290 EXPERIMENT STATION RECORD. 



imiiilH'f is a convenient means of measuring the progress of this process. By the 

 hydrolytic processes the fats are split up into glycerin and free fatty acids, and 

 accordingly the progress of the process is measured by the amount of free fatty acids, 

 expressed in acid number. It has been shown by numerous investigators that the 

 unpalatability of fat is not always in proportion to the content of free fatty acids, 

 whicli appears to be due to the fact that only those fatty acids which are volatile or 

 soluble affect the senses. Fats which contain no glycerids of volatile fatty acids can 

 not, like butter, become rancid as the result of hydrolytic cleavage, but only as the 

 result of oxidation. 



The author enumerates four possible causes to which rancidity has been assigned, 

 namely, the oxygen or the carbonic acid of the air, the enzyms originally present in 

 the milk, and micro-organisms. These jjossibilities were each investigated, the action 

 of a long list of micro-organisms l)eing studied. 



It is shown that several forms of micro-organisms commonly occurring in l)utter are 

 capable of decomposing the butter, with the formation of volatile fatty acid and 

 esters. This, the author believes, is sufficient proof that the rancidity of butter may 

 be due to micro-organisms, and that the hypothesis of the action of a milk enzym is 

 very improbable. It is thought very possible, however, that the micro-organisms 

 effect the cleavage of the fat by means of a ferment (steapsin) which they form. 

 The author added to filtered and sterilized butter a quantity of an old milk culture 

 of the organisms to be tested which had been jiassed through a Chaml»erland filter. 

 Samples of butter were treated in this way with cultures of Bacillus ^fiuorcsrcns lirjue- 

 fnciens and of Oidium lartis. These experiments showed that the acid number 

 increased, i. e.. that the rancidity progressed, and the author concludes that this 

 was due to enzyms (steapsin). 



The author sums up the principal results of his investigations as follows: 



The air plays a direct part in the spoiling of butter only when the latter is exposed 

 to sunlight or to a high temperature. The butter is then oxidized and takes oji a 

 very disagreeable odor and taste, but is not rancid. 



Butter becomes rancid only through the action of certain micro-organisms. As 

 these are all aerobic, rancidity progresses from the surface inward. For the preser- 

 vation of butter it is, therefore, advisable to seal it up hermetically, or at least to 

 leave as little surface exposed as possible, i. e., to keep it in large pieces rather 

 than in small. 



The micro-organisms which under ordinary conditions cause ranciditj^ are Oldimn 

 Idciis, Cladosporium Imti/ri, Bacillus fluorencens liqui'faciois, and occasional Bacillus 

 prodigiosus also. All of these cleave the butter fat. 



The volatile fatty acids are formed at first by the bacteria and later by the two 

 molds working together. By the latter action butyric acid ester is also formed. By 

 means of salt the formation of volatile fatty acids can be reduced, and by means of 

 milk sugar the ester formation checked. Whether or not rancidity can ])e entirely 

 prevented by the combined action of these two substances has not been determined. 



BiiciUns fluorencens liquefaciens and B. prodigiosus are often introduced into the 

 butter in the water used, the former the more frequently. The Oidium lactis and 

 Cladosporium butyri are believed to come from the air. The former is always present 

 in the air of creameries in large quantities. In order to make butter which keeps 

 well, therefore, the milk, cream, and butter should come into contact as little as pos- 

 sible with water, and be excluded, so far as possible, from the air. Experiments 

 have shown that the danger of infection through water is materially diminished by 

 ripening the cream, provided actual pure cultures of lactic acid ferments are used, 

 since with an impure starter there is much danger of infection with mold. Pasteur- 

 ization of the cream at 85° C. destroys all of the micro-organisms injurious to the 

 keeping quality of butter. If the cream is cooled, however, l)y allowing it to spread 

 out in a thin layer over the cooler under the free access of air, and is then ke]>t in 



