DETERMINATION OF ALCOHOL PRECIPITATE. 21 



time clarif 3^ with lead subacetate and alumina cream," make up to 100 cc, 

 filter, and polarize in a 200 mm tube. A pure fruit jelly will show a 

 rotation of not more than a few tenths of a degree either to the right 

 or to the left. If a Schmidt and Haensch polariscope be used and a 10 

 per cent solution is polarized in a 200 mm tube, the number of degrees 

 read on the sugar scale of the instrument multiplied by 0.8755 will 

 give the percentage of dextrin; or the following formula maybe used; 



Percentage of dextrin = ^^^^Lx^^ 



in which — 



C = degrees of circular rotation. 



V = volume in cubic centimeters of solution polarized. 

 L = length of tube in centimeters. 

 W = weight of sample in solution in grams. 



DETERMINATION OF ALCOHOL PRECIPITATE. 



Evaporate to 20 cc 100 cc of a 20 per cent solution of jelly, or 200 cc 

 of the washings from the determination of insoluble solids; then add 

 slowh^ and with constant stirring 200 cc of 95 per cent alcohol by vol- 

 ume, and allow the mixture to stand overnight. Filter and wash with 

 80 per cent alcohol b}^ volume. Wash this precipitate ofi' the filter 

 paper with hot water into a platinum dish; evaporate to dr^^ness; dry 

 at 100° C. for several hours and weigh; then burn the organic matter 

 and weigh the residue as ash, after treating with ammonium carbonate 

 as directed under determinations of ash (p. 13). The loss in weight 

 upon ignition is called alcohol precipitate. 



The ash should be largely lime, and not more than 5 per cent of the 

 total weight of the alcohol precipitate. If it is larger than this it is 

 due to the presence of salts of the organic acids. Titrate the water- 

 soluble portion of this ash with decinormal acid, as any potassium 

 bitartrate precipitated by the alcohol can thus be estimated. 



The general appearance of the alcohol precipitate of fruit products 

 is one of the best indications as to the presence of glucose. Upon the 

 addition of alcohol to a pure fruit product a flocculent precipitate is 

 formed with no turbidity, while in the presence of glucose a white 

 turbidity appears at once upon adding the alcohol, and a thick, gummy 

 precipitate forms. In fresh fruit juices there is often a marked 

 turbidity, closely resembling the precipitate given by glucose, which 

 is caused by the starchv matters present. 



''Wiley, Principles and Practice of Agricultural Analysis, Vol. Ill, p .100, gives the 

 following method for preparation of alumina cream : A solution of alum is treated with 

 ammonium hydroxid and the precipitate washed until it is freed from ammonia. 

 The hydroxid is then suspended in water in proportions necessary to produce a 

 creamy liquid. 



