228 OUTLINES OF BACTERIOLOGY 



milk to stiffen to a jelly-like consistency. After standing a while this 

 jelly contracts slightly and a watery liquid the whey oozes out of 

 it. Then, still later, firm clots called curds are found floating in the 

 whey. These clots, which are the crude cheese, are massed together, 

 pressed, and then set aside for several weeks, sometimes for months, 

 to undergo a process of ripening. The taste of freshly-made cheese 

 is not pleasant, and its value depends on the changes that take 

 place during the ripening, of which, unfortunately, we know as yet 

 very little. That the ripening is due to bacteria is shown by the 

 following facts : 



1. It takes place best at temperatures which are most favourable 



for the growth of bacteria. 



2. Cheese prepared from sterilised milk will not ripen ; it possesses 



the same taste for months after being made. 



3. Bacteria undoubtedly grow and multiply in cheese during 



ripening. 



4. The substances formed during ripening resemble in some cases 



the decomposition products of bacterial action. 



It is therefore not surprising that sometimes, when apparently the 

 ripening has proceeded in a normal manner, it is found that a cheese 

 has become abnormal and worthless. The bacteria present in cream 

 are never exactly the same in any two samples, and sometimes disease- 

 bacteria gain entrance into the cream and subsequently into the cheese. 

 These during ripening multiply slowly, producing substances which 

 render the cheese worthless. The ripening of cheese is a slow process, 

 because the consistency is very dense and there is very little moisture 

 present, so that rapid multiplication of the contained bacteria is not 

 possible. Further, these bacteria are very probably aerobic, so that the 

 lack of a sufficient quantity of oxygen is probably another reason for 

 the absence of rapid multiplication. Of late years much attention has 

 been bestowed on the bacteria that are found in ripening cheese. 

 These are found to be of four kinds : 



1. Lactic-acid bacteria. 



2. Casein-digesting bacteria. 



3. Gas-producing bacteria. 



4. Extraneous bacteria accidentally present. 



There is some evidence to show that in most cases ripening is 

 accomplished by several of the lactic-acid and the casein-digesting 

 bacteria and not by any one organism. It is probable that we shall, 

 before long, be able to estimate the exact role played by each of the 



