Outlines of dairy bacteriology, 10th edition / A concise manual for the use of students in dairying

by H. L. Russell; E. G. Hastings

Other factors concerned in cheese ripening. There are other factors that are also concerned in the complex series of ripening changes noted in cheddar cheese. All animal fluids and tissues, if kept under perfectly sterile conditions at ordinary temperatures, will undergo a certain amount of decomposition, due apparently to their content in enzymes that have a digestive action. Meat kept in storage becomes more tender due to the softening of the connective tissue. Milk, derived as it is from actively secreting cell tissue, gives certain reactions that are common to living material. If chloroform, which restrains the action of bacteria, but does not prevent the activity of enzymes, is added to it, it will curdle in the course of a few weeks and will become partially digested. This digesting ferment found in milk is known as galactase. Compounds are formed in milk thus preserved that are similar to those found in a ripe cheddar cheese. Many experiments have been made with pasteurized milk, but it has not been possible to produce typical, normal cheese from thoroughly pasteurized milk. Such cheese are markedly deficient in the typical flavor of cheddar cheese. From this fact it is believed that the inherent enzymes of milk are a factor of some importance in the ripening of this type of cheese at least, if not of all types.

In the past, other factors have been thought to be of importance. Duclaux, a French bacteriologist, considered that the enzymes formed by the digesting bacteria are responsible for the ripening. It is now known that they can have but little if any part in the process, since they are not present in all cheese in sufficient numbers to have any marked effect, and since the acidity of the cheese mass will not permit of their development.

Other types of bacteria have been considered by bacteriologists to be of importance in the ripening process, but it is certain that the purely digestive change in the mass of the cheese can be accounted for through the action of the factors already noted.

Flavor production. The flavor of any type of cheese is the most important characteristic, just as it is in butter, for it is largely the flavor that determines the selling value of the product, and is the most difficult thing to control. It has been thought that the flavor-producing substances were derived from the paracasein of the curd and were produced by the factors that are concerned in the digestion of the paracasein. It has been shown that a cheese may be thoroughly ripened as far as its physical properties are concerned; that it may contain the end products of casein digestion, and yet be low in flavor. From recent researches it seems probable that the production of flavor is connected with the change that the sugar undergoes in the acid fermentation, as volatile acids, acetic, formic, etc., as well as alcohols and esters are formed in increasing amounts as the ripening progresses. These may have come from the decomposition of the milk sugar, or from a secondary change in the products of the lactic fermentation. There are organisms in both milk and cheese that do not grow on the ordinary culture media used by the bacteriologist, and it may well be that some of these are of importance in flavor production. Their destruction in pasteurization is likely to be one of the reasons for the failure of cheese made from pasteurized milk to develop typical flavor.

Effect of temperature on ripening. The temperature at which the ripening cheese is kept has been found to be of the greatest importance in determining the quality of the product. If the cheese is kept at high temperatures, the ripening proceeds rapidly; the cheese is short lived, and has a sharp, strong flavor, and generally a more or less open texture. Unless the cheese is made from the best quality of milk, it is likely to undergo undesirable fermentations when ripened at high temperatures.

Within recent years it has been found possible to ripen cheese at temperatures that were previously thought to be certain to spoil the product. Much of the cheese is now ripened at temperatures below 50° F. The ripening goes on more slowly than at higher temperatures, but the flavor of the cheese is clean and entirely devoid of the sharp undesirable tang that is so frequently noted in old cheese, and the texture is solid and meaty. Ripening at low temperatures, when the milk is not of the best quality, is certain to result in a much better product than when higher temperatures are employed.

Abnormal fermentations in cheese. As has been previously shown, it is necessary to have an abundant supply of acid-forming bacteria in the milk from which cheese is to be made. If these bacteria are supplanted by other kinds, the product will be more or less abnormal either in texture or in flavor, or possibly in both. Many of these abnormal fermentations have been studied and the organisms concerned in the changes found.

If the milk is handled carelessly, it will contain many bacteria able to form acid and gas. As noted previously, these organisms form products in milk that have an offensive odor and a disagreeable taste. In cheese the gases cause the formation of holes, more or less numerous, depending on the number of the gas-forming bacteria in the milk. Where these bacteria are abundant, gas may appear while the curd is in the vat, causing it to float in the whey, when it is known as a "floater." Again, the gas may not become evident until the cheese is in the press or on the curing shelf, when it becomes apparent by the swelling or bulging of the cheese. Such cheese is termed "huffed" or "swelled." The internal pressure may be so great as to cause the cheese to crack and to force out some of the curd. The presence of gas holes is indicative of a poor cheese, because the formation of gas is always accompanied by the presence of other undesirable compounds.

Pure culture starters are often used to overcome gassy fermentations. In cheese a certain amount of acid can be produced by the acid-forming bacteria. When the pure lactic bacteria alone are present, the cheese is very likely to be of good quality. If the sugar is fermented by gas-forming organisms, the curd will be full of holes and the flavor poor, while if the sugar is fermented by a mixture of the desirable and undesirable bacteria, the quality of the product will depend on the relation of the two types. If through the addition of a pure-culture starter, the proportion of desirable bacteria is increased, the gas will be lessened in amount and the cheese improved. It was formerly supposed that the lactic bacteria had an injurious effect on the gas-forming organisms. There is no good reason to believe that this is the case, but that both grow in the milk and cheese, but since only a certain amount of acid can be produced, it is important to have as much of it formed by the lactic bacteria as possible, since the amount of injurious products in the cheese will thus be limited.