1969]

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ORGANIC ACID, AMINO ACID, AND ASCORBIC ACID C O N T E N T O F P O T A T O E S AS A F F E C T E D B Y STORAGE CONDITIONS J. P. SWEENEY, P. A. HEPNER, AND S. Y. LIBECK 1 ABSTRACT

A study of the changes in organic acids and amino acids occurring during storage of tubers at 70 and 55 F was made on several varieties grown in 1965 and 1966. Citric acid tended to increase and malic acid usually decreased. Pungo potatoes showed decreases in aspartic and glutamic acids and increases in several other amino acids. Increases or decreases in both organic and amino acids were usually greater in potatoes stored at 70 F than those stored at 55 F. Temperature of storage appeared to have a greater effect on organic and amino acids than would be indicated by results of previous workers. Considerable amounts of pyroglutamic acid were found in some potato varieties. There were significant decreases in aseorbie acid in potatoes stored at both 70 F and 55 F. RESUMEN

Se hizo un estudio de los cambios en/tcidos org~nicos y amino ~cidos que tienen lugar en tub6rculos almacenados a 70 y 55 F de algunas variedades cultivadas en 1965 y 1966. Acido citrico tendia a aumentar y el ~icido m~iIico generalmente disminuia. Papas Pungo dieron indicacioues de disminuci6n en ficidos asp~rtico y glut/tmico y de incrementos en varios otros amino /tcidos. Tanto los incrementos como las disminuciones en ~.cidos orgfi.nicos y anfino ~icidos fueron generalmente mayores en papas almacenadas a 70 F q u e e n las almacenadas a 55 F. La temperatura de almacenamiento pareci6 tener un efecto myor sobre los ~icidos org~micos y amino ~tcidos del indicado por los resultados obtenidos por los investigadores hasta la fecha. Cantidades considerables de fi~cido piroglufftmico fueron encontradas en algunas variedades de papa. H u b o una disminuci6n significante en /tcido asc6rbico en papas almacenadas tanto a 70 F como a55F. The preservation of the palatability and nutritive value of potatoes during storage depends to a considerable extent on storage conditions. Studies of changes in composition of potatoes may provide an insight into storage problems. Previous investigations of compositional changes during storage have dealt chiefly with carbohydrates and total solids (Schwimmer et al 15), or with nitrogenous matter (Cotrufo and Levitt 5). An indication of metabolic changes occurring in potatoes is provided by a study of organic acid and amino acid content. Organic acids are known to play a central role in metabolism (Burris 4) and may also be involved in nonenzymic browning reactions and after-cooking discoloration (Heisler et al 8). Amino acids are important because of their nutritive value and because they may be involved in nonenzymic browning (Talley et al 16). 1Human Nutrition Research Division, Agricultural Research Service, U. S. Department of Agriculture, Beltsville, Maryland 20705. Accepted for publication April 28, 1969.

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Except for ascorbic acid, the organic acids in potatoes have not been extensively studied. Schwartz et al (14), working with Maine Katahdin potatoes stored eight months at 38 F, followed by one month at 45 F and an additonal month at 50 F, have related several organic acids to specific gravity and storage time and presented evidence for the conversion of malic acid to citric acid during storage. tteisler (8) found the stem end of several potato varieties to be lower in organic acids than the bud end and reported an association of organic acid content to after-cooking discoloration. Minina (10) investigated the concentrations of both citric and malic acids during 7 months' storage. H e concluded that there was an initial decrease in citric acid content followed by an increase, and a decrease in malic acid toward the end of the storage period. Very little information is available on changes in free amino acids during storage. Talley et al (16) working with the same potato extracts used by Schwartz et al. (14) have reported an increase in proline to be the only significant change in potatoes stored at 38 F and classified according to specific gravity. Potatoes are generally stored in the home at approximately 70 F. Investigations have revealed that potatoes stored at 50 to 60 F have better color, texture and flavor when cooked or made into chips than do those stored at lower temperatures (Heinze et al. 7). But commercially potatoes are usually stored at temperatures near 40 F in order to prevent sprouting over a period of several months. Much of the previous research on organic and amino acid content has been carried out on potatoes stored at 38 F. Storage changes at higher temperatures might be expected to differ from those obtained at 38 F. The work described here was carried out to obtain more information regarding changes in organic acids and amino acids during storage at 70 and 55 F. Possible relationships between increases in organic acid content and increases or decreases in amino acid content were also considered. MATERIALS AND METHODS Potato varieties studied in 1965 were Pungo from Virginia, Rosemont Cobbler from Minnesota and Russet Burbank fronl Washington. In 1966 Pungos were obtained from Virginia, and Irish Cobblers from Minnesota. The Pungo potatoes were transported to the laboratory on the day of harvest. Other varieties investigated were received in the laboratory from 7 to l0 days after harvest. Determinations were nlade on the potatoes when received, after 1 month at 70 F, and after periods of 2 and 5 months at 55 F. Changes during storage were investigated by determination of titratable acids, citric, malic and pyroglutamic acids and ascorbic acid. Amino acids were determined only on the Pungos grown in 1966. Preparation of sample extracts: Sample extracts were prepared by blending 50 g samples, taken from 15 to 20 pared potatoes, with 200 ml cold 80% ethyl alcohol. An electric blender was used. These extracts were filtered and stored at 0 F until used for determination of organic and titratable acids.

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Non volatile organic acids : A 50 ml aliquot for analysis was evaporated under reduced pressure to remove alcohol. The sample was passed through Dowex 50 W - X 8 ( H + form, 20 to 50 mesh) and diluted to 200 ml. A 50 ml aliquot was chromatographed on Dowex l - X 8 (formate form, 200-400 mesh), and eluted by gradient elution with formic acid. Acids determined were citric, malic and pyroglutamic. The procedure used was a modification of that of Palmer (11) and Hulrne and Wooltorton (9). Titratable acids: The alcohol was removed from a 50 ml aliquot of the sample extract, the sample rediluted with water and titrated to p H 8, as determined with a p H meter. Ascorbic acid: The indophenol titration procedure of Bessey and King (3) was used for ascorbic acid determination. Amino acids and nitrogen: Amino acids and nitrogen were determined by a commercial laboratory. Extracts for amino acids were prepared by blending 100 g potatoes with 190 ml 100% ethyl alcohol. The mixture was filtered and the pulp reextracted four times with 70% alcohol. The extract and washings were combined and diluted to 1000 ml with 70% alcohol. The chromatographic procedure of Moore et al (12) was used for amino acids. Nitrogen was determined as described in Association Official Agricultural Chemists (1). Total solids: A procedure similar to that described in Association Official Agricultural Chemists (2) was used for total solids determination. Samples were heated to constant weight in a vacuum oven at 70 F. Specific gravity: Specific gravity was determined by loss of weight in water. RESULTS AND DISCUSSION

Non volatile organic acids The potatoes stored 1 month at 70 F tended to show an increase in citric acid. Malic acid generally decreased (Table 1). In potatoes stored at 55 F citric acid content also tended to increase while that of malic acid usually decreased. However, changes in acid content at 55 F were not as large as at 70 F. Citric was the predominant acid at all times. These results differ somewhat from those obtained by Schwartz et al. (14), who worked with potatoes stored at lower temperatures than those described above. They reported that potatoes stored at 38 F, followed by storage at 45 and 55 F had an initial decrease in citric acid, followed by an increase. Malic acid followed a reverse pattern, first increasing and then decreasing. Raising the storage temperature, they reported, made no significant difference in the pattern of organic acid change. Schwartz et al. (14) have speculated on a possible interconversion of malic and citric acids. Our results on potatoes stored at 70 F show generally decreases in malic acid and increases in citric acid. However, the citric acid changes at both 55 and 70 F were too large to be accounted for entirely by malic acid changes. Also for the 1966 Pungos there were significant increases in both malic and citric acids during the same storage period. This would seem to indicate that factors other than citric-malic interconversion were also involved.

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[ Vol. 46

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S~,VEENEY, et al:

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Pyroglutamic acid content did not appear to follow any regular pattern. In the Pungo potatoes it was present in considerable amounts after 2 months storage and declined thereafter. In the other varieties the amount present generally increased with storage time. Schwartz et al. (14) have reported consistent decreases in this acid with storage time of potatoes at 38, 45 and 50 F. Several workers have postulated that this acid may be formed from glutamine. Titratable Acids

Titratable acids tended to increase in potatoes stored 1 month at 70 or 2 months at 55 F. During the storage interval between 2 and 5 months at 55 F there was usually a decrease. Since titratable acids were determined by direct titration, only free acids would be measured. In contrast, extracts for non volatile acids were passed through Dowex 50, and titration values would include salts of the acids as well as the free acids. Free amino acids:

Free amino acids content, specific gravity and total solids of Pungo potatoes harvested in 1966 are shown in Table 2. Because of the interval between sampling times there is a possibility that fluctuations in amino acids occurred that were not detected by determinations carried out after 0, 1, 2 and 5 months storage. During the first 2 months storage at 55 F there were increases in all free amino acids investigated except glutamic. During 1 month's storage at 70 F all except aspartie and glutamic acids showed increases. During the interval between 2 and 5 months storage at 55 F all amino acids except leucine decreased in amount. In general the pattern of change during 1 month at 70 F or 2 months at 55 is similar to that reported by Gee et al. (6) for foam-lnat dried tomato powders during storage. The considerable changes in amino acid content reported here are ia contrast to those reported by Talley et al. (16). Investigating potatoes stored at 38, 45 and 50 F they found that proline content appeared to increase during storage, but changes in other amino acids were quite indefinite. Increases or decreases in amino acids were of interest because of their possible relationship to increases or decreases in organic acid content of the potatoes. Results on the 1966 Pungo potatoes show that both organic and amino acid increases or decreases were generally greater at 70 than at 55 F. The results obtained would seem to show that temperature of storage has a greater effect on both organic acid and amino acid content than would appear from the work of Schwartz et al. (14) on organic acids, and that of Talley et al. (16) on amino acids. Ascorbic acid:

There were significant decreases in ascorbic acid with each storage period (Table 1). Decreases in ascorbic acid content of potatoes with storage have been reported by several investigators.

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TABLE 2.--Free amino acids, nitrogen, specific gravity and total solids in Pungo potatoes as affected by storage time and temperatures (1966). 1 As Received

Alanine ................ Arginine .............. Aspartic acid ...... Cystine 2 .............. Glutamic acid .... Glycine ................ Histidine .............. Isoleucine ............ Leucine ................ Lysine .................. Methionine ........ Phenylalanine .... Proline ................ Serine3 t ...... Threonine } Tryptophan 4 ...... Tyrosine .............. Valine .................. Nitrogen, % ...... Specific gravity .. Total solilds, %_.

Two Months 55 F

Five Months 55 F

1.01 5.00 11.38

6.71 8.29 31.97

3.94 5.88 8.96

5.53 7.99 5.59

14.71 1.36 2.96 2.90 0.87 3.92 2.10 2.11 5.38

9.80 2.62 4.80 8.01

6.29 3.35 4.37 6.13

4.01 3.54 7.40 13.82

4.33 1.62 3.08 4,00 2,68 3.50 2,39 3.69 12.68

4.92 2.43 8.12 8.95

14.54

25.13

20.29

14.73

2.22 5.96 0.15 1.088 24.0

5.27 15.27 0.24 1.089 23.6

4.55 13.04 0.19 1.095 25.4

5.38 13.99 0.20 1.089 24.9

2.57

One month 70 F

222

1Amino acids expressed as micromoles per gram potato (dry weight). 2None found. 3Serine and Threonine not separated. Calculated as serine. 4Not determined.

Specific gravity and total solids: S p e c i f i c g r a v i t y a n d t o t a l s o l i d s c o n t e n t of t h e p o t a t o e s d i d n o t a p p e a r to v a r y g r e a t l y o v e r t h e s t o r a g e i n t e r v a l s i n v e s t i g a t e d . R e s u l t s s h o w n for the 1966 Pungo potatoes (Table 2) were typical.

LITERATURE CITED 1. A.O.A.C. 1965. Official methods of analysis. Assoc. Official Agricultural Chemists. Washington, D. C. 10th ed., p. 16. 2. A.O.A.C. ~1965. Official methods of analysis. Assoc. Official Agricultural Chemists. Washington, D. C. 10th ed., p. 308. 3. Bessey, O. A. and C. G. King. 1933. The distribution of vitamin C in plant and animal tissues and its determination. J. Biol. Chem. 103, 687, 4. Burris, R. H. 1953. Organic acids in plant metabolism. Ann. Rev. Plant Physiol. 4, 91, 5. Cotrufo, C. and J. Levitt. 1958. Investigations of the cytoplasmic particulates and proteins of potato tubers. IV. Nitrogen changes associated with emergence of potato tubers from the rest period. Physiol. Plantarum 11, 240. 6. Gee, M., R. P. Graham and A. I. Morgan. 1967. Storage changes in the free amino acids of foam-mat dried tomato powders. J. Food Sci. 32, 78. 7. Heinze, P. H., M. E. Kirkpatrick and E. F. Dochterman. ,1955. Cooking quality and compositional factors of potatoes of different varieties from several commercial locations. Tech. Bull. No. 1106. U.S. Dept. Agr. 69 pp.

1969] 8. 9. 10. 11. 12. 13. 14. 15. 16.

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469

Heisler, E. G., J. Siciliano, C. F. Woodward and W. L. Porter. 1964. Aftercooking discoloration of potatoes. Role of organic acids. J. Food Sci. 29, 555. Hulme, A. C. and L. S. C. Wooltorton. d958. Determination and isolation of the non volatile acids of pome fruits and a studuy of acid changes in apples during storage. J. Sci. Food Agr. 9: 150. Minina, A. K. 1953. Changes in organic acid content of potato leaves and tubers. Biokhimiya 18: 718. Palmer, J. K. 1955. Chemical investigations of the tobacco plant. X. Determination of organic acids by ion exchange chromatography. Conn. Agr. Exp. Sta. Bull. 589 (New Haven) 31 pp. Moore, S., D. H. Spackman and W. H. Stein. 1958. Chromatography of amino acids on sulfonated polystyrene resins. Anal. Chem. 30:1185. Reynolds, T. M. ~1965. "Chemistry of nonenzymatic browning." Advances in Food Research 14: 167. Schwartz, J. H., R. B. Greenspun and W. L. Porter. 1961. Chemical composition of potatoes. II. Relationship of organic acid concentrations to specific gravity and storage time. Food Technol. 15: 364. Schwimmer, S., A. Bevenue, \V. J. Weston and A. L. Potter. ~954. Potato composition. Survey of major and minor sugar and starch components of the white potato. J. Agr. Food Chem. 2 : 1284. Talley, E. A., T. J. Fitzpatrick and W. L. Porter. 1964. Chemical composition of potatoes. IV. Relationship of the free amino acid concentrations to s~)ecific gravity and storage times. Amer. Potato J. 41 : 357.