gg Biological, Chemical and Physical Characteristics of Milk. 



Enzymes with the action of rennet have been found in various plants and parts 

 of plants, such as the artichoke, branches of fig trees, candytuft (Iberis pinnata), 

 yellow mustard (Isatis tinctoria), etc., also in bacteria (proteolytic) and in yeast. 



The individual kinds of rennets vary considerably in their sensitiveness to various 

 influences. 



Whereas the rennet of calves is very susceptible to heat, and exerts its action 

 readily in alkaline solutions, the parachymosin is less influenced by the harmful action 

 of heat, but is greatly affected in its action by the presence of alkalies. 



The rennet enzymes obtained from plants act in an optimal way at high tem- 

 peratures (sykochymas at 65-70 deg. C. for raw, at 85 deg. C. for sterilized milk). 



Aside from casein, milk contains proteids which are coagulable 

 by heat. 



(1) Lactalbumin which is related to the serum albumin but is 

 not identical with it (it has a slight optical polarization: 36.4 to 

 38 against 60.1 to 62.6, Sebelien). 



(2) Lacto-globulin may be precipitated with the aid of mag- 

 nesium sulphate. It is contained in milk in quantities of about . 1 

 per cent, of the total proteids. The lacto-albumin is obtained from 

 the residual solution after saturation with magnesium sulphate and 

 acidifying it, or by almost complete saturation with ammonium sul- 

 phate. 



3. Lacto-mucin has been also demonstrated in milk by Storch, Siegfeld, Voltz 

 and Eosengren, whereas other proteid substances such as albumose, peptone, albuminose, 

 lacto-protein, gelatin, galactozymase and opalisin, are considered more recently as 

 products of the preparation of other proteid bodies, at least so far as their appearance 

 in ripe milk is concerned. 



The proteids which remain in the fluid after precipitation with 

 acid and boiling are collected under the term "lacto-protein." 



The milk fat consists of a mixture of triglycerides, choles- 

 terin, lecithin, and a coloring substance, and distinguishes itself 

 considerably from the fat of the body and from the nutritive fat 

 by its chemical and physical characteristics. Although the milk 

 fats manifest considerable dependence upon the nutritive fat, as 

 will be seen from the later chapters, nevertheless a transition of 

 the nutritive fat into milk fat cannot be asserted. The same state- 

 ment would also apply to the transition of body fat, although in 

 this instance a closer relationship between the substances must be 

 admitted. 



It is possible that transitory relations exist, by means of which split up body 

 fat may be converted in the milk gland into milk fat, and thus the nutritive fat 

 takes part indirectly in the formation of milk fat after first having been deposited 

 as body fat. 



It should be considered however, that the specific activity of the cell builds up 

 the fat from the constituents at hand, and utilizes whatever material is placed at its 

 disposition, such as nutritive fat, when such is present, or body fat in emergencies. 

 The product will approach in its properties the material which has been utilized, but 

 will always remain peculiar to the species of animal producing it. 



A formation of fat from proteid is possible, as may be seen 

 when cows are fed with substances free of fat, and after the body 

 fat deposits have been used up. It is probable that the carbohy- 

 drates of the food here take part in the formation of fat. 



