced. Such curds are tough and rubbery. In some cases a bad flavor or odor is apparent even though the texture of the curd is not open and full of holes. The curd, the surface of which is slimy indicates undesirable organisms. A solid curd of agreeable odor is indicative of the presence of the desirable acid-forming bacteria. Such a milk is excellent from the standpoint of the butter or cheese maker, but may not be so desirable from the standpoint of the milk dealer on account of its poor keeping qualities. On the other hand a milk suitable from the standpoint of the milk dealer, on account of its low germ content, and hence good keeping quality, may give a poor curd test. It is certain to contain some bacteria, especially those from the interior of the udder while it may contain none of the desirable acid-forming organisms without which a curd of good texture and flavor can not be obtained. The bacteria in the clean milk will grow rapidly at the high temperatures at which the curds are kept and the changes they will produce as to flavor and odor may be undesirable. The milk might be judged as poor when in reality it might be a most excellent sample, and if kept at the ordinary storage temperatures, it might keep for days. The test when used for market milk should be interpreted with this in mind.

If the results are to be of any value, the test must be made with care to avoid all sources of error; the tester must know that the bacteria causing the gas and bad flavors in the sample were originally present in the milk at the time the sample was taken, and that they have not come from the containers used or from other sources. To insure these conditions the jars must be thoroughly cleaned and then sterilized just before use by placing them in cold water and bringing them to the boiling point, or sterilized by a thorough steaming. The sample of milk of a patron must be taken so as to avoid contamination from the milk of the other patrons. This can best be done by filling the jars as the milk is poured from the patron's can into the weigh can. In cutting the curds, the knife used must be dipped in hot water between each test to cleanse the same. In short, the test should be carried out with great care so that the tester is certain of the results obtained.

Other tests for the bacteriological condition of milk will be described in Chapter IX.

Overcoming abnormal fermentations. The lactic acid bacteria are often looked upon as normal to milk, and it is certain that they are to be classed as harmful, only as they injure the keeping qualities of milk. In milk designed for butter and cheese their presence is necessary. At times these desirable forms of bacteria may disappear, and be replaced by less desirable types. In one case it was observed that the usual lactic bacteria had been replaced in a cheese factory supply by an acid-forming organism that produced an intensely bitter taste in the milk, thus rendering the cheese of no value. When such harmful forms appear, they must be overcome, and the normal types of bacteria replaced. A thorough cleaning of the milk utensils, attention to the cattle and all places from which such bacteria may find their way into the milk is often sufficient to cause a disappearance of the trouble. If the acid-forming bacteria have disappeared, the inoculation of the milk with cultures in ways later to be discussed is often of advantage. At times more stringent measures must be employed in order to destroy the harmful bacteria, such as the use of strong disinfectants.

Disinfection and disinfectants. If any building or room becomes infected with disease-producing bacteria, or if organisms causing abnormal fermentations become established in a factory, the use of a disinfectant that will destroy with great rapidity the life of bacteria is necessary. The disinfection of all types of dairy apparatus and utensils can be accomplished by thorough cleansing, and by the use of steam or boiling water. The disinfection of rooms and stables cannot be so readily accomplished.

Consideration must always be given to the resistance of the organism it is desired to destroy. Those that form spores are very resistant toward all chemical agents, while those that do not produce these resistant bodies are easily killed. In the dairy and factory, it is often necessary to destroy the organisms that develop in decomposing organic matter. Here, as in all disinfection, a thorough cleaning should precede the application of any disinfectant. Some chemicals act as deodorants, i.e., destroy the offensive odor, without removing the cause. It is impossible effectually to destroy bacteria embedded in a mass of organic matter, and through the removal of the material itself, the larger part of the bacteria will be removed. The disinfectant then comes in direct contact with the surface to be disinfected, consequently destroys the bacteria not removed in the cleaning.

All places in which dairy work of any kind is done should be provided with an abundance of light and air. The direct rays of the sun have a powerful disinfecting action, and light makes evident accumulations of dirt that in a darker room would be unnoticed. Ventilation keeps the rooms dry and thus prevents the growth of mold and the development of a musty odor.

Disinfectants are divided into two classes: (1) solid materials used in suspension, or in watery solutions; (2) gaseous substances. The latter are preferable for room disinfection when their use is permissible, for the gas penetrates to every part of the space, even into the cracks. Gaseous disinfectants can only be used when the space is tightly closed, for the gas must be confined for several hours in the room, in order to make the process effective. Such disinfectants can often be used to advantage in the treatment of refrigerators and cheese rooms to destroy mold spores. In less tightly closed spaces, reliance must be placed on the use of the solid or liquid disinfectants.

Lime. Quick lime or stone lime has a considerable disinfecting action. On exposure to the air, quick lime becomes air slaked, and then has no disinfecting action whatever. Water-slaked lime used in the form of white wash, lime water, or the powder is effective. Air-slaked and water-slaked lime are similar in appearance, but a difference can be noted by placing a particle of each on the tongue; the air-slaked tastes like chalk while the water-slaked material causes the tongue to burn.

White wash is one of the most effective agents that can be used in the disinfection of barns, milkrooms, etc. Besides being a fairly strong disinfectant, it has a tendency to absorb odor, it encrusts the walls and lightens the interior of rooms. It can be applied with a brush or with a spray pump.

Carbolic acid and cresol compounds. These substances are among the cheapest and best disinfectants, but their use in the dairy is not advisable, on account of the penetrating and lasting odor. They can be used to advantage on the farm. Some of the proprietary compounds, as Zenoleum, Kresol, etc., are easily applied, since they mix readily with water in all proportions, forming a milky-white emulsion that can be easily applied. They are less caustic and less poisonous than carbolic acid.

Corrosive sublimate. Corrosive sublimate is the most efficient disinfectant under ordinary conditions. It is such an intense poison that it must be used with caution in places to which stock have access, or in the dairy. A solution of one part of the salt to a thousand parts of water (half ounce to 4 gallons of water) is the standard generally used.

For gutters, drains, and waste pipes in factories, ferrous sulphate (green vitriol), and copper sulphate (blue vitriol), can be used to advantage. They are to be classed as deodorants rather than as true disinfectants. Since they have no odor of their own, they can be used in any amount in the dairy.

Sulphur can be used to advantage in the destruction of mold spores in cheese rooms, but the effect of the vapors of burning sulphur on germ life is relatively slight, unless there is an abundant supply of moisture in the air of the enclosed space, in which case sulphurous acid is formed which has a much greater effect. To have the desired effect sulphur should be burned at the rate of three pounds to each one thousand cubic feet of space, and the room kept sealed for at least twelve hours. If the sulphur is placed in an iron kettle which is set in a vessel of water, danger from fire will be avoided, and the heat generated by the burning sulphur will evaporate sufficient water to increase the effect of the fumes.

Formalin. Another disinfectant that may be used as a liquid or as a gas is formalin, which is a watery solution of the gas, formaldehyde. It is much more powerful in its action than sulphur, and has a great advantage over corrosive sublimate and other strong disinfectants in that it is not so poisonous to animals as it is to bacteria and fungi.

It can be used as a solution (one to five per cent) for the washing of woodwork, or for the treatment of any object, since it has no corrosive action. It can also be employed as a gaseous disinfectant for the treatment of rooms. It is most conveniently applied by suspending large cloths in the room and spraying them with the solution, then closing the room for a number of hours.

Bleaching powder. Chloride of lime, or bleaching powder as it is often called, is a good disinfectant, as well as a deodorant. It is used as a wash in the proportion of four to six ounces to a gallon of water. It must be used with care in factories since the free chlorine that is given off has a penetrating odor.

CHAPTER VI.

It might naturally be supposed that any method by which dirt is removed from milk would improve the keeping quality of milk, due to the reduction of bacteria, yet while the straining of the milk at the time of milking removes dirt of various kinds, it does not appreciably enhance the keeping quality, owing to the fact that the bacteria adherent to the dirt particles are washed off in straining, and pass through the pores of the strainer.

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.

Within recent years a method for the preservation of milk was introduced by a Danish engineer, Budde, which consists of adding to milk a very small amount of peroxid of hydrogen which is a very efficient antiseptic. The peroxid is decomposed by some substance in the milk; the products of decomposition being water and free oxygen. The peroxid together with the application of heat at a comparatively low temperature (122° F.) is sufficient to destroy the larger part of the bacteria in the milk. Practical difficulties are encountered in the commercial application, so that it is probable the process will never be a commercial success.

For the preservation of composite samples of milk for analytical purposes, such as the Babcock test, strong disinfectants, as corrosive sublimate, are employed. This material is very poisonous, and leaves the milk unchanged in appearance. Some coloring matter is therefore usually mixed with the sublimate in making the preservative tablets, so as to render their use more conspicuous. Corrosive sublimate not only stops all bacterial growth, but quickly destroys the life of the cells. Bichromate of potash is generally employed in the preservation of composite samples for the Hart casein test.

Destruction of bacteria in milk. Actual destruction of the life of bacterial cells by heat is one of the most important ways for preserving milk. Heat easily destroys the vegetating, growing bacteria, while the spores, of which there are always a number in milk, are very resistant. If, however, the growing organisms are destroyed, the milk will keep much longer than if it had not been so treated.

The process of pasteurization was first used by the French bacteriologist, Pasteur, for the treatment of the wines of his native district which were likely to undergo undesirable types of fermentations due to bacteria. From the wine industry it was applied in the brewing industry, and was later found to be of the greatest service in the dairy industry. The process of pasteurization may be briefly defined, as the heating of milk to temperatures, varying from 140° F. and upward for a longer or shorter time, and subsequently cooling to a low temperature, so as to prevent the germination of the spores that are not destroyed by the heating.

Effect of heat on milk. When milk is heated it undergoes more or less profound changes, depending on the temperature and time of heating. Some of these changes are of practical importance, since they are more or less evident, and objectionable to the consumer.

In raw milk the fat globules are largely found in larger or smaller aggregates, rather than uniformly distributed throughout the serum. The surface of a mass of fat globules is smaller in proportion to the volume of the mass than is the case with single globules, hence globule clusters encounter less resistance in their passage through the serum, either as they rise to the surface in gravity creaming, or in the separator bowl. If these clusters are broken up, so that the globules are uniformly distributed, the milk will cream much less rapidly and completely. In the process known as "homogenization" of milk, the individual fat globules are broken into such small globules, that they cannot overcome the viscosity of the serum, and they remain distributed throughout the milk. In such cases, no cream rises, and even the cream separator is unable to remove the fat from such milk.

In selling bottled milk, it is highly desirable that the cream line should show distinctly. In normal milk, this line forms in a few hours, but where milk is heated to a high temperature, and agitated at the same time, the clusters of fat globules are broken apart and the creaming power injured. This physical change is dependent not only on the temperature, but also on the time of exposure. A momentary exposure at 160° F., or for 20 minutes at 145° F., is about the maximum limit which can be applied to milk without material injury to the creaming property.

The body or consistency of pasteurized cream may be restored by allowing the cream to stand for several days at low temperatures, or by the addition of a small amount of sucrate of lime. This substance, known to the dairy trade as "viscogen," is made by adding to a thick solution of cane sugar, some freshly slaked lime. The sugar solution permits of the dissolving of a much larger amount of the lime than is possible in water. When the liquid is allowed to settle, the clear solution is then decanted off and is used at the rate of about one part to 100 to 150 parts of cream. The fat globules are, by its action, brought into aggregates and the body of the cream thus restored. Viscogen contains nothing that is at all harmful, but milk and cream to which it is added must be sold under some distinctive name as "visco-cream," since the laws of practically all states do not allow the addition of any substance whatever to milk or cream.

Heated milk has a taste unlike that of raw milk; to one not accustomed to it the taste is objectionable. This change is due to some extent to the expulsion of the carbon dioxide from the milk. The insipid taste of boiled water is, in part, due to its freedom from carbon dioxide. The production of this cooked flavor is dependent upon the time and temperature of exposure. It has been claimed that heated milk is less digestible than raw, and a considerable amount of experimental work has been done, both on animals and children, in order to determine the relative digestibility of heated and raw milk. The results obtained have been contradictory. It is claimed that heated milk causes such diseases as rickets, scurvy and marasmus in children. It is probably true that milk heated to the boiling point is less fitted as food for the young child than raw milk, but, on the other hand, it has not been proven that properly pasteurized milk is an unsuitable food for children. The best evidence has been accumulated in recent years, in many of the large cities of this country and of Europe, where pasteurized milk has been used with the greatest success in the feeding of children of all ages.

The heated milk does not curdle readily when rennet is added due to the precipitation of the lime salts by heat. The curdling power can be restored by the addition of soluble lime salts or of acids.

Purpose of pasteurization. There are two reasons for the pasteurization of milk: (1) To improve the keeping quality; (2) To destroy any pathogenic bacteria it may contain. The first may be called the economic reason; the second, the hygienic reason fur pasteurization. In the selection of a proper pasteurizing temperature, two factors must be taken into account: First, the effect of heat on milk, and second, the temperature necessary to destroy those forms of bacteria that are of the greatest importance, as far as the keeping properties are concerned, and the pathogenic bacteria that might possibly be present in the milk. The lactic acid bacteria are non-spore-bearing and are not resistant to heat. Most of them are destroyed when the milk is heated to 140° F. for fifteen minutes or to 160° F. for a moment. To insure proper keeping quality, somewhat higher temperatures must be employed, such as 145° to 150° F. for fifteen to twenty minutes.

Milk pasteurized at these temperatures will, as a rule, undergo an acid fermentation in much the same manner as will raw milk. The rate with which the acid develops is of course much slower than in the raw milk, due to the destruction of 95 to 99 per cent of the acid-forming bacteria. If the milk has been pasteurized at higher temperatures, the acid fermentation may not appear. The spores of the spore-bearing organisms will be left; these may germinate and cause their characteristic change in milk, which, as previously noted, is usually a sweet-curdling or a digesting fermentation. Since the changes they produce in the milk are not evident at first, it might be used as food even though it was so far advanced in decomposition as to be undesirable or even harmful as food. Indeed one of the objections urged against pasteurization is that it destroys the natural safe guard, the acid-forming bacteria. Many people are so accustomed to use this as the indication of spoiled milk that they will use milk long after it should be used if it does not show an acid fermentation.

The butyric acid organisms are spore forming and may at times produce their characteristic fermentation in pasteurized milk. The milk shows gas formation and develops an objectionable odor.

The pathogenic bacteria most likely to be present in the milk are the typhoid and the tubercle organisms. The typhoid bacillus is no more resistant to heat than the ordinary acid-forming bacteria, and all milk that has been heated, so as to impart to it satisfactory keeping properties, will certainly be free from typhoid bacilli. It has sometimes been asserted that the tubercle bacillus is very resistant to heat; some claiming that it is necessary to heat milk to 200° F. in order to destroy it. Other experimenters have asserted that lower temperatures would suffice, but the temperatures were still above those at which the milk is physically and chemically changed by the heating process. More recent work has shown that not all sources of error were avoided in the earlier attempts to determine the thermal death point of the tubercle bacillus, as, for example, it has been shown by the authors that the "scalded film" that forms on the surface of milk when heated in an open vessel will protect the bacteria imbedded in it. It has also been shown by the authors that a temperature of 140° F., for twenty minutes or 160° F. for one minute will destroy the tubercle bacilli in milk, in case the heating is done with sufficient thoroughness to insure all particles of the milk being heated to the same temperature for these periods of time.

The pasteurization of milk can be done in such a manner as to impart to it good keeping qualities and to insure its freedom from pathogenic bacteria, and yet not impair its physical and chemical properties, but much of the so-called pasteurized milk placed on the market is not treated in accordance with proper hygienic methods.

Methods of pasteurization. In order to destroy the bacteria in milk, it is necessary that the milk be heated for a varying time dependent upon the temperature employed. A lower temperature for a considerable period may exert the same effect on the bacteria as a higher temperature for a shorter time. In practice, two types of pasteurizing machines are employed, depending on the temperature at which the milk is to be treated. The discontinuous machines or intermittently operated pasteurizers are those in which the milk is heated for any desired time at any temperature. Such machines consist of jacketed containers the inner receptacle being filled with milk, while the outer space between the walls is filled with circulating hot water or steam. The milk is kept agitated by the rotation of the machine. After it is heated, it is cooled in the same container by replacing the hot water first with cold water, then ice water. The disadvantage of this process is that the capacity of the machine is limited which precludes its use in places where large quantities of milk or cream are handled; for the pasteurization of limited quantities, it is very successful, as every particle of milk or cream is under the direct control of the operator and may be thoroughly and efficiently treated.

As pasteurization was introduced for the treatment of market milk, and for the preparation of cream for butter, machines have been devised which permit large quantities, as thousands of pounds, to be handled per hour. It is evident under these conditions that the milk must be heated for only a short time, and hence a higher temperature must be employed. These machines are called "continuous flow" pasteurizers since the milk passes through them in a constant stream. The period of exposure is very short, in some only a few seconds; hence, they are sometimes called "flash" pasteurizers.

All machines of this type possess the obvious disadvantage that it is impossible to heat all of the milk for a uniform period. The milk in contact with the walls of the machine flows much more slowly than in the middle of the stream, just as the current near the bank is less rapid than in mid-stream. In none of the machines yet devised have the designers been able to overcome this disadvantage. In a test of one of the most widely used pasteurizers of this type, it was found that some of the milk passed through the machine in 15 seconds, while the larger part of it was held for about 30 seconds, and some as long as forty-five to sixty seconds. If the temperature employed had been such as to destroy the bacteria in that part of the milk heated for the minimum time, hygienic safety would be assured, but in order to avoid injuring the physical properties of the milk, the tendency is to use as low a temperature as possible, so that the milk heated for the minimum time may often contain organisms that have passed through the machine uninjured.

Many devices have been proposed for the heating and cooling of the milk. In many of the pasteurizers, the milk flows in a thin stream over a metal surface, on the opposite side of which is the heating agent, usually steam; while in others, the milk is allowed to flow through a vat in which revolve a series of discs into which steam is passed. The discs are of considerable size; thus, making a large heating surface; the milk is thus heated quickly, and is constantly stirred by the rotation of the heating discs. In other types the milk passes into the bottom of a chamber in which a dasher revolves at a rapid rate. This catches the milk, throwing it in a thin film onto the wall of the chamber, which is heated with steam on the opposite side. From such machines, of which the Fjord, the Jensen, and the Reid machines are types, the milk may be forced to a considerable height. These are widely used in this country for the pasteurization of milk and cream for butter making.

Milk that has been heated must be cooled at once by the use of cold water and ice. In order to economize in the use of both steam and cooling agents, the so-called regenerative machines were devised. The essential feature of these machines lies in the fact that the cold milk inlet and the hot milk outlet are on opposite sides of a single partition; thus the inflowing cold milk is partially heated by means of the already treated hot milk which it is desired to cool.

In order to avoid the disadvantages of the continuous machines, viz., lack of control, an apparatus has recently been devised which can handle large quantities of milk, heating the same to any temperature for any desired time. In such a machine the milk is first heated in a continuous heater, and is then passed into large tanks in which it is allowed to remain for the desired time, and from which it flows over the coolers. Such an apparatus is called a "holding" machine, and is probably the most feasible type of pasteurizer now on the market, when all factors are considered. In some of the continuous machines, an attempt is made to accomplish the same result, by building the machine so that the milk requires fifteen to twenty minutes for passage through the machine, but in all such cases the same disadvantage of variation in rate of flow, as in other continuous flow type of machines obtains.

Tests of pasteurizing machines. It is possible for the operator to test the rate of flow in a machine, so as to determine whether all of the milk is heated for a uniform time. This is done most easily in the following manner: The machine is first filled with water, heating the same to the desired temperature, and regulating the rate of flow as it would be if milk was used. The flow of water is then turned off, and a stream of milk containing a known per cent of fat admitted to the machine. The time elapsing between the admission of milk to the machine, and that at which the first sign of turbidity is noted at the outlet, will be the minimum period necessary for any portion of the milk to flow through the machine. At frequent intervals thereafter, samples of the outflowing liquid may be collected, noting the time at which each sample is taken. The percentage of fat in the various samples is determined by the Babcock test; at the moment when all of the water has been removed, the sample taken will show the same fat content as the milk used. The samples taken previous to this will show a lower fat test, dependent upon the relative amount of water and milk. In this manner, the minimum, the maximum, and the average period of exposure of milk in the machine tested, can be determined with exactness.

The accompanying table gives results that were obtained in the testing of one of the continuous types of machines. The machine in question required about three hundred pounds of milk to fill it and was supposed to handle 1,000 pounds per hour. Thus theoretically it should require twenty minutes for any portion of the milk to pass through the machine. As will be seen from the data, some of the milk passed through within seven minutes after the water was shut off and the milk turned on. The figures also show that not all of the water had been replaced by the milk in even 45 minutes. In actual practice like results will be obtained, and a portion of the milk will be heated to the temperature employed but a short time. In this, the vegetating bacteria will not be wholly destroyed.

Pasteurization of small quantities of milk. It is often desirable to treat a small quantity of milk for home use, in which case the commercial types of pasteurizers are out of the question. This treatment can be done in a number of ways, consideration always being paid to the manner of heating which should be done under such conditions, as have been shown to be necessary for efficient pasteurization. Milk may be heated in tall, narrow cans which are placed in hot water. In the household, milk may be treated by placing the filled bottle in a pail having a false bottom so the bottle shall not be broken when the pail is placed on the stove. The pail should be filled with water so that its level is about the same as that of the milk. The water is then heated to the desired temperature, maintained for the requisite period of time, and is then cooled as rapidly as possible. During the heating, the mouth of the bottle should be covered, either with an inverted glass tumbler, or the paper cap may be left in place, simply punching a small hole through it so as to permit of the insertion of a thermometer.

Efficiency of pasteurizing. It is easy to destroy over 99 per cent of the bacteria present by the use of any of the modern types of machines. The number remaining after treatment will be largely dependent, other things being equal, upon the number of bacteria before pasteurization. The pasteurizing process is not one by which poor milk can be changed into good milk, nor is it legitimate to use the process in place of cleanliness, as is sometimes done. There is a legitimate field for the process in the handling of market milk, as well as in the creamery; but it should be used to improve the keeping quality, and to insure the freedom of the milk from pathogenic bacteria, when other protective measures have been carried as far as possible under the prevailing conditions.

Details of process. If the process is to be successful, due attention must be given to certain details. In the treatment of market milk, care should be taken to use only that in which the acidity has not materially increased. A fair standard is about 0.2 per cent. High acid milk usually means old milk or dirty milk, either of which is very likely to contain many more spore-bearing bacteria than clean, fresh milk. The greater the number of spores, the more rapidly will the pasteurized milk spoil. If it is possible to exercise any selection of milk prior to pasteurization, the rapid test for determination of acidity will prove of great advantage.

Care should be taken to prevent fluctuations in the temperature to which the milk is heated. With varying steam pressure and variations in the rate of flow of milk, these fluctuations may be very considerable. Regulators are now made that will control the temperature within narrow limits.

In all pasteurized milk as it flows from the machine, there will remain some living bacteria. The spores will not be destroyed by any pasteurizing process, and under commercial conditions, vegetating bacteria are also present. If the milk is not quickly chilled after heating, these forms will grow, and their development is particularly hastened by the destruction of the lactic bacteria, the acid of which would otherwise hold them in check. The result is that, unless immediately chilled, pasteurized milk spoils almost as rapidly as though it had not been heated at all. Efficient and rapid cooling are, therefore, as essential a portion of the process as the heating itself.

Care should also be taken to protect the milk from contamination after treatment. Every utensil with which it comes in contact should be sterilized. The bottles should be thoroughly washed and sterilized and subsequently protected from dust until used.

Sterilization of milk. It is possible to render milk sterile by the use of temperatures above the boiling point of water, where it is heated in a closed vessel, in which steam under pressure is generated. Such milk is often found in the European markets. In our own country, the only milk of this kind is the so-called "evaporated milk." In this process sweet fresh milk is evaporated in vacuum pans to about one-third of the original volume. This is then placed in tin cans, which are treated, as in the canning of such vegetables as peas and corn, by heating the milk to 230° or 240° F. for a few minutes. In this process, the bacteria (spores as well as vegetating forms) are completely killed, and the milk acquires a brownish tint, due to the caramelization of the sugar. The appearance of the product is very similar to cream, and previous to the passage of the pure food law, it was sold as evaporated cream.

Condensed milk is not wholly free from bacteria, but is sufficiently thick, by reason of its treatment so that the contained bacteria cannot grow. They remain dormant in the milk, but as soon as it is diluted to a normal consistency, growth takes place, and the milk rapidly spoils. Condensed milk is prepared by adding cane sugar to fresh sweet milk, then evaporating the mixture to one-third the original volume, forming a semi-solid product. Syrups owe their keeping qualities to the same factor, as condensed milk, i.e., the high consistency.

Milk is also preserved by wholly evaporating the water, thus leaving a dry powder, which on being mixed with water again will have much the same properties as the original milk. Various methods have been devised for the preparation of these milk powders, all of which have been patented by the inventors. If the powder is to be kept for long periods, skim milk must be used, since the fat slowly undergoes changes which cause it to have a rancid odor. These dry preparations are largely used by bakers in place of fresh milk.

CHAPTER VII.

Methods of creaming. In the shallow-pan method of creaming, the milk is kept at ordinary room temperatures. These temperatures favor especially the growth of the acid-forming bacteria. The milk is usually sour by the time the cream is removed from it; consequently, the bacterial content of the cream is high. Moreover, the cream is exposed to air contamination, and is thus seeded with molds, and those forms of bacteria that are always found in the air. The cream obtained in this manner is likely to contain not only numerous bacteria, but a great variety of forms, some of which undoubtedly are the cause of the poor keeping qualities of butter made from such cream.

In the more modern method of gravity creaming, in which the milk is placed in deep narrow cans kept in cold water, the conditions are not favorable for the growth of acid-forming bacteria. If the milk is produced under clean conditions, and is placed in cold water at once, the bacterial content of the cream will be low, and it will be less likely to contain undesirable forms than the cream which is obtained from the shallow pans.

In separator cream the bacteria will be represented by the kinds present in the milk at time of separation. If this milk is quite old, the cream will contain large numbers of bacteria; if, however, early separation is made and the milk is clean, the bacterial content of the cream will be low.

Types of butter. Butter may be divided into two types—acid or sour-cream, and sweet-cream, depending upon whether the cream is allowed to undergo the acid fermentation or not before it is churned. In southern Europe, it is the custom to churn the cream as sweet as possible, and the resulting product possesses only the natural, or primary milk flavor. To one accustomed to butter made from sour or ripened cream, this taste is flat, and if the butter is free from salt, may remind one of grease. Sweet-cream butter has a delicate flavor when it is made from good milk, and the taste for it is rapidly acquired. In some centers, as in Paris, the market demands this type of butter quite exclusively.

If the cream is allowed to undergo the acid fermentation before churning, the butter has a much higher degree of flavor and one that differs materially in kind. Under primitive methods, it was difficult to keep the cream sweet until it could be churned. On the small farm with gravity creaming in shallow vessels and infrequent churning, the cream was certain to be sour when churned. Undoubtedly, the making of butter from sour cream came into use because of its greater convenience; people became accustomed to sour-cream butter, and at the present time it is used in the greater part of the world, and is the type made in all of the great dairy countries.

Ripening of cream. In modern dairy practice the souring of the cream is called the ripening process, and is, where the best methods are employed, largely under the control of the butter maker. The changes that go on in the ripening process are the same as have been discussed in the acid fermentation of milk. The increase in acid is accompanied by an enormous increase in the number of bacteria; the ripe cream will contain hundreds of millions of bacteria in each cubic centimeter. The effect of this germ life is to improve or injure the butter, depending upon the class of bacteria to which it belongs. The problem of the modern butter maker is to control the kinds of bacteria growing in the cream.

The temperature at which cream is held during the ripening process is favorable to the growth of the acid-forming bacteria; hence, in ripe cream, they are practically the only kind of bacteria to be found. It must be remembered however, that there are different classes of acid-forming organisms, some of which produce desirable flavors, while others are distinctly harmful.

The intensity of flavor of butter is, in a general way, directly related to the amount of acid that is formed in the cream. A low acidity at time of churning is usually associated with a mild flavor, while a higher degree of acidity, up to a certain point, imparts a more pronounced flavor to the product. If cream is over-ripened, the quality of the flavor is seriously impaired.

In determining the acidity of cream, a definite volume is taken, and the acidity determined by titration, expressing the results as such a per cent of lactic acid. Manifestly, the amount of fat in the cream influences the apparent per cent of acidity. The acidity will not usually exceed 0.5 to 0.7 per cent, but in reality the serum will contain more than this, as the acid is formed in the serum, the butter fat having no role whatever. In a very rich cream, 40 to 50 per cent fat, it is impossible to develop more than 0.4 to 0.5 per cent of acidity, and the flavor of the butter will be low, because of the relation between the amount of acid and fat, while in a thin cream having the same acidity, the ratio between the amounts of fat and acid will be very different. For example, in one hundred pounds of 50 per cent cream of 0.5 per cent acidity there will be one-half pound of acid and fifty pounds of fat; in the same quantity of cream containing 20 per cent of fat and having an acidity of 0.5 per cent there will be one-half pound of acid to twenty pounds of fat. The flavor of the butter from the rich cream will be quite different in intensity from that made from the thinner cream.

The acidity of cream cannot be determined with any degree of accuracy by the taste or odor. Every butter maker should have some method of determining the degree of acidity in his cream, so that he may better control the flavor of his product. Several methods have been devised for this purpose and the necessary apparatus is sold by all dairy supply houses.

The effect of the ripening of the cream is shown not only in the flavor of the product, but in a number of other ways. Sour cream churns more easily, and more exhaustively than does sweet cream. It is supposed that the fat globules are surrounded by a film of albuminous material which prevents their coalescing readily. During the ripening process, the action of the acid apparently dissolves this enveloping substance, and the globules cohere more easily in the churning process.

When raw cream is used the ripened-cream butter keeps better than that made from sweet cream. In sweet cream there are few lactic bacteria, the majority of the bacteria present being of various kinds, many of which may be injurious, so far as the keeping quality is concerned. In sour-cream butter the lactic bacteria make up over 99 per cent of the bacteria present, and their presence tends to prevent the development of undesirable non-acid forms.

Source of butter flavor. The flavor of ripened-cream butter has been shown to be directly connected with the acid-fermentation of the cream. The amount of lactic acid formed from the sugar fermented is dependent upon the kind of bacteria present. The acid-producing organisms that are desirable from the standpoint of the butter maker form comparatively small amounts of other by-products, but these undoubtedly affect the flavor of the butter. As fats have the power of absorbing odors, the butter fat absorbs some of the by-products of the acid fermentation, thus acquiring a certain aroma and flavor.

It is not necessary that the cream be ripened, in order to have the fat acquire a flavor, for if sweet cream is churned with a considerable proportion of sour milk, the butter will have much the same flavor, both as to intensity and kind, as though the cream had been allowed to sour naturally. A process of butter making known as the LeClair method is based on this principle. The flavor-producing substances can also be absorbed by the butter after it is churned, by working the butter in contact with sour milk. Attempts have been made to add pure lactic acid to the cream, instead of allowing the acid to be formed by the bacteria, but while the physical effect on the cream is the same, the flavor and aroma of the butter are deficient, because the acid itself does not supply the necessary aromatic products. This emphasizes the importance of the by-products of the acid fermentation other than the lactic-acid.

In the past numerous attempts have been made to find organisms that might be added to the cream, in order to produce the delicate flavor characteristic of the best type of butter. Some bacteriologists have claimed that the source of the flavor-giving substance was to be found in the decomposition products of the nitrogenous constituents of the milk. None of these attempts have stood the test of practical use in creameries, and it has been demonstrated that the finest type of butter can be made by the use of lactic bacteria alone. Formerly, when butter was made wholly from cream soured under natural conditions, a much higher degree of flavor was developed. Under present market demands, a less pronounced flavor is desired, a condition more readily met by the use of modern methods.

Importance of butter flavor. The importance of flavor in determining the commercial value of butter is evidenced by the relatively high value placed upon this factor in scoring, viz., flavor, 45 points; body or texture, 25 points; color 15; salt 10; and package 5 points. The factors on which butter is judged, are with the exception of flavor, wholly under the control of the maker, but as the production of flavor is dependent on the kind of bacteria present in the cream, it is a far more difficult matter to control, and yet it is of the utmost importance in determining the value of the product.

The flavor of the butter is dependent on the quality of the cream. If this is dirty and sour, the maker has little control over the type of fermentation, and hence, little control of the flavor of the butter. This has led in some cases to the grading of the cream, basing the division on the acidity, flavor, and fat content. Such practice is entirely justifiable, as a better quality of butter can be made from fresh, sweet cream than from that already fermented. It is noteworthy that the quality of butter has not improved since the introduction of the centralizer system, in which cream is shipped for long distances.

Control of the type of fermentation. In the older methods of butter making, there was little or no control of the type of fermentation that took place in the cream. Where milk is produced under clean conditions, and kept at ordinary temperatures, it will generally undergo fermentation changes, due to the desirable type of acid-forming organisms. In milk, which is less carefully handled, the undesirable bacteria are more abundant and the quality of the butter of lower grade. When butter was made on the farm, before the development of the factory system, it was not a question of vital importance whether the product was uniform from day to day, but with the advent of the modern creamery, turning out thousands of pounds of butter per day, and with the extension of the markets for the product, the question of uniformity came to be of much importance. A uniform product can be secured only by the control of the type of fermentation in the cream, or by the control of the kinds of bacteria that cause the souring of the cream. Modern methods of butter making have been devised on the basis of an improvement in the ripening process.

Starters. From the earliest practice of allowing the cream to stand until sufficient quantity had accumulated for churning, it was only a step, but a most important one, to the addition of sour milk, sour cream, or butter milk, to hasten the ripening process. This was the beginning of the modern starter. Experience demonstrated that the addition of these already fermented liquids exercised a desirable effect upon the production of butter flavor, even though, at that time, the phenomenon of milk fermentation was not satisfactorily understood, and the relation of bacterial by-products to the production of flavor in butter was not recognized.

As a result of experience alone, improvements in the development of the "home made" starter took place. By careful selection of clean milk, and the natural fermentation of this under carefully controlled conditions, as well as the control of the temperature of the cream during the ripening, improvement in the technique of cream ripening gradually developed. More and more attention was given to the preparation of the starter, and its propagation from day to day, under conditions which would prevent its deterioration. This method of utilizing naturally fermented milk or cream was gradually extended, until it became almost universal in the larger butter-producing districts.

In 1890 a more refined and scientific process was introduced by the Danish bacteriologist, Storch. Recognizing the fact that butter flavor was attributable to the development of the bacteria present in the ripening cream, he conceived the idea of isolating the various types of organisms found in milk and testing them as to their effect on the quality of flavor. Selection was then made of the most favorable flavor-producing types, and these were propagated in suitable culture media, such as skim milk, which was rendered more or less perfectly sterile by pasteurization or sterilization. Under such conditions the addition of a selected ferment could be made to the fresh cream, and so control the type of fermentation which occurred therein. An essential requisite in any organism used for this purpose must be the ability to produce relatively large amounts of acid rapidly at ordinary ripening temperatures, and also to form sufficient quantities of the proper flavor-producing substances to impart a suitable flavor to the butter fat. Such starters are known as pure culture or commercial starters, and are prepared in both liquid and dry form. At present they are used to a greater or less extent in all of the leading dairy districts.

Liquid starters consist of a mass of sterile nutrient medium, milk or beef broth, inoculated with the pure culture. The dry starters are made by adding liquid cultures, containing the growing bacteria, to some absorbing material, such as milk sugar, milk powder, or starch, the whole mass being dried at low temperatures, so as not to injure the bacteria. Under such conditions the bacteria, exist in a dormant state, and are protected from their own by-products, to which they would be exposed if maintained in liquid cultures. The keeping quality, therefore, of dry cultures, is much better than that of liquid cultures.

By the use of the pure-culture starters, the butter maker is able to add to his cream the same kind of bacteria from day to day, and the butter will be more uniform than when the less constant home-made starter is employed. In cream to which the starter is added, there are present a greater or less number of acid-forming bacteria, depending upon the age of the cream, and upon the condition under which it was produced. These will grow during the ripening process, and the flavor of the product will be the result of the mixture of the bacteria in the cream. The maker can not, therefore, be certain that the addition of a pure culture to raw cream will effectively control the type of fermentation. This can be secured only by first destroying the existing bacteria in the cream, before the selected culture is added. Heating the cream accomplishes this; and in cream thus freed from the various kinds of bacteria, the butter maker can insure the dominance of the desirable types, contained in the pure-culture starter. If the cream can be obtained in a sweet condition, the maker through this process of pasteurization, and the use of pure cultures, secures almost perfect control over the type of fermentation that occurs in the cream, and thus exercises control over the degree and kind of flavor of the product. This most scientific type of butter making is now used by the most progressive butter makers in the leading butter-producing regions of the world.

Pasteurization of the cream also distinctly improves the keeping quality of butter, a condition doubtless due to the freedom of the same from organisms other than the lactic bacteria. This is a factor of as much importance as uniformity, because under modern business conditions, the surplus production must be kept in storage, and it is essential that the quality should not deteriorate materially during this time.

Process of pasteurization for butter making. In the pasteurization of market milk, it is necessary to take into account the effect of heating on the physical and chemical properties of the milk, and the degree of heat that can be employed is limited. In pasteurizing cream for butter, there is no such limitation, and the cream may be heated to any temperature desired. In Denmark where the process of pasteurization has been used most extensively, temperatures ranging from 176° F. to 190° F. are used. The machines are of the "continuous flow" type, and the cream rather than the whole milk is treated. To prevent the spread of tuberculosis and other diseases, the Danish government requires that all cream and milk be heated to 176° F., before the skim milk or butter milk is returned to the farms.

The heating of the butter fat to high temperatures has an injurious effect on the texture of the butter, unless the cream is cooled to 50° F., for a period of at least two hours previous to churning.

Propagation of starters. As has been previously shown, the quality of butter depends on the kind of bacteria in the cream or in the starter added. The commercial starters contain lactic acid bacteria that have been selected with especial care; most of the starters now sold contain but a single kind of bacteria; hence, are often called pure-culture starters. The package purchased contains but a small quantity, and before the starter can be used in the ripening of cream, it must be increased in amount. It must also be propagated from day to day so that a fresh starter shall be available daily for addition to the cream. The propagation of the starter must be done with especial reference to keeping it in good condition and in as high a state of purity as possible.

In the past the starter was propagated, by adding the contents of the bottle purchased to a small amount of milk that had been heated and cooled; this, if kept in a warm place, would be curdled in twenty-four hours, and could be used for the inoculation of a large mass of milk, that had been treated in a like manner, and which, when curdled, was added to the cream; a small amount was saved for the purpose of again inoculating a mass of milk that had been heated and cooled. Following this method it was very difficult to keep the culture from becoming contaminated with other forms of bacteria. More recently the most successful butter makers have propagated the so-called "mother starters" in small vessels, and have used the larger mass of starter for the inoculation of the cream alone.

Glass vessels are preferable for the propagation of the mother starters since they are impervious and through the transparent wall the condition of the ripened starter can be more easily determined than in a metal or earthenware vessel. An ordinary milk bottle with an inverted tumbler for a cover, to protect the starter from contamination from the air, is a most convenient vessel.

The starters may be propagated either in whole or skim milk; the former is preferable since, in most creameries, it can be more easily selected. The quality of the milk used has much to do with the quality of the starter; it should be as fresh and clean as it is possible to obtain. The clean bottle should be filled half to two-thirds full, covered and heated in some manner so that the milk shall be at a temperature close to the boiling point for fifteen to twenty minutes. The heating may be done by placing the bottles in water, which is heated on a stove or by steam, or the bottles may be subjected to streaming steam. The milk is cooled quickly and the contents of the package purchased added and well mixed with the milk. In the case of the dry starters, the mixing should be done with especial care. The bottle is kept in a warm place and in twenty-four to thirty-six hours, the milk should be curdled. A second bottle must be treated as before and inoculated from the first, and the process repeated daily since the bacteria must have fresh food, if they are to be maintained in good condition.

In order to accomplish this, the maker must be able to maintain constant conditions from day to day, especially with reference to the amount of the ripened starter that is transferred to the fresh bottle of milk, and the temperature at which the bottles are kept. A spoon, arranged as shown in Fig. 31, enables one to carry a definite amount of the ripened starter to the bottle of milk to be inoculated and a constant temperature box (Fig. 32) permits of the maintenance of the same temperature from day to day. Through careful supervision of these points, and by taking care at every step to avoid the introduction of contaminating organisms, the purity of the culture can be maintained, and the bacteria kept in a healthy condition.

The starter is used because of the acid-forming bacteria it contains; it is said to be ripe and in the best condition for use at the time it contains the greatest number of living bacteria. It has been found by experiment that this is at the time the milk curdles at ordinary temperature, or when the acidity is about 0.6-0.7 per cent. If the acidity is allowed to increase to 0.8 or 0.9 per cent, the number of bacteria will be less and a larger amount of the starter must be used in order to ripen a definite amount of cream in the desired time. The use of an overripe starter may also have an injurious effect on the flavor.