142 DAIRY BACTERIOLOGY 



bacteria) could not be demonstrated in large numbers, a natural 

 result of the ability of these organisms to overgrow all other 

 bacteria under circumstances favourable to themselves. Thus, 

 they also obtain predominance by slow degrees in the Danish 

 dairy cheese, although this cheese, like Cheddar, is freely inocu- 

 lated with lactic acid-producing streptococci by the addition of 

 buttermilk. 



Barthel ascribes to the streptococci a more important part in 

 the ripening of cheese than has hitherto been done. He has 

 shown tfrat certain strains form, at ordinary temperatures, quite 

 appreciable amounts of Sol. N. (see table, p. 143). The author 

 has also found, with fair regularity, streptococci, especially strains 

 of Sc. cremoris, which are conspicuously able to split casein into 

 soluble products ; as these bacteria gradually lose this power 

 when cultivated on artificial media, it may be surmised that 

 streptococci which do not hydrolyse casein to any appreciable 

 extent may acquire this property when cultivated in milk and 

 cheese. It must also be pointed out that many lactic acid bacteria 

 grow better in milk to which rennet has been added than in 

 ordinary milk ; that is, their action is promoted by the rennet 

 just as, conversely, the action of the rennet is promoted by the 

 lactic acid. 



In addition to the action of certain lactic acid streptococci 

 which hitherto have not been further investigated, the chief 

 factors in the ripening of the hard rennet curd cheeses are 

 seen to be the rennet, the exotrypsin of the peptonising tetracocci 

 and the endoerepsin of the lactic acid rod bacteria. The relative 

 importance of each of these factors varies considerably in different 

 cheeses, and determines the characteristic properties of each variety. 

 Further differences arise owing to the fact that the above-men- 

 tioned groups of lactic acid bacteria include many different 

 species, an illustration of the production of special characteristics 

 owing to the action of a particular species of organism being seen 

 in the case of Emmental cheese. The table on page 143 has 

 been drawn up to illustrate the action of the several factors in 

 ripening ; the amounts of soluble nitrogen (Sol. N.), nitrogen of 

 decomposition products (Dec. N.), and ammonia nitrogen (Am. N.) 

 formed in milk after two months are given (cf. table, p. 133). All 

 the cultures were treated with chalk and shaken regularly so that 

 the lactic acid produced was neutralised. 



Taking a wider view of the ripening process, as including not 

 only the decomposition of -the casein, we must also consider the 

 propionic acid bacteria as a ripening factor. Weigmann is of the 

 opinion that the aromas of the various kinds of cheese are princi- 

 pally due to the action of bacteria other than the lactic acid 



