156 MISC. PUBLICATION 5 40, U. S. DEPT. OF AGRICULTURE 



at 70° to 80° O. for 30 minutes. This should give a starch that is reasonably 

 pure. Analyses can be made to establish its purity if such accuracy is desired. 

 Note : If approximately half of the water is added to the sample in the blender 

 and the instrument run at low speed controlled with a rheostat until the sample 

 is well disintegrated, and then the remainder of the water is added and the 

 blender gradually run faster until maximum speed is reached, a more nearly 

 homogeneous product is obtained. The volumes of the first and second dilutions 

 depend upon the starch content of product and the internal diameter of the 

 colorimeter tubes. 



Determination of Sulfur (Dried Fruits and Vegetables) 



It is now common practice to treat cabbage with a solution of sodium 

 sulfite or a mixture of the normal sulfite and bisulfite prior to dehy- 

 dration. Future specifications may provide for similar treatment of 

 other vegetables (carrots, potatoes, and possibly others) with sulfite 

 solutions and control of this operation involves the determination of 

 the sulfite in the dehydrated products. Dried and dehydrated fruits 

 are commonly exposed to sulfur-dioxide gas in sulfuring chambers 

 before sun-drying or dehydration. The Monier- Williams procedure 

 (4, pp- 463-465) can be used satisfactorily for both sulfured fruits and 

 vegetables. It is a distillation method and is time-consuming and not 

 especially well adapted to field work where a minimum of equipment 

 is desired. Kecently a direct titration method has been devised by 

 Prater and others (39a) especially for use on dehydrated foods. The 

 method as applied to dehydrated cabbage follows. Little modifica- 

 tion is necessary for other vegetables or fruits but reference to the 

 discussion in the paper by Prater and others (39a) is desirable before 

 undertaking the assay of such materials. 



Apparatus: Blender, analytical balance, burette (50 ml.), flasks (1-liter Erlen- 

 meyer), graduated pipettes (10 ml.). 



Reagents: Sodium hydroxide (5 N), hydrochloric acid (5 N), acetone, starch 

 solution (1 percent soluble), iodine solution (0.05 N). 



Procedure : The shredded sample should be ground dry in the blender im- 

 mediately before the test is begun. From the ground material two 8-gm. samples 

 are weighed (to 10 mg.) and labeled A and B. Each sample is transferred to a 

 1-liter flask and 400 ml. of water and 5 ml. of 5 N sodium hydroxide added. 

 The mixtures are allowed to stand for 20 minutes with occasional shaking. 

 Each is acidified with 7.5 ml. of 5 N hydrochloric acid, which should reduce the 

 solution to a pH of about 2.0. To sample B, 40 ml. of acetone is immediately 

 added and the mixture let stand for 10 minutes. To sample A, 10 ml. of 1-percent 

 soluble starch solution is added and the sample titrated to the starch end point 

 with 0.05 N iodine solution. After the required 10 minutes, sample B is titrated 

 in the same manner. 



In order to obtain the blank values for the reagents, titrations A arid B are 

 repeated without the cabbage and the respective blanks subtracted from the first 

 titrations for A and B. Blank determinations need be run only once a day or 

 when fresh solutions are employed. The sulfur dioxide in parts per million is 

 obtained by multiplying the corrected titration for A, minus that for B, by 20O. 



The inexperienced analyst will find it desirable to run a preliminary titration, 

 adding the iodine rapidly, about 0.5 to 1.0 ml. at a time to determine the 

 approximate end point. In titration A, with a titer of 4 to 15 ml., a double 

 end point may be observed, the first where the whole solution becomes blue 

 but fades rapidly. The second is. permanent for several minutes, and is obtained 

 on addition of 0.5 to 1 ml. more iodine solution. A double end point is also 

 observed with sample B, but both are much less permanent, owing to slow 

 liberation of S0 2 from the acetone. It is recommended that in each case the 

 first or flash end point be taken. Titration A yields total S0 2 , both free and 

 bound, and other reducing substances. Titration B yields only the other re- 

 ducing substances, because a complex is formed by the acetone and S0 2 which 

 does not immediately react with the iodine, but only gradually, and on standing. 

 The acetone concentration must be controlled, because an unsatisfactory end 



