22 SCIENCE AND PRACTICE OF DAIRYING. 



volatile groups, The individual members of both groups exhibit such 

 slight differences in their chemical behaviour and distinctive properties, 

 that as yet it has been found impossible to separate them from one another, 

 or to determine their composition. On this account the proportion of 

 stearin, palmitin, and olein in milk-fat, generally stated in the literature 

 of the subject, is practically unreliable. At the most, by determining the 

 so-called iodine coefficient of milk-fat, which is proportional to the amount 

 of olein it contains, it can be ascertained which of two given samples of 

 milk-fat contains most olein. For the approximate determination of the 

 amount of the individual volatile fatty acids in milk-fat, Duclaux has 

 devised an ingenious method of determination. By this method the fatty 

 acids are reckoned as triglyce rides, and the probable average composition 

 of milk-fat is calculated approximately as follows : 



Palmitin, stearin, olein, and traces of myristin and butin, 91 '50 



Butyrin, 4'20 



Capronin, 2*50 



Caprylin, caprinin, and traces of lauriii, 1'80 



1001)0 



The percentage of insoluble and soluble fatty acids varies according to 

 the length of time after lactation, the amount of the soluble fatty acids 

 gradually diminishing, and that of the insoluble acids increasing, with the 

 increase of the duration of this period. To a certain extent the amount is 

 influenced by the individuality of the animal, and by the breed, probably 

 also by the age of the cow; but the influence of feeding has not yet been 

 proved with certainty. According to Adolf Mayer the percentage of the 

 volatile fatty acids in milk-fat is distinctly increased by feeding with fresh 

 meadow hay, and is diminished by feeding with straw and poppy-cake. 

 The percentage of olein in milk-fat appears to increase with the lapse of 

 time after lactation. Butter-fat, containing a small percentage of volatile 

 fatty acids, contains, as a rule, a correspondingly larger percentage of non- 

 volatile fatty acids. 



Lecithin, a substance containing nitrogen and phosphorus, may be 

 mentioned as a characteristic constituent of milk-fat. It is further to be 

 noticed with regard to the chemical composition of milk-fat, that it contains 

 less carbon than other kinds of fat. Milk-fat, freshly separated from cows' 

 milk, is, at ordinary temperatures, a soft yellowish mass, which soon 

 assumes a granular structure, and possesses a mild taste and very slight 

 odour. If melted butter-fat be allowed to cool gradually, it occasionally 

 occurs that a separation of the mass into two parts takes place, viz. a solid 

 portion, and a portion called butter-oil, which remains liquid at ordinary 

 temperatures. Milk-fat melts usually between 31 C. and 36 C. ; occasion- 



