Bacterial Activity in Cheese.
—Modern science has led to the conclusion that the ripening of cheese is due principally to bacterial activity. The changes which take place in the chemical and physical properties of cheese materials, the flavor and aroma which are developed, the production of mould and other growths are marks of the activity of organisms of different character, living and unorganized. Due credit must be given to the enzymic (unorganized) action in these processes and the enzymes are not regarded as living organisms but, on the other hand, as catalytic agents inducing chemical changes similar to those produced in starch by the action of diastase. The peculiar flavors of cheeses which are found in different kinds have been ascribed in late years almost exclusively to the character of bacterial activity. This assumption is perhaps correct, but it must not be forgotten in this connection that the same species of bacteria, in changed environments, does not always produce the same results. The activities of bacteria are peculiarly sensitive to the environment, such as change of temperature, physical conditions of different kinds, locality, and other factors of a complex nature, making up the total conditions in which the organisms live. For this reason the attempts to produce peculiar cheeses which belong in particular localities in other localities have not been gustatorily even if technically successful. It is true that cheeses may be made of the types mentioned, having some of the general characteristics but lacking that indescribable something which after all gives true character. Just as it is impossible to make a Rhine wine in California or a Bordeaux wine in New York so is it impossible to make a Cheddar cheese in Ohio or a Camembert cheese in Connecticut.
Number of Bacteria.
—The number of bacteria, per gram, which appear in cheese varies according to the age of the cheese, conditions under which it is made, temperature, etc. The usual number of bacteria in one gram of cheese varies from five hundred thousand to nearly one hundred million (21st Annual Report of the Wisconsin Agricultural Experiment Station).
Ageing does not seem to increase the number of organisms, since it has been found by some observers that the maximum number present in cheese is found at the time it is taken from the press. It is difficult also to properly sample a cheese for the number of bacteria, since they are unequally distributed in different parts thereof, and the trier, by means of which the sample is secured, may show largely differing numbers in different parts of the same cheese. During the process of curing, especially if the curing be at a high temperature, the number of organisms decreases. At first the decrease is very rapid and then becomes slower as the cheese becomes riper. The decrease in the number of bacteria when the temperature of curing is raised is somewhat contrary to expectations. It has been found that when a cheese is taken from cold storage, say at 24 degrees F., and placed in a temperature of 60 degrees F., the decline in the number of bacteria is always greater than when the cheese is retained at the lower temperature. This may be due to the fact that bacteria which have been developed at a low may lose their vitality at a higher temperature. On the contrary, the development of flavor does not seem to depend upon the number of organisms since the peculiar flavor of cheese is more rapidly developed at the higher temperature, provided it be not too high, although this be attended with a diminution in the number of organisms. Evidently the conditions which favor the metabolic activities of organisms also favor their destruction, since when they have performed their functions they undergo natural disintegration. The character of cheese is such that when it is once formed there is no more opportunity given for a rapid proliferation of the organisms.
It may be found, however, that the development of bacterial life is not the sole or perhaps not the dominant factor in the development of flavors and aromas in cheeses but that this process is due very largely to the enzymic activities obtained from the rennet and which pre-exist in the milk.
Chemical Changes Which Take Place During the Ripening of the Cheese.—Loss of Weight.
—During the process of ripening of cheese there is considerable loss of weight, amounting to from 15 to 20 percent of the total weight of the fresh product. This loss is due chiefly to the evaporation of water, while in the fermentation which takes place volatile bodies are formed which also escape with the water. For instance, any free gas, either carbon dioxid, hydrogen, or nitrogen, which is produced will escape, likewise any alcohol which is formed will at least partially volatilize. There may be also a slight loss due to mechanical attrition, but that is not of any consequence. Owing to the loss of water some of the constituents which may diminish in actual quantity have their percentages proportionately increased. These changes are illustrated by the following analytical data:
| Water. | Protein. | Fat. | Milk Sugar. | Ash. | |
|---|---|---|---|---|---|
| Fresh cheese, | 40.42 | 24.80 | 28 | 1.65 | 5.43 |
| In the dry substance, | .... | 41.62 | 46.99 | .... | .... |
| Same cheese one year old, | 33.12 | 27.35 | 31.70 | 2.96 | 4.87 |
| In the dry substance, | .... | 40.89 | 47.40 | .... | .... |
The quantity of water which is lost in part depends upon the temperature of the store house and the dryness of the air. The loss of water should not be too great, otherwise the cheese would be dry and the ripening process would not go on in a proper manner. In some of the processes which take place during the ripening of cheese water is formed. If, therefore, there is no loss of weight during the process of ripening, the ripened cheese would have more water than the fresh cheese and this would impair the quality of the product. The loss of a certain part of water, namely, from 15 to 20 percent must be regarded as an advantage in the production of cheese.