Studies on browning of grated Chinese yam. I

Abstract

Browning of grated Chinese yam has been one of the most serious problems in production of yam for early market. In this study, properties of the browning substances and factors affecting their content were investigated.At first, it was acertained that the browning phenomenon of grated Chinese yam was due to Fenzymatic oxidation of polyphenolic substances. In ascending paper chromatography of 50% methanol extract of tuber, using butanol-acetic acid-water (4:1:1) as developing solvent, were found four spots with Rf. values of 0.07, 0.29, 0.43 and 0.62 (Fig. 1). In the same way, two spots with Rf. values tof 0.07 and 0.43 were isolated from the extract of aerial tuber. Among these spots, the substance with Rf. value 0.43 was considered to play the most important role in the browning phenomenon. Ultraviolet absorption spectrum of this substance has a maximum at 277mμ and a minimum at 260 mμ (Fig. 2).Analyses of tubers harvested at different stages of maturity, from July 20 to November 30, revealed that polyphenol content was higher in young tubers than in old ones, but a slight increase was found in the last two harvests (Fig. 3). It was observed that the earlier the digging date, the greater the degree of discoloration of grated tuber (Fig. 5).Contents of polyphenolic substances in various portions of tuber are shown in Fig. 4. In general, polyphenol content is higher in apical portion of tuber than in central portion and is intermediate in basal portion.Influence of storage on polyphenol content of tubers dug up at different dates is shown in Fig. 7. It shows that the content increased during storage from harvesting time till November 27, but decreased on February 10. Incerase during storage was remarkable in the early digging tubers, especially in their apical portion.Polyphenol content in aerial tubers was higher than that in tubers, and was similar in its pattern of distribution as in the tuber (Fig. 10 and 11). It decreased markedly after one month of storage in sand.Polyphenol content in leaves was higher than that in tuber or aerial tuber, and increased regularly from basal leaves to apical ones (Fig. 12).