BACTERIA IN FOODS 22$ 



to avoid some contamination of the milk previous to the 

 addition of the culture. The continual uncontaminated 

 supply of pure culture is by no means an easy matter. The 

 maintenance of a low temperature to prevent the rapid mul- 

 tiplication of extraneous bacteria will, in some localities, be 

 a serious difficulty. These difficulties have, however, not 

 proved insurmountable, and by various workers in various 

 localities and countries culture-ripening is being carried on. 



Abnormal Ripening. Unfortunately, from one cause or 

 another, faulty fermentations and changes are not infre- 

 quently 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 " cheese, on account of its sub- 

 stance being split up by innumerable cavities and holes con- 

 taining carbonic acid gas, or sometimes ammonia or free 

 nitrogen. Some twenty-five species of micro-organisms 

 have been shown by Adamety to cause this abnormal swell- 

 ing. In severe cases of this gaseous fermentation the pro- 

 duct 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. 



We may remark that 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 numer- 

 ous. (Swiss cheese having this characteristic is known as 

 Nissler cheese.) 



Many of these gas 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 

 15 



