PARTICIPATION OF ASCORBIC ACID IN THE DESTRUCTION OF ANTHOCYANIN IN STRAWBERRY JUICE AND MODEL SYSTEMS^a

Abstract

That ascorbic acid may play a significant role in the red color deterioration of fruit juices was first pointed out by Beattie, Wheeler, and Pederson (2) and by Pederson, Beattie, and Stotz (6) who observed parallel losses of ascorbic acid and anthocyanins during storage. The best correlation between ascorbic acid and red color losses was obtained with strawberry juice. Pederson and coworkers postulated that an interaction between ascorbic acid and anthocyanins may occur. Similar observations and conclusions were reported by Nebesky, Esselen, McConnel, and Fellers ( 5 ) .The availability of crystalline pelargonidin 3-monoglucoside isolated from strawberries (10) prompted us to study these interactions in simplified artificial systems as well as in strawberry juice. I n the presence of ascorbic acid, maximum anthocyanin destruction was observed under those conditions which favor the aerobic oxidation of ascorbic acid. EXPERIMENTALPelargonidin 3-monoglucoside chloride was obtained by a previously described procedure (20). The anthocyanin picrate was recrystallized six times from hot dilute picric acid solution before conversion to the chloride. The model systems contained 28 mg. pigment per 100 ml. of Sorensen's disodium citrate-hydrochloric acid buffer of pH 3.40. The strawberry juice used in these experiments was expressed from Dresden strawberries grown a t Geneva, N. Y., during the 1950 season. This juice had a pH of 3.4 and an ascorbic acid and anthocyanin content of 49 mg. and 23 mg. per 100 ml., respectively. All samples were heated to 9O"C., and held for 5 minutes in order to expel most of the dissolved gases. The solutions which were to be held under essentially anaerobic conditions were rapidly poured into 2-dram (7.39 ml.) screw-capped vials, leaving no headspace. They were cooled in a 30°C. constant-temperature bath and sealed with paraffin wax. In experiments where a fairly constant air supply was desired, 50 ml. of solution in a 250-ml. volumetric flask was shaken continuously in an automatic shaker in which the temperature could be controlled. Anthocyanin losses were followed by a previously described method (9) based on the difference in absorption at 500 mp produced by a change in pH from 3.4 to 2.0. Ascorbic acid was determined by the method of Robinson and Stotz (7) which involves oxidation by indophenol under specific conditions. The above authors warn against the possible interference of sulfhydryl compounds in the ascorbic acid determination. We found that thiourea does tend to give high ascorbic acid values but that the error is small if the thiourea content does not exceed 0.1% and the excess indicator is extracted rapidly. RESULTS AND DISCUSSION

Strawberry juice colour: The effect of some processing variables on the stability of anthocyanins

“…They found that the loss of colour in strawberry juice stored at 50°C under nitrogen was several-fold less than that in juice stored under air. In contrast with the results of Sondheimer and Kertesz (1953), a colour loss still occurred in the juice stored under nitrogen. The role of oxygen and nitrogen was also studied in model systems containing pelargonidin 3-glucoside, catechin and ascorbic acid .…”

“…This presumably contributed also to the loss of colour in the clarified juice prepared for storage. Furthermore, the unavoidable extra oxidation during clarification allowed more breakdown of anthocyanins as a result of the interaction with ascorbic acid and oxygen, as previously reported (Sondheimer and Kertesz 1953 ; Markakis et a/ 1957). Additionally, other breakdown mechanisms prevalent under oxidative conditions such as the degradation of anthocyanins by strawberry polyphenol oxidase may have contributed to this loss.…”

“…Specifically the interaction of ascorbic acid with anthocyanins and subsequent mutual degradation is an important problem, since most fruit products contain high concentrations of ascorbic acid. Conditions favouring aerobic oxidation of ascorbic acid in strawberry juice were found to cause maximal loss of anthocyanins, but when oxygen was excluded no colour deterioration was observed (Sondheimer & Kertesz 1953). A synergistic effect between ascorbic acid and oxygen on pigment degradation has been reported (Markakis et a1 1957).…”

Degradation of various fruit juice anthocyanins by hydrogen peroxide

“…Furthermore, the high amount of ascorbic acid in strawberry juice may have also contributed to the degradation of anthocyanins. In fact, the adverse effects of ascorbic acid on anthocyanins have been shown in strawberry juice (Sondheimer & Kertesz, 1953) and model systems containing strawberry anthocyanins (Markakis, Livingston, & Fellers, 1957). Contrary to strawberry juice, both sour cherry (Herrmann, 1978) and pomegranate juices …”

“…The accelerated degradation of anthocyanins in the presence of both H 2 O 2 and ascorbic acid can be attributed to the degradation products of ascorbic acid. In fact, Sondheimer and Kertesz (1953) showed the maximum loss of strawberry anthocyanins under conditions most favorable to ascorbic acid degradation. This indicates that the degradation products of ascorbic acid, not ascorbic acid itself, are responsible for the anthocyanin degradation (Jackman, Yada, Tung, & Speers, 1987).…”