78 PEG VISIONAL METHODS FOR ANALYSIS OF FOODS. 



12. POLARIZATION. 



Dissolve half the normal weight of jelly or other fruit product, or the normal 

 weight of juices or fresh fruits, in a sufficient quantity of water in a 100-cc sugar 

 flask, add an excess of lead subacetate (from 5 to 10 cc, see footnote, p. 84), filter, and 

 polarize in a 200-mm tube, observing the temperature of the solution. Invert 50 cc 

 of this solution using 5 cc of hydrochloric acid and heating to 68 C. in 15 minutes. 

 Polarize in a 220-mm tube at the same temperature as was employed in making the 

 direct reading. 



On account of the large amounts of invert sugar usually found in these products 

 it is necessary that the direct and invert readings should be made at the same tem- 

 perature. 



13. DETERMINATION OF CANE SUGAR. 



(a) BY POLARIZATION. 



Calculate cane sugar from the direct and the invert readings according to Clerget's 

 formula: 



g 100 (a -b) 

 144- 1 



(b) BY INVERSION. 



Where only a small amount of cane sugar is present it is best determined by calcu- 

 tion from the increase in reducing sugars after inversion. For this purpose treat 

 5 grams of jelly, sirup, or other fruit product, or 25 grams of juice or fresh fruit with 

 lead subacetate in excess, and after making up to 100 cc and filtering invert 50 cc in 

 a 100-cc flask with 5 cc of hydrochloric acid. After inversion neutralize the acid 

 w r ith sodium hydroxid, precipitate excess of lead with sodium sulphate and increase in 

 volume to 100 cc. Filter and dilute so that the solution does not contain more than 

 1 per cent of reducing sugar. The per cent increase in reducing sugar after inversion 

 multiplied by 0.95 equals per cent of cane sugar. 



14. DETERMINATION OF REDUCING SUGARS. 



Treat 5 grams of jelly (25 cc of a 20 per cent solution [2 (a)] may be employed), 

 sirup, or other fruit product, or 25 grams of juice or fresh fruit with lead subacetate 

 in excess (2 to 5 cc); make up to 100 cc and filter. Transfer from 25 to 50 cc 

 depending upon the per cent of reducing sugar present to a 100-cc flask and add a 

 saturated solution of sodium sulphate in sufficient amount to precipitate the excess of 

 lead; complete the volume to 100 cc and use the filtered solution for the determina- 

 tion of reducing sugars. The approximate amount of reducing sugar present may be 

 readily ascertained from the polarizations and from the percentage of solids. Use 

 Allihn's method for the determination (p. 49). a 



15. DETERMINATION OF DEXTRIN. 



Dissolve 10 grams of the sample b in a 100-cc flask; add 20 mg of potassium fluorid 

 and then about one-quarter of a cake of compressed yeast. c Allow the fermentation 

 to proceed below 25 C. for 2 or 3 hours to prevent excessive foaming, and then place 

 in an incubator at a temperature of from 27 to 30 C. for 5 days. At the end of that 

 time, clarify with lead subactetate 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 



U. S. Department of Agriculture, Division of Chemistry, Bulletin 46 revised, page 35. 



b ln the case of jellies, 50 cc of a 20 per cent solution, prepared as directed under 1 (a), may be used. 



e Bigelow and McElroy. Jour. Am. Chem Soc. 1893, 16, 668. 



