118 DAIRY BACTERIOLOGY 



for keeping the temperature of the milk as constant as possible 

 during the souring period. 



Before leaving the subject of cream ripening, it may be pointed 

 out that the so-called souring defects need by no means be ascribed 

 to infections of totally foreign groups of microorganisms such as 

 pseudo lactic acid bacteria, yeasts and moulds ; as often as not 

 they may originate from the true lactic acid bacteria. Thus 

 Storch once isolated a lactic acid bacterium which was able to 

 produce a tallowy taste in cream and butter, and C. O. Jensen 

 found, almost simultaneously, lactic acid bacteria, some of which 

 gave the butter an oily taste, and others which imparted a burnt 

 or malty taste 1 . The last-mentioned taste is often produced by 

 strains of Sc. lactis. From the foregoing it will be seen that the 

 species of lactic acid bacterium chosen for the culture is by no 

 means a matter of indifference. 



Buttermilk is obtained as a by-product from butter making ; 

 its good qualities have already been mentioned. It becomes 

 particularly good when the cream is pasteurised and soured with 

 a pure starter. As the most favourable stage of the lactic acid 

 fermentation has always been reached before the churning, the 

 buttermilk quickly deteriorates in taste and throws out large 

 lumps of casein unless it is kept quite cold. It follows as a matter 

 of course that water should not be added to buttermilk which is 

 to be retailed in towns ; under these circumstances all washing 

 and rinsing of the butter should be done with ice-cooled butter- 

 milk from a previous churning. Separated milk is not so suitable 

 for this purpose, as it causes the buttermilk to curdle more quickly. 

 The undiluted buttermilk will always have a lower acidity than 

 the starter which has been used in souring the cream from which 

 it is made, even if the cream and the starter milk have been 

 soured in exactly the same way. Tholstrup Pedersen 2 has shown 

 that the difference is largely due to the fact that the buttermilk 

 loses its carbonic acid during churning ; the sour starter will also 

 show a lower acidity if it is shaken before titration. 



The Souring of Separated Milk. In the manufacture of mar- 

 garine, separated milk is soured and churned into an emulsion 

 with melted fat. The aroma of the soured milk is taken up by 

 the fat in much the same way as occurs in butter making ; the 

 difference between the two cases lies in the fact that while the 

 aroma is taken up by the fat globules in the cream during the 

 ripening process, this occurs in margarine only during the churn- 

 ing, or emulsifying process, and the ensuing operations. As 

 regards the actual souring of the separated milk, this subject has 



1 " Forsogslaboratoriets,'' 22 de Beretning, 1891. 

 a " Maelkeritidende," 1916, p. 65. 



