152 DAIRY BACTERIOLOGY 



etc. ; not only are the hard cheeses submitted to great pressures 

 in the press, but any tendency towards expansion from within 

 will meet with considerable resistance, owing to the close texture 

 of the curd, and particularly the rind. By storing at a low 

 temperature, a tendency towards sponginess may be kept in check 

 in two ways, for not only is the absorption of carbon dioxide by 

 water increased by lowering the temperature, but the development 

 of the gas-forming bacteria is checked. It will thus be understood 

 why sometimes bacterial growth may appear to commence afresh 

 when the cheese is taken out of a cold cellar on a warm day, even 

 though no new development of gas-forming bacteria actually 

 takes place. It will also be understood why eyes are more readily 

 formed in Emmental cheese when the curd has been saturated 

 with carbon dioxide during the fermentation of the lactose ; 

 under these conditions the distinction between normal and 

 abnormal eyes may not become so sharp as might be expected, 

 even though the two processes are due to totally different bacteria. 

 Unlike carbon dioxide, hydrogen is only very sparingly absorbed 

 by water ; it follows that those organisms which produce the most 

 hydrogen are capable of doing the most harm. Thus the butyric 

 acid organisms may transform the cheese into a large-holed^ 

 spongy mass in the course of two days ; the non-motile butyric 

 acid bacteria are especially dangerous, owing to their ability to 

 ferment calcium lactate, and thus to cause damage after all the 

 lactose has been fermented. The aerogenes bacteria, especially 

 the colon bacteria, also produce hydrogen. On the other 

 hand, these organisms are able not only to effect respiration by 

 means of atmospheric oxygen, but they will transfer loosely-bound 

 oxygen from oxidising agents, like saltpetre, to sugar, and thus 

 consume the sugar completely, so that no hydrogen is liberated 

 or lactic acid formed. Saltpetre is thus an excellent preventative 

 of the harmful effects of these bacteria ; as a rule it will be 

 sufficient to add 30 to 50 grams of potassium nitrate per 100 litres 

 of milk 1 . As the propionic acid bacteria and the lactose fer- 

 menting yeasts do not form other gases than carbon dioxide, 

 they are less dangerous to the fresh curd. The yeasts can, 

 however, blow the soft cheeses in which there are particularly 

 large amounts of sugar to be fermented ; their growth is pro- 

 moted by small amounts of free lactic acid. The most natural 

 means of preventing sponginess, as well as most other cheese 

 defects, is the use of a vigorous lactic acid starter consisting, as 



1 According to Eosengren, it is dangerous to use saltpetre in Emmental 

 cheese, even in small amounts ; 10 grams per 100 litres may produce an 

 unclean taste and turn the cheese red. Feebly acid cheeses, like Gouda, 

 are best able to stand the addition of larger amounts of saltpetre. 



