Part 5 (2/2)

Under the conditions in which milk is drawn, it is practically impossible to secure the same without bacterial contaanises its character materially as most bacteria cause the production of more or less pronounced fermentative processes Under normal conditions, milk sours, i e, develops lactic acid, but at times this es which are marked by the production of soe in appearance

In referring to these changes, it is usually custonate them after the most proenerally some other decoanises prevail or not depends upon the initial seeding, and the conditions under which the row so luxuriantly in the milk that they overpower all competitors and so determine the nature of the fermentation; but occasionally the milk becoe numbers and the conditions may be especially suitable to the develop the course of the nores 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 es of a chees do not pernition

2 This class erowth, 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 feres, or the appearance of the es 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 decoe is well nigh universal, it does not occur without a pre-existing cause, and that is the presence of certain living bacterial forreat rapidity, and the deco are due to the by-products of their developoes ferh various other by-products, as other organic acids (acetic, foraseous products, as CO_{2}, H, N and methane (CH_{4}) are produced in sins to be evident to the taste when it reaches about 03 per cent, calculated as lactic acid As the foroes on, the casein is precipitated and incipient curdling or lobbering of the ins to be apparent when the acidity is about 04 per cent, but the curd beco acidity The rapidity of curdling is also dependent upon the temperature of the ht rerowth of the bacteria is continued until about 08 to 10 per cent acid is forh the maximum amount fluctuates considerably with different lactic acid species Further forar 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 seruer proportion of its volulobules This factof crea 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 nu numerously represented Still the preponderance of evidence is in favor of the view that a few types are responsible for es The es the as This type has been described by various workers on European as well as Anated 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 draith special care is exaanisms 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 aniain access to the milk In this medium they find an exceptionally favorable environrowth, so that by the time milk is consumed, either in the forer portion of the bacteria present

Another widely disseh nuenes_ This type foras in as bubbles It also grows more luxuriantly in contact with the air

Other types occur more or less sporadically, so 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 quantities 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, h, according to McDonnell,[52] they are rowth of these lactic fore in temperature, appreciable quantities of acid are not produced except very slowly at temperatures below 50 F[53]

Froes that occur incharacter are developed in connection with acids, fro bad flavors usually accoas production These fermentations are of most serious import in the cheese industry, as they are especially prone 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 alanisms takes place while the milk is on the farm Numerous varieties of bacteria possess this property of producing gas (H and CO_{2} are h N and methane (CH_{4}) are sometimes produced) The enes_ and the common fecal type, _B coli commune_ The ordinary habitat of this type is dirt and intestinal filth Hence careless e 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 storanisms

There is no reason to believe but that the pheno is wholly related to the development of bacteria Sterile milks are never affected by the action of electric storas bacteria abound, the aenerally reduced, due to the splitting up of soerm life does not seem to be able to develop well in the presence of the typical lactic acid non gas-for bacteria

[Illustration: FIG 16 Cheese esting fer 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 enzyrowth of various species of bacteria Later the whey separatesa ”wheyed off” condition Generally the coagulum in these cases is soft and somewhat sliradual digestion or peptonization of the casein by proteid-dissolving enzyms (tryptic type) that are also produced by the bacteria causing the change

A large nu elatin (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 class The hay and potato forenerally associated with filth and manure, and find their way into the milk from the accumulations on the coat of the animal

Conn[55] has separated the rennet enzym from bacterial cultures in a relatively pure condition, while Ferestive feriven to this digesting enzym the name _casease_ or cheese ferment These isolated fer the characteristic curdling and subsequent digestion quite independent of cell development The quantity of ferment produced by different species differs estive fermentations, the chemical transfor broken down into albumoses, peptones, amido-acids (tyrosin and leucin) and ammonia as well as fatty acids

Not infrequently these fere, 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 pasteurizedspecies develop