328 THE FERMENTATION OF CHEESE. 



oxygen to the strongly fermenting milk. The existence of this 

 gaseous stratum can be detected by the sense of smell on carefully 

 raising the lid. 



The bitter flavour developed in the milk was formerly as- 

 cribed e.g. by R. KRUEGER (II.) to the chief product formed by 

 these bacteria, viz., butyric acid, until WEIGMANN (IX.) in 1890 

 showed that no bitter taste is produced in milk by the addition of 

 butyric acid. Like HUEPPE (VII.), he attributes the bitter flavour 

 to the peptone formed from the albuminoids in milk. Later re- 

 searches on this point have led, in the main, to the same results. 

 Consequently the subject falls \vithin the present section, which 

 deals with the decomposition of albumin. 



Nageli was the first to attribute the development of bitter 

 flavour in milk to bacterial activity, and since then many attempts 

 have been made to find and prepare pure cultures of bacteria 

 possessing such properties. Some of these are capable of pro- 

 ducing bitter principles both in milk and cheese, whilst others 

 are injurious solely to the former; but all liquefy gelatin, and 

 consequently produce a peptic ferment. 



Of the first group two species are known up to the present, 

 viz., (i.) Tyrothrix geniculatus obtained as a pure culture by 

 DUCLAUX (XIII.) from Cantal cheese, and already noticed in 

 176 produces a bitter substance in both milk and soft cheese. 

 (2.) Micrococcus casei amari was isolated to a pure culture by 

 FREUDENREICH (X.) in 1894 from bitter, hard, Swiss cheese. This 

 organism, which measures about i ^ in diameter, is endowed with 

 the somewhat rare dual property of forming lactic acid and lique- 

 fying gelatin. In milk and cheese but not in bouillon it gives 

 rise to a strongly bitter flavour, which Freudenreich only partly 

 ascribes to the peptone produced, since after the latter has been 

 thrown down by alcohol from milk cultures of the coccus, the 

 filtered liquid leaves, on evaporation, a bitter residue. Here the 

 widespread experience of practical cheese-makers, viz., that the 

 bitter taste generally makes its appearance at the stage of semi- 

 ripeness, vanishing again as the cheese increases in age, may be 

 mentioned. 



The capacity of developing a bitter flavour in milk alone is 

 possessed by the following organisms : i. Weigmann described in 

 his above-named treatise a sporogenic bacillus, 1.51.8 & long and 

 0.9-1.1 /u, broad, which does not produce gas, but gives rise to a 

 casein-dissolving enzyme and a volatile acid (differing from butyric 

 acid). 2. The micrococcus of bitter milk, of CONN (IV.), is 

 aerobic and non-motile, forms butyric acid, and develops a repul- 

 sive bitter flavour in milk, cream, and butter. 3. M. BLEISCH (I.) 

 obtained from milk, which had become decomposed after sterilisa- 

 tion by the Neuliauss process, a pure culture of a facultatively 

 anaerobic motile bacillus, whose endogenous spores were able to 

 stand exposure for six hours at a temperature of 100 C. in milk. 



