264 THE CHEMICAL CONTROL OF THE DAIRY. 



fine muslin or swandown is always practised in dairies ; this 

 removes the grosser impurities — i.e., hairs, large vegetable 

 fibres, etc. — but the quantity removed in this way does not 

 exceed 0"0025 per cent. In Denmark and Germany, and in a 

 few dairies in England, filtration through layers of gravel and 

 sand is practised, though the method appears now to be dying 

 out ; this method, which adds considerably to the labour of 

 handling the milk, owing to the necessity of washing the gravel 

 and sand with caustic soda, followed by water, sterilising, and 

 drying, fails to remove appreciably more from the milk than 

 simple straining or upward filtration through muslin or swans 

 down. 



Another method which is considerably used is filtration through 

 a thin layer of cotton wool ; this method is fairly efficient, especi- 

 ally if practised as soon as possible after milking, and before 

 the particles of dirt have had time to disintegrate and yield 

 their soluble matters and micro-organisms to the milk ; a special 

 advantage of this method is that the cotton wool is very cheap, 

 and it is impossible to wash it, and, therefore, it is thrown away 

 and not used a second time. 



The separator is also used as a cleaner for milk ; for this 

 purpose the separated milk and cream are either all made to come 

 out of one outlet or they are mixed immediately after separation. 

 This method is, of course, perfectly effi.cient in removing solid 

 impurities, but it necessitates the milk being warmed and after- 

 wards cooled, and makes the milk very frothy, and may even 

 lead to incipient churning. 



Cream — Composition. — The name cream is given to the 

 layer which rises to the surface when milk is allowed to stand. 

 This layer consists essentially of the fat globules, together with 

 a proportion of the aqueous portion of milk (Fig. 37). 



Qualitatively, it has the same composition as milk ; quantita- 

 tively, it contains a higher proportion of fat, the other consti- 

 tuents being correspondingly depressed. 



It is by many accepted as a fact that cream contains a larger 

 proportion of solids not fat to water than the milk from which 

 it was derived ; and various explanations of this have been put 

 forward. Thus a membrane round each fat globule has been 

 alleged to exist by some {e.g., Storch and Bechamp) ; others have 

 considered that the proteins are concentrated in the aqueous 

 layer formed round each globule by surface tension. The author's 

 experiments have indicated that the ratio of solids not fat to 

 water in cream is the same as that in milk, and Weibull and 

 Smith and Leonard have confirmed this conclusion. It is true 

 that in some cases a distinctly higher ratio has been found, but 

 it has been noticed that in these cases ample opportunity for 



