_3. No diminution in cream "body."_ Coincident with this change which takes place in the creaming of the milk is the change in body or consistency which is noted where cream is pasteurized at too high a temperature. For the same reason as given under (2) cream heated above these temperatures is reduced in apparent thickness and appears to contain less b.u.t.ter-fat. Of course the pasteurizing process does not change the fat content, but its "body" is apparently so affected. Thus a 25 per cent. cream may seem to be no thicker or heavier than an 18 per cent. raw cream. This real reduction in consistency naturally affects the readiness with which the cream can be whipped.

~Biological requirements.~ _1. Enhanced keeping quality._ In commercial practice the essential biological requirement is expressed in the enhanced keeping quality of the pasteurized milk. This expresses in a practical way the reduction in germ life accomplished by the pasteurizing process. The improvement in keeping quality depends upon the temperature and time of exposure, but fully as much also on the way in which the pasteurized product is handled after heating. The lowest temperature which can be used with success to kill the active, vegetative bacteria is about 140 F., at which point it requires about ten minutes exposure. If this period is curtailed the temperature must be raised accordingly. An exposure to a temperature of 175 F. for a minute has approximately the same effect as the lower degree of heat for the longer time.

The following bacteriological studies as to the effect which a variation in temperature exerts on bacterial life in milk are of importance as indicating the foundation for the selection of the proper limits. In the following table the exposures were made for a uniform period (20 minutes):

_The bacterial content of milk heated at different temperatures._

Number of bacteria per cc. in milk.

45 C. 50 C. 55 C. 60 C. 65 C. 70 C.

Unheated 113 F. 122 F. 131 F. 140 F. 149 F. 158 F.

Series I. 2,895,000 ---- 1,260,000 798,000 32,000 5,770 3,900 Series II. 750,000 665,000 262,400 201,000 950 700 705 Series III. 1,350,000 1,100,000 260,000 215,000 575 610 650 Series IV. 1,750,000 ---- 87,360 ---- 4,000 3,500 3,600

It appears from these results that the most marked decrease in temperature occurs at 140 F. (60 C.). It should also be observed that an increase in heat above this temperature did not materially diminish the number of organisms present, indicating that those forms remaining were in a spore or resistant condition. It was noted, however, that the developing colonies grew more slowly in the plates made from the highly heated milk, showing that their vitality was injured to a greater extent even though not killed.

_2. Destruction of disease bacteria._ While milk should be pasteurized so as to destroy all active, multiplying bacteria, it is particularly important to destroy any organisms of a disease nature that might find their way into the same. Fortunately most of the bacteria capable of thriving in milk before or after it is drawn from the animal are not able to form spores and hence succ.u.mb to proper pasteurization. Such is the case with the diphtheria, cholera and typhoid organisms.

The organism that is invested with most interest in this connection is the tubercle bacillus. On account of its more or less frequent occurrence in milk and its reputed high powers of resistance, it may well be taken as a standard in pasteurizing.

~Thermal death limits of tubercle bacillus.~ Concerning the exact temperature at which this germ is destroyed there is considerable difference of opinion. Part of this arises from the inherent difficulty in determining exactly when the organism is killed (due to its failure to grow readily on artificial media), and part from the lack of uniform conditions of exposure. The standards that previously have been most generally accepted are those of De Man,[137] who found that thirty minutes exposure at 149 F., fifteen minutes at 155 F., or ten minutes at 167 F., sufficed to destroy this germ.

More recently it has been demonstrated,[138] and these results confirmed,[139] that if tuberculous milk is heated in closed receptacles where the surface pellicle does not form, the vitality of this disease germ is destroyed at 140 F. in 10-15 minutes, while an exposure at 160 F. requires only about one minute.[140] If the conditions of heating are such that the surface of the milk is exposed to the air, the resistance of bacteria is greatly increased. When heated in open vessels Smith found that the tubercle organism was not killed in some cases where the exposure was made for at least an hour. Russell and Hastings[141] have shown an instance where the thermal death-point of a micrococcus isolated from pasteurized milk was increased 12.5 F., by heating it under conditions that permitted of the formation of the scalded layer.

It is therefore apparent that apparatus used for pasteurization should be constructed so as to avoid this defect.

~Methods of treatment.~ Two different systems of pasteurization have grown up in the treatment of milk. One of these has been developed from the hygienic or sanitary aspect of the problem and is used more particularly in the treatment of cream and relatively small milk supplies. The other system has been developed primarily from the commercial point of view where a large amount of milk must be treated in the minimum time. In the first method the milk is heated for a longer period of time, about fifteen minutes at a relatively low temperature from 140-155 F.; in the other, the milk is exposed to the source of heat only while it is pa.s.sing rapidly through the apparatus. Naturally, the exposure under such conditions must be made at a considerably higher temperature, usually in the neighborhood of 160 F.

The types of apparatus used in these respective processes naturally varies. Where the heating is prolonged, the apparatus employed is built on the principle of a _tank_ or _reservoir_ in which a given volume of milk may be held at any given temperature for any given period of time.

When the heat is applied for a much shorter period of time, the milk is pa.s.sed in a continuous stream through the machine. Naturally the capacity of a continuous-flow apparatus is much greater than a machine that operates on the intermittent principle; hence, for large supplies, as in city distribution, this system has a great advantage. The question as to relative efficiency is however one which should be given most careful consideration.

~Pasteurizing apparatus.~ The problems to be solved in the pasteurization of milk and cream designed for direct consumption are so materially different from where the process is used in b.u.t.ter-making that the type of machinery for each purpose is quite different. The equipment necessary for the first purpose may be divided into two general cla.s.ses:

1. Apparatus of limited capacity designed for family use.

2. Apparatus of sufficient capacity to pasteurize on a commercial scale.

~Domestic pasteurizers.~ In pasteurizing milk for individual use, it is not desirable to treat at one time more than will be consumed in one day; hence an apparatus holding a few bottles will suffice. In this case the treatment can best be performed in the bottle itself, thereby lessening the danger of infection. Several different types of pasteurizers are on the market; but special apparatus is by no means necessary for the purpose. The process can be efficiently performed by any one with the addition of an ordinary dairy thermometer to the common utensils found in the kitchen. Fig. 24 indicates a simple contrivance that can be readily arranged for this purpose.

The following suggestions indicate the different steps of the process:

1. Use only fresh milk.

2. Place milk in clean bottles or fruit cans, filling to a uniform level, closing bottles tightly with a cork or cover. If pint and quart cans are used at the same time, an inverted bowl will equalize the level. Set these in a flat-bottomed tin pail and fill with warm water to same level as milk. An inverted pie tin punched with holes will serve as a stand on which to place the bottles during the heating process.

3. Heat water in pail until the temperature of same reaches 155 to 160 F.; then remove from source of direct heat, cover with a cloth or tin cover, and allow the whole to stand for half an hour. In the preparation of milk for children, it is not advisable to use the low-temperature treatment (140 F.) that is recommended for commercial city delivery.

[Ill.u.s.tration: FIG. 24. A home-made pasteurizer.]

4. Remove bottles of milk and cool them as rapidly as possible without danger to bottles and store in a refrigerator.

~Commercial pasteurizers.~ The two methods of pasteurization practiced commercially for the preservation of milk and cream have been developed because of the two types of machinery now in use. Apparatus constructed on the reservoir or tank principle permits of the retention of the milk for any desired period of time. Therefore, a lower temperature can be employed in the treatment. In those machines where the milk flows through the heater in a more or less continuous stream, the period of exposure is necessarily curtailed, thereby necessitating a higher temperature.

~Reservoir pasteurizers.~ The simplest type of apparatus suitable for pasteurizing on this principle is where the milk is placed in shotgun cans and immersed in water heated by steam. Ordinary tanks surrounded with water s.p.a.ces can also be used successfully. The Boyd cream ripening vat has also been tried. In this the milk is heated by a swinging coil immersed in the vat through which hot water circulates.

In 1894 the writer[142] constructed a tank pasteurizer which consisted of a long, narrow vat surrounded by a steam-heated water chamber. Both the milk and the water chambers were provided with mechanical agitators having a to-and-fro movement.

[Ill.u.s.tration: FIG. 25. Pott's pasteurizer.]

Another machine which has been quite generally introduced is the Potts'

rotating pasteurizer. This apparatus has a central milk chamber that is surrounded with an outer sh.e.l.l containing hot water. The whole machine revolves on a horizontal axis, and the cream or milk is thus thoroughly agitated during the heating process.

~Continuous-flow pasteurizers.~ The demand for greater capacity than can be secured in the reservoir machines has led to the perfection of several kinds of apparatus where the milk is heated momentarily as it flows through the apparatus. Most of these were primarily introduced for the treatment of cream for b.u.t.ter-making purposes, but they are frequently employed for the treatment of milk on a large scale in city milk trade. Many of them are of European origin although of late years several have been devised in this country.

The general principle of construction is much the same in most of them.

The milk is spread out in a thin sheet, and is treated by pa.s.sing it over a surface, heated either with steam directly or preferably with hot water.

Where steam is used directly, it is impossible to prevent the "scalding on" of the milk proteids to the heated surface.

In some of these machines (Thiel, Kuehne, Lawrence, De Laval, and Hochmuth), a ribbed surface is employed over which the milk flows, while the opposite surface is heated with hot water or steam. Monrad, Lefeldt and Lentsch employ a centrifugal apparatus in which a thin layer of milk is heated in a revolving drum.

In some types of apparatus, as in the Miller machine, an American pasteurizer, the milk is forced in a thin sheet between two heated surfaces, thereby facilitating the heating process. In the Farrington machine heated discs rotate in a reservoir through which the milk flows in a continuous stream.

One of the most economical types of apparatus is the regenerator type (a German machine), in which the milk pa.s.ses over the heating surface in a thin stream and then is carried back over the incoming cold milk so that the heated liquid is partially cooled by the inflowing fresh milk.

In machines of this cla.s.s it requires very much less steam to heat up the milk than in those in which the cold milk is heated wholly by the hot water.

A number of machines have been constructed on the principle of a reservoir which is fed by a constantly flowing stream. In some kinds of apparatus of this type no attempt is made to prevent the mixing of the recently introduced milk with that which has been partially heated. The pattern for this reservoir type is Fjord's heater, in which the milk is stirred by a stirrer. This apparatus was originally designed as a heater for milk before separation, but it has since been materially modified so that it is better adapted to the purposes of pasteurization. Reid was the first to introduce this type of machine into America.

~Objections to continuous flow pasteurizers.~ In all continuous flow pasteurizers certain defects are more or less evident. While they fulfill the important requirement of large capacity, an absolute essential where large volumes of milk are being handled, it does not of necessity follow that they conform to all the hygienic and physical requirements that should be kept in mind. The greatest difficulty is the shortened period of exposure. The period which the milk is actually heated is often not more than a minute or so. Another serious defect is the inability to heat _all_ of the milk for a uniform period of time. At best, the milk is exposed for an extremely short time, but even then portions pa.s.s through the machine much more quickly than do the remainder. Those portions in contact with the walls of the apparatus are r.e.t.a.r.ded by friction and are materially delayed in their pa.s.sage, while the particles in the center of the stream, however thin, flow through in the least possible time.

The following simple method enables the factory operator to test the period of exposure in the machine: Start the machine full of water, and after the same has become heated to the proper temperature, change the inflow to full-cream milk, continuing at the same rate. Note the exact time of change and also when first evidence of milkiness begins to appear at outflow. If samples are taken from first appearance of milky condition and thereafter at different intervals for several minutes, it is possible, by determining the amount of b.u.t.ter-fat in the same, to calculate with exactness how long it takes for the milk to entirely replace the water.

Tests made by the writer[143] on the Miller pasteurizer showed, when fed at the rate of 1,700 pounds per hour, the minimum period of exposure to be 15 seconds, and the maximum about 60-70 seconds, while about two-thirds of the milk pa.s.sed the machine in 40-50 seconds. This manifest variation in the rate of flow of the milk through the machine is undoubtedly the reason why the results of this type of treatment are subject to so much variation. Naturally, even a fatal temperature to bacterial life can be reduced to a point where actual destruction of even vegetating cells does not occur.

~Bacterial efficiency of reservoir pasteurizers.~ The bacterial content of pasteurized milk and cream will depend somewhat on the number of organisms originally present in the same. Naturally, if mixed milk brought to a creamery is pasteurized, the number of organisms remaining after treatment would be greater than if the raw material was fresh and produced on a single farm.

An examination of milk and cream pasteurized on a commercial scale in the Russell vat at the Wisconsin Dairy school showed that over 99.8 per cent of the bacterial life in raw milk or cream was destroyed by the heat employed, i. e., 155 F. for twenty minutes duration.[144] In nearly one-half of the samples of milk, the germ content in the pasteurized sample fell below 1,000 bacteria per cc., and the average of twenty-five samples contained 6,140 bacteria per cc. In cream the germ content was higher, averaging about 25,000 bacteria per cc. This milk was taken from the general creamery supply, which was high in organisms, containing on an average 3,675,000 bacteria per cc. De Schweinitz[145]

has reported the germ content of a supply furnished in Washington which was treated at 158 to 160 F. for fifteen minutes. This supply came from a single source. Figures reported were from 48-hour-old agar plates. Undoubtedly these would have been higher if a longer period of incubation had been maintained. The average of 82 samples, taken for the period of one year, showed 325 bacteria per cc.

[Ill.u.s.tration: FIG. 26. Effect of pasteurizing on germ content of milk.

Black square represents bacteria of raw milk; small white square, those remaining after pasteurization.]

~Bacterial efficiency of continuous-flow pasteurizers.~ A quant.i.tative determination of the bacteria found in milk and cream when treated in machinery of this cla.s.s almost always shows a degree of variation in results that is not to be noted in the discontinuous apparatus.

[Ill.u.s.tration: FIG. 27. Reid's Continuous Pasteurizer.]

Harding and Rogers[146] have tested the efficiency of one of the Danish type of continuous pasteurizers. These experiments were made at 158, 176 and 185 F. They found the efficiency of the machine not wholly satisfactory at the lower temperatures. At 158 F. the average of fourteen tests gave 15,300 bacteria per cc., with a maximum to minimum range from 62,790 to 120. Twenty-five examinations at 176 F. showed an average of only 117, with a range from 300 to 20. The results at 185 F.

showed practically the same results as noted at 176 F. Considerable trouble was experienced with the "scalding on" of the milk to the walls of the machine when milk of high acidity was used.

Jensen[147] details the results of 139 tests in 1899, made by the Copenhagen Health Commission. In 66 samples from one hundred thousand to one million organisms per cc. were found, and in 22 cases from one to five millions. Nineteen tests showed less than 10,000 per cc.