116 DAIRY BACTERIOLOGY 



previous treatment of the milk (perhaps a maximum capacity 

 for souring is found in that heated to 80 C.). On the other hand, 

 the two thermobacteria which are distinctly anaerobic in character 

 are greatly influenced by the previous treatment of the milk, as 

 is seen from the acidity and also/ in the case of Thermobacterium 

 bulgaricum (of Yoghurt), by direct microscopic examination (cf. 

 the illustrations, p. 30). Both these organisms grow best in raw 

 milk, where the oxygen is quickly consumed by other organisms, 

 and in sterile milk which is quite free from oxygen. Thermo- 

 bacterium helveticum also shows a maximum of souring in the milk 

 heated to 75 C. in which the bactericidal substances have been 

 destroyed, but which still contains a little dissolved albumin. Of 

 the lactic acid bacteria employed in these experiments, this was 

 the only one which was influenced by the albumin, all the others 

 growing better in milk which had been heated to 80 C. or above. 

 From what has been said we may draw the conclusion that no 

 harm will be done by pasteurising the starter milk thoroughly. 

 All that is necessary is to avoid discolouration or burning, which 

 may cause difficulty in judging the aroma of the starter or impart 

 a cooked flavour to the butter in cases where large amounts of 

 starter are used. Experience shows that pasteurisation for one 

 hour at 85 gives good results. The milk is heated in a water 

 bath, stirring frequently, and it is cooled as quickly as possible to 

 the souring temperature, stirring carefully, the pail or can being 

 placed in running water. Aeration by pouring the milk from 

 vessel to vessel is unnecessary ; the risk of infection is only 

 increased by exposure to the air, and especially by contact with 

 more vessels than is strictly necessary. It is a great mistake to 

 pass the pasteurised milk over a cream cooler. As in the case of 

 cream, undesirable fermentations are best avoided by keeping the 

 souring temperature low, and moreover, it is unsound as a matter 

 of principle to accustom the bacteria to a sensibly higher tem- 

 perature in the starter than that at which they are required to act 

 in the cream. A temperature of 22 to 23 C. is quite satisfactory 

 if only this is kept constant by placing the starter cans in a 

 sufficiently large water-bath at the right temperature. Boekhout's 

 and Starch's aroma bacteria, it may be noted, do not thrive well 

 at higher temperatures. On the other hand, it is not advisable 

 to lower the temperature further, as Sc. cremoris nearly always 

 degenerates on propagation several times at temperatures below 

 22 C. As cream is always soured under this temperature, we find 

 here a new and hitherto unknown reason why the use of butter- 

 milk for this process is unsatisfactory in the long run. The water- 

 bath may be made of wood or galvanised or tinned iron, and 

 covered in in the same way as the culture apparatus, so that only 



