52 DAIRY BACTERIOLOGY 



better in sour than in fresh milk. It is always this mould which 

 forms the velvety layer on sour milk which has been kept too long. 

 On sugar gelatine it only shows luxuriant development of aerial 

 threads in places where lactic acid bacteria are also growing, 

 which shows that it thrives best in presence of lactic acid. It 

 liquefies gelatine very slowly. As already mentioned, it has no 

 specialised conidiophores. It is mostly found on the surface of 

 many soft cheeses, and contributes towards the development of 

 rancidity in butter. Several allied forms are known, some of 

 which, according to Weigmann, will produce a smell of turnips 

 and other feeding stuffs in milk. A red form, Oidium aurantiacum, 



FIG. 61. Two varieties of Penicittivm brcvicaule. 



will, according to Adametz, produce orange or red spots on the 

 surface of cheese. 



3. The Penicillia. These are represented by numerous species, 

 difficult to distinguish from one another. Their distinguishing 

 feature is a sheaf of conical cells, growing out from the upper end 

 of the conidiophore^ from which the conidia (usually green) are 

 formed by constriction. The name of this group of moulds is 

 derived from Penicillium, a brush, owing to the characteristic 

 tassel or brush-like appearance presented by the arrangement of 

 the spores. The commonest species is Penicillium glaucum 

 (Figs. 3 and 9), which liquefies gelatine and produces the well- 

 known mouldy or musty smell. A nearly related form, which 

 thrives particularly well on apples, produces an ethereal fruity 

 odour. In Roquefort. Gorgonzola and Stilton cheeses, a special 

 species, Penicillium roqueforti, is found, which forms shorter 

 conidiophores and somewhat larger conidia. On sugar gelatine 

 its colonies develop an uneven and often fairly wide white rim ; 



