250 BACTERIA IN MILK AND MILK PRODUCTS 



Abnormal Cheese-Ripening- 



Unfortunately, from one cause or another, faulty fermentations 

 and changes are not infrequently set up. Many of these may be 

 prevented, being due to lack of cleanliness in the process or in the 

 milking ; others are due to the gas-producing bacteria being present 

 in abnormally large numbers. When this occurs we obtain what is 

 known as "gassy" or inflated cheese, on account of its substance 

 being split up by innumerable cavities and holes containing carbonic 

 acid gas or sometimes ammonia or free nitrogen. Some twenty-five 

 species of micro-organisms have been shown by Adametz to cause 

 this abnormal swelling. In severe cases of this gaseous fermentation 

 the product is rendered worthless, and even when less marked the 

 flavour and value are much impaired. Winter cheese contains more 

 of this species of bacteria than summer. Acid and salt are both 

 used to inhibit the action of these gas-producing bacteria and yeasts, 

 and with excellent results. The character of the gas holes in cheese is 

 not of import in the differentiation of species. If a few gas bacteria 

 are present, the holes will be large and less frequent ; if many, the 

 holes will be small, but numerous. (Swiss cheese having this 

 characteristic is known as Nissler cheese.) Many of these gas- 

 producing germs belong to the lactic acid group, and are susceptible 

 to heat. A temperature of 140° F. maintained for. fifteen minutes 

 is fatal to most of them, largely because they do not form 

 spores. 



The sources of the extensive list of bacteria found in cheese are 

 of course varied, more varied indeed than is the case with milk. JTor 

 there are, in addition to the organisms contained in the milk brought 

 to the cheese factory, the following prolific sources, viz., the vats 

 and additional apparatus ; the rennet (which itself contains a great 

 number) ; the water that is used in the manufacture. 



In addition to the abnormalities due to gas, there are also other 

 faulty types. The following chromogenic conditions occur: red 

 cheese, due to a micrococcus ; Hue cheese, produced, according to 

 Vries, by a bacillus ; and hlack cheese, caused by a copious growth 

 of low fungi. Bitter cheese is the result of Tyrothrix geniculafus 

 (Duclaux), or the Micrococcus casei amari of Freudenreich, a closely 

 allied form of Conn's micrococcus of bitter milk. Sometimes cheese 

 undergoes a putrefactive decomposition, and becomes more or less 

 putrid. At other times it becomes " tainted." These latter condi- 

 tions, like the gassy cheeses, are due to the intrusion of bacteria 

 from without, or from udder disease of the cow.* Healthy cows, 

 clean milking, and the introduction of pure cultures, are the methods 



* For a discussion on the duration of life of the tubercle bacillus in cheese, see 

 Nineteenth Ann. Rep. Bureau of Animal Industry, 1902, p. 217. 



