325] Estimation of Foreign Fats in Butter 195 



evaporate on the water bath. Now pour the contents of the 

 flask into a separating funnel (preferably made of thin glass, 

 see fig. 48). Wash the flask well with hot water, and add the 

 washings. Whilst the liquid in the separating 

 funnel is still hot, add hot dilute hydrochloric 

 acid (1-3) until the liquid is acid. Shake well, 

 and allow to stand over night. This will separate 

 all the fatty acids, which will collect in a wax-like 

 film above the water. This long standing is 

 necessary to allow the complete separation of the 

 insoluble acids, which, when once thoroughly col- 

 lected on the surface of the water, give no trouble 

 in the subsequent washings. When the separation has taken 

 place, shake the apparatus to detach the film from the walls of 

 the bulb, and run off the liquid by means of the stopcock 

 below. Now pour about 50 c.c. of boiling water into the bulb, 

 and shake to wash the fatty acids. They will melt and rapidly 

 collect on the surface of the water, which will be left perfectly 

 clear, and may be run off. This washing must be repeated 

 several times until the whole of the washing liquid, together 

 with the original acid liquor, measures 300 c.c. In the final 

 washing run off the water as completely as possible, then run 

 the acids into a tared shallow dish. Wash out the acids which 

 adhere to the funnel with a little ether. Place the dish, with its 

 contents, in the water bath, and heat until it ceases to lose weight. 

 325. Calculation. — All animal fats contain about 95*5 

 per cent, of insoluble fatty acids. Butter fat contains on the 

 average 87*5 per cent. It is, therefore, easy to calculate the 

 approximate percentage of foreign fat in a sample of butter by 



the formula 



x'm 12-5 (a - 87*5), 



where x = percentage of foreign fat and a — percentage of 

 insoluble fatty acids found. 



