80 METHODS OF ANALYSIS. 



12. Dextrin. Provisional. 



Dissolve 10 grams of the sample in a 100 cc flask, add 20 mg of potassium 

 fluorid, and then about one-quarter of a cake of compressed yeast. Allow the 

 fermentation to proceed below 25 C. for two or three hours to prevent 

 excessive foaming, and then place in an incubator at a temperature of from 27 

 to 30 C. for five days. At the end of that time, clarify with lead subacetate 

 and alumina cream, make up to 100 cc, 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 polariscope having the Ventzke scale be used 

 and a 10 per cent solution be 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 may be used : 



CXIOO 

 Percentage of dextrin = 198XLxW 



in which 



C=degrees of circular rotation. 

 L length of tube in decimeters. 

 W=weight of sample in 1 cubic centimeter. 



13. Alcohol Precipitate. Provisional. 



Evaporate 100 cc of a 20 per cent solution of jelly (see 1 (a), p. 77), 

 diluted sirup, or of the washings from the determination of insoluble solids, to 

 20 cc ; add slowly and with constant stirring 200 cc of 95 per cent alcohol and 

 allow the mixture to stand over night. Filter and wash with 80 per cent alcohol 

 by volume. Wash this precipitate off the filter paper with hot water into a 

 platinum dish ; evaporate to dryness ; dry at 100 C. for several hours and 

 weigh; then burn off the organic matter and weigh the residue as ash. 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 some of the salts 

 of the organic acids have been brought down. Titrate the water-soluble por- 

 tion of this ash with tenth-normal acid, as any potassium bitartrate precipitated 

 by the alcohol can thus be estimated. 



The general appearance of the alcohol precipitate is one of the best indica- 

 tions as to the presence of glucose and dextrin. 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. 



14. Tartaric, Citric, and Malic Acids (Schmidt-Hiepe Method Modified). <* 



Official. 



Use the filtrate from the alcohol precipitate in this determination. After 

 evaporating the alcohol and taking up the acids with water add lead subacetfrte 

 until the solution is alkaline, then filter and wash the precipitate until only a 

 slight amount of lead remains in the washings. Wash the precipitate off the 

 filter paper into a beaker with hot water, precipitate the lead by hydrogen 

 sulphid, and filter off the lead sulpbid while hot, washing with hot water. 

 Evaporate the filtrate which contains the free organic acids to about f>o <<. 

 neutralize with potassium hydroxid. add an excess of strong solution of neutral 



Zts. anal. Chem., 1882, 21 : 534-541. 



