XIII] FRUITS AXD FRUIT PRODUCTS 179 



11 FREE MINERAL ACIDS.— TENTATIVE. 

 Proceed as directed under XIX, 26, 27 or 28. 



12 PROTEIN.— OFFICIAL. 



Proceed as directed under I, 18, 21 or 23, using 5 grams of jell}' or other fruit 

 product containing a large amount of sugar, or 10 grams of juice or fresh fruit and 

 a larger quantity of the sulphuric acid if necessary for complete digestion. Multi- 

 ply the percentage of nitrogen by 6.25 to obtain the percentage of protein. 



SUCROSE. 



13 By Polarization. — Official. 



Determine by polarizing before and after inversion, as directed under IX, 22 or 23. 



14 By Reducing Sugars Before and After Inversion. — Tentative. 

 Proceed as directed under VIII, 18. 



15 REDUCING SUGARS.-TENTATIVE. 



Proceed as directed under VIII, 25, expressing the results as invert sugar. 



16 COMMERCLA.L GLUCOSE.— TENTATIVE. 

 Proceed as directed under IX, 25. 



1 7 DEXTRIN.— TENTATIVE. 



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

 and then about one fourth of a cake of compressed yeast. Allow the fermentation 

 to proceed below 2o°C. for 2-3 hours to prevent excessive foaming, and then incu- 

 bate at 27°-30°C. for 5 days. At the end of that time, clarify with basic lead acetate 

 solution and alumina cream, make up to 100 cc. and polarize in a 200 mm. tube. 

 A pure fruit jelly will show a dextro or laevo-rotation of not more than a few tenths 

 of a degree. If a polariscope having the Ventzke scale be used and a 10% solution 

 polarized in a 200 mm. tube, the number of degrees read on the sugar scale of the in- 

 strument multiplied by 0.8755 will give the percentage of dextrin, or the following 

 formula may be used: 



C X 100 



Percentage of dextrin = in which 



198 X L X W 

 C = degrees of circular rotation; 

 L = length of tube in decimeters; 

 W = weight of sample in 1 cc. 



18 ALCOHOL PRECrPITATE.-TENTATIVE. 



Evaporate 100 cc. of a 20% solution of jelly or diluted sirup, 1 (b), or of the wash- 

 ings from the determination of insoluble solids, 4, to 20 cc. ; add slowly, with constant 

 stirring, 200 cc. of 95% alcohol by volume and allow the mixture to stand overnight. 

 Filter and wash with 80% alcohol by volume. Wash the precipitate from the 

 filter paper with hot water into a platinum dish; evaporate to dryness; dry at lOCC. 

 for several hours and weigh; then burn off the organic matter and weigh the residue 

 as ash. Designate the loss in weight upon ignition as the alcohol precipitate. 



The ash should be chiefly lime and not more than 5% of the total weight of the 

 alcohol precipitate. If it is greater than this, some of the salts of the organic acids 

 have been brought down. Titrate the water-soluble portion of this ash with N'lO 

 acid, as any potassium bitartrate precipitated by the alcohol can thus be estimated. 



