From the examination of many experimental cheeses it appears that this treatment does not influence the flavor to any appreciable extent if at all. The body of the cheese is more firm than where the cheese is made in the usual manner. There are cases where this treatment fails to show improvement; in most cases, however, the results are extremely gratifying. The cause of this improvement in Swiss cheese might be one of several things, but as far as our observations show the improvement seems to be due to the removal of dirt or other cellular elements from the milk.

In initial attempts to manufacture cheese from centrifuged milk difficulty was experienced with niszler cheese, due to the fact that it was necessary to hold the milk at a high temperature for a considerable period of time prior to centrifuging. However, this difficulty has been overcome by cooling the milk. This spring 20 centrifuged cheeses were made without a niszler or No. 2 cheese.

In the small factories which receive their milk while it is still warm, the problem of making cheese by this process is more difficult. These difficulties may be specified as follows:

1. Centrifuging the milk requires extra labor, time, and expense. 2. Warm milk appears more subject to foaming and the niszler fermentation.

3. Very few factories are provided with suitable facilities for cooling and holding the milk.

Despite these difficulties, several factories have had good success by this treatment. One of our Swiss cheese specialists in Wisconsin reports that from July 29 to September 8, when this treatment was given trial, there was but one No. 2 cheese, and 70 per cent of the remainder were fancy. The factories received from 3 to 5 cents more per pound for the treated than for the untreated cheese, and the centrifuged-milk cheese had fewer and larger eyes than the untreated cheese. The centrifuged cheese during the last year has won first prizes at the cheese exhibits of Wisconsin and Ohio.

THE INFLUENCE OF SECONDARY HEATING ON GAS FORMATION IN
SWISS CHEESE.

Orla-Jensen, in working with Swiss cheese, found that there was less danger of niszler and pressler cheese when the higher cooking temperatures were employed. His comparisons, however, were based on cheese made on one day compared with that made on another day.

In order to study the effect of high heat in suppressing gas, cheese was made by inoculating milk with definite quantities of liquid culture of a coli-aerogenes type originally isolated from an inflated cheese. Thus two cheeses were made daily in the same manner, from the same milk, with the single exception of a variation in the cooking temperature. Approximately 300 pounds of milk were employed for each cheese. The temperature in each kettle was carefully watched; the final cooking temperature probably did not vary 0.5° C., as a cheese maker was stationed at each kettle so that the temperature could be carefully regulated.

Normally about 0.5 per cent of the bulgaricus culture is used. In order to make the difference in cooking temperature more pronounced, 0.1 per cent of bulgaricus was used in conjunction with

1,000 cubic centimeters of the gas-producing organism. When a cooking temperature of 52° was compared with 55° C., a distinct difference was noted. In some cases where the curd was cooked to 52° C. the cheese would show niszler or pressler tendencies on the surface of the cheese, while at other times indications of gas occurred throughout the cheese, whereas the cheese cooked to 55° would seldom show indications of gas.

Somewhat similar results were observed when 0.5 per cent of bulgaricus was employed in conjunction with 3,000 cubic centime

FIG. 5.-Influence of high heat on uality of Swiss cheese. (18) Cheese cooked to 52° C. (18-1) Cheese cooked to 55° C.

ters of the gas-producing organism. Figure 5 shows the effect of high heat in controlling the fermentation.

This is a cheese cut the day after making. Cheese 18 was cooked to 52° C., whereas cheese 18-1 was cooked to 55° C.

There have been very few cases in our experience where the use of a high cooking temperature has checked niszler trouble in the field. However, the results of our experiments in the laboratory with the use of high cooking temperatures and gas-producing organisms of the coli-aerogenes type has led us to believe that there is some advantage to be gained by the use of a high secondary cooking temperature.

The gassy fermentation commonly known as "niszler cheese" causes a loss of several hundred thousand dollars annually, and is the most serious difficulty that confronts the manufacturer of Swiss cheese in the United States. Some factories do not have more than half a dozen niszler cheeses during the entire season, but others run as high as 50 per cent during certain months. During the last year the percentage of niszler cheese in six culture factories in Ŏhio averaged 7.4 per cent on the basis of 2,463 cheese made. Incomplete figures for the present year indicate that the percentage of niszler cheese will be considerably lower than last year.

While conducting some filtering experiments in which we passed milk through several layers of cotton, it was observed that the cheese made from milk so treated was normal, whereas the cheese made from the same milk untreated became inflated. We failed to get the same results upon repeating the experiment; however, the first experiment

led us to the opinion that possibly the beneficial results were from the aeration received. In making Cheddar cheese, it is recalled, when gas is in evidence it is the custom to stir, air, and pile the curd until the gas disappears. One book, in the discussion of gassy curd, says: "This shows that the gas-producing organisms have weakened and will not cause any more holes." It occurred to us that by treating the milk known to be subject to a gassy fermentation with oxygen we might overcome that fermentation.

FIG. 6. First pair of cheeses showing effect of oxygen upon quality of Swiss cheese. (3) Treated with oxygen. (4) Not treated with oxygen.

FIG. 7.-Second pair of cheeses showing effect of oxygen upon quality of Swiss cheese. (8) Treated with oxygen. (9) Not treated with oxygen.

With this purpose in view, we added a pure culture of a vigorous gas-producing spore-forming anaerobe to milk, and then treated onehalf of it with ozone. The organism used was isolated from a typical niszler cheese and when added to milk in sufficient quantities the development of gas could not be checked by high heat nor by any other means known to us. By this treatment we succeeded fairly well in checking this fermentation in our preliminary work with ozone. However, the use of ozone, though it had no apparent injurious effect upon eye formation, did seem responsible for the strong disagreeable flavor in the cheese so treated.

We next tried the use of oxygen, with favorable results. It was found that if oxygen was run directly into milk which had pre