EFFECTS OF HEAT ON MILK 9 



coagulation of milk, which is an altogether different change. 

 Coagulation occurs when milk is brought, either in the stomach or 

 outside the body, into contact with rennin, the active ferment of 

 rennet. Rennet is produced in the stomachs of mammals, and is 

 generally prepared from the fourth stomach of the calf It acts on 

 casein only in neutral or acid solution. The larger the amount of 

 acidity which the milk possesses, the more will be the action. It 

 requires for its efficient action the presence of calcium salts, 

 generally calcium phosphate (Hammarsten), from which latter body 

 it is with difficulty separated. " The action of rennet is to split up 

 the casein into a dyscaseose, the calcium compound of which is 

 insoluble and which forms curd and a soluble caseose " (Richmond). 

 If calcium salts be completely removed, no insoluble dyscaseose 

 is produced. The curd carries down with it the fat entangled with 

 the casein : the whey contains the other proteids, sugar and salts 

 of milk. Probably the entire fermentation consists of two stages, 

 the first, by which the ferment changes caseinogen into casein ; the 

 second, by which the calcium salt precipitates the casein as curd. 

 The clotting of caseinogen is due then to a specific action of the 

 enzyme and not to the formation of lactic acid from milk-sugar. 

 When milk is clotted the separation is so complete that no casein 

 is found in the whey.^ 



Effects of Heat. — When milk is heated various changes occur. 

 Coagulation is facilitated, changes occur in the fat globules, the 

 flavour and colour are altered and a skin forms on the surface at 

 about 60° C. This " scum " consists in part of coagulated lact- 

 albumin and in part of caseinogen and salts. These latter bodies 

 would appear to become separated by heat, and the caseinogen 

 becomes entangled in the fat and carried to the surface where, 

 by evaporation, it forms part of the dried wrinkled scum or skin. 

 The brown colour which occurs is probably due to the burning of 

 the sugar (caramel). 



So many and varied seemed to be the views accepted on this 

 question that we made a series of simple investigations which we 

 may briefly mention. Our only justification for differing from 

 various authorities is that we have experimented again and again 

 and arrived at certain results. We found that milk taken off the 

 fire as "boiling" by the ordinary cook or nurse rarely exceeds 

 90° C, and often falls short of this, the custom being to judge by the 



* The Chemical Basis of the Animal Body, by Sheridan Lea, F.R.S., pp. 21-22. 

 See 3\so four, of Physiology, 1890, pp. 307, 369, 378, 448 ; d^oAfour. of Anatomy 

 and Physiology, 1894-95, p. 188. 



