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Noboru Muramatsu, Toshio Takahara and Tatsushi Ogata

To compare to two types of Citrus fruit rind [i.e., soft type (satsuma mandarin, Citrus unshiu Marc.) and firm type (Hassaku, C. Hassaku Hort. Tanaka)], rind firmness and contents of cell wall polysaccharides were measured from August to January. In August, firmness was measured by a puncture test and found to be ≈3000g in both species. Firmness of satsuma mandarin decreased drastically with time from August to September and decreased slightly thereafter. In contrast, Hassaku firmness increased slightly from August to September, decreased from September to November, and fluctuated. Hassaku firmness, therefore, was significantly higher than satsuma mandarin firmness after September. We measured sugar content in each fraction after fractionalizing cell wall polysaccharides. In flavedo tissue, sugar content in cellulose fraction was the highest, followed by hot-water and EDTA fraction; hemicellulose fraction was the lowest. Although both species were almost the same in sugar content in cellulose and EDTA fraction in August, satsuma mandarin was significantly higher than Hassaku in January. These data showed that changing of rind firmness in citrus was related to the sugar content of cellulose and EDTA fraction in flavedo tissue. In albedo tissue, sugar content in the cellulose fraction was the highest, followed by hemicellulose and hot-water fraction, and EDTA fraction was the lowest. However the extent of seasonal fluctuation in albedo tissue was smaller than that of flavedo tissue, not having any relation to the changing of the firmness.

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Noboru Muramatsu, Toshio Takahara, Kiyohide Kojima and Tatsushi Ogata

Various species and cultivars of citrus were studied to determine the relationship between texture and cell wall polysaccharide content of fruit flesh. Among those tested cultivars, navel orange (Citrus sinensis Osbeck) and hassaku (C. hassaku Hort. ex Tanaka) were firmest, `Fukuhara orange' (C. sinensis Osbeck) was intermediate, and satsuma mandarin (C. unshiu Marc.) was softest. There was a 3-fold difference in firmness among the 12 citrus cultigens measured. Cohesiveness values ranged from 0.30 to 0.49 and were not correlated with fruit firmness. Sugar content in each cell wall fraction was highest in the water and EDTA fractions, followed by the hemicellulose fraction, and was lowest in the cellulose fraction. Correlation coefficients between firmness and sugar content ranged from 0.69 to 0.88 and were highest in the cellulose fraction. This study suggests that firmness of fruit flesh among the cultigens is influenced by cell wall polysaccharide composition. Chemical name used: ethylenediaminetetraacetic acid (EDTA).

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Noboru Muramatsu, Toshio Takahara, Tatsushi Ogata and Kiyohide Kojima

Changes in rind firmness and cell wall polysaccharide composition were measured in fruit with a) a soft rind, (`Satsuma' mandarin, Citrus unshiu Marc., cv. Aoshima), and b) a firm rind (hassaku, C. hassaku Hort. ex Tanaka), from August to January of the following year. Rind firmness was similar in both species in August, but hassaku had significantly firmer rind than did mandarin from September to January. Both flavedo and albedo tissues were extracted, and the extracts were hydrolyzed and fractionated to yield four fractions: (hot water, EDTA, hemicellulose, and cellulose). In flavedo tissue, sugar concentration was highest in the cellulose fraction, and lowest in the hemicellulose fraction. The concentration in all fractions decreased as the fruit developed and matured. Although the sugar concentration in the cellulose and EDTA fractions of both species was similar in August, it was significantly higher in both fractions in hassaku than in mandarin in January. The sugar concentration of each fraction from albedo tissue was in the order: cellulose > hemicellulose > hot water > EDTA. The range of variation in cell wall sugars in albedo tissue was smaller than that in flavedo tissue. Chemical name used: ethylenediaminetetraacetic acid (EDTA).