Other organic acids are undoubtedly formed as well as certain aromatic products. While the production of acid as a result of fermentative activity is usually accompanied with a development of flavor, the flavor is not directly produced by the formation of acid. If cream is treated in proper proportions with a commercial acid, as hydrochloric,[154] it assumes the same churning properties as found in normally ripened cream, but is devoid of the desired aromatic qualities. Lactic acid[155] has also been used in a similar way but with no better results.
The amount of acidity that should be developed under natural conditions so as to secure the optimum quality as to flavor and aroma is the most important question in cream ripening. Concerning this there have been two somewhat divergent views as to what is best in practice, some holding that better results were obtained with cream ripened to a high degree of acidity than where a less amount was developed.[156] The present tendency seems to be to develop somewhat more than formerly, as it is thought that this secures more of the "high, quick" flavor wanted in the market. On the average, cream is ripened to about 0.5 to 0.65 per cent. acidity, a higher percentage than this giving a strong-flavored butter. In the determination of acidity, the most convenient method is to employ the Farrington alkaline tablet, which permits of an accurate and rapid estimation of the acidity in the ripening cream. The amount of acidity to be produced must of necessity be governed by the amount of butter-fat present, for the formation of acid is confined to the serum of the cream; consequently, a rich cream would show less acid by titration than a thinner cream, and still contain really as much acid as the other. The importance of this factor is evident in gathered-cream factories.
The rate of ripening is dependent upon the conditions that affect the rate of growth of bacterial life, such as time and temperature, number of organisms in cream and also the per cent of butter fat in the cream. Some years ago it was customary to ripen cream at about 50° to 60° F., but more recently better results have been obtained, it is claimed, where the ripening temperature is increased and the period of ripening lessened. As high a temperature as 70° to 75° F. has been recommended. It should be said that this variation in practice may have a valid scientific foundation, for the temperature of the ripening cream is undoubtedly the most potent factor in determining what kind of bacteria will develop most luxuriantly. It is well known that those forms that are capable of producing bitter flavors are able to thrive better at a lower temperature than some of the desirable ripening species.
The importance of this factor would be lessened where a pure culture was used in pasteurized cream, because here practically the selected organism alone controls the field.
It is frequently asserted that better results are obtained by stirring the cream and so exposing it to the air as much as possible. Experiments made at the Ontario Agricultural College, however, show practically no difference in the quality of the butter made by these two methods. The great majority of the bacteria in the cream belong to the facultative class, and are able to grow under conditions where they are not in direct contact with the air.
Flavor and aroma. The basis for the peculiar flavor or taste which ripened cream-butter possesses is due, in large part, to the formation of certain decomposition products formed by various bacteria. Aroma is a quality often confounded with flavor, but this is produced by volatile products only, which appeal to the sense of smell rather than taste. Generally a good flavor is accompanied by a desirable aroma, but the origin of the two qualities is not necessarily dependent on the same organisms. The quality of flavor and aroma in butter is, of course, also affected by other conditions, as, for instance, the presence or absence of salt, as well as the inherent qualities of the milk, that are controlled, to some extent at least, by the character of the feed which is consumed by the animal. The exact source of these desirable but evanescent qualities in butter is not yet satisfactorily determined. According to Storch,[157] flavors are produced by the decomposition of the milk sugar and the absorption of the volatile flavors by the butter fat. Conn[158] holds that the nitrogenous elements in cream serve as food for bacteria, and in the decomposition of which the desired aromatic substance is produced. The change is unquestionably a complex one, and cannot be explained as a single fermentation.
There is no longer much doubt but that both acid-forming and casein-digesting species can take part in the production of proper flavors as well as desirable aromas. The researches of Conn,[159] who has studied this question most exhaustively, indicate that both of these types of decomposition participate in the production of flavor and aroma. He has shown that both flavor and aroma production are independent of acid; that many good flavor-producing forms belong to that class which renders milk alkaline, or do not change the reaction at all. Some of these species liquefied gelatin and would therefore belong to the casein-dissolving class. Those species that produced bad flavors are also included in both fermentative types. Conn has found a number of organisms that are favorable flavor-producers; in fact they were much more numerous than desirable aroma-yielding species. None of the favorable aroma forms according to his investigations were lactic-acid species,—a view which is also shared by Weigmann.[160]
McDonnell[161] has found that the production of aroma in certain cases varies at different temperatures, the most pronounced being evolved near the optimum growing temperature, which, as a general rule, is too high for cream ripening.
The majority of bacteria in ripening cream do not seem to exert any marked influence in butter. A considerable number of species are positively beneficial, inasmuch as they produce a good flavor or aroma. A more limited number are concerned in the production of undesirable ripening changes. This condition being true, it may seem strange that butter is as good as it is, because so frequently the requisite care is not given to the development of proper ripening. In all probability the chief reason why this is so is that those bacteria that find milk and cream pre-eminently suited to their development, e. g. the lactic-acid class, are either neutral or beneficial in their effect on butter.
Use of starters. Experience has amply demonstrated that it is possible to control the nature of the fermentative changes that occur in ripening cream to such an extent as to materially improve the quality of the butter. This is frequently done by the addition of a "starter." While starters have been employed for many years for the purpose mentioned, it is only recently that their nature has been understood. A starter may be selected from widely divergent sources, but in all cases it is sure to contain a large number of bacteria, and the presumption is that they are of such a nature as to produce desirable fermentative changes in the cream.