CHAPTER VII. 



THE ESTIMATION OF TOTAL SOLIDS AND ASH. 



THE total solids of milk are estimated by evaporating the water 

 and weighing the residue. 



Wanklyn's Method. Wanklyn proposed limiting the time 

 of drying to three hours at the temperature of boiling water ; 

 he weighed 5 grammes in a platinum basin, kept it for three 

 hours on a briskly boiling water-bath, and, after cooling in a 

 desiccator, weighed the residue. This method has now fallen 

 entirely into disuse, as the residue thus obtained still contained 

 a quantity of water, which could be driven off by further 

 evaporation. 



Method of Society of Public Analysts. A very obvious 

 modification of this is to continue the drying on a water-bath or 

 in an oven at 100 C. till the weight is constant. This method 

 has been adopted by the Society of Public Analysts. It is not, 

 however, possible to attain absolute constancy, as a decomposition 

 due to heating takes place, and the weight continually diminishes 

 slightly on further heating ; for this reason the weight is 

 usually considered constant when less than 1 milligramme per 

 hour is lost on further drying. The method gives very good 

 results, and duplicates agree closely ; but it is doubtful whether 

 the results represent accurately the true total solids of the milk. 

 First, an absolutely constant weight is not attained ; and, next, 

 the residue is markedly brown, indicating decomposition ; the 

 point taken as constancy is really a point where it may be assumed 

 that the bulk of the water is driven off, and comparatively little 

 decomposition has taken place. 



The author has found by taking smaller quantities of milk 

 about 1 gramme and spreading this over a large surface 

 during evaporation, that a nearly white residue is obtained, and 

 constancy of weight can be attained. It appears probable that 

 the decomposition to which the browning is due takes place 

 during the heating of the milk before evaporation is concluded. 

 In support of this view it may be noted that Cazeneuve and 



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