[155] Milch Zeit., 1889, p. 7; 1894, p. 624; 1895, p. 383.

[156] Dean, Ont. Agr. Coll., 1897, p. 66.

[157] Storch, Nogle, Unders. over Floed. Syrning, 1890.

[158] Conn, 6 Storrs Expt. Stat., 1893, p. 66.

[159] Conn, 9 Storrs Expt. Stat., 1896, p. 17.

[160] Weigmann, Milch Zeit., 1891, p. 793

[161] McDonnell, u. Milchsaure Bakterien (Diss. Kiel, 1899), p. 43.

[162] Storch, Milch Zeit., 1890, p. 304.

[163] Conn, 9 Storrs Expt. Stat., 1896, p. 25.

[164] Milch Zeit., 1891, p. 122; 1894, p. 284; 1895, p. 56; 1896, p.

163.

[165] McKay, Bull. 32, Iowa Expt. Stat., p. 47

[166] Weigmann, Landw. Woch. f. Schl. Hol., No. 2, 1890.

[167] Weigmann, Cent. f. Bakt., II Abt., 3:497, 1897.

[168] At the National Creamery b.u.t.termakers' a.s.sociation for 1901, 193 out of 240 exhibitors used starters. Of those that employed starters, nearly one-half used commercial cultures. There was practically no difference in the average score of the two cla.s.ses of starters, but those using starters ranked nearly two points higher in flavor than those that did not.

[169] Russell, Bull. 143, Wis. Expt. Stat., Feb. 1907.

[170] Melick, Bull. 138, Kansas Expt. Stat., June 1906.

[171] Reinmann, Cent. f. Bakt., 1900, 6:131; Jensen, Landw. Jahr. d.

Schweiz, 1901.

[172] Jensen, Cent. f. Bakt., 1891, 11:409.

[173] Jensen, Milch Zeit., 1892, 6, Nos. 5 and 6.

[174] Pammel, Bull. 21, Iowa Expt. Stat., p. 803.

[175] Fischer, Hyg. Rund., 5:573.

[176] Storch, 18 Rept. Danish Agric. Expt. Stat., 1890.

[177] Rogers Bull. 57, B. A. I. U. S. Dept Agric., 1904.

CHAPTER VIII.

BACTERIA IN CHEESE.

The art of cheese-making, like all other phases of dairying, has been developed mainly as a result of empirical methods. Within the last decade or so, the subject has received more attention from the scientific point of view and the underlying causes determined to some extent. Since the subject has been investigated from the bacteriological point of view, much light has been thrown on the cause of many changes that were heretofore inexplicable. Our knowledge, as yet, is quite meager, but enough has already been determined to indicate that the whole industry is largely based on the phenomena of ferment action, and that the application of bacteriological principles and ideas is sure to yield more than ordinary results, in explaining, in a rational way, the reasons underlying many of the processes to be observed in this industry.

The problem of good milk is a vital one in any phase of dairy activity, but it is pre-eminently so in cheese-making, for the ability to make a first-cla.s.s product depends to a large extent on the quality of the raw material. Cheese contains so large a proportion of nitrogenous const.i.tuents that it is admirably suited, as a food medium, to the development of bacteria; much better, in fact, than b.u.t.ter.

INFLUENCE OF BACTERIA IN NORMAL CHEESE PROCESSES.

In the manufacture of cheddar cheese bacteria exert a marked influence in the initial stages of the process. To produce the proper texture that characterizes cheddar cheese, it is necessary to develop a certain amount of acid which acts upon the casein. This acidity is measured by the development of the lactic-acid bacteria that normally abound in the milk; or, as the cheese-maker expresses it, the milk is "ripened" to the proper point. The action of the rennet, which is added to precipitate the casein of the milk, is markedly affected by the amount of acid present, as well as the temperature. Hence it is desirable to have a standard amount of acidity as well as a standard temperature for coagulation, so as to unify conditions. It frequently happens that the milk is abnormal with reference to its bacterial content, on account of the absence of the proper lactic bacteria, or the presence of forms capable of producing fermentative changes of an undesirable character.

In such cases the maker attempts to overcome the effect of the unwelcome bacteria by adding a "starter;" or he must vary his method of manufacture to some extent to meet these new conditions.

~Use of starters.~ A starter may be employed to hasten the ripening of milk that is extremely sweet, so as to curtail the time necessary to get the cheese to press; or it may be used to overcome the effect of abnormal conditions.

The starter that is employed is generally one of domestic origin, and is usually taken from skim milk that has been allowed to ferment and sour under carefully controlled conditions. Of course much depends upon the quality of the starter, and in a natural starter there is always the possibility that it may not be perfectly pure.

Within recent years the attempt has been made to control the effect of the starter more thoroughly by using pure cultures of some desirable lactic-acid form.[178] This has rendered the making of cheese not only more uniform, but has aided in repressing abnormal fermentations particularly those that are characterized by the production of gas.

Recently, pure cultures of Adametz's _B. n.o.bilis_, a digesting organism that is claimed to be the cause of the breaking down of the casein and also of the peculiar aroma of Emmenthaler cheese, has been placed on the market under the name _Tyrogen_. It is claimed that the use of this starter, which is added directly to the milk and also rubbed on the surface of the cheese, results in the improvement of the curds, a.s.sists in the development of the proper holes, imparts a favorable aroma and hastens ripening.[179]

Campbell[180] states that the discoloration of cheese in England, which is due to the formation of white spots that are produced by the bleaching of the coloring matter in the cheese, may be overcome by the use of lactic-acid starters.

The use of stringy or slimy whey has been advocated in Holland for some years as a means of overcoming the tendency toward gas formation in Edam cheese which is made from practically sweet milk. This fermentation, the essential feature of which is produced by a culture of _Streptococcus Hollandicus_,[181] develops acid in a marked degree, thereby inhibiting the production of gas.

The use of ma.s.ses of moldy bread in directing the fermentation of Roquefort cheese is another ill.u.s.tration of the empirical development of starters, although in this instance it is added after the curds have been prepared for the press.

~Pasteurizing milk for cheese-making.~ If it were possible to use properly pasteurized milk in cheese-making, then practically all abnormal conditions could be controlled by the use of properly selected starters.

Numerous attempts have been made to perfect this system with reference to cheddar cheese, but so far they have been attended with imperfect success. The reason for this is that in pasteurizing milk, the soluble lime salts are precipitated by the action of heat, and under these conditions rennet extract does not curdle the casein in a normal manner.

This condition can be restored, in part at least, by the addition of soluble lime salts, such as calcium chlorid; but in our experience, desirable results were not obtained where heated milks to which this calcium solution had been added were made into cheddar cheese.

Considerable experience has been gained in the use of heated milks in the manufacture of certain types of foreign cheese. Klein[182] finds that Brick cheese can be successfully made even where the milk is heated as high as 185 F. An increased weight is secured by the addition of the coagulated alb.u.min and also increased moisture.

~Bacteria in rennet.~ In the use of natural rennets, such as are frequently employed in the making of Swiss cheese, considerable numbers of bacteria are added to the milk. Although these rennets are preserved in salt, alcohol or boric acid, they are never free from bacteria.

Adametz[183] found ten different species and from 640,000 to 900,000 bacteria per cc. in natural rennets. Freudenreich has shown that rennet extract solutions can be used in Swiss cheese-making quite as well as natural rennets; but to secure the best results, a small quant.i.ty of pure lactic ferment must be added to simulate the conditions that prevail when natural rennets are soaked in whey, which, it must be remembered, is a fluid rich in bacterial life.

Where rennet extract or tablets are used, as is generally the case in cheddar making, the number of bacteria added is so infinitesimal as to be negligible.

~Development of acid.~ In the manufacture of cheddar cheese, the development of acid exerts an important influence on the character of the product. This is brought about by holding the curds at temperatures favorable to the growth of the bacteria in the same. Under these conditions the lactic-acid organisms, which usually predominate, develop very rapidly, producing thereby considerable quant.i.ties of acid which change materially the texture of the curds. The lactic acid acts upon the casein in solutions containing salt, causing it to dissolve to some extent, thus forming the initial compounds of digestion.[184] This solution of the casein is expressed physically by the "stringing" of the curds on a hot iron. This causes the curds to mat, producing a close, solid body, free from mechanical holes. Still further, the development of this acid is necessary for the digestive activity of the pepsin in the rennet extract.

In some varieties of cheese, as the Swiss, acid is not developed and the character of the cheese is much different from that of cheddar. In all such varieties, a great deal more trouble is experienced from the production of "ga.s.sy" curds, because the development of the gas-producing bacteria is held in check by the rapid growth of the lactic acid-producing species.

~Bacteria in green cheese.~ The conditions under which cheese is made permit of the development of bacteria throughout the entire process. The cooking or heating of curds to expel the excessive moisture is never so high as to be fatal to germ life; on the contrary, the acidity of the curd and whey is continually increased by the development of bacteria in the same.

The body of green cheese fresh from the press is, to a considerable extent, dependent upon the acid produced in the curds. If the curds are put to press in a relatively sweet condition the texture is open and porous. The curd particles do not mat closely together and "mechanical holes," rough and irregular in outline, occur. Very often, at relatively high temperatures, such cheese begin to "huff," soon after being taken from the press, a condition due to the development of gas, produced by gas-generating bacteria acting on the sugar in the curd. This gas finds its way readily into these ragged holes, greatly distending them, as in Fig. 30.

[Ill.u.s.tration: FIG. 30. _L_, a sweet curd cheese direct from the press.