Outlines of Dairy Bacteriology Part 13

CHAPTER VII.

BACTERIA AND b.u.t.tER-MAKING.

In making b.u.t.ter from the b.u.t.ter fat in milk, it is necessary to concentrate the fat globules into cream, preliminary to the churning process. The cream may be raised by the gravity process or separated from the milk by centrifugal action. In either case the bacteria that are normally present in the milk differentiate themselves in varying numbers in the cream and the skim-milk. The cream always contains per cc. a great many more than the skim-milk, the reason for this being that the bacteria are caught and held in the ma.s.ses of fat globules, which, on account of their lighter specific gravity, move toward the surface of the milk or toward the interior of the separator bowl. This filtering action of the fat globules is similar to what happens in muddy water upon standing. As the suspended particles fall to the bottom they carry with them a large number of the organisms that are in the liquid.

~Various creaming methods.~ The creaming method has an important bearing on the kind as well as the number of the bacteria that are to be found in the cream. The difference in species is largely determined by the difference in ripening temperature, while the varying number is governed more by the age of the milk.

_1. Primitive gravity methods._ In the old shallow-pan process, the temperature of the milk is relatively high, as the milk is allowed to cool naturally. This comparatively high temperature favors especially the development of those forms whose optimum growing-point is near the air temperature. By this method the cream layer is exposed to the air for a longer time than with any other, and consequently the contamination from this source is greater. Usually cream obtained by the shallow-pan process will contain a larger number of species and also have a higher acid content.

_2. Modern gravity methods._ In the Cooley process, or any of the modern gravity methods where cold water or ice is used to lower the temperature, the conditions do not favor the growth of a large variety of species. The number of bacteria in the cream will depend largely upon the manner in which the milk is handled previous to setting. If care is used in milking, and the milk is kept so as to exclude outside contamination, the cream will be freer from bacteria than if carelessness prevails in handling the milk. Only those forms will develop in abundance that are able to grow at the low temperature at which the milk is set. Cream raised by this method is less frequently infected with undesirable forms than that which is creamed at a higher temperature.

_3. Centrifugal method._ Separator cream should contain less germ-life than that which is secured in the old way. It should contain only those forms that have found their way into the milk during and subsequent to the milking, for the cream is ordinarily separated so soon that there is but little opportunity of infection, if care is taken in the handling.

As a consequence, the number of species found therein is smaller.

Where milk is separated, it is always prudent to cool the cream so as to check growth, as the milk is generally heated before separating in order to skim efficiently.

Although cream is numerically much richer in bacteria than milk, yet the changes due to bacterial action are slower; hence milk sours more rapidly than cream. For this same reason, cream will sour sooner when it remains on the milk than it will if it is separated as soon as possible.

This fact indicates the necessity of early creaming, so as to increase the keeping quality of the product, and is another argument in favor of the separator process.

~Ripening of cream.~ If cream is allowed to remain at ordinary temperatures, it undergoes a series of fermentation changes that are exceedingly complex in character, the result of which is to produce in b.u.t.ter made from the same the characteristic flavor and aroma that are so well known in this article. We are so accustomed to the development of these flavors in b.u.t.ter that they are not generally recognized as being intimately a.s.sociated with bacterial activity unless compared with b.u.t.ter made from perfectly fresh cream. Sweet-cream b.u.t.ter lacks the aromatic principle that is prominent in the ripened product, and while the flavor is delicate, it is relatively unp.r.o.nounced.

In the primitive method of b.u.t.ter-making, where the b.u.t.ter was made on the farm, the ripening of cream became a necessity in order that sufficient material might be acc.u.mulated to make a churning. The ripening change occurred spontaneously without the exercise of any especial control. With the development of the creamery system came the necessity of exercising a control of this process, and therefore the modern b.u.t.ter-maker must understand the principles which are involved in this series of complex changes that largely give to his product its commercial value.

In these ripening changes three different factors are to be taken into consideration: the development of acid, flavor and aroma. Much confusion in the past has arisen from a failure to discriminate between these qualities. While all three are produced simultaneously in ordinary ripening, it does not necessarily follow that they are produced by the same cause. If the ripening changes are allowed to go too far, undesirable rather than beneficial decomposition products are produced.

These greatly impair the value of b.u.t.ter, so that it becomes necessary to know just to what extent this process should be carried.

In cream ripening there is a very marked bacterial growth, the extent of which is determined mainly by the temperature of the cream. Conn and Esten[153] find that the number of organisms may vary widely in unripened cream, but that the germ content of the ripened product is more uniform. When cream is ready for the churn, it often contains 500,000,000 organisms per cc., and frequently even a higher number. This represents a germ content that has no parallel in any natural material.

The larger proportion of bacteria in cream as it is found in the creamery belong to the acid-producing cla.s.s, but in the process of ripening, these forms seem to thrive still better, so that when it is ready for churning the germ content of the cream is practically made up of this type.

~Effect on churning.~ In fresh cream the fat globules which are suspended in the milk serum are surrounded by a film of alb.u.minous material which prevents them from coalescing readily. During the ripening changes, this enveloping substance is modified, probably by partial solution, so that the globules cohere when agitated, as in churning. The result is that ripened cream churns more easily, and as it is possible to cause a larger number of the smaller fat-globules to cohere to the b.u.t.ter granules, the yield is slightly larger--a point of considerable economic importance where large quant.i.ties of b.u.t.ter are made.

~Development of acid.~ The result of this enormous bacterial multiplication is that acid is produced in cream, lactic being the princ.i.p.al acid so formed.

Other organic acids are undoubtedly formed as well as certain aromatic products. While the production of acid as a result of fermentative activity is usually accompanied with a development of flavor, the flavor is not directly produced by the formation of acid. If cream is treated in proper proportions with a commercial acid, as hydrochloric,[154] it a.s.sumes the same churning properties as found in normally ripened cream, but is devoid of the desired aromatic qualities. Lactic acid[155] has also been used in a similar way but with no better results.

The amount of acidity that should be developed under natural conditions so as to secure the optimum quality as to flavor and aroma is the most important question in cream ripening. Concerning this there have been two somewhat divergent views as to what is best in practice, some holding that better results were obtained with cream ripened to a high degree of acidity than where a less amount was developed.[156] The present tendency seems to be to develop somewhat more than formerly, as it is thought that this secures more of the "high, quick" flavor wanted in the market. On the average, cream is ripened to about 0.5 to 0.65 per cent. acidity, a higher percentage than this giving a strong-flavored b.u.t.ter. In the determination of acidity, the most convenient method is to employ the Farrington alkaline tablet, which permits of an accurate and rapid estimation of the acidity in the ripening cream. The amount of acidity to be produced must of necessity be governed by the amount of b.u.t.ter-fat present, for the formation of acid is confined to the serum of the cream; consequently, a rich cream would show less acid by t.i.tration than a thinner cream, and still contain really as much acid as the other. The importance of this factor is evident in gathered-cream factories.

The rate of ripening is dependent upon the conditions that affect the rate of growth of bacterial life, such as time and temperature, number of organisms in cream and also the per cent of b.u.t.ter fat in the cream.

Some years ago it was customary to ripen cream at about 50 to 60 F., but more recently better results have been obtained, it is claimed, where the ripening temperature is increased and the period of ripening lessened. As high a temperature as 70 to 75 F. has been recommended.

It should be said that this variation in practice may have a valid scientific foundation, for the temperature of the ripening cream is undoubtedly the most potent factor in determining what kind of bacteria will develop most luxuriantly. It is well known that those forms that are capable of producing bitter flavors are able to thrive better at a lower temperature than some of the desirable ripening species.

The importance of this factor would be lessened where a pure culture was used in pasteurized cream, because here practically the selected organism alone controls the field.

It is frequently a.s.serted that better results are obtained by stirring the cream and so exposing it to the air as much as possible. Experiments made at the Ontario Agricultural College, however, show practically no difference in the quality of the b.u.t.ter made by these two methods. The great majority of the bacteria in the cream belong to the facultative cla.s.s, and are able to grow under conditions where they are not in direct contact with the air.

~Flavor and aroma.~ The basis for the peculiar flavor or taste which ripened cream-b.u.t.ter possesses is due, in large part, to the formation of certain decomposition products formed by various bacteria. Aroma is a quality often confounded with flavor, but this is produced by volatile products only, which appeal to the sense of smell rather than taste.

Generally a good flavor is accompanied by a desirable aroma, but the origin of the two qualities is not necessarily dependent on the same organisms. The quality of flavor and aroma in b.u.t.ter is, of course, also affected by other conditions, as, for instance, the presence or absence of salt, as well as the inherent qualities of the milk, that are controlled, to some extent at least, by the character of the feed which is consumed by the animal. The exact source of these desirable but evanescent qualities in b.u.t.ter is not yet satisfactorily determined.

According to Storch,[157] flavors are produced by the decomposition of the milk sugar and the absorption of the volatile flavors by the b.u.t.ter fat. Conn[158] holds that the nitrogenous elements in cream serve as food for bacteria, and in the decomposition of which the desired aromatic substance is produced. The change is unquestionably a complex one, and cannot be explained as a single fermentation.

There is no longer much doubt but that both acid-forming and casein-digesting species can take part in the production of proper flavors as well as desirable aromas. The researches of Conn,[159] who has studied this question most exhaustively, indicate that both of these types of decomposition partic.i.p.ate in the production of flavor and aroma. He has shown that both flavor and aroma production are independent of acid; that many good flavor-producing forms belong to that cla.s.s which renders milk alkaline, or do not change the reaction at all. Some of these species liquefied gelatin and would therefore belong to the casein-dissolving cla.s.s. Those species that produced bad flavors are also included in both fermentative types. Conn has found a number of organisms that are favorable flavor-producers; in fact they were much more numerous than desirable aroma-yielding species. None of the favorable aroma forms according to his investigations were lactic-acid species,--a view which is also shared by Weigmann.[160]

McDonnell[161] has found that the production of aroma in certain cases varies at different temperatures, the most p.r.o.nounced being evolved near the optimum growing temperature, which, as a general rule, is too high for cream ripening.

The majority of bacteria in ripening cream do not seem to exert any marked influence in b.u.t.ter. A considerable number of species are positively beneficial, inasmuch as they produce a good flavor or aroma.

A more limited number are concerned in the production of undesirable ripening changes. This condition being true, it may seem strange that b.u.t.ter is as good as it is, because so frequently the requisite care is not given to the development of proper ripening. In all probability the chief reason why this is so is that those bacteria that find milk and cream pre-eminently suited to their development, e. g. the lactic-acid cla.s.s, are either neutral or beneficial in their effect on b.u.t.ter.

~Use of starters.~ Experience has amply demonstrated that it is possible to control the nature of the fermentative changes that occur in ripening cream to such an extent as to materially improve the quality of the b.u.t.ter. This is frequently done by the addition of a "starter." While starters have been employed for many years for the purpose mentioned, it is only recently that their nature has been understood. A starter may be selected from widely divergent sources, but in all cases it is sure to contain a large number of bacteria, and the presumption is that they are of such a nature as to produce desirable fermentative changes in the cream.

In the selection of these so-called natural starters, it follows that they must be chosen under such conditions as experience has shown to give favorable results. For this purpose, whole milk from a single animal is often used where the same is observed to sour with the production of no gas or other undesirable taint. A skim-milk starter from a mixed supply is recommended by many. b.u.t.ter milk is frequently employed, but in the opinion of b.u.t.ter experts is not as suitable as the others mentioned.

It not infrequently happens that the practical operator may be misled in selecting a starter that is not desirable, or by continuing its use after it has become contaminated.

In 1890[162] a new system of cream ripening was introduced in Denmark by Storch that possesses the merit of being a truly scientific and at the same time practical method. This consisted in the use of pure cultures of specific organisms that were selected on account of their ability to produce a desirable ripening change in cream. The introduction of these so-called culture starters has become universal in Denmark, and in parts of Germany. Their use is also rapidly extending in this country, Australia and New Zealand.

~Principles of pure-culture cream-ripening.~ In the proper use of pure cultures for ripening cream, it is necessary first to eliminate as far as possible the bacteria already present in cream before the culture starter is added. This result is accomplished by heating the cream to a temperature sufficiently high to destroy the vegetating organisms. The addition of a properly selected starter will then give the chosen organism such an impetus as will generally enable it to gain the ascendency over any other bacteria and so control the character of the ripening. The principle employed is quite like that practiced in raising grain. The farmer prepares his soil by plowing, in this way killing the weeds. Then he sows his selected grain, which is merely a pure culture, and by the rapid growth of this, other forms are held in check.

The attempt has been made to use these culture starters in raw sweet cream, but it can scarcely be expected that the most beneficial results will be attained in this way. This method has been justified on the basis of the following experiments. Where cream is pasteurized and no starter is added, the spore-bearing forms frequently produce undesirable flavors. These can almost always be controlled if a culture starter is added, the obnoxious form being repressed by the presence of the added starter. This condition is interpreted as indicating that the addition of a starter to cream which already contains developing bacteria will prevent those originally present in the cream from growing.[163] This repressive action of one species on another is a well-known bacteriological fact, but it must be remembered that such an explanation is only applicable in those cases where the culture organism is better able to develop than those forms that already exist in the cream.

If the culture organism is added to raw milk or cream which already contains a flora that is well suited to develop in this medium, it is quite doubtful whether it would gain the supremacy in the ripening cream. The above method of adding a culture to raw cream renders cream-ripening details less burdensome, but at the same time Danish experience, which is ent.i.tled to most credence on this question, is opposed to this method.

~Reputed advantages of culture starters.~ _1. Flavor and aroma._ Naturally the flavor produced by pure-culture ferments depends upon the character of the organism used. Those which are most extensively used are able to produce a perfectly clean but mild flavor, and a delicate but not p.r.o.nounced aroma. The "high, quick" flavor and aroma that is so much desired in the American market is not readily obtained by the use of cultures. It is quite problematical whether the use of any single species will give any more marked aroma than normally occurs in natural ripening.

_2. Uniformity of product._ Culture starters produce a more uniform product because the type of fermentation is under more complete control, and herein is the greatest advantage to be derived from their use. Even the best b.u.t.ter-maker at times will fail to secure uniform results if his starter is not perfectly satisfactory.

_3. Keeping quality of product._ b.u.t.ter made from pasteurized cream to which a pure-culture starter has been added will keep much better than the ordinary product, because the diversity of the bacterial flora is less and the milk is therefore not so likely to contain those organisms that produce an "off" condition.

_4. Elimination of taints._ Many defective conditions in b.u.t.ter are attributable to the growth of undesirable bacteria in the cream that result in the formation of "off" flavors and taints. If cream is pasteurized, thereby destroying these organisms, then ripened with pure ferments, it is generally possible to eliminate the abnormal conditions.[164] Taints may also be present in cream due to direct absorption from the cow or through exposure to foul odors.[165] Troubles of this sort may thus be carried over to the b.u.t.ter. This is particularly true in regions where leeks and wild onions abound, as in some of the Atlantic States. The heating of the cream tends to expel these volatile taints, so that a fairly good article of b.u.t.ter can be made from what would otherwise be a relatively worthless product.

~Characteristics desired in culture starters.~ Certain conditions as the following are desirable in starters made from pure cultures:

1. Vigorous growth in milk at ordinary ripening temperatures.

2. Ability to form acid so as to facilitate churning and increase the yield of b.u.t.ter.

3. Able to produce a clean flavor and desirable aroma.

4. Impart a good keeping quality to b.u.t.ter.

5. Not easily modified in its flavor-producing qualities by artificial cultivation.

These different conditions are difficult to attain, for the reason that some of them seem to be in part incompatible. Weigmann[166] found that a good aroma was generally an evanescent property, and therefore opposed to good keeping quality. Conn has shown that the functions of acid-formation, flavor and aroma production are not necessarily related, and therefore the chances of finding a single organism that possesses all the desirable attributes are not very good.

In all probability no one germ possesses all of these desirable qualities, but natural ripening is the resultant of the action of several forms.[167] This idea has led to the attempt at mixing selected organisms that have been chosen on account of certain favorable characteristics which they might possess. The difficulty of maintaining such a composite culture in its correct proportions when it is propagated in the creamery is seemingly well nigh insuperable, as one organism is very apt to develop more or less rapidly than the other.

A very satisfactory way in which these cultures are marketed is to mix the bacterial growth with some sterile, inert, dry substance. This is the method used in most of the Danish cultures. In this country, some of the more prominent cultures employed are marketed in a liquid form.