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. nobilis, 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, assists 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 masses of moldy bread in directing the fermentation of Roquefort cheese is another illustration 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 albumin 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 quantity 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.