112 DAIRY BACTERIOLOGY 



temperature after pasteurisation 1 . On the usual type of surface 

 cooler the cream is exposed to great risk of infection from the air, 

 but, on the other hand, certain evil-smelling substances may be 

 eliminated, while oxygen, will be absorbed. Opinion is divided as 

 to whether the latter is advantageous or not. Aeration tends to 

 inhibit the development of lactic acid bacteria, but it may possibly 

 promote the production of aroma ; at any rate the amount of 

 volatile acids is increased thereby 2 . It will be best to pour the 

 starter into the cream tanks before the cream, so that the souring 

 of the cream may commence immediately after the pasteurisation ; 

 the surviving microorganisms (chiefly bacterial spores) will then 

 have no time to develop. It follows as a matter of course that the 

 starter must be properly distributed throughout the whole bulk 

 of the cream, and the cream should be stirred occasionally so that 

 it may become uniformly sour. 



As the amount of starter which is necessary to sour the cream in 

 a given time depends on the souring temperature, these two 

 factors may be discussed simultaneously. As the harmful 

 bacteria which have resisted pasteurisation grow best at high 

 temperatures, the cream should be soured at the lowest possible 

 temperature. The author has found the optimum for vigorous 

 cultures of Streptococcus cremoris to be 25 C., so that there is no 

 reason why this temperature should be exceeded. On the other 

 hand, the temperature should not be so low that the lactic acid 

 bacteria do not thrive well, for then the full aroma will not be 

 obtained. Without doubt, 18 to 20 C. is the best temperature, 

 and if, as is sometimes the case in Denmark, a somewhat lower 

 temperature of 15 C. to 17 is frequently employed, it is either 

 because the dairies have not the equipment necessary to cool the 

 cream to a low temperature after souring, or else for the reason 

 that it is obviously more convenient to carry out the process in 

 such a way that the cream will be as nearly as possible at the churn- 

 ing temperature when it is ripe. In order to secure a pure 

 fermentation, and especially to obtain a good firm butter, Rosen- 

 gren has recommended in Sweden, and Weigmann in Germany, 

 to sour the cream at a still lower temperature, i.e., at 8 to 12 C., 

 and to add 10 to 20 per cent, of starter. It will be seen that many 

 different methods are used in practice, and it is hardly possible to 

 lay down any hard and fast rule except the following : The 

 fresher and cleaner the milk, the higher should be the souring tem- 

 perature, and the lower the percentage of starter used, and conversely, 



1 See the author's article " Can a Cleaner Churning be attained by a 

 more thorough Cooling of the Cream." ( '' Maelkeritidende," 1910, p. 801). 



2 Kayser, " Contribution a 1'etude de la fermentation lactique," Doctoral 

 thesis, Paris, 1894. 



