REPRINTS, COPIES

Corresponding figures on a dry weight basis are 20.1, 11.0, and 13.9 mg/100 g. The decrease due to two days of additional ripening averaged 53.3 percent on a fresh weight basis (52.9 percent dry weight). After four days of additional ripening, the decrease in ascorbic acid was only 26. 9 percent on a fresh weight basis (20. 0 percent dry weight). This decrease was significantly less (5 percent level) than that observed in bananas held to ripen for an additional two days (Table 2). It must be kept in mind that analyses were done on different bananas and biological variability may have resulted in slower metabolism of ascorbic acid in those particular bananas that were held for four days of additional ripening compared to those held for two days. An unlikely alternative is that ascorbic acid may decrease and later increase in ripening bananas. Harris and Poland (1939) reported a decrease in ascorbic acid in bananas as they matured from ripe to overripe.

The data on ripening bananas presented thus far show the extent of browning and the quantitative changes in the factors which might be involved. To determine whether interrelationships between the variables studied were significant, the Student-Fischer's t-test for zero correlation was employed (Table 10). The strongest relationship found was between dopamine content and susceptibility of the bananas to discoloration, i. e. , the change in percent transmittance over 30 minutes. The correlation between dopamine content and extent of browning, i. e. , initial percent transmittance of filtrates, was also significant at the 5 percent level. As the content of dopamine decreased with ripening, the initial percent transmittance decreased and the change in percent transmittance over 30 minutes increased. The decrease in dopamine as bananas ripened may have been due to oxidation by polyphenol oxidase, according to the mechanism proposed by Palmer (see p. 19). One intermediate in this oxidation is the red compound, 2, 3-dihydroindole-5, 6-quinone. Measurements of percent transmittance were made at 475 nm which would have reflected the presence of this compound or the presence of melanin which has a general absorption. If some of the dopamine had been converted to either the red intermediate or to melanin, this could account for both the lower dopamine content and the lower initial percent transmittance with ripening. Moreover, if some of the dopamine had been converted to intermediates closer to the formation of the red quinone or to melanin, this could account for the increase in susceptibility to discoloration over 30 minutes, as more of these compounds could have been formed during this time. The latter appears to have been more predominant because the change in percent transmittance over 30 minutes was more positively correlated to the disappearance of dopa- mine than was the initial percent transmittance. If much of the dopamine had been converted to melanin during ripening, the rate of browning would have been limited as compared to the initial percent transmittance. This was the case as shown in Table 2. The increase in browning in the intact fruit with ripening was much greater than was the increase in susceptibility to browning.

Weurrnan and Swain (1955) claimed that browning was not to related to phenolic content in apples. They based their argument on posedthe fact that the content of substrate per apple increased with ripening, redand this did not parallel the decrease in extent of browning of the apple filtrate. However, the apples gained weight with ripening, giving a decrease in phenolic content per grarn of apple which did parallel the decrease in extent of browning. toDeSwardt et al. (1967) suggested that unpolymerized tannins may both inhibit enzymes, thus influencing ripening. In this study, no significant correlation was found between the concentration of dopamine and the activity of polyphenol oxidase (Table 10).

Significant relationships (5 percent level) were found between the concentration of ascorbic acid and both the extent of browning and the susceptibility to discoloration (Table 10). As ascorbic acid decreased with ripening in individual bananas, the rate and extent of browning increased. Palmer (1964) reported that ascorbic acid delayed the enzymatic oxidation of dopamine, the delay being less with decreasing levels of ascorbic acid. The inhibitory effect of ascorbic acid on browning may be due to its reducing capacity, as suggested by Kreuger (1950) who studied its effects on the oxidation of tyrosine and DOPA. Baruah and Swain (1953), who studied the effects of ascorbic acid on potato polyphenol oxidase, suggested that the effects of ascorbic acid may be due to a direct inhibition of the enzyme. Pierpoint (1966) suggested that ascorbic acid acted both as a reducing agent and as an inhibitor of the enzyme in the oxidation of chlorogenic acid in tobacco leaves. In the present study, the concentration of ascorbic acid was more significantly correlated with the concentration of dopamine than with the activity of the enzyme (Table 10). The specific activity of banana polyphenol oxidase was not significantly related to the concentration of ascorbic acid or to either the extent of browning or the susceptibility to discoloration. This suggests that polyphenol oxidase was not the limiting factor. However, a significant correlation between the concentration of ascorbic acid and the concentration of dopamine was found. As the level of ascorbic acid diminished, so did the dopamine. If the ascorbic acid were oxidized, quinones would accumulate instead of being reduced to dopamine. Because the strongest relationship is between the concentration of dopamine and the susceptibility of ripening bananas to browning and because the concentration of dopamine is significantly related to the concentration of ascorbic acid, the concentration of dopamine as influenced in part by the concentration of reduced ascorbic acid appears to be the limiting factor in the browning of bananas.

While presence of both substrate and enzyme is essential in the susceptibility of a fruit to discoloration, of equal importance may be the location of each and the ability of the enzyme to come in contact with the substrate. Dopamine was located histochemically in a few isolated parenchyma cells and in the vacuoles of the latex vessels, as indicated by an arrow in Figure 3A. This photomicrograph confirms the location of dopamine in banana tissue which Barnell and Barnell (1945) previously illustrated by drawings. Melanin granules produced by polyphenol oxidase appeared to occur in the pulp ubiquitously, as illustrated in Figure 3B. Buckley (1964) reported that polyphenol oxidase is of"general distribution" in the cells of banana roots. Because the melanin granules produced by the action of the enzyme on application of dopamine were so large, it was impossible to determine whether the enzyme was associated with the vacuole, the cytoplasm, or with the cell walls, as Palmer has suggested (1963). Harel et al. (1964, 1965) suggested that polyphenol oxidases in apples are associated with organelles in green fruit but are released to the cytoplasm during ripening. Polyphenol oxidases may also occur in the latex vessels, but at least most of the enzyme and substrate are compartmentalized.

Ripening may be the result of a progressive increase in cell permeability leading to an increased contact between enzymes and substrates already present in tissue, and thereby to an enhanced metabolism. Alternately, ripening may be a differentiation process under genetic control involving the de novo synthesis of proteins. Two pertinent studies have been done on bananas. Sacher (1966, 1967) studied permeability in ripening bananas and found that an increase in permeability occurred two days prior to and rose exponentially during the respiratory climacteric of bananas. At the respiratory peak, he found the tissue was 100 percent permeable to mannitol, sucrose, fructose, and chloride. Support for this theory is given by Palmer and McGlasson (1969) who found an increase in respiration after cutting banana tissue. A breakdown of compartmentalization may have caused the increase in respiration. No change in levels of protein or amino acids during the climacteric was reported by Sacher. On the other hand, Brady et al. (1970) reported an increase in the incorporation of amino acids into proteins and a change in the pattern of proteins in bananas undergoing a climacteric rise in respiration as observed by gel electrophoresis.

If Sacher (1966, 1967) is correct in assuming that ripening results from an increase in permeability, banana polyphenol oxidase and dopamine may be allowed to come into contact and, thereby, cause an increase in discoloration in ripening bananas. On the other hand, an increase in protein was observed with ripening in this study which supports the differentiation hypothesis of Brady et al. (1970). Perhaps both of these processes are involved in the ripening of the banana fruit.

Updated: Wednesday, June 20, 2007.