154 MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



has been found rapid and very satisfactory for both fresh and de- 

 hydrated products. 



Apparatus: Blender, analytical balance, volumetric flasks (50 ml.), burette 

 (10 ml.). 



Reagents : Ethyl alcohol, saturated solution of neutral lead acetate, saturated 

 solution of disodium phosphate, dilute hydrochloric acid (mix 100 ml. concen- 

 trated HCl with 1,100 ml. water), sodium hydroxide solution (24 gm. NaOH 

 made to 100 ml. of solution with distilled water), alkaline potassium ferricy- 

 anide solution (8.75 gm. K 3 Fe(CN) 6 plus 10.6 gm. anhydrous sodium carbonate 

 made to 1,000 ml. with distilled water and stored in refrigerator), sulfuric acid 

 solution (5 N), standardized eerie sulfate solution (0.01 N), phenolphthalein (0.1 

 percent in 90-percent alcohol), S'etopalin C indicator ^0.1 percent in water), 

 Mohr's salt solution (0.01 N), dextrose solution (1 percent in 0.25-percent 

 aqueous benzoic acid). 



Procedure : Before the test is applied dehydrated materials must be rehy- 

 drated by heating in distilled water at 50° to 60° C. for 2 hours. An amount of 

 dehydrated material corresponding to 60 gm. of fresh carrots, 75 gm. of fresh 

 corn, or 150 gm. of fresh potatoes (or approximately equivalent amounts of other 

 products, based on their estimated sugar content) is added to' 150 ml. of water 

 plus an additional amount calculated to bring the dehydrated material to the 

 fresh basis. With fresh products only the 150 ml. of water is added. The 

 mixture, fresh or rehydrated, is thoroughly broken up by agitation for 2 to 4 

 minutes in a blender. A 15-gm. portion of the resultant suspension (keep well 

 mixed to maintain uniformity) is transferred to a 50-ml. volumetric flask with 

 a minimum amount of ethyl alcohol. Add 2 ml. of a saturated solution of 

 neutral lead acetate, mix, and add 4 ml. of a saturated solution of disodium 

 phosphate. Make up to volume by addition of water and alcohol in such pro- 

 portion as to yield a final mixture of about 50-percent alcohol. Mix thoroughly 

 and filter through a folded filter paper. 



For total sugars, transfer 5 ml. of filtrate to a 50-ml. volumetric flask. Add 

 10 ml. of the dilute hydrochloric acid. The sample can then, according to con- 

 venience, be heated in a boiling-water bath for 10 minutes, cooled, made up to 

 50 ml. with distilled water, and mixed ; or it can be allowed to stand 24 hours at 

 room temperature (approximately 25° C), made to mark with distilled water, 

 and mixed. 



Transfer 5 ml. of the solution to a 22 x 175-mm. test tube; add 1 drop of 

 phenolphthalein and the NaOH solution until the pink color develops. Add HCl 

 until pink color disappears. Use a dropping tube for additions of NaOH and 

 HCl, as only a few drops of each are required. 



Add 10 ml. of the alkaline potassium ferricyanide solution and heat in a 

 boiling-water bath 15 minutes. Cool and then add 8 ml. of 5N H 2 S0 4 . Add 

 7 to 10 drops of S'etopalin C indicator and titrate with eerie sulfate solution from 

 a 10-ml. burette. The end point is indicated by the sharp change from greenish 

 yellow to golden brown. 



Ceric sulfate may be standardized against 0.01 N solution of Mohr's salt. An 

 exact dextrose equivalent can be obtained with 5-ml. aliquots of dilute dextrose 

 solution made up from the 1 percent stock solution by dilution so that 5 ml. 

 contains 2.5 mg. of dextrose. It is advisable to standardize the ceric sulfate with 

 the dextrose solution at least once a month. If it is more convenient, pure sodium 

 oxalate may be used for the standardization instead of dextrose. In this instance 

 the equivalent weight of invert sugar is 33.8 gm. and of sucrose 32.1 gm. 



It is usually advisable to run titrations in duplicate and to take the average. 

 Generally the agreement is within 0.05 ml. of the ceric sulfate. A blank run 

 should be made on alcohol and the reagents added as in a regular determination. 

 This value, usually 0.1 to 0.2 ml. of ceric sulfate, is subtracted from the values 

 for the samples to obtain true values. By means of the dextrose value of the ceric 

 sulfate as found by titration of pure dextrose, and the amount of aliquot as run, 

 the percentage of sugar, expressed as dextrose, is readily found. The aliquots 

 titrated should not have more than about 7.0 mg. of dextrose. 



