BACTERIA AND CHEESE 119 



found, in the same quantity of Swiss cheese one million, in other sorts still 

 more. In the preparation of Roquefort cheese a mould fungus (Peni- 

 cillium glaucum) is introduced with mouldy bread, and forms the well- 

 known green patches in the cheese. In other cases, forms like Oidium 

 and also yeast fungi play a part. 



Not all the bacteria of cheese are equally active in the ripening 

 process ; many are only useless intruders, others perhaps contribute to 

 the finer shades of flavour. The main part of the process is the work 

 of the lactic and butyric ferments. Forms that were once supposed to 

 be specific 'cheese bacteria' (Tyrothrix, Duclaux, allied to B. subtilis) 

 do not play the important part that was attributed to them. How 

 difficult it is to determine the share which the various species have in the 

 process, is shown by the fact that in some cheeses no less than nineteen 

 different bacteria and three yeasts have been found. In other kinds 

 the flora is still more varied, and it changes, moreover, with the different 

 stages of the ripening process. This is the reason that, in spite of numerous 

 and careful investigations, the bio-chemistry of cheese has but few results 

 to show, and those contradictory. Even the qualitative composition of 

 cheese varies, and a quantitative analysis is as yet no more possible than 

 in the case of putrefactive processes. Emmenthaler cheese contains lactic, 

 butyric, and phenylamidopropionic acids, leucin, tyrosin, ammonia, casein 

 (partly unchanged, partly in the form of soluble albumoses), fats, and fatty 

 acids. The fresh curds from which cheese is prepared contain chiefly three 

 substances, whose gradual alteration constitutes the ripening: i. Carbo- 

 hydrates (milk sugar) ; 2. Proteids (casein and paracasein) ; 3. Fat. The 

 milk sugar is converted first by the lactic bacteria, and afterwards by the 

 butyric bacteria, into organic acids, CO 2 , and free hydrogen. It is these 

 gases which cause the cavities in the cheese. The casein is changed, by the 

 action of the enzyme secreted by the bacteria, into an albumose-like body 

 (improperly called caseoglutin), and a small portion of it further split up 

 into tyrosin, leucin, ammonia, and phenylamidopropionic acid ; but genuine 

 putrefactive products, like skatol or indol, are not formed. The ripening 

 of cheese cannot therefore be regarded as a true putrefaction, but only as 

 an allied process in which fatty acids arise. The conversion of the casein 

 goes on very gradually until, by the time the ripening is finished, there 

 is none left in an unchanged state. It is not known precisely what species 

 of bacteria are instrumental in effecting this alteration. Fat is not formed 

 from casein, and the butter remaining in the curds is for a long time 

 unaffected, but towards the end of the process it is split up into glycerine 

 and fatty acids. Of these phenomena the two of chief importance are the 

 fermentation of the milk sugar and the alteration of the casein. 



Another derivative of milk, kephir, is produced by the united action 

 of lactic acid bacteria and a yeast (Saccharomyces). Kephir is a weakly 



