236 MILK 



not detrimental, but heating for ten to fifteen minutes at 72° C. 

 and instantaneous heating at 76° C. are destructive. 



Peroxydases act best in neutral or slightly acid media; they 

 are not precipitated with the casein, but remain in the filtrate. 

 Excess of hydrogen peroxid is detrimental, but formaldehyd 

 intensifies the reaction and protects the enzym to some extent 

 against heat destruction. Milk that has been heated and has 

 become inactivated is reactivated by addition of formaldehyd. 

 The guaiac reaction is not influenced by formaldehyd. 



Reductases. — Fresh milk reduces sulphur to hydrogen sulphid 

 and decolorizes pigments, such as indigo, litmus, methylene-blue, 

 etc. This property is due to the presence of reductases. The 

 reducing property is lost when milk is heated above a certain tem- 

 perature. Three kinds of reductases have been distinguished, 

 namely: 



1. Hydrogenase, which reduces sulphur to hydrogen sulphid. 



2. Reductase, which reduces methylene-blue. 



3. Aldehyd-catalase, which reduces methylene-blue with for- 

 maldehyd. 



It is probable that all these reactions are due to the presence . 

 of but one enzym. 



Reductases are present in cream in greater quantity than in 

 milk; they are precipitated with the casein and do not pass through 

 a porcelain filter. They cannot be recovered from cream by water 

 or sodium chlorid solution. They act best in slightly alkaline 

 media. Reductases are constantly present in cow's milk, but 

 have not been found in human and goat's milk. 



Reductases are probably bacterial products, since sterile raw 

 milk does not reduce sulphur to hydrogen sulphid. Furthermore, 

 the reducing power increases when milk stands for some time, 

 and finally, boiled milk is reactivated by inoculation with raw 

 milk. 



Schardinger's reductase, which acts on methylene-blue and 

 formaldehyd, has been also called indirect reductase and aldehyd 

 reductase. Seligmann assumes that formaldehyd plays a role with 

 the reductases similar to that played by hydrogen peroxid with 

 the indirect oxydases. Excess of formalin injures the enzym. 



It is clear that up to the present it has not been possible to 

 show with certainty that there are any true milk enzyms. It is pos- 

 sible that bacteria are able to produce all the enzyms that so far 

 have been found in milk, and it is difficult to exclude bacterial 

 action from milk. To prove the presence of inherent milk enzyms 

 it is necessary to obtain milk directly from the mammary glands 

 in sufficient quantity to carry on experiments. This is evidently 

 a difficult problem. 



