CHAPTEE XVIII. 



THE PHYSICAL EXAMINATION OF BUTTER FAT. 



THE most important physical properties are microscopic exam- 

 ination under polarised light, density, refractive index, viscosity, 

 and behaviour on melting. 



Microscopic Examination under Polarised Light. This 

 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 will 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 

 with a good emulsor, separating the cream, and churning this 

 with ordinary cream, behaves as butter, and Pizzi has succeeded 

 in adding 30 per cent, of foreign fat to butter in this way without 

 being able to distinguish it. Finally, rancid butter, and butter 

 which has been at once churned from pasteurised cream at a 

 low temperature, may sometimes give an appearance resembling 

 margarine. Butter prepared from clotted cream shows many 

 crystalline particles (Fig. 35). 



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