PHENOLIC COMPOUNDS IN POTATO TISSUE^a

Abstract

It has long been known that the darkening of cooked potatoes involves an oxidation to produce the blue-gray-black color especially prominent at the stem-end portion of a tuber. It has been suggested that the colorless precursor was produced by tyrosinase (phenolase) action (1,8,9). Tyrosine or other free alpha amino acids, catechols or dihydroxy-phenylalanine have been listed as possible substrates (1, 4, 7). More recently, Juul ( 4 ) reported that ortho diphenols (probably caffeic acid) were involved in the stem-end darkening of potatoes after cooking, and Johnson and Schaal (3) have identified tyrosine and chlorogenic acid in potato tissue.The widespread occurrence of darkening of cooked potatoes grown in the northern regions of the United States and of Europe warrants further studies of the components involved in this discoloration. The plan of study was to compare the total tannin (tannin and tyrosine) content of various portions of potato tubers and of a number of varieties and to correlate such information with the degree of darkening. Paper chromatographic and spectro-photometric identification of some phenolic compounds were made. EXPERIMENTAL PROCEDUREThe samples for total tannin content included a group of Wisconsin market Chippewa, Russet Burbank, and Sebago potatoes which had been stored from 4 to 6 months.Market samples of Russet Burbanks grown in Idaho and Triumphs grown in Florida were also used. The potatoes were stored at a temperature of 38" to 42°F. and held a t room temperature for one week prior to the tests.A judging panel of 6 or 7 trained judges scored the degree of darkening of a quarter of each potato, using a scale of 1 to 10, with the score of 10 for perfect white or cream color and 7 for an acceptable color. These quarters of potatoes had been boiled approximately 30 minutes to an internal temperature of 98-1OO0C., skinned, and exposed to the air for one hour t o permit maximum color development before scoring. The cooking was done in tap water of 20 degrees hardness and in similar aluminum saucepans, as is common household procedure.Other portions of the same tubers were extracted in a Waring blender for 5 minutes according to the method of Johnson and Schaal (3), using 95% ethyl alcohol in the amount of 3 ml. to 1 g. of potato. The extract was filtered and 1 ml. was removed for the tannin determination. The rest of the extract was concentrated under reduced pressure t o dryness and in order to remove any fatty or waxy material, the resulting powder was extracted three times with 5 ml. petroleum ether in a 15 ml. centrifuge tube, centrifuged and the petroleum ether decanted each time. The dry powder was dissolved with distilled water (1 ml. per 100 g. equivalent potato tissue) and then stored in a refrigerator until used for the paper chromatography. The same procedures were used for the potato peeling, except that 0.3 ml. of extract was removed for the total tannin determinations.On some other tubers, comparison was made of the amount of tannin present in the areas which tended to d...

THE EFFECT OF MATURITY AND STORAGE ON PHENOLIC CONTENT, ENZYMATIC ACTIVITY AND DISCOLORATION OF POTATOESa

“…Changes in phenolic content of potatoes after storage, as found in this study, are not in agreement with the findings of Craft et al (6) who reported no significant change in phenolic Content of two varieties of potatoes stored for five months at 40" F. Differences in findings may be due to differences in potato varieties studied, or in the part of the tuber analyzed for phenolic content. Craft et al sampled the entire tuber; in this study only the cortex tissue was analyzed, since the cortex had been found to be the area highest in phenolic content (4).…”

Effect of Some Natural Extracts, Indolbutiric Acid and Naphthalene Acetic Acid on Rooting of Picual Olive Cuttings

“…The ethyl extract was transformed into aqueous phase by evaporation at 30 ± 2°C under vacuum for determining total phenols. Folin-Denis Spectrophotometry (750 mu) was used to estimate phenols concentrations as pyrogallol on dry weight basis of cutting tissue (g/100 g) (Cheng and Hanning, 1955).…”

Effect of Ozone Application on Postharvest Quality and Microbiological State of “Zaghloul” Date Palm Fruits

“…Optical density was measured at 750 nm and total phenols were calculated from a standard curve of gallic acid. These data were expressed as the mg gallic acid equivalents per gram of fresh weight basis according to Cheng and Hanning (1955).…”