12 MASS. EXPERIMENT STATION BULLETIN 354 



ADAPTATION OF TITRATION METHOD TO THE DETERMINATION OF 

 ASCORBIC ACID IN TOMATOES AND TOMATO PRODUCTS < 



Method of Extraction 



Harris and Ray (1933) found that enzymatic oxidation of ascorbic acid takes 

 place in neutral solutions. At the normal pH of most plant materials, this also 

 occurs when the tissue cells are macerated and exposed to the air. They suggested 

 that the method of Tillmans, Hirsch, and Hirsch (1932) be modified by lowering 

 the pH of the medium to 2.5 by the addition of strong acetic acid. This acidifica- 

 tion reduces enzymatic oxidation of ascorbic acid. Bessey and King (1935) 

 found that trichloroacetic acid is preferable to acetic acid as an extracting medium. 



The comparative efficiencies of hot 8 percent acetic and 3 percent trichloroacetic 

 acids for extracting ascorbic acid from tomato tissue were determined, with 2, 

 6-dichlorophenolindophenol dye as the oxidizing agent to determine the amount 

 of ascorbic acid extracted. The more constant and representative results were 

 obtained by using trichloroacetic acid. The average maximum variation on five 

 samples of tomato juice extracted with trichloroacetic acid was 1.89 percent and 

 for hot 8 percent acetic acid was 5.24 percent. 



Fujita and Iwatake (1935) have demonstrated the advantage of 2 percent 

 metaphosphoric acid for the extraction of ascorbic acid. The advantage is the 

 elimination of the slow oxidation of ascorbic acid which occurs when trichloro- 

 acetic acid is the sole acidifying medium. Musulin and King (1936) and Mack and 

 Tressler (1937) indicated consistent results with a mixture of 3 percent trichlo- 

 roacetic acid and 2 percent metaphosphoric acid. This combination has the 

 advantages of both acids as an ascorbic acid extracting medium. With this 

 mixture, the cellular structures in some tissues are disrupted more rapidly 

 b}' trichloroacetic acid while the metaphosphoric acid prevents the slow oxidation 

 by the trichloroacetic acid and also exerts a better deproteinizing action. 



The substitution of IN, 2N, 3N, or 4N sulfuric acid for trichloroacetic acid 

 in the acid mixture was suggested by Thornton (1938) after the present investiga- 

 tion had been completed. Basing his work on that of Mack and Tressler (1937), 

 he finds that only strong acids inactivate the ascorbic acid oxidizing enzymes 

 found in plant tissue. Also, suitable concentrations of sulfuric acid eliminate the 

 necessity of later treating the tissue extract with hydrogen sulfide to recover 

 ascorbic acid which may have been oxidized. 



Number of Extractions 



The usual method of extracting ascorbic acid from plant tissue consists of a 

 contact period with the acid mi.xture, separating the color by centrifugal force, 

 and decanting the clear serum. It is desirable to know whether one extraction 

 of this serum is sufficient to obtain all of the ascorbic acid present in the tomato. 

 The residual ascorbic acid extracted from 24 samples of tomatoes on making a 

 second extraction was 8.3 percent. A third extraction, although usually imprac- 

 tical on routine samples, showed residue of about 1.5 percent. 



A somewhat higher ascorbic acid content was found in tomato juice after one 

 extraction. The average increase in 8 juice samples was 15.6 percent. 



*For details of experimental methods and results see Maclinn (1938). 



