318 MILK SUGAR 



A globulin (Eugling) and a fibrin (Babcock) have also been described 

 as occurring in small quantities in milk. For all ordinary purposes, 

 however, the proteids of milk may be considered as being composed 

 mainly of casein or caseinogen and lact-albumin, the amount of the 

 latter being usually about one-seventh of that of the former. 



According to some investigators l milk contains a substance carnic 

 acid, C 10 H 15 N 3 O 5 in union with phosphoric acid, the amount in cows' 

 milk being 0*056 per cent, in human milk 0*124 per cent, of phos- 

 phorcarnic acid or nucleon. The compound phosphorcarnic acid is 

 said also to occur in plants, especially during germination and blos- 

 soming. 



Milk Sugar. Lactose or Lacto-biose occurs in the milk of animals 

 in varying quantities. It has not, with certainty, been detected in 

 plants. 



The hydrated substance, C 12 H 2 . 2 O n + H 2 O, forms large transparent 

 rhombic or monoclinic crystals, which possess well-marked cleavage. 

 Its specific gravity is 1*534. It possesses a faint sweet taste. The 

 crystals are stable at 100, but at about 130 they begin to lose water, 

 and decompose at 180 with partial charring. 



Milk sugar dissolves in 5*87 parts of water at 10 or in 25 parts at 

 100. The solution saturated at 10 contains 14*5 per cent of sugar 

 and has a specific gravity of 1*055. By spontaneous evaporation, 

 the solution becomes supersaturated and does not deposit crystals 

 until it contains over 21 per cent of sugar. On cooling hot saturated 

 solutions down to ordinary temperatures in closed vessels, no crystal- 

 lisation occurs and a highly supersaturated solution is obtained. 

 The crystals are insoluble in alcohol or ether, but very soluble in hot 

 acetic acid. 



Lactose resembles glucose in possessing reducing properties, as 

 shown by its action upon ammoniacal silver solution in the cold and 

 upon alkaline copper solutions on heating. This is in consequence of 

 its containing an aldehyde group, and on hydrolysis, either by dilute 

 acids or by an enzyme known as lactase, it yields glucose and galactose. 



Its constitution has already been given (vide p. 182). 



Milk sugar does not readily undergo alcoholic fermentation, but 

 by the action of certain yeasts, aided perhaps by the hydrolysing 

 enzyme, lactase, it can be induced to do so. 



It is much more prone to undergo the lactic fermentation. This 

 is brought about by micro-organisms, which are always abundant in 

 dairies, etc., though probably all are not of the same kind. The 

 chemical change involved is apparently of a very simple character 



C 12 H 22 O n + H 2 0. = 4C 2 H 4 (OH)COOH, 



but in many cases other products are formed and much more compli- 

 cated reactions must occur. 



Milk sugar is prepared from whey or from milk by removing the 

 nitrogenous matter and fat by means of mercuric nitrate, precipitating 



1 Wittmaick and Siegfried, Jour. Che.n. Soc., 1897, Abstracts, ii. 220, and Stok- 

 lasa, ditto, 578. 



