MODIFIED MILK AND MILK PRODUCTS 115 



may be directly injected into guinea-pigs. A chemical examina- 

 tion for preservatives should always be carried out before the 

 bacteriological examinations are undertaken. 



IV. Butter. 



The bacteriology of butter comes under consideration from 

 two points of view. Part of the changes which occur in the 

 conversion of the cream into butter are bacterial in origin, and 

 much investigation has taken place as to the precise bacteria 

 involved, the changes they cause and the influence of the other 

 kinds of bacteria present. This economic bacteriology of butter 

 is of the utmost importance in relation to the production of 

 good butter, but can only be briefly mentioned here. The other 

 point of view is concerned with the part butter may play as a 

 vehicle for the transmission of pathogenic bacteria. 



As pointed out in the preceding sub-section, bacteria are 

 very numerous in cream, and are of many different varieties. 

 Some of these are active participators in butter ripening, others 

 are prejudicial and may cause abnormal flavours, etc. Butter can 

 be made from either sweet or soured cream. Sweet cream butter 

 is prepared at once, while to make the latter the cream is 

 allowed to become sour, by the development and chemical 

 activities of the lactic acid group of bacilli, before it is converted 

 into butter. 



If the cream is pasteurised first and the requisite bacteria 

 subsequently added in the form of the so-called " starters," the 

 process is better controlled, and therefore more uniform quality 

 butter is produced, abnormalities of taste, odour, etc., due to 

 foreign bacteria are prevented, while the keeping qualities of 

 the butter are improved. 



In America to some extent, and in Denmark for the most 

 part, the butter is made from pasteurised cream to which starters 

 are added, while in England it would appear that the greater 

 part of the butter is made naturally, that is by trusting to the 

 suitable bacteria being naturally present. 



The " starters " are more or less pure cultures of lactic acid 

 bacteria. 



8-2 



