Outlines of Dairy Bacteriology Part 14

~Culture vs. home-made starters.~ One great advantage which has accrued from the use of culture or commercial starters has been that in emphasizing the need of closer control of the ripening process, greater attention has been paid to the carrying out of the details. In the hands of the better operators, the differences in flavor of b.u.t.ter made with a culture or a natural starter are not marked,[168] but in the hands of those who fail to make a good product under ordinary conditions, an improvement is often secured where a commercial culture is used.

~Pasteurization as applied to b.u.t.ter-making.~ This process, as applied to b.u.t.ter making, is often confounded with the treatment of milk and cream for direct consumption. It is unfortunate that the same term is used in connection with the two methods, for they have but little in common except in the use of heat to destroy the germ life of the milk. In pasteurizing cream for b.u.t.ter-making, it is not necessary to observe the stringent precautions that are to be noted in the preservation of milk; for the addition of a rapidly developing starter controls at once the fermentative changes that subsequently occur. Then again, the physical requirement as to the production of a cooked taste is not so stringent in b.u.t.ter-making. While a cooked taste is imparted to milk or even cream at about 158 F., it is possible to make b.u.t.ter that shows no permanent cooked taste from cream that has been raised as high as 185 or even 195 F. This is due to the fact that the fat does not readily take up those substances that give to scalded milk its peculiar flavor.

Unless care is taken in the manipulation of the heated cream, the grain or body of the b.u.t.ter may be injured. This tendency can be overcome if the ripened cream is chilled to 48 F. for about two hours before churning. It is also essential that the heated cream should be quickly and thoroughly chilled after being pasteurized.

The Danes, who were the first to employ pasteurization in b.u.t.ter-making, used, in the beginning, a temperature ranging from 158 to 167 F., but owing to the prevalence of such diseases as tuberculosis and foot-and-mouth disease, it became necessary to treat all of the skim milk that was returned from the creameries. For this purpose the skim milk is heated to a temperature of 176 F., it having been more recently determined that this degree of heat is sufficient to destroy the seeds of disease. With the use of this higher temperature the capacity of the pasteurizing apparatus is considerably reduced, but the higher temperature is rendered necessary by the prevailing conditions as to disease.

When the system was first introduced in Denmark, two methods of procedure were followed: the whole milk was heated to a sufficiently high temperature to thoroughly pasteurize it before it was separated, or it was separated first, and the cream pasteurized afterwards. In the latter case, it is necessary to heat the skim milk after separation to destroy the disease organisms, but this can be quickly done by the use of steam directly. Much more care must be used in heating the cream in order to prevent injury to the grain of the b.u.t.ter. In spite of the extra trouble of heating the cream and skim milk separately, this method has practically supplanted the single heating. With the continual spread of tuberculosis in America the heating of skim milk separately is beginning to be introduced.[169]

~Use of starters in pasteurized and unpasteurized cream.~ In order to secure the beneficial results presumably attributable to the use of a starter, natural as well as a pure culture, it should be employed in cream in which the bacteria have first been killed out by pasteurization. This is certainly the most logical and scientific method and is the way in which the process has been developed in Denmark.

Here in this country, the use of pure cultures has been quite rapidly extended, but the system of heating the cream has been used in only a slight measure. The increased labor and expense incurred in pasteurizing the cream has naturally militated somewhat against the wide-spread use of the process, but doubtless the main factor has been the inability to secure as high a flavor where the cream was heated as in the unheated product. As the demands of the market change from a high, quick flavor to one that is somewhat milder but of better keeping quality, doubtless pasteurization of the cream will become more and more popular. That such a change is gradually occurring is already evident, although as yet only a small proportion of b.u.t.ter made in this country is now made in this way. Where the cream is unheated, a considerable number of species will be found, and even the addition of a pure culture, if that culture is of the lactic acid-producing species, will to some extent control the type of fermentation that occurs. Such would not be the case with a culture composed of the casein-digesting type of bacteria. Only those forms could thus be used which are especially well suited to development in raw cream. For this reason the pure culture ferments that are generally employed in creamery practice are organisms of the lactic acid type, able to grow rapidly in cream and produce a pure cream flavor in the b.u.t.ter.

~Purity of commercial starters.~ Naturally the b.u.t.ter maker is forced to rely on the laboratory for his commercial starter, and the question will often arise as to the purity and vigor of the various ferments employed.

As there is no way for the factory operator to ascertain the actual condition of the starter, except by using the same, the greatest care should be taken by the manufacturer to insure the absolute purity of the seed used.

A bacteriological examination of the various cultures which have been placed on the market not infrequently reveals an impure condition. In several cases the writer has found a not inconsiderable number of liquefying bacteria mixed with the selected organism. Molds not infrequently are found in cultures put up in the dry form. Doubtless the effect of these accidental contaminations is considerably less in the case of a starter composed of a distinctively lactic acid-producing organism than with a form which is less capable of thriving vigorously in milk, and it should be said that these impurities can frequently be eliminated by continued propagation.

The virility and vigor of the starter is also a fluctuating factor, dependent in part at least, upon the conditions under which the organism is grown. In some cases the germ is cultivated in solutions in which acid cannot be formed in abundance. Where the conditions permit of the formation of acid, as would be the case if sugar was present with a lactic acid-producing species, the vitality of the culture is often impaired by the action of the gradually acc.u.mulating acid. Some manufacturers attempt to minimize this deleterious condition by adding carbonate of lime which unites with the acid that is formed.

~Propagation of starters for cream-ripening.~ The preparation and propagation of a starter for cream-ripening is a process involving considerable bacteriological knowledge, whether the starter is of domestic origin or prepared from a pure-culture ferment. In any event, it is necessary that the starter should be handled in a way so as to prevent the introduction of foreign bacteria as far as possible. It should be remembered at all times that the starter is a live thing and must be handled throughout its entire history in a way so as to retain its vitality and vigor unimpaired. The following points should be taken into consideration in growing the starter and transferring it from day to day:

1. If a commercial starter is used, see that it is fresh and that the seal has not been broken. If the culture is too old, the larger part of the organisms may have died out before it is transferred, in which case the effect of its addition to the sterilized milk would be of little value.

When the commercial ferment is received, it should be stored in the refrigerator pending its use so as to r.e.t.a.r.d as much as possible the changes that naturally go on in the culture liquid. Be careful that the bottle is not exposed to the influence of direct sunlight for in a transparent medium the organisms may be readily killed by the disinfecting action of the sun's rays.

2. If a home-made starter is employed, use the greatest possible care in selecting the milk that is to be used as a basis for the starter.

3. For the propagation and perpetuation of the starter from day to day, it is necessary that the same should be grown in milk that is as germ-free as it is possible to secure it. For this purpose sterilize some fresh skim-milk in a covered can that has previously been well steamed. This can be done easily by setting cans containing skim-milk in a vat filled with water and heating the same to 180 F. or above for one-half hour or more. Steam should not be introduced directly. This process destroys all but a few of the most resistant spore-bearing organisms. This will give a cooked flavor to the milk, but will not affect the cream to which the starter is added. Dairy supply houses are now introducing the use of starter cans that are specially made for this purpose.

4. After the heated milk is cooled down to about 70 or 80 F., it can be inoculated with the desired culture. Sometimes it is desirable to "build up" the starter by propagating it first in a smaller volume of milk, and then after this has developed, adding it to a larger amount.

This method is of particular value where a large amount of starter is needed for the cream-ripening.

5. After the milk has been inoculated, it should be kept at a temperature that is suitable for the rapid development of the contained bacteria, 65-75 F., which temperature should be kept as uniform as possible.

This can best be done by setting the covered can in a vat filled with warm water. The starter cans are often arranged so that temperature can be controlled by circulating water.

6. The starter should not be too thoroughly curdled when it is needed for use, but should be well soured and only partially curdled for it is difficult to break up thoroughly the curd particles if the starter is completely curdled. If these curd ma.s.ses are added to ripening cream, white specks may appear in the b.u.t.ter.

7. The vigor of the starter is in all probability stronger when the milk is on the point of curdling than it is after the curd has been formed some time. The continued formation of lactic acid kills many of the bacteria and thus weakens the fermentative action. It is therefore highly important that the acidity of the starter should be closely watched.

8. Do not refrigerate the starter when it has reached the proper stage of development, as this r.e.t.a.r.ds the bacterial growth in the same manner as cold weather checks the growth of grain. It is preferable to dilute the starter, if it cannot be used when ready, with sufficient freshly sterilized sweet milk to hold the acidity at the proper point and thus keep the bacteria in the starter in a condition which will favor vigorous growth.

9. The starter should be propagated from day to day by adding a small quant.i.ty to a new lot of freshly prepared milk. For this purpose two propagating cans should be provided so that one starter may be in use while the other is being prepared.

~How long should a starter be propagated?~ No hard-and-fast rule can be given for this, for it depends largely upon how carefully the starter is handled during its propagation. If the starter is grown in sterilized milk kept in steamed vessels and is handled with sterile dippers, it is possible to maintain it in a state of relative purity for a considerable period of time; if, however, no especial care is given, it will soon become infected by the air, and the retention of its purity will depend more upon the ability of the contained organism to choke out foreign growths than upon any other factor. Experience seems to indicate that pure-culture starters "run out" sooner than domestic starters. While it is possible, by bacteriological methods, to determine with accuracy the actual condition of a starter as to its germ content, still such methods are inapplicable in creamery practice. Here the maker must rely largely upon the general appearance of the starter as determined by taste and smell. The supply houses that deal in cultures of this cla.s.s generally expect to supply a new culture at least every month.

~Bacteria in b.u.t.ter.~ As ripened cream is necessarily rich in bacteria, it follows that b.u.t.ter will also contain germ life in varying amounts, but as b.u.t.ter-fat is not well adapted for bacterial food, the number of germs in b.u.t.ter is usually less than in ripened cream.

Sweet-cream b.u.t.ter is naturally poorer in germ life than that made from ripened cream. Grotenfelt reports in sweet-cream b.u.t.ter, the so-called "Paris b.u.t.ter," only a few bacteria while in acid cream b.u.t.ter the germ content runs from scores to hundreds of thousands.

~Effect of bacteria in wash water.~ An important factor in contamination may be the wash water that is used. Much carelessness often prevails regarding the location and drainage of the creamery well, and if same becomes polluted with organic matter, bacterial growth goes on apace.

Melick[170] has made some interesting studies on using pasteurized and sterilized well waters for was.h.i.+ng. He found a direct relation to exist between the bacterial content of the wash water and the keeping quality of the b.u.t.ter. Some creameries have tried filtered water but under ordinary conditions a filter, unless it is tended to with great regularity, becomes a source of infection rather than otherwise.

~Changes in germ content.~ The bacteria that are incorporated with the b.u.t.ter as it first "comes" undergo a slight increase for the first few days. The duration of this period of increase is dependent largely upon the condition of the b.u.t.ter. If the b.u.t.termilk is well worked out of the b.u.t.ter, the increase is slight and lasts for a few days only, while the presence of so nutritious a medium as b.u.t.termilk affords conditions much more favorable for the continued growth of the organisms.

While there may be many varieties in b.u.t.ter when it is fresh, they are very soon reduced in kind as well as number. The lactic acid group of organisms disappear quite rapidly; the spore-bearing species remaining for a somewhat longer time. b.u.t.ter examined after it is several months old is often found to be almost free from germs.

In the manufacture of b.u.t.ter there is much that is dependent upon the mechanical processes of churning, was.h.i.+ng, salting and working the product. These processes do not involve any bacteriological principles other than those that are incident to cleanliness. The cream, if ripened properly, will contain such enormous numbers of favorable forms that the access of the few organisms that are derived from the churn, the air, or the water in was.h.i.+ng will have little effect, unless the conditions are abnormal.

BACTERIAL DEFECTS IN b.u.t.tER.

~Rancid change in b.u.t.ter.~ Fresh b.u.t.ter has a peculiar aroma that is very desirable and one that enhances the market price, if it can be retained; but this delicate flavor is more or less evanescent, soon disappearing, even in the best makes. While a good b.u.t.ter loses with age some of the peculiar aroma that it possesses when first made, yet a gilt-edged product should retain its good keeping qualities for some length of time. All b.u.t.ters, however, sooner or later undergo a change that renders them worthless for table use. This change is usually a rancidity that is observed in all stale products of this cla.s.s. The cause of this rancid condition in b.u.t.ter was at first attributed to the formation of butyric acid, but it is now recognized that other changes also enter in.[171] Light and especially air also exert a marked effect on the flavor of b.u.t.ter. Where b.u.t.ter is kept in small packages it is much more p.r.o.ne to develop off flavors than when packed in large tubs. From the carefully executed experiments of Jensen it appears that some of the molds as well as certain species of bacteria are able to incite these changes. These organisms are common in the air and water and it therefore readily follows that inoculation occurs.

Practically, rancidity is held in check by storing b.u.t.ter at low temperatures where germ growth is quite suspended.

~Lack of flavor.~ Often this may be due to improper handling of the cream in not allowing it to ripen far enough, but sometimes it is impossible to produce a high flavor. The lack of flavor in this case is due to the absence of the proper flavor-producing organisms. This condition can usually be overcome by the addition of a proper starter.

~Putrid b.u.t.ter.~ This specific b.u.t.ter trouble has been observed in Denmark, where it has been studied by Jensen.[172] b.u.t.ter affected by it rapidly acquires a peculiar putrid odor that ruins it for table use.

Sometimes, this flavor may be developed in the cream previous to churning.

Jensen found the trouble to be due to several different putrefactive bacteria. One form which he called _Bacillus foetidus lactis_, a close ally of the common feces bacillus, produced this rotten odor and taste in milk in a very short time. Fortunately, this organism was easily killed by a comparatively low heat, so that pasteurization of the cream and use of a culture starter quickly eliminated the trouble, where it was tried.

~Turnip-flavored b.u.t.ter.~ b.u.t.ter sometimes acquires a peculiar flavor recalling the order of turnips, rutabagas, and other root crops. Often this trouble is due to feeding, there being in several of these crops, aromatic substances that pa.s.s directly into the milk, but in some instances the trouble arises from bacteria that are able to produce decomposition products,[173] the odor and taste of which strongly recalls these vegetables.

~"Cowy" b.u.t.ter.~ Frequently there is to be noted in milk a peculiar odor that resembles that of the cow stable. Usually this defect in milk has been ascribed to the absorption of impure gases by the milk as it cools, although the gases and odors naturally present in fresh milk have this peculiar property that is demonstrable by certain methods of aeration.

Occasionally it is transmitted to b.u.t.ter, and recently Pammel[174] has isolated from b.u.t.ter a bacillus that produced in milk the same peculiar odor so commonly present in stables.

~Lardy and tallowy b.u.t.ter.~ The presence of this unpleasant taste in b.u.t.ter may be due to a variety of causes. In some instances, improper food seems to be the source of the trouble; then again, b.u.t.ter exposed to direct sunlight bleaches in color and develops a lardy flavor.[175]

In addition to these, cases have been found in which the defect has been traced to the action of bacteria. Storch[176] has described a lactic-acid form in a sample of tallowy b.u.t.ter that was able to produce this disagreeable odor.

~Oily b.u.t.ter.~ Jensen has isolated one of the causes of the dreaded oily b.u.t.ter that is reported quite frequently in Denmark. The specific organism that he found belongs to the sour-milk bacteria. In twenty-four hours it curdles milk, the curd being solid like that of ordinary sour milk. There is produced, however, in addition to this, an unpleasant odor and taste resembling that of machine oil, a peculiarity that is transmitted directly to b.u.t.ter made from affected cream.

~Bitter b.u.t.ter.~ Now and then b.u.t.ter develops a bitter taste that may be due to a variety of different bacterial forms. In most cases, the bitter flavor in the b.u.t.ter is derived primarily from the bacteria present in the cream or milk. Several of the fermentations of this character in milk are also to be found in b.u.t.ter. In addition to these defects produced by a biological cause, bitter flavors in b.u.t.ter are sometimes produced by the milk being impregnated with volatile, bitter substances derived from weeds.

~Moldy b.u.t.ter.~ This defect is perhaps the most serious because most common. It is produced by the development of a number of different varieties of molds. The trouble appears most frequently in packed b.u.t.ter on the outside of the ma.s.s of b.u.t.ter in contact with the tub. Mold spores are so widely disseminated that if proper conditions are given for their germination, they are almost sure to develop. In some cases the mold is due to the growth of the ordinary bread mold, _Penicillium glauc.u.m_; in other cases a black mold develops, due often to _Cladosporium butyri_. Not infrequently trouble of this character is a.s.sociated with the use of parchment wrappers. The difficulty can easily be held in check by soaking the parchment linings and the tubs in a strong brine, or paraffining the inside of the tub.

~Fishy b.u.t.ter.~ Considerable trouble has been experienced in Australian b.u.t.ter exported to Europe in which a fishy flavor developed. It was noted that the production of this defect seemed to be dependent upon the storage temperature at which the b.u.t.ter was kept. When the b.u.t.ter was refrigerated at 15 F. no further difficulty was experienced. It is claimed that the cause of this condition is due to the formation of trimethylamine (herring brine odor) due to the growth of the mold fungus _Oidium lactis_, developing in combination with the lactic-acid bacteria.

A fishy taste is sometimes noted in canned b.u.t.ter. Rogers[177] has determined that this flavor is caused by yeasts (_Torula_) which produce fat-splitting enzyms capable of producing this undesirable change.

FOOTNOTES:

[153] Conn and Esten, Cent. f. Bakt., II Abt., 1901, 7:746.

[154] Tiemann, Milch Zeit., 23:701.