LACTOSE 5 



The most important products derived from lactose, in connec- 

 tion with the bacteriological examination of milk, are the lactic 

 acids. Lactic acid (CsHeOs) exists as four different isomers, 

 three having the constitutional formula CH3-CH(0H)-C00H 

 or alpha hydroxy propionic acid, and one CH2(0H) ■ CH2 • COOH 

 hydracrylic acid or beta hydroxy propionic acid. As the latter 

 is not produced during the bacterial decomposition of lactose 

 no further description of this acid is necessary in this work. 

 Alpha hydroxy propionic acid, or lactic acid- as it is usually 

 known as, contains an asymmetric carbon atom 



H 



I 

 CH3— C— COOH 



I 

 OH 



and exists, therefore, in three different forms, viz., dextro, 

 Isevo, and racemic or inactive lactic acids. The dextro and 

 Isevo rotatory acids are both produced by micro-organisms, but 

 unless pure cultures are employed the majority of the acid 

 produced is of the racemic (d+l) variety. 



15° 



The density of lactic acid is 1.2485 -jo and the refractive 

 index M/320° 1.4469. On evaporation of aqueous solutions of 

 lactic acid dehydrolactic acid CeHioOs is produced, and, ulti- 

 mately, at higher temperatures, lactide C6Hs04 is formed. The 

 boiling point of lactic acid is 83° C. at 1 mm. pressure and 119° 

 C. at 12 mm. pressure. Lactic acid, though insoluble in petro- 

 leum ether, is soluble in, and miscible with alcohol and ether 

 in all proportions. 



Lactic acid forms weU-defined salts with various metals 

 and these may be used for the separation of the acid. The 

 calcium salt which crystallises with 5 molecules of water is 

 soluble to the extent of 9.5 per cent in cold water: zinc lactate 

 (ZnCeHioOi • 3H2O) is less soluble, 1.3 per cent in cold water 

 and 13 per cent in hot, and forms well-defined monoclinic 

 prisms. 



