14 DAIRY BACTERIOLOGY 



less readily, being usually referred to as liquefying organisms. 

 The proteolytic enzymes of the gastric juice, chymosin (rennet) 

 and pepsin, only carry the cleavage of proteins as far as the peptone 

 stage ; the trypsin of the pancreas effects complete hydrolysis to 

 amino acids. The erepsin of the intestinal juice is particularly 

 thorough in its action, but only attacks proteins which have 

 already been partially broken down by other enzymes. Casein 

 is exceptional in being directly attacked by erepsin 1 . 



Oxidising and reducing enzymes are known as oxidases and 

 reductases ; they are of prime importance in connection with the 

 breathing of animals and the reduction of carbohydrates to fats. 

 Storch's well-known reaction, by which it is possible to ascertain 

 whether milk has been heated to over 80 C. or not, depends 

 on the presence in milk of an oxidase which is destroyed 

 at 80 C., and can transfer the loosely bound oxygen of 

 hydrogen peroxide to paraphenylene diamine or certain other 

 colourless substances giving coloured products. Paraphenylene 

 diamine gives a violet or, in the presence of casein, a blue 

 colour. Many vegetable products, such as potatoes, fruits or 

 fungi, contain oxidases as well as substances which yield coloured 

 products on oxidation ; hence the darkening in colour which 

 takes place when they are cut into pieces and left exposed to 

 air ; if the material has been boiled, the oxidase will have been 

 destroyed, and no darkening occurs. Reductases, on the other 

 hand, are generally recognised by taking advantage of the fact that 

 many substances, such as methylene blue, are decolorised on 

 reduction. Catalase occupies a position intermediate between 

 oxidases and reductases ; it decomposes hydrogen peroxide, but 

 the oxygen so liberated will not act on paraphenylene diamine or 

 similar oxidisable substances. Catalase only occurs in small 

 quantities in milk, but plentifully in blood ; as constituents of 

 the blood always pass into milk, which is drawn from diseased 

 udders, an abnormally copious evolution of oxygen in the catalase 

 test (see p. 159) is to be regarded as a bad sign. 



Certain poisons, easily destroyed by heat, known as toxins, are 

 closely related to the enzymes. They are found in a few plants, 

 in poisonous reptiles and other animals, and are frequently 

 secreted by microorganisms. The various toxins are generally 

 the active agents in diseases due to pathogenic organisms. Healthy 

 living tissues are highly resistant towards enzyme action, and are 

 provided with special poisons, bactericidal substances, which repel 



1 It follows that microorganisms which secrete no other enzymes than 

 erepsin attack casein, but do not liquefy gelatine. Conversely, the author 

 has found that micrococci which liquefy alkaline, but not neutral or acid 

 gelatine, do not peptonise casein. 



