14 MASS. EXPERIMENT STATION BULLETIN 354 



Bessey and King (1935) preferred the standardization of the unstable aqueous 

 solutions of ascorbic acid with iodine, using the iodine solution as a stock ref- 

 erence. Iodine solutions stored in dark glass containers at cool temperatures are 

 relatively stable; at least, they are less subject to error than the fresh ascorbic 

 acid solutions prepared daily. Lemon juice, an acid medium that protects the 

 ascorbic acid of the fruit from atmospheric oxidation, is standardized with the 

 stock iodine solution and in turn standardizes the 2, 6-dichlorophenolindophenol 

 dye. This appears to be a complicated procedure and subject to possible error, 

 but, with the small probable error of titration and the stability of the acid lemon 

 juice, the standardization of the dye solution is considered reliable. This is 

 the method finally adopted for this work. However, very recently Buck and 

 Ritchie (1938) and also Menaker and Guerrant (1938) have shown that the dye 

 is very easily and accurately standardized by the use of standard thiosulfate 

 and iodine solutions. 



Interfering Substances 



Harris and Ray (1933) contended that acidifying the sample prevents re- 

 duction of the dye by substances other than ascorbic acid and the amino acid, 

 cysteine. However, Szent-Gyorgyi and Svirbely (1932, 1933) found that when 

 titration is rapid cysteine does not interfere with the determination. It is there- 

 fore assumed that ascorbic acid can be determined in the tomato without special 

 preparation to remove possible interfering materials. 



Not all investigators are content to depend on rapid titration. Tauber and 

 Kleiner in 1935 proposed a method in which ascorbic acid is destroyed by a 

 specific enzyme. This is an alcohol-soluble oxidase found in Hubbard and 

 summer squash. The method is based on the theory that the reducing value 

 after the ascorbic acid has been destroyed is due to interfering substances. 



Crystalline ascorbic acid was used experimentally to test the accuracy of this 

 method. In brief, the results obtained were neither consistent nor accurate and 

 the method was discarded as unsuitable. 



Reversibly Oxidized Ascorbic Acid 



Tillmans, Hirsch, and Hirsch (1932) found that ascorbic acid may be oxidized 

 to a reversible form which is antiscorbutically active but is not determinable by 

 the usual chemical titration method. Tillmans, Hirsch, and Dick (1932) reduced 

 this reversible form of oxidized ascorbic acid with hydrogen sulfide, scrubbed out 

 the latter with an inert gas, and then continued the titration by the regular method. 



Using commercially canned tomato juice and the juice of raw tomatoes, ex- 

 perimental results show the absence of oxidized ascorbic acid in both products. 

 Storage of these juices for 26 hours in open containers resulted in an apparent 

 loss of 30 percent of the ascorbic acid. On reduction of the stored juices with 

 hydrogen sulfide, the actual loss of ascorbic acid was found to be only about 10 

 percent. That is, approximately 20 percent of the ascorbic acid present in these 

 juices was reversibly oxidized by standing in open containers at room tempera- 

 ture for 26 hours. 



Effect of Dissolved Iron and Tin 



Acid foods preserved in tin cans normally contain stannous and ferrous ions 

 that have been dissolved from the metal container. Since this study includes 

 preserved as well as fresh products, it is desirable to know whether these ions 



