カキ果汁の脱渋性に関する研究 : とくに甘ガキの自然脱渋について

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Many earlier resarchers have studied the removal of astringency in Japanese persimmon fruits, and have indicated that decrease of tannin content in the fruits is associated with ethanol and acetaldehyde accumulation. These volatiles, especialiy acetaldehyde, allow soluble tannins to coagulate and change into insoluble complexes, thereby leading to the loss of astringency. This mechanism of deastringency has been obtained mainly from studies of astringent-type fruits from which astringency waS removed artificially. However, the mechanism for removal of astringency in nonastringent-type fruits natural growth process has not been understood. The present study was undertaken to elucidate this mcehanism, especially focusing on pollination-constant nonastringent（PCNA）type fruits. 1） It was investigated first whether or not ethanol and acetaldehyde are responsible for natural removal of astringency in nonastringent-type fruits on trees. In pollination-variant nonastrigent （PVNA） type fruits, ethanol and acetaldehyde greatly increased in accordance with rapid decrease in soluble tannins. On the contrary, in PCNA type fruits, no accumulation of these volatiles in the fruits was measured even before the time when soluble tannins were no longer detected. Moreover, another experiment on trees revealed that treatment of young astringent fruits of a PCNA cultivar, 'Fuyu', with either ethanol or acetaldehyde was ineffective for removal of their astringency. These results evidently indicate that neither ethanol nor acetaldehyde is closely associated with deastringency in PCNA fruits under natural conditions. 2） Qualitative and quantitative comparisons of tannins were made among 4 fruit types : pollination-constant nonastringent （PCNA） and astringent （PCA）, and poilination-variant nonastringent （PVNA） and astringent （PVA） type. Cntechin and gallic acid were commonly major components of phenolics in ethyl acetate extracts of fruits. In PCNA cultivars, catechin was detected throughout the growing period of fruits, while gallic acid was detected only in earlier stages. On the oher hand, in PVNA, PVA, an PCA cultivars, catechin disappeared rapidly in June, while level of gallic acid enhanced remarkably in the same month to reach a maximum in late June, followed by the gradual decline to an extremely low level in late July. Subsequenty, variances of molecular weight of tannins in apueous aetone extracts of fruits were examined by means of size exclusion chromatography, using controlled-pore glass media. In PVNA, PVA, and PCA cultivars, tannins of high moleeular weight were predominant, while in PCNA cultivars those of low molecular weight were prevalent. 3） Chemical properties of tannin were examined for cv. 'Fuyu' as a PCNA type and cv. 'Hiratanenashi' as a PVA type. Tannin fractions obtained by size exclusion chromatography were incubated at 40 ℃. At proper intervals, sedimentation coefficients of components in these fractions were determined by ultracentrifugation method. Sedimentaion coefficients of respective components were relativeiy constant in 'Fuyu', while those in 'Hiratanenasi' showed a great variation during incubation. To make clear the reactivity of tannins to acetaldehyde in both varieties, time required for coagulation of fruit juice which was exposed in acetaldehyde vapor was compared at different tannin concentrations and pHs. The fruit juice from 'Fuyu' coagulated more slowly than that from 'Hiratanenashi' without exception. These results suggest that tannins in PCNA type fruits have milder chemical property than other type fruits, giving a reasonable explanation why the treatment with ethanol and acetaldehyde failed to remove astringency from young astringent fruits of a PCNA cultivar on trees. 4） Fruits of 41types from incipient through mature stage were observed by light microscopy with the main purpose of elucidating the mechanism of natural removal of astringency in PCNA type fruits. No differerences were found in manner of occurrence of tannnin cells in ovary. Thereafter, the tannin cells in flesh tissues of PCNA type fruits gradually increased in size and stopped their enlargement at the end of June, whereas those in other types continued to increase in size until the end of July. Thus, final size of tannin cells was much smaller in the former fruits than the latter. Number of tannin cells per unit area showed almost no difference in all 4 type fruits at any stage. Thus, in PCNA cultivars the area occupied by tannin cells gradually reduced after tannin cells stopped their development. Such a decreasing process was coincident with the decreasing process of soluble tannins in PCNA fruits. Moreover, observation of tannin cells of fresh pericarp sections stained with ferric chloride revealed that tannins in PCNA type fruits did not completely coagulate in early August, although soluble tannins were negligibly low in contents. These results suggest that the decrease of astringency of PCNA fruits on trees may be mainly attributed to the reduction in relative area occupied by tannin cells, but not their coagulation. Such a reduction in area occupied by tannin cells starts when development of tannin cells stops, as mentioned above. 5） To study timing of coagulation of tannin cells in more detail, tannin cells were observed by scanning electron microscopy. The comparative observation between ethanol-treated fruits and untreated fruits revealed that vacuoles of tannin cells had very rough surface. A number of protrusions and large pores which might correspond to plasmodeamata were present cell walls of tannin cells, when tannins had not yet coagulated in fruits. These protrusions of vacuole surfaces and the large pores of cell walls are probably formed by the outgrowth of tonoplasts at the entry of plasmodeamata in cell walls. On the contrary, when tannins coagulated and browning of flesh tissues became visible to the naked eye, vacuole surfaces turned smooth and large pores in the cell walls became invisible. It indicates that the time of coagulation of tannins can be determined from these morphological changes of tannin cells. So, when seasonal changes in morphology of tannin cells in 4 type fruits were observcd, it was verified that in PCNA cultivars coagulation of tannins occured at late growth tage of fruits.
三重大学農学部の論文

著者

米森敬三
京大農

カキ果汁の脱渋性に関する研究 : とくに甘ガキの自然脱渋について

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