XIII] FRUITS AND FRUIT PRODUCTS 181 



MALIC ACID. 



25 Method I.— Tentative. 



(For fruit juices and similar products containing no tartaric acid and not 

 over 15% of sugars and in which the color does not interfere 

 with polarization.) 



Filter the sample, if necessary to secure a solution which can be readily polar- 

 ized, and polarize with white light and a dichromate cell, using a 200 mm. tube if 

 possible. 



If the sample contains free mineral acid, transfer a measured portion (75 cc. is a 

 convenient volume) to a 100 cc. graduated flask, add enough standard alkali, cal- 

 culated from the acidity as determined in 9, to neutralize the total acidity, dilute 

 to the mark, mix well and filter. If no free mineral acids are present, it is unneces- 

 sary to neutralize the sample. If neutralized, proper correction must be made for 

 dilution in making the final calculation. 



Transfer 25 cc. of the sample, or a neutralized solution, to a flask graduated at 25 

 and 27.5 cc, add about 2.5 grams of powdered uranyl acetate, and shake vigorously 

 at frequent intervals for 3 hours, keeping the mixture well protected from light. 

 If all of the uranyl acetate dissolves, add more so that a small amount remains 

 undissolved at the end of 3 hours. Dilute the solution to the 27.5 cc. mark with 

 saturated uranyl acetate solution, mix well and filter, if necessary, through a folded 

 filter. Polarize, if possible, in a 200 mm. tube. If the solution is too dark to polarize 

 in a 200 mm. tube, a 100 or 50 mm. tube may be used. Multiply the reading by 1.1 

 to correct for the dilution. 



Multiply the algebraic difference in degrees Ventzke between the 2 readings cal- 

 culated to the basis of a 200 mm. tube by the factor 0.036 to obtain the weight of 

 malic acid in the sample in grams per 100 cc. 



Make all polarizations at the same room temperature with white light and with 

 a dichromate cell. Make at least 6 readings in each case and take an average of 

 these. 



In the case of dark colored fruit juices which cannot be polarized readily, approxi- 

 mately quantitative results may be obtained by adding to the solutions a few drops 

 of bromin, shaking thoroughly and filtering just before polarization. 



Method II. — Tentative. 



(Approximate determination for fruit juices and similar products con- 

 taining no tartaric acid and more than 15% of sugars.) 



26 PREPARATION OF SOLUTION^ 



Weigh out 25 grams of the sample and transfer to a 600 cc. beaker with a little 

 95% alcohol by volume. Add alcohol a little at a time until 200 cc. have been added, 

 stirring the mixture well, and warming, if necessary, to insure solution of all alcohol- 

 soluble substances. Filter on a BUehner funnel, using suction, and thoroughly 

 wash the precipitated pectins and insoluble matter with 95% alcohol, disregarding 

 any slight turbidity which may appear in the filtrate after the washings have 

 been added. From 9, calculate the amount of N/4 barium hydroxid required 

 nearly to neutralize the acidity in the 25 grams of sample taken. To the com- 

 bined filtrate and washings in an Erlenmeyer flask add the calculated quantity of 

 barium hydroxid solution, stir until reaction is complete and then add 3-5 drops, or 

 more if required, of 50% barium acetate solution to insure an excess of barium. 

 Make up the volume of the mixture to about 375 cc. (not less) with alcohol, and re- 

 flux until the precipitate settles readily after being shaken. This may require 3—1 



