I70 FERMENTATION IN MILK 



excreting both enzymes, the casein precipitant and the casein 

 solvent. These species liquefy nutrient gelatine. 



The ratio between these two ferments differs in the various 

 species. A few produce the casein solvent alone, and when sown 

 in milk do not precipitate the casein, but decompose it direct into 

 soluble fission products, among which leucin and ty rosin have been 

 identified. [In proportion as the casein disappears the milk 

 becomes clearer, and is finally quite transparent. The essential 

 phenomenon of this change is digestion of the casein. Milk which 



has undergone lactic fermentation cannot be so digested.'''^^hllst 



rennet acts only within narrow limits of temperature, casease has 

 a wider sphere of activity, and it has been shown that some 

 organisms produce both enzymes, at first curdling the milk and 

 subsequently digesting it. 



The constitution of casease has not yet been accurately deter- 

 mined, neither has any one succeeded in ascertaining in what 

 precise respect this enzyme differs from pepsiji__and trypsin — 

 which it greatly resembles in'actibh^^^iToF^'hether the caseTir-HTs"- 

 solving enzyme produced by different species of bacteria is the 

 same in all cases. Weigmann states that he has isolated casease 

 from bacterial cultures, and that this substance favours and accel- 

 erates ripening when added to fresh cheese. Occasionally these 

 non-acid coagulative fermentation organisms occur in dairies and 

 curdle milk. They have also been found (because of their spores) 

 to withstand sterilisation. Organisms which digest casein liquefy 

 gelatine, and for this reason are readily demonstrable in culture. 

 This feature of liquefaction has been commonly used as an indica- 

 tion of the quality of a milk. If a milk contains few liquefiers, 

 though many bacteria, the milk, though not fresh, is probably not 

 greatly contaminated. If, on the other hand, milk contains many 

 organisms, most of which are liquefying, its quality is at once open 

 to suspicion. As we have pointed out, these species are as a rule 

 overpowered by the lactic acid organisms. 



The spontaneous coagulation of milk without the co-operation 

 of micro-organisms, the possibility of which was maintained by 

 early workers, and finally established as a fact by Meissner, was 

 more closely examined in 1887 by A. Levy, who found that a very 

 faint coagulation can be detected in all milk that has been left to 

 stand for some time.^ He pointed out that the sediment deposited 



1 The rapid curdling of milk so frequently observed during thunderstorms 

 has been explained in various fantastic ways. The opinion expressed by J. 

 Liebig in 1890 will not bear investigation, and the assumption put forward, on 



