442 BUTTER, CHEESE, ETC. 



precipitates a further amount of proteins. The whey is then 

 boiled down in vacuum pans, and the sugar allowed to crystallise 

 out. Milk-sugar is purified by re-crystallisation from water or, 

 where alcohol is cheap, by dissolving it in water and precipi- 

 tating with alcohol. 



The milk-sugar of commerce is usually in the form of fine 

 powder, but it is also sold in crystals ; it consists of essentially 

 pure sugar, but may contain sensible amounts of lactic acid, ash, 

 and, sometimes, proteins. Its chief use is in the preparation 

 of infants' food, and it is also employed in medicine, especially 

 in homoeopathic preparations, entering largely into the com- 

 position of the triturates ; it is official in most pharmacopoeias. 

 It has been used in the manufacture of penta-nitro -lactose, 

 which forms a part of some high explosives. 



Junkets. This preparation is made by adding cane sugar and 

 flavouring agents to milk and curdling by rennet at a low 

 temperature. It is a sweetish gelatinous substance, and is 

 usually eaten with nutmeg and cream. 



Casein. Many preparations of this protein are now on the 

 market, and, besides being consumed largely as foods, they are 

 employed in the arts for purposes such as sizing paper, as a 

 mordant, and for clarifying wines. 



Casein is prepared by precipitating the protein from separated 

 milk by means of an acid ; sulphuric or hydrochloric acids are 

 generally employed, but sometimes acetic acid or the lactic acid 

 of strongly acid whey is used. If a moderately pure protein 

 is desired the precipitated protein is dissolved in a small quantity 

 of alkali, and reprecipitated, but many of the preparations on 

 the market consist of once precipitated casein, which has not 

 even been washed with water. Rennet caseins are prepared 

 by purifying the curd precipitated by rennet in the same way. 



To prepare a casein soluble in water, a small quantity of alkali 

 (sodium carbonate) is added to dissolve the protein, and the 

 solution is dried on hot rollers, as a fine spray, or in thin layers, 

 and the resulting solid ground. A casein' nearly insoluble in 

 water, but easily soluble in dilute alkali solutions, may be pre- 

 pared by dissolving in ammonia, and evaporating the solution ; 

 practically all the ammonia passes off on drying. The curd 

 produced by rennet may also be used, but it is more difficult to 

 dissolve than that precipitated by acids. If the precipitated 

 casein is directly dried a hard, horny mass is produced, and as, 

 in the process of drying it is often overheated, it does not dissolve 

 easily in dilute alkali solutions. 



The following products of casein are commercial substances : 



Plasmon, Tilia, etc. The sodium compounds of casein, 

 containing the bulk of the salts of the milk. 



