TABLE XVI
From Wis. Bul. 150.
The large number of bacteria in the cheese is very striking. The number as given in the accompanying table is not that actually in the cheese, as it is very difficult to obtain the sample in suitable condition for plating.[115]
The principal action of the lactic acid-forming bacteria in the cheese ripening is the changing of the milk-sugar or lactose into lactic acid and the formation of small amounts of other substances, such as acetic, succinic and formic acids, alcohol, aldehydes and esters and some gases, carbon dioxide and hydrogen. While the amount of these substances other than lactic acid is small, it is thought that the effect of these on the cheese may be important. Heinemann shows[116] that lactic acid exists in two optical modifications, the levorotatory and dextrorotary acids. In cheese they are usually found in the inactive or racemic form, the levorotatory and dextrorotary acids being present in equal amounts. What importance the question of optical activity of the lactic acid may assume is not definitely known. Just as some groups of bacteria have a specific effect on the lactose, producing only one modification of lactic acid, so bacteria attacking lactic acid may exercise a selective action and use only one or the other optically active modification. In other words, the early flora of cheese-ripening bacteria may determine the later flora by the production of a form of lactic acid attacked by one group of bacteria and not by another, and the effect on the flavor will differ accordingly. The amount of lactic acid in the cheese increases for a time, then decreases.
The errors in determining lactic acid are considerable. It seems that the tendency is toward an increase of lactic acid in the cheese long after the lactose has disappeared. Two explanations are offered: one, that in the lactic acid fermentation an intermediate compound or compounds are formed which exist for some time, the conversion into lactic acid being complete at about three months; the other is that lactic acid is formed as a product of paracasein proteolysis.
The lactic acid formed in cheese ripening does not exist in a free state but reacts with the calcium salts in the cheese and forms calcium lactates. It is thought that there is sufficient of these salts to neutralize all the acid formed, and therefore the acid does not enter into combination with the paracasein salts. It has been found that lactates are the principal source of acetic and propionic acids. These are supposed to have some effect on the flavor of the cheese.
The effect of lactic acid as a determinant of bacterial and enzymic changes is very important. Early in the ripening process, lactic acid suppresses the growth of undesirable micro-organisms. It also furnishes the acid medium necessary for the best action of both the coagulating and peptic enzymes.
The importance of the lactic acid bacteria in cheese ripening has been summed up by Hastings[117] as follows: "The functions of this group of bacteria in Cheddar cheese are through their by-product lactic acid as follows: (a) To favor the curdling of milk by rennet. (b) The bacteria of the milk are held in great part in the curd. Through the acid they influence the shrinkage of the curd and expulsion of the whey, (c) The acid so changes the nature of the curd as to cause 'matting,' or 'cheddaring' of the curd, (d) The acid activates the pepsin of the rennet extract, (e) The acid prevents the growth of putrefactive bacteria in the cheese. (f) It has been shown that Bacterium lactis acidi is able to form acid in the absence of the living cell. (g) The development of Bacterium lactis acidi is followed by the growth of another group of acid-forming bacteria, the Bacillus Bulgaricus group. They reach numbers comparable with those of the first group, reaching their maximum number within the first month of ripening. Since they develop after the fermentation of the milk-sugar, they must have some other source of carbon and of energy than milk-sugar." It is also probable that other groups constantly present contribute to the changes.
From the preceding discussion it is evident that each of the ripening agents has its important part to play in the ripening process and a normal ripening of the cheese is a composite result of these various agencies.