Chapter VIII: Bacteria and Cheese Making

Butter, such as that of the sweet-cream type that is highly esteemed in many parts of the world, may be made without the aid of bacteria, but no important kind of cheese can be made under commercial conditions without them.

=Types of cheese.= Cheese consists of the fat and the precipitated casein of milk, together with a large amount of water and the salts found in milk. The numerous types of cheese may be divided into two groups, depending on the manner in which the curdling of the milk is brought about. Sour-milk cheese is made from curd, formed as a result of the acid fermentation of the milk. Thus, at the very first stage in the making of this type, the importance of bacteria is apparent.

The second type is that made from curd, which is precipitated by the addition of rennet to the milk. This type may also be divided into two groups, depending upon their texture; the hard cheese, and the soft cheese. The ordinary cheddar, the common American type, is the most important example of the hard cheese; Limburger, of the soft cheese. Cheese are designated as hard or soft, depending upon the amount of whey that is retained in them during the making process. The moisture content has an important influence on the type and amount of life that develops on and in the curd mass, and as will be seen, the ripening and flavor of the cheese are dependent upon these biological factors.

The two groups of hard and soft cheese have no sharply defined limits, but merge into each other. The extreme types of the hard cheese are so dry and firm that they can be cut only with difficulty. Such cheese are used primarily as condiments to impart a flavor to certain dishes, as macaroni, and for this purpose are grated. The extreme type of soft cheese is a soft, pasty mass and can be easily spread with a knife.

Hard cheese, because the ripening process goes on uniformly throughout the entire mass of cheese, may be made of any size which permits of commercial handling. They can also be kept for long periods and preserve their good qualities. Soft cheese are made in small sizes, since on account of their consistency, they could not otherwise be handled, and also because of the manner of ripening. The ripening is due to the action of organisms developing on the surface, the by-products of which diffuse into the curd. If the cheese are too large, the outer layers become overripe, while the interior remains more or less unchanged, or insufficiently changed. Soft cheese mature much more rapidly than hard cheese; consequently they are short lived.

Although made from the same substance, milk, it is noteworthy that there are over four hundred varieties of cheese produced. Most of these find only a local market where made. Less than a dozen varieties are to be regarded as general articles of commerce.

=Quality of milk.= In the making of butter there are a number of processes that the maker can use when he finds himself obliged to utilize poor milk. The milk can be pasteurized and the harmful bacteria thus destroyed; desirable kinds can then be added in the form of a pure-culture starter. Pasteurization also drives off some of the volatile by-products of the first acid fermentation. By the use of these means, the maker can prepare a very good product from poor material.

In the making of most kinds of cheese, especially those of the greatest commercial importance, the cheese maker can call to his help no such aids, but must use the milk as it is brought to him. It is possible to prepare certain kinds of soft cheese from pasteurized milk that differ in no essential point from the same cheese made from raw milk. Hard cheese are also made from pasteurized milk, but in most cases such cheese differ, especially in the degree of flavor, from that made from unheated milk. It is quite probable that, as the factors concerned in the ripening of cheese become better known, methods will be evolved for the successful production of many kinds of cheese from pasteurized milk.

It has been shown that the quality of milk is almost wholly dependent upon the number and kinds of bacteria it contains. These bacteria pass into the cheese, and there produce the same products as they would have done in the milk itself. In butter making, practically all processes are under the control of the maker, until the product is ready for the market; but cheese, on the other hand, passes through a complicated series of changes after it has left the maker's control. During the manipulation of the milk and the curd in the vat, he can exert some influence on the quality of the product, but he is much more dependent on the quality of the milk than is the case in butter making.

Every effort should therefore be made to furnish to the cheese maker the quality of milk from which he can prepare fine cheese. In other words, the milk should be produced under clean conditions and carefully cooled and handled until delivered to the maker. Poor milk from a single farm may have such an effect upon the cheese made from the milk of twenty farms as to depreciate the selling value of the entire product several cents per pound.

The tests that have been previously described (p. 105) have been devised especially for testing the quality of the milk for cheese making purposes, and are of the greatest service to the maker in tracing the source of poor milk.

=Cheddar cheese.= The first step in the making of cheddar cheese is the "ripening" of the milk, or the development of a small amount of acid. In this fermentation, the development of acid is preceded by an enormous increase in the number of acid-forming bacteria. Milk for cheese making should show an acidity of about 0.2 per cent or slightly more than in fresh milk. In other words, the maker wishes the milk to be in such condition, bacteriologically, that if kept at a temperature favorable for the growth of the acid-forming bacteria, the acidity will increase rapidly.

The curdling of the milk to precipitate the cheese solids is produced by the addition of rennet, which is obtained by extracting the fourth stomach of the young calf with a solution of common salt. In the past the maker prepared his own rennet solution from the dried stomachs ("rennets"), but at present, the extract is prepared commercially, in a much more uniform manner. The rapidity of the curdling is dependent upon the acidity of the milk. In order to secure proper rennet action, a slight increase of acid over that found in fresh milk is usually necessary; thus at the very beginning of the process of making cheddar cheese, the bacteria are of importance.

As the milk curdles, the bacteria are enclosed in the curd as are the fat globules. The curd is cut into small fragments by means of a curd knife, and as the mass is warmed, the acid develops, causing the curd particles to shrink, thus expressing the whey. Within a short time, the volume of the curd is not more than one-eighth that of the milk, but in the curd are held over 75 per cent of the bacteria of the milk. To secure rapid curdling in the vat, the milk is warmed to 85 deg. to 90 deg. F., a temperature that is most favorable for the growth of the lactic bacteria. Since there is a large number of bacteria concentrated in a small volume, and the temperature, as well as all other conditions, is favorable to growth, multiplication of the bacteria goes on rapidly, and as a consequence, acid is formed in large amounts, as is shown by the following figures given by Publow for the manufacture of the export type of cheddar cheese:

Acidity of milk before adding rennet .2 to .21 per cent
Acidity of whey before heating curd .14 to .145 "
Acidity of whey before removing from curd .16 to .18 "
Acidity of whey coming from the curd after
removal of whey and curd is packed .24 to .30 "
Acidity of whey coming from curd before milling .65 to .75 "
Acidity of whey coming from curd before salting .90 to 1.10 "

If the milk had been kept at the same temperature as the curd, the acidity would have increased much more slowly since the acid would have been distributed through a larger volume. In the cheese curd the same amount of acid is probably formed, as would have been produced in the total amount of milk during the same interval.

The acid produced by this bacterial activity has a most marked effect on the curd. At first the curd masses are tough and firm, the particles showing no tendency to adhere to each other. As the acid increases in amount, the curd becomes plastic, the outer surface of the particles adhering or "matting," as the maker expresses it. The result is a solid coalescent mass of curd, which is cut into small pieces, _i.e._, "milled," before it is put to press. The acid allows the blending of the pieces under the influence of the pressure so that a cheese is one single mass. Under certain abnormal conditions, the development of acid may be interfered with and the particles of curd fail to mat, in which case, the cheese will be crumbly when it is cut. The determination of the proper time for pressing is made by the application of what is known as the hot iron test. This is made by determining the length of the "strings" or "threads" which can be drawn from a mass of curd when it is brought in contact with a hot iron at a cherry red heat, the length of the curd threads being a measure of the amount of acid that has been formed in the curd.

The rate of acid formation within the curd particles is also measured by determining the acidity of the whey as it comes from the curd at different stages in the making. This test, which is often used in place of the "hot iron" test is carried out in the same manner, as in determining the acidity of milk or cream. The quality of the cheese, both as to texture and flavor, is dependent to a great degree upon the amount of acid that is formed during the various stages in making; hence, the successful maker must follow closely by some means the acid formation in the curd until it is put to press.

It is very necessary that the milk shall contain a sufficient number of acid-forming bacteria to produce the required amount of acid. If a sufficient number of bacteria are not present in the milk as it is received, as is the case with very sweet milk, they must be added by the maker in the form of a starter, or the process of making will be much prolonged.

A photomicrograph of curd just after curdling has taken place. Note the few lactic acid bacteria embedded in the curd.]

=Starters in cheese making.= The starters used in cheese making, are identical with those employed in butter making and the same precautions should be observed in their propagation. It is important that the starters should not be such as to form a hard curd that cannot be mixed uniformly with the milk, since the curd particles would appear as white specks in the cheese. The starter should be added to the milk through a hair sieve, and well mixed with the milk, so as to distribute the bacteria uniformly. Amounts varying from 0.5 to 2 per cent are used. In butter making, it is essential that the bacteria of the starter be able to form not only acid, but sufficient flavor-forming substances to impart to the butter a desirable flavor. In cheese making it is not probable that this latter characteristic is of any particular importance.

A photomicrograph of curd at the time the salt is added. The lactic acid bacteria have increased materially in numbers.]

It is desirable that the process of cheese making shall conform as closely as possible to that which experience has shown to give the best results. The rate at which acid is developed in the curd and the rapidity with which the whey is expelled therefrom should bear a certain ratio to each other. If the milk has too high a degree of acidity, _i.e._, is overripe, the acidity developed in the curd will be too high before the curd is sufficiently firm; with a very sweet milk, the reverse may be true. It is desirable for the cheesemaker to obtain as good an idea as possible of the condition of the milk with reference to its bacterial content, since this will determine the rate at which acid will be formed in the curd. If the milk is too sweet, _i.e._, too low in acid-forming bacteria, a starter should be added. The only methods by which this information can be obtained by the maker is by determining the acidity by the usual method or better by the use of the rennet test by which is ascertained the time required for a given amount of rennet to curdle a definite quantity of milk at a standard temperature. The varying factor in the test will be the acidity of the milk. Very slight differences influence profoundly the time of curdling. If, working under standard conditions, it is found that the time of curdling of one sample is 10 seconds and of another sample, 20 seconds, it is proof that the acidity of the first is higher than that of the second, that its bacterial content is greater and that acidity will develop in the curd more rapidly. The first may need a small amount of starter, the second a larger quantity. Working with milk from the same source, the maker, from his experience, will know how much starter should be added to milk that has given a certain result with the rennet test in order that the acid shall be developed in the curd at a desired rate.

=Ripening of cheese.= The curd at the time it is put to press is tough and rubbery, and has none of the characteristic flavor of cheddar cheese; it is also quite insoluble and indigestible. Before the cheese is fit to eat it must pass through a complex series of changes which are collectively known as _ripening_. In these changes there is not only a breaking down of the casein into soluble compounds, which process makes the cheese soft and plastic under pressure, but the characteristic flavor is developed in greater or less degree. A very considerable part of the cheese thus becomes soluble in water, and it is much more easily digested than in an unripened condition.

The different factors that are operative in the ripening changes are not yet fully known, but in recent years as a result of scientific study, material progress in the study of the changes has been made.

=Rennet.= The commercial rennet extract when in condition for use contains very few bacteria. A preservative, boric acid, is added by the manufacturer to restrain the bacteria, otherwise the extract would soon be unfit for use. The bacteria in the commercial rennet extract are too few to be of any importance whatever in the ripening process.

Rennet extract contains an enzyme, rennin, that causes the milk to curdle; also another enzyme, pepsin, that exerts a digestive action on the curdled casein. Pepsin is always found in the stomach juices of all animals, but no digestive action takes place, unless the reaction is distinctly acid, as is the ease under normal conditions, since hydrochloric acid is excreted by the walls of the stomach. Outside of the stomach, the same conditions must obtain with reference to the presence of acid, if pepsin is to exert a digestive effect. In the cheese curd, the milk sugar is rapidly changed into lactic acid by the action of the bacteria. This gives the proper chemical reaction for peptic action, and the enzyme is then able to act on the paracasein, the nitrogenous part of the cheese. If milk contains no acid-forming bacteria, conditions will not permit of peptic action, and as a consequence, the ripening processes do not take place. If the sugar is fermented by some organism that does not form acid, as the lactose-fermenting yeasts, the cheese does not ripen. The lactic bacteria are therefore an essential factor in inaugurating the ripening changes in all types of rennet cheese.

=Preservative action of acid.= In a previous chapter it was shown that raw milk does not undergo putrefaction because of the restraining effect of the acid formed by the lactic bacteria on the putrefactive organisms. This same phenomenon is noted in cheese. Milk always contains putrefactive bacteria which pass into the cheese, but they cannot grow therein because of the high acidity. In the absence of the acid-forming organisms in the cheese, the cheese may remain tough and rubbery, on account of the lack of suitable conditions for the action of the pepsin of the rennet extract, or when the milk contains large numbers of digesting organisms, the cheese may develop a putrefactive condition, as noted by the offensive odor and soft pasty texture.

=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 deg. 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.

Such a cheese is worthless on account of its poor flavor. The irregular holes are mechanical. The crack on the upper side is due to the pressure of the gas which has caused the cheese to bulge at this point.]

The gas formed in the curd before the cheese is put to press can be gotten rid of by proper manipulation of the curd. While this treatment may improve the appearance of the cheese, it does not eliminate the substances that impart to the cheese undesirable qualities.

Gassy curds have also been treated by washing the curd with cold water. Care must be taken in applying this method for the removal of too much of the sugar and acid from the curd by the washing will permit the growth of injurious forms of bacteria. The addition of salt or of saltpeter has also been made to the milk in order to overcome gassy conditions in the milk. In the handling of gassy milk, the usual practice has been to develop a larger amount of acid before drawing the whey than in the case of good milk. This was done with the idea that acid suppressed gas formation. It has been shown previously that this is not the case. It has also been shown by Doane that the development of too much acid before drawing the whey is likely to result in undesirable flavors, producing what is known as "high-acid" or "sour" cheese.

The gas-forming bacteria grow best at high temperatures; hence, cheese kept under these conditions are more likely to be affected by this trouble than are those kept at lower temperatures.

The most successful method of preventing trouble with gassy milk in cheese making is to eliminate undesirable milk by frequent testing of the supply of the different patrons by means of the Wisconsin curd test.

Not only gas-forming bacteria may be the cause of gassy cheese, but the lactose-fermenting yeasts may cause similar trouble. If these are abundant in the milk, a considerable part of the sugar may be fermented by them, in which case, carbon dioxide gas is abundantly formed. The cheese thus rendered gassy will present the same appearance to the eye as where the gas is formed by bacteria, but will have a different flavor. The odor of alcohol may be evident, and if most of the sugar has been fermented by the yeast, the acidity of the cheese may not be sufficient for the pepsin to exert its digestive action.

Milk containing many gas-forming bacteria occurs most frequently in summer. It is claimed by some that the milk of cattle pastured on low lands is more likely to contain the gas-forming organisms than that from cattle running on higher lands. If this is true, it must be due to the bacterial content of the soil; the udders of the animals become soiled as they lie on the ground, and during the milking, the dust finds its way into the pail. Many cheese makers think that the milk from an animal suffering from a garget may be the cause of the huffing of cheese. This belief is undoubtedly well founded, as some of the bacteria known to be the cause of garget are gas-forming.

=Bitter cheese.= In a previous chapter the bitter fermentation of milk has been discussed. If milk containing large numbers of such organisms is made into cheese, the bitterness is very likely to be noted in it. Cheese made from milk containing few or no lactic bacteria is likely to develop a bitter taste, due to the growth of the digestive bacteria that are able to grow through the lack of acid in the cheese.

If the milk contains considerable numbers of yeasts, a sweet or fruity flavor is apt to develop, due to the products of the fermentation of the sugar by the yeast. This flavor resembles that of fermented fruit, or the bouquet of certain kinds of wine.

=Putrid cheese.= In the absence of acid-forming bacteria, the cheese may develop a putrid or rotten odor, due to the growth of some types of putrefactive or digesting bacteria. This trouble is very infrequent in cheddar cheese, since this is made from ripened milk, but occurs more frequently in those types in which no acid is developed.

Bacteria develop in the cheese in colonies or masses, just as they do in the plate cultures of the bacteriologist, made with transparent media, such as gelatin. Cheese is opaque; therefore, the growing colonies cannot be readily discovered, but when pigment-forming bacteria grow in the cheese, their presence is likely to be noted, because of the colored spots that are formed.

=Rusty spot.= The "rusty spot" that has been encountered in New York and Canada is due to one of the colored bacteria which produces an orange or yellowish-red pigment. Various other pigment-forming organisms have been met in cheese, each producing its colored colony which differentiates itself from the mass of the cheese. If the pigment is produced in considerable quantities, and is soluble in any of the constituents of the cheese, the color will not appear in spots but will be more diffuse, or may impart a color to the entire mass.

Cases of acute poisoning arising from the ingestion of cheese are not infrequently reported; similar instances result from the use of ice cream. In both cases it is believed that poisonous products have been formed by bacteria, probably by some of the putrefactive forms.

From what has been said with reference to the abnormal fermentations of cheese, it will be seen that they are always due to the lack of acid-forming bacteria, or to their partial replacement by other types. In order to prevent such troubles, it is necessary to insure that the milk has been produced under clean conditions, from healthy cows, and has been handled in such a manner as to reach the maker in as sweet and fresh condition as possible. The maker can, by the use of proper starters, control the kinds of bacteria essential for the ripening process. A well trained maker should be able to prepare from such milk a uniform product of the highest quality. The effort of cheese makers at the present time is to handle milk of more or less objectionable quality so as to secure from it as good cheese as is possible. But cheese is so sensitive as to character of milk used that greater effort should be spent in securing an improved supply.

=Moldy cheese.= In the case of the cheddar cheese and other types of hard cheese, it is essential that their surfaces be kept clean, and not discolored by the growth of molds, which find favorable conditions for growth on the surface of the cheese in the moist atmosphere of the curing room. The molding of cheddar cheese can be prevented by covering the cheese with a layer of paraffin which stops the development of the mold spores, by shutting off the necessary supply of oxygen. For this purpose the cheese are dipped in melted paraffin when a few days old.

In the case of types of cheese which are salted by applying the salt to the surface, or with soft cheese which ripen from the outside, other methods of mold prevention are employed, such as rubbing and washing the cheese. The curing room itself may be freed from the mold spores by the use of such standard disinfectants as formalin or sulphur.

=Swiss cheese.= One of the most important kinds of hard cheese, is the Swiss or Emmenthaler, so named, from the country and valley in which the cheese was first made. In America, this type was introduced by Swiss immigrants, and is being made in constantly increasing quantities in Ohio and Wisconsin.

Swiss cheese is a hard firm type, appearing in the markets in the form of the flat circular "drum" cheese, two to three feet in diameter, and six to eight inches thick, or in the smaller "block" form. In this country the cheese is prepared twice a day, since it is necessary to work up the milk while it is perfectly sweet. Indeed, the milk is received at the factories while it is still warm, and within five or six hours after it is drawn from the cow the cheese is on the press. If the attempt is made to prepare Swiss cheese from the kind of milk that is best suited for cheddar purposes, _i.e._, milk in which the acidity has increased to some extent, the flavor of the resulting product is likely to approximate a cheddar cheese rather than that of a Swiss.

In the salting process, the salt is not mixed with the curd before it is pressed, but is applied by immersing the cheese for a few days in a saturated brine, and then rubbing salt over the surface of the cheese. In this way the salt gradually diffuses quite uniformly through the cheese. The method of salting has apparently a marked influence on the ripening process, since if the salt is added in the same way, and in amounts used in the cheddar process, the flavor will not be that of a Swiss cheese but will resemble a cheddar.

In cheddar cheese, the whey is expelled from the curd by means of the acid which is developed in the curd, and by heating the curd to a temperature of 95 deg. F. to 100 deg. F. In Swiss cheese the development of acid during the making process is prevented, because of the smaller number of acid-forming bacteria in the milk; other factors must therefore contribute to the expulsion of the whey to secure a firm curd. This is accomplished by cutting the curd into very small pieces and by briskly stirring it during the making, heating it during this process for a period of 20 to 30 minutes at 130 deg. to 140 deg. F. It might be thought that this high temperature, which is approximately that used in pasteurization would destroy the acid-forming bacteria, but these are apparently protected as they are within the curd. During the time the cheese is being pressed, the contained bacteria begin to grow and the whey coming from the cheese toward the end of the pressing shows a high acidity. If it does not show such a development of acid, the maker has reason to believe that the cheese may never ripen in a typical manner.

It has been mentioned that the milk contains but few acid-forming bacteria. The maker, however, attempts to insure the presence of a sufficient number by the use of "home-made" rennet. This is prepared by placing a piece of dried rennet, _i.e._, the stomach of the calf, in whey, keeping the same in a warm place for twenty-four to thirty-six hours. As the rennet contains acid-forming organisms, these grow rapidly in the warm whey, so that by adding this sour whey to the milk, the maker is not only adding rennet, that is to curdle the milk, but also a small starter of lactic bacteria. If the rennet thus prepared contains no harmful bacteria and the milk is of good quality, the cheese is likely to ripen in a normal manner. The rennet should be prepared with due regard to bacteriological principles, a condition that is rarely met in Swiss factories in this country.

Swiss cheese has two striking characteristics, the flavor and the presence of holes or "eyes." The flavor is sweetish rather than the sharp and pungent flavor of cheddar cheese. The bacteria concerned in its production are not known, but it is certain that specific organisms play some role, since if the flora of the cheese is changed by salting the curd or by the use of milk containing large numbers of lactic bacteria, the flavor will also be changed. This role of the acid-forming bacteria in Swiss is the same as in cheddar, _i.e._, through the acid, conditions are established for peptic action, the curd being partially digested while at the same time the curd mass is protected from putrefactive processes.

In Swiss cheese during the ripening process, holes about the size of a large cherry develop which should be quite uniformly distributed throughout the cheese. The inner surface of the hole is glistening and, in a well-ripened cheese, a small quantity of clear brine, _i.e._, "tears" may be noted. These holes or "eyes" may be called the trade mark of the Swiss cheese, since without them the product has a lessened commercial value, even if it possesses the typical flavor. The "eyes" are caused by bacteria that ferment the lactic acid produced by the lactic bacteria, forming from it propionic acid and carbon dioxide, the latter gas being the cause of the hole or "eye."

Normal development of "eyes" in a Swiss cheese. The eyes are generally as large as a cherry.]

The "eye"-forming organisms cannot grow in the presence of any amount of salt, hence, if salt is added directly to the curd, the cheese is likely to be "blind" or free from holes. The eyes are formed not at the time gas holes are produced in a cheddar cheese, _i.e._, early in the ripening process, but after a lapse of three or four weeks. They are most abundant in the middle of the cheese since the manner of salting is such as to inhibit their formation near the surface. The eye-forming bacteria may have some effect on the flavor of the cheese.

The Swiss maker encounters the same troubles as does the cheddar maker. Gassy cheese is more prevalent in the Swiss than in the cheddar industry, since the maker cannot call to his aid the methods used by the cheddar maker, viz., the addition of a heavy starter, the washing of the curd, etc. It is especially important that the quality of the milk be first class in every respect, and yet customs prevail in the Swiss industry that are directly inimical to the production of good milk. The grossest carelessness prevails at the factories in the matter of handling the whey. It is often kept in individual barrels for each patron. (See Fig. 8.) These are not kept thoroughly clean and the result is that the whey taken back to the farm in the cans that are used to bring the fresh milk is often in an advanced stage of fermentation.

There are many other kinds of hard cheese; but in each, so far as is known, the role of the acid-forming bacteria is identical with that noted in cheddar and Swiss cheese, viz, in preparing conditions favorable for peptic action, and preventing the development of putrefactive bacteria present in the curd.

=Roquefort cheese.= Among the more important foreign types of cheese that are characterized by the development of mold is Roquefort, so named from the district in France in which it is made. This cheese is made from sheep's milk, in much the same manner as cheddar. The characteristic process in its preparation is the inoculation of the curd, at the time it is put to press, with the spores of a particular kind of mold, a type closely related to the ordinary green mold of bread and cheese. The mold for inoculation is grown on bread, the whole mass being dried so that it can be powdered; then the ground-up material is sprinkled on the curd as it is placed in the press hoops. The first stage in the ripening of Roquefort is probably identical with that of the types of hard cheeses already considered, the breaking-down of the curd being due to the pepsin of the rennet used, which action is made possible by the acid formed by the bacteria.

The second stage in ripening, and one in which the characteristic flavor of the cheese is developed, is due to the growth of the mold with which the cheese is seeded. Molds can grow only in the presence of air, and in order to provide this condition, the cheese are run through a machine having a series of needle-like projections which fills the cheese with fine holes. This allows the air to penetrate the cheese and the mold to grow, the fruiting of which develops the characteristic flavor. The changes produced by the mold are not well understood, but the flavor is evidently connected with its development since in the absence of mold, it does not appear. The cheese must be cured under carefully controlled conditions, as to temperature and moisture; in France these are secured by curing the cheese in limestone caves that are highly saturated with moisture. Attempts have been made to make Roquefort cheese in other parts of the world, but they have never been successful, due undoubtedly to the fact that the proper environment and conditions for the development of the various types of organisms necessary in the ripening process have not been met. This cheese is sold for 50 to 75 cents per pound in the markets of the world.

There are two other kinds of cheese that are closely related to Roquefort, as to the manner of ripening, viz., the Gorgonzola of Italy and the Stilton of England, both of which possess their characteristic flavors by reason of the development of molds. In Stilton cheese the mold is not intentionally added, the maker relying on the contamination that comes from the factory for the usual seeding. If this does not develop, it is sometimes inoculated by exchanging plugs with a well-ripened Stilton. This method is not so certain as in the inoculation of Roquefort.

=Camembert cheese.= A typical example of soft cheese is one of the French types, known as Camembert. This cheese is prepared from cow's milk which is curdled by rennet. The curd is not cut but is dipped into the forms, which condition, taken with the absence of pressure in forming the cheese, accounts for the large quantity of whey in it. The finished cheese are about one inch in thickness and three inches in diameter. In the ripening, the moisture and temperature of the curing room are very carefully regulated.

The first stage in the ripening is due to the rennet and the lactic bacteria. Later there appears on the surface of the moist cheese, a moldy growth. In this, there are at least two kinds of molds, the ordinary mold that appears on sour milk, _Oidium lactis_, and another that is related to the bread mold but which has a white instead of a green fruiting stage. These molds are confined to the surface of the cheese but the enzymes which they produce diffuse into the substance, changing the color from a dull, opaque white to a translucent yellow. The acid that has been formed by the lactic bacteria is gradually used up by the growth of the mold, and conditions then become favorable for the growth of putrefactive bacteria which digest the curd. The cheese is ready for use when the action of the mold has penetrated to the center of the cheese, and before any pronounced putrefaction has taken place. The production of the typical flavor is dependent upon there being a definite relation between the growth of the molds and bacteria. This relation is dependent largely upon the moisture and temperature of the curing room. These cannot always be regulated with exactness; and hence, much of this type of cheese is not of first quality, and must be sold for a low price. While such fancy cheeses, as Camembert, bring fifty cents and upward per pound, and the yield from the milk is much greater than with the hard type of cheese, yet the difficulties of successful manufacture are such as to make success less easily attained than with the other types.

There are many other kinds of soft cheese that depend for their ripening upon factors similar to those concerned in the ripening of Camembert; most of them are, however, of small importance from a commercial standpoint.

=Limburger cheese.= A very famous cheese is one originally made in Germany to which the name Limburger is given. It is classed as a soft cheese although it is much firmer than Camembert. This cheese is made from cow's milk and is pressed very lightly or not at all, which condition accounts for its high per cent (50 per cent) of moisture. The surface is kept moist by repeated washing of the cheese, and by keeping the air of the curing room very moist. A yellowish, slimy, bacterial layer soon develops on the surface under these conditions. The enzymes produced by this external growth gradually diffuse to the center of the cheese, when it is regarded as ripe. The odor of the matured product is somewhat putrefactive, but is not so offensive as is usually supposed.

Definite knowledge concerning the types of organisms concerned in the surface layer is very limited. It is not certain whether the same kinds of organisms must always be present. Limburger is much easier to make than Camembert, due possibly to the fact that there are not needed definite forms of life and that the balance between them is not so delicate.

A cheese known as brick is closely related to Limburger in its method of making and of ripening but is less pronounced in flavor.

In the manufacture of all of these types of cheese, troubles are likely to develop, due to an abnormal bacterial condition of the milk.

It will be seen from what has been said that the bacteria are essential factors in cheese ripening, and that the cheese industry, like the butter industry, may be called a true fermentation industry. Close co-operation must exist between the milk producer, and the maker so that the type of fermentation that goes on in the milk can be controlled. A recognition of the fundamental principles governing these fermentations, both normal as well as abnormal, is now regarded as an essential part of the training of the dairy manufacturer of today.