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L.D. Melton and L.M. Davies

Cell wall changes during ripening have a major effect on fruit texture. The cell walls isolated using phenol-Tris buffer were sequentially extracted to give polysaccharide fractions that contained mainly water-soluble pectin, chelator-soluble (CDTA) pectin, hemicelluloses (0.05 M Na2CO3 followed by 1M and 4M KOH) and cellulose. The fractions were analyzed colorimetrically for uronic acid, total neutral sugar and cellulose contents. The component sugars of each fraction were determined as their alditol acetates by GC. Then was a decrease in the two pectin fractions during ripening. The pectins appear to have arabinan and galactan side chains. Pectic galactose decreases during ripening. The weight of the combined hemicellulose fractions did not change during ripening, nor did the cellulose level. At least two types of arabinan are present. Pectins were found in all cell wall fractions. Nashi cell walls contain a relatively large amount of xylan compared to other fruit.

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D.M. Dawson, C.B. Watkins, and L.D. Melton

Cell wall changes in `Fantasia' nectarines [Prunus persica (L) Batsch var. nectarina (Ait) maxim] were determined after storage at 0C with or without intermittent warming (at 20C at 2-week intervals) and after ripening. For comparison, fruit were examined at harvest and after ripening without storage. Fruit stored continuously at 0C for 6 weeks became mealy during ripening, whereas fruit subjected to intermittent warming ripened normally. Ripening immediately after harvest was associated with solubilization and subsequent depolymerization of pectic polymers and a net loss of galactosyl residues from the cell wall. No solubilization of pectic polymers from the cell wall occurred during storage of fruit at 0C. Mealy fruit, ripened after continuous storage at 0C, showed only limited solubilization of pectins and depolymerization, high relative molecular weight (M) polymers being predominant. During ripening after storage, pectic polymer solubilization was not as extensive in intermittently warmed fruit as in fruit undergoing normal ripening but solubilized polymers were depolymerized, low M uronic acid-rich polymers becoming predominant. Intermittent warming of fruit resulted in significant softening during storage, alleviating the development of mealiness by promotion of cell wall changes associated with normal ripening.

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D.M. Dawson, L.D. Melton, D.M. Dawson, and C.B. Watkins

Nectarine fruit (Prunus persica (L) Batsch) cv. Fantasia, were ripened immediately after harvest (normal ripening), or stored for 6 weeks either continuously at 0°C or were intermittently warmed (IW) for 48 h at 20C after 2 and 4 weeks, and then ripened. Fruit subjected to IW ripened normally, whereas the continuously stored fruit developed mealiness during ripening. Normal ripening was associated with solubilization and depolymerization of pectic polymers and a net loss of galactose. Only limited pectic solubilization and removal of side chains occurred during ripening of mealy fruit. Pectic polymer polymerization occurred at each IW occasion continued during ripening after storage, but was not as extensive as in normally ripened fruit. Mealy fruit had high autolytic capacity, probably as a result of insoluble pectic polymers in the cell wall that were not solubilized during ripening. The release of uronic acid suggests that cool storage temperatures do not irreversibly inhibit polygalacturonase activity.