338 BUTTEE. 



with the acid to be used in a small bottle, and a 5 c.c. pipette, 

 are placed in an incubator kept at 30° C. for at least half an 

 hour, and the temperature noted. 5 c.c. of acid are added, and 

 the mixture well stirred with the thermometer, till the tempera- 

 ture ceases to rise. The difference between the initial tempera- 

 ture and that finally attained is taken as the rise of temperature, 

 and the Relative Molecular Maumene figure is calculated from 

 this. 



The E.M.M. of butters varies from 33-0° to 34-5°, with a mean. 



value of 34 "0°. The ratio .— V — '-^ — -, — ^ has been about 0'633,. 



lodme absorbed 



varying from 0'615 to 0'649. Any increase in this ratio may 



be taken to indicate adulteration by vegetable oils. 



This method is occasionally useful, but is rather troublesome, 

 and cannot be well recommended, except as an additional test in 

 cases of doubt. It is very important that the fat be well dried. 



The Physical Examination of Butter Fat. — The most important 

 physical properties are microscopic examination under polarised 

 light, density, refractive index, viscosity, and behaviour on 

 melting. 



Microscopic Examination under Polarised Light. — Thi& 

 method is founded on the fact that when a crystalline substance 

 is placed between two crossed Nicol prisms the light undergoes 

 rotatory polarisation ; the rays that would normally vibrate in 

 the plane, which would cause total reflection, are caused to 

 vibrate in a plane inclined to this, and the light consequently 

 passes through the second Nicol prism. Substances which have 

 no crystalline structure do not cause any interference with the 

 plane of vibrations. 



This method was first applied by Campbell Brown to detect 

 adulteration of butter with foreign fat. The fat of milk when 

 churned into butter is devoid of crystalline structure. The fats 

 of which margarine is composed, having been melted and cooled, 

 usually acquire a more or less pronounced crystalline form. 



It has been studied by Taylor, Pizzi, and others, and is fairly 

 reliable. The following are the sources of error : — The presence 

 of salt, salicylic acid, and other crystalline substances added to 

 butter as preservatives, or accidentally mixed with it, will cause 

 the light to pass, and may be mistaken for crystalline fat ; but 

 a simple microscopical examination will usually reveal the nature 

 of particles of this nature, and an experienced observer will 

 rarely be misled. Butter which has been melted, re-emulsified, 

 and rechurned wiU behave to this test as margarine, though no 

 similar appearance is noticed in butter which has been kept just 

 below the melting point for some length of time. Margarine 

 which has been prepared by emulsifying the fat with skim milk 



