386 OTHER MILK PRODUCTS. 



readings every minute till the specific rotatory power begins to 

 diminish. If the temperature at which the solution is polarised 

 is kept at 15° C, or below, there is no difficulty in obtaining 

 several readings which are nearly constant, and the mean of 

 these are taken as the initial rotation. Allow the tube to stand 

 for twenty-four hours, and polarise again at the same tempera- 

 ture ; this is the normal rotation. The initial rotation divided 

 by the normal rotation will give the " birotation ratio." The 

 amount of milk-sugar in 100 c.c. of this solution is estimated, 

 either by drying 5 c.c. at 100° C, when a residue of anhydrous 

 sugar will be left, or by deducing it from the normal rotation. 

 This is done by dividing the reading in angular degrees by 1 "106. 

 The two figures should agree closely. 



About 10 grammes of sugar are weighed out into a 100 c.c. 

 flask and boiled with about 80 c.c. of water for a few minutes. 

 The solution is cooled to 20°, made up to 100 c.c, and polarised 

 in a 200 mm. tube. The reading in angular degrees multiplied 

 by 100 and divided by the weight of sugar taken multiplied by 

 1 05 will give the percentage of mUk-sugar in the sample. 



Five grammes are weighed out in a platinum basin, ignited 

 over a moderate flame and the ash weighed. 



Ten grammes are dissolved in 100 c.c. of milk ; this is brought 



to the boil ; the milk should not be curdled. If it is, the acidity 



should be estimated by titrating 5 grammes dissolved in water 



N 

 with Y^^ alkali and calculated as lactic acid. 



To a solution in water (10 per cent.) a little mercuric nitrate 

 is added ; the solution should not show more than the faintest 

 turbidity. 



Good commercial milk-sugar crystallised from water should 

 give the following figures : — 



Milk-sugar per cent., . 99-6 to 99-9. 



Birotation ratio, . . .1-6, 



ITall of temperature, . . 0-5° C. 



Solubility at 15° C, . . 7-0 grammes per 100 c.c. (anhy- 



drous sugar), each 1° increase 

 of temperature raises this figure 

 about 0-1 gramme jjer 100 c.c. 



Ash, ..... not more than 0-05 per cent. 



Milk-sugars which have been precipitated with alcohol usually 

 polarise slightly over 100 per cent. ; have a birotation ratio 

 below 1-6 and, usually, above 1-5; cause a slightly greater 

 rise of temperature ; and have a rather higher solubility in 

 water. 



Detection of Adulteration. — Milk-sugars which are adulter- 

 ated with other sugars will show marked divei-gence from the 



