Casein. 35 



Casein is also soluble in other salts, but not, or only to a very 

 slight extent in NaCl, Na 2 S0 4 , NaN0 3 , KC1 and others. 



In the presence of an excess of acid the casein which is first 

 precipitated is again dissolved into a syrup-like mass, but may be 

 again recovered as casein after neutralization. Neutral calcium 

 casein suspensions do not coagulate in boiling, but they form a 

 pellicle on the surface. (The nature of this manifestation is not 

 entirely clear, but depends probably on the drying and transforma- 

 tion of the casein into a more solid form.) 



Casein is precipitated even in the presence of relatively small 

 quantities of acid while boiling under this condition changes it 

 slowly into a body not susceptible to the action of rennet. In over- 

 heating and likewise in boiling and over-heating with small excesses 

 of alkali, casein is split up through hydrolysis. 



Even under the action of water, casein is split up into a pro- 

 teid body which is coagulated by heat, passes through a filter and 

 is probably identical with whey casein. 



The latter substance is formed after the precipitation of the 

 cheesy substance, through the action of rennet, and is a mixture of 

 reduction products of the casein originating through the action of 

 the rennet (Raudnitz). 



One characteristic property of casein is its precipitation by 

 rennet in the presence of earthy alkali salts. The precipitation of 

 casein has no connection with the action of the rennet as such. This 

 may occur even without having precipitation as a result. If for 

 instance a casein solution is mixed with active rennet, and another 

 solution mixed with inactive boiled rennet, then in the mixture con- 

 taining active rennet, para-casein is formed without any action 

 being noticeable. Only after the addition of soluble calcium salts 

 will precipitation of the para-casein calcium result in the glass 

 which contains the active rennet, but not in the glass containing 

 rennet which has been inactivated by heating. 



In the change of casein by the rennet ferment, there results in addition to the 

 substance designated as para-casein, another proteid body free of phosphorus, with the 

 properties of albumose, the whey -proteid (Hammarsten). 



The change of the casein to para-casein, and whey proteid may 

 be a splitting up of the casein, or it may depend on a change in 

 the grouping of the molecules, or it may correspond to a change in 

 its physical condition. 



The action of rennet in the curdling of milk is practically the 

 same as in casein solutions ; however it is influenced by the other 

 (dissolved) substances, by the other proteids and salts, and pos- 

 sibly also by the physical condition of the fatty emulsion. 



Curdling with calf rennet develops in accordance with definite 

 laws. In milk that has been brought to low temperatures (refrigera- 

 tor) the action of the rennet may be established by subsequent heat- 

 ing; the precipitation, however, will not take place until the mix- 

 ture is heated to 37 deg. C. (Morgenroth.) 



