NUTRITIVE VALUE OF POTATOES 7 



in ascorbic acid content between individual potatoes within the same variety 

 has been found by Ijdo (22). Effects of cooking on the ascorbic acid content 

 vary from a gain (Richardon, Davis, and Mayfield, 37), and a very slight loss 

 (Thiessen, 48; Armentano, 1; and Levy, 27) for potatoes boiled in the skins, 

 to a large loss for baked or fried potatoes (Levy, 27). Ijdo (22) reported that 

 the amount of ascorbic acid present is but little affected by the locality of pro- 

 duction, and is independent of tuber size; that there is uniform distribution 

 throughout ithe potato; and that there is practically no oxidized ascorbic acid 

 present. H^ygaard and Rasmussen (21) found that the addition of 1 percent 

 sodium chloride to the cooking water increased the retention of ascorbic acid. 

 Thiessen (48), Mayfield, et al. (31), Smith and Paterson (44), and Zilva and Barker 

 (58), found a decrease in ascorbic acid after storage. Woods (56) states that new 

 immature potatoes contain twice as much vitamin C as mature potatoes, and 

 that common storage of the mature potato for three to eight months does not 

 change the vitamin C content to any marked degree. 



Methods 



The method of Tillmans, Hirsch, and Hirsch (49), as modified by Bessey 

 and King (5) and Mack and Tressler (30), was used for the determination of 

 ascorbic acid in potato tubers. Twice normal sulfuric acid and 2 percent met- 

 aphosphoric acid were used in the extraction of the potatoes. The method of 

 Buck and Ritchie (6) was used to standardize the 2,6-dichlorophenolindophenol 

 dye solution. 



At the start of this investigation the accuracy of the dye titration method for 

 determining vitamin C in potatoes was checked against the biological method. 

 Raw, baked, and boiled potatoes were assayed for vitamin C using the guinea 

 pig method of Sherman, LaMer, and Campbell (42) as modified by Eddy (12). 

 The results of the two methods were in close agreement in all cases, thus indicating 

 the accuracy of the chemical test for this purpose. For example, 4.5 grams of 

 raw potatoes or 5.3 grams of boiled potatoes had an antiscorbutic value in the 

 guinea pig assay of 0.5 milligrams of ascorbic acid, that is 10 international units 

 of vitamin C (28). When these same samples of potatoes were examined by the 

 indophenol titration procedure, identical values were obtained. Hence, through- 

 out this investigation the dye-titration method was used. 



Eight varieties of potatoes, which were grown under similar conditions on 

 experimental plots at the Massachusetts Agricultural Experiment Station, were 

 used. These were compared with identical varieties obtained from several other 

 state agricultural experiment stations very soon after harvest. All of the Massa- 

 chusetts potatoes were dug at the same time the first part of October. 



Dehydroascorbic Acid in Potatoes 



Since an oxidized, but still physiologically active form of ascorbic acid, called 

 dehydroascorbic acid, is sometimes present in plant tissues, eight samples of raw 

 potatoes were treated with hydrogen sulfide for 30 minutes after extraction in 

 order to reduce any oxidized ascorbic acid which might be present. This was 

 followed by a two-hour treatment with carbon dioxide to remove all traces of 

 hydrogen sulfide. An ascorbic acid value of 0.20 milligram per gram, both 

 before and after this treatment, indicated that there was no dehydroascorbic 

 acid present in the raw potato. These findings are in agreement with those 

 of Ijdo (22) and Rolf (39). 



