Outlines of Dairy Bacteriology Part 5
[32] Dunbar and Kister, Milch Zeit., p. 753, 1899. Harrison and Streit, Trans. Can. Inst., 7: 488, 1902-3.
[33] Doane, Bull. 88 Md. Expt. Stat., May 1903.
[34] Eckles, h.o.a.rd's Dairyman, July 8, 1898.
[35] Fraser, Bull. 91, Ill. Expt. Stat.
[36] Fraser, Bull. 91, Ill. Expt. Stat., Dec. 1903.
[37] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
[38] Backhaus. Ber. Landw. Inst. Univ. Konigsberg 2: 12, 1897.
[39] De Schweinitz, Nat. Med. Rev., April, 1899.
[40] Conn, Proc. Soc. Amer. Bacteriologists, 1902.
[41] Freudenreich, Ann. de Microg., 2:115, 1890.
[42] Conn, Bull. 26, Storrs Expt. Stat.
[43] New York City is supplied with milk that is shipped 350 miles.
[44] Park, N. Y. Univ. Bull., 1: 85, 1901.
[45] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
[46] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
[47] Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104.
[48] Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Bot.).
[49] Connell, Rept. of Commissioner of Agr., Canada, 1897, part XVI, p.
15.
[50] Harrison, h.o.a.rd's Dairyman, March 4, 1898.
CHAPTER IV.
FERMENTATIONS IN MILK AND THEIR TREATMENT.
Under the conditions in which milk is drawn, it is practically impossible to secure the same without bacterial contamination. The result of the introduction of these organisms often changes its character materially as most bacteria cause the production of more or less p.r.o.nounced fermentative processes. Under normal conditions, milk sours, i. e., develops lactic acid, but at times this more common fermentation may be replaced by other changes which are marked by the production of some other more or less undesirable flavor, odor or change in appearance.
In referring to these changes, it is usually customary to designate them after the most prominent by-product formed, but it must be kept in mind that generally some other decomposition products are usually produced.
Whether the organisms producing this or that series of changes prevail or not depends upon the initial seeding, and the conditions under which the milk is kept. Ordinarily, the lactic acid organisms grow so luxuriantly in the milk that they overpower all compet.i.tors and so determine the nature of the fermentation; but occasionally the milk becomes infected with other types of bacteria in relatively large numbers and the conditions may be especially suitable to the development of these forms, thereby modifying the course of the normal changes that occur.
The kinds of bacteria that find it possible to develop in milk may be included under two heads:
1. Those which cause no appreciable change in the milk, either in taste, odor or appearance. While these are frequently designated as the inert bacteria, it must not be supposed that they have absolutely no effect on milk. It is probably true in most cases that slight changes of a chemical nature are produced, but the nature of the changes do not permit of ready recognition.
2. This cla.s.s embraces all those organisms which, as a result of their growth, are capable of producing evident changes. These transformations may be such as to affect the taste, as in the sour milk or in the bitter fermentations, or the odor, as in some of the fetid changes, or the appearance of the milk, as in the slimy and color changes later described.
~Souring of milk.~ Ordinarily if milk is allowed to stand for several days at ordinary temperatures it turns sour. This is due to the formation of lactic acid, which is produced by the decomposition of the milk-sugar.
While this change is well nigh universal, it does not occur without a pre-existing cause, and that is the presence of certain living bacterial forms. These organisms develop in milk with great rapidity, and the decomposition changes that are noted in souring are due to the by-products of their development.
The milk-sugar undergoes fermentation, the chief product being lactic acid, although various other by-products, as other organic acids (acetic, formic and succinic), different alcohols and gaseous products, as CO_{2}, H, N and methane (CH_{4}) are produced in small amounts.
In this fermentation, the acidity begins to be evident to the taste when it reaches about 0.3 per cent., calculated as lactic acid. As the formation of acid goes on, the casein is precipitated and incipient curdling or lobbering of the milk occurs. This begins to be apparent when the acidity is about 0.4 per cent., but the curd becomes more solid with increasing acidity. The rapidity of curdling is also dependent upon the temperature of the milk. Thus milk which at ordinary temperatures might remain fluid often curdles when heated. The growth of the bacteria is continued until about 0.8 to 1.0 per cent. acid is formed, although the maximum amount fluctuates considerably with different lactic acid species. Further formation then ceases even though all of the milk-sugar is not used up, because of the inability of the lactic bacteria to continue their growth in such acid solutions.
As this acidity is really in the milk serum, cream never develops so much acid as milk, because a larger proportion of its volume is made up of b.u.t.ter-fat globules. This fact must be considered in the ripening of cream in b.u.t.ter-making where the per cent. of fat is subject to wide fluctuations.
The formation of lactic acid is a characteristic that is possessed by a large number of bacteria, micrococci as well as bacilli being numerously represented. Still the preponderance of evidence is in favor of the view that a few types are responsible for most of these changes. The most common type found in spontaneously soured milk changes the milk-sugar into lactic acid without the production of any gas. This type has been described by various workers on European as well as American milks, and is designated by Conn as the _Bact. lactis acidi_ type.[51] It is subject to considerable variation under different conditions.
Curiously enough if milk which has been drawn with special care is examined immediately after milking, the lactic organisms are not usually found. They are incapable of development in the udder itself, as shown by injections into the milk cistern. They abound, however, on hay, in dust, in the barn air, on the hairy coat of the animal, and from these sources easily gain access to the milk. In this medium they find an exceptionally favorable environment and soon begin a very rapid growth, so that by the time milk is consumed, either in the form of milk or milk products, they make up numerically the larger portion of the bacteria present.
Another widely disseminated, although numerically less prevalent, type is _B. lactis aerogenes_. This type forms gas in milk so that the soured milk is torn by the presence of gas bubbles. It also grows more luxuriantly in contact with the air.
Other types occur more or less sporadically, some of which are capable of liquefying the casein of milk while at the same time they also develop lactic acid. Conn and Aikman refer to the fact that over one hundred species capable of producing variable quant.i.ties of lactic acid are already known. It is fair to presume, however, that a careful comparative study of these would show that simply racial differences exist in many cases, and therefore, that they are not distinct species.
As a group these bacteria are characterized by their inability to liquefy gelatin or develop spores. On account of this latter characteristic they are easily destroyed when milk is pasteurized. They live under aerobic or anaerobic conditions, many of them being able to grow in either environment, although, according to McDonnell,[52] they are more virulent when air is not excluded.
While growth of these lactic forms may go on in milk throughout a relatively wide range in temperature, appreciable quant.i.ties of acid are not produced except very slowly at temperatures below 50 F.[53]
From the standpoint of frequency the most common abnormal changes that occur in milk are those in which gases of varying character are developed in connection with acids, from the milk sugar. Other volatile products imparting bad flavors usually accompany gas production. These fermentations are of most serious import in the cheese industry, as they are especially p.r.o.ne to develop in the manufacture of milk into certain types of cheese. Not often is their development so rapid that they appear in the milk while it is yet in the hands of the milk producer, but almost invariably the introduction of the causal organisms takes place while the milk is on the farm. Numerous varieties of bacteria possess this property of producing gas (H and CO_{2} are most common although N and methane (CH_{4}) are sometimes produced). The more common forms are those represented by _B. lactis aerogenes_ and the common fecal type, _B. coli commune_. The ordinary habitat of this type is dirt and intestinal filth. Hence careless methods of milk handling invite this type of abnormal change in milk.
It is a wide-spread belief that thunder storms cause milk to sour prematurely, but this idea has no scientific foundation. Experiments[54]
with the electric spark, ozone and loud detonations show no effect on acid development, but the atmospheric conditions usually incident to a thunder storm are such as permit of a more rapid growth of organisms.
There is no reason to believe but that the phenomenon of souring is wholly related to the development of bacteria. Sterile milks are never affected by the action of electric storms.
~"Ga.s.sy" milks.~ Where these gas bacteria abound, the amount of lactic acid is generally reduced, due to the splitting up of some of the sugar into gaseous products. This type of germ life does not seem to be able to develop well in the presence of the typical lactic acid non gas-forming bacteria.
[Ill.u.s.tration: FIG. 16. Cheese made from "ga.s.sy" milk.]
~"Sweet curdling" and digesting fermentations.~ Not infrequently milk, instead of undergoing spontaneous souring, curdles in a weakly acid or neutral condition, in which state it is said to have undergone "sweet curdling." The coagulation of the milk is caused by the action of enzyms of a rennet type that are formed by the growth of various species of bacteria. Later the whey separates more or less perfectly from the curd, producing a "wheyed off" condition. Generally the coagulum in these cases is soft and somewhat slimy. The curd usually diminishes in bulk, due to the gradual digestion or peptonization of the casein by proteid-dissolving enzyms (tryptic type) that are also produced by the bacteria causing the change.
A large number of bacteria possess the property of affecting milk in this way. So far as known they are able to liquefy gelatin (also a peptonizing process) and form spores. The Tyrothrix type of bacteria (so named by Duclaux on account of the supposed relation to cheese ripening) belongs to this cla.s.s. The hay and potato forms are also digesters.
Organisms of this type are generally a.s.sociated with filth and manure, and find their way into the milk from the acc.u.mulations on the coat of the animal.
Conn[55] has separated the rennet enzym from bacterial cultures in a relatively pure condition, while Fermi[56] has isolated the digestive ferment from several species.
Duclaux[57] has given to this digesting enzym the name _casease_ or cheese ferment. These isolated ferments when added to fresh milk possess the power of causing the characteristic curdling and subsequent digestion quite independent of cell development. The quant.i.ty of ferment produced by different species differs materially in some cases. In these digestive fermentations, the chemical transformations are profound, the complex proteid molecule being broken down into alb.u.moses, peptones, amido-acids (tyrosin and leucin) and ammonia as well as fatty acids.
Not infrequently these fermentations gain the ascendency over the normal souring change, but under ordinary conditions they are held in abeyance, although this type of bacteria is always present to some extent in milk.
When the lactic acid bacteria are destroyed, as in boiled, sterilized or pasteurized milk, these rennet-producing, digesting species develop.