154 DAIRY BACTERIOLOGY 



evening milk is often kept for eighteen hours. In the case of iron 

 the colour is due to ferrous salts, for which reason the outer 

 portion of the cheese will not be coloured, while the colour 

 disappears from a slice of the cheese which is exposed to the air. 

 In the case of copper the conditions are reversed ; as the colour 

 is due to the green cupric salts, the outer portions will be the most 

 affected, and a slice cut out of the cheese will only develop the 

 full colour after exposure to air for some time. Metallic sulphides, 

 stable in air, will be produced where hydrogen sulphide is formed. 

 The minute coloured spots which are sometimes found distributed 

 throughout the whole mass are of greater interest from the 

 bacteriological point of view, as they are colonies of chromogenic 

 organisms, which develop in the same way as on other solid media, 

 such as nutrient gelatine or agar x . The conditions of bacterial 

 growth consequent on the cheese being a solid medium are not 

 so strikingly illustrated in the case of the lactic acid bacteria, 

 which grow rapidly throughout the fresh curd, and thus appear 

 to be evenly distributed ; but the slow-growing organisms which 

 appear at a later stage will appear in this characteristic manner. 

 Thus Bacillus cyaneofuscus (which, however, dies out before the 

 cheese has fully ripened) forms blue spots in Edam cheese, while 

 the chromogenic propionic acid bacteria form red and brown spots 

 in Emmental cheese. In all probability the eyes in Emmental 

 cheese are formed in those places where the colonies of propionic 

 acid bacteria are particularly abundant. According to Connel 2 . 

 the so-called rusty spots in Cheddar are caused by Bacillus rudensis, 

 an acid-producing organism, which may possibly belong to the 

 propionic acid group, as it is generally found in or near the eyes 

 of Emmental cheese ; being particularly prevalent in spring, 

 it is supposed to originate from the fresh grass ; if once established 

 in the dairy, it will appear in successive batches of cheese unless 

 all the appliances are sterilised. 



The form of discoloration resulting in the production of a red 

 colour just inside the rind, but not in the rind itself, may be 

 regarded as intermediate between interior and exterior discolora- 

 tion. It is said to be due to the diffusion of colouring matter from 

 the shelves into the cheese ; shelves of white pine, but not of red 

 pine or fir, are said to be objectionable in this respect. This 

 explanation hardly holds good in all cases, for the red zone may 

 spread after the cheeses have been removed to another place, and 



1 The staining of cheese sections to show the natural position of the 

 bacteria was first accomplished by Miss Gerda Troili-Petersson, 1904 

 ("Centralblatt f. Bakt.," 2 Abt., Bd. XI., p. 212). 



2 "Discoloration of Cheese," Canadian Dept. of Agric. Bull., 1897, and 

 Harding and Smith, New York Agric. Exper. Station, Geneva, 1902, 

 Bull. No. 225, 



