24 GENERAL MICROBIOLOGY 



reduce the hydrogen peroxid to E^O and nascent oxygen 

 which reoxidizes the reduced litmus (showing by the reac- 

 tion of the milk the type of microorganisms present). Re- 

 oxidation takes place slowly under natural conditions. 

 Reduction may take place when milk is acid, alkaline or 

 neutral. 



(d) Curdling through Acid Production. The casein, like 

 most proteins, is amphoteric, i.e., it is capable of reacting both 

 as a weak acid and a weak base. The otherwise insoluble 

 casein is found to be in the milk in a partially dissolved 

 state (colloidal), due to its combination with the calcium 

 salts: the calcium that was formerly combined with the 

 casein, through the production of acid by certain micro- 

 organisms, now combines with the lactic acid; as a result 

 the casein precipitates, causing curdling (coagulation). Lit- 

 mus is turned decidedly red. Milk having an acid curd 

 will titrate above +50. 



(e) Rennet Curd. Coagulation may also take place when 

 the medium is neutral or only slightly^ acid. This pro- 

 duction of curd is due to a rennet-like enzyme produced by 

 microorganisms, and is similar to the action of the rennet 

 used to curdle milk in cheese factories. 



Many spore-forming species are found under the group 

 of rennet-producing organisms. Rennet curd is usually 

 followed by peptonization. 



(/) Peptonization. The curd produced by acid or ren- 

 net-forming microorganisms may gradually disappear, leav- 

 ing only a whey-like liquid. This is caused by certain 

 bacteria which produce proteolytic enzymes that digest the 

 curd and render it soluble. This liquefaction of solid pro- 

 teins like gelatin, fibrin, boiled egg white, milk curd, etc., is 

 due to two groups of enzymes, pepsin and trypsin. 



The pepsin of the animal body acts only in an acid 

 medium (present in the stomach). 



The trypsin of the animal body acts only in alkaline 

 medium (present in the intestine). 



