Polysaccharide composition of dietary fiber and cell walls from 'Rocha' pear at two ripening stages

Cell wall polysaccharides and dietary fiber were extracted from 'Rocha' pear (Pyrus communis L.) at two ripening stages. Fruit were sampled with a flesh firmness of 45 or 20 N, corresponding to crispy and soft textures, respectively. Total dietary fiber content of 'Rocha' pear, as determined by the enzymic-gravimetric method, was 2.3 g/100 g at both ripening stages. Soluble fiber content was 0.9±0.1 g/100 g in crispy pears (45 N) and 1.2±0.1 g/100 g in soft pears (20 N) whereas insoluble fiber content was 1.4±0.0 and 1.1±0.0 g/100 g, in crispy and soft pears, respectively. No significant differences were found in the concentrations of pectins, hemicelluloses or cellulose in the dietary fiber obtained from pears at the two ripening stages. Pectins, hemicelluloses, and cellulose accounted for 23, 10, and 36% of total fiber dry weight, respectively. Ethanol insoluble solids contained more pectin (744.8 mg/100 g) than the dietary fiber extracted from the same fruit (495.5 mg/100 g). Although the total pectin content was similar at both ripening stages, an increase in the water soluble fraction at the expense of the chelator-soluble fraction was observed during ripening. These differences in pectin solubility were not observed in the dietary fiber obtained from the same pears. The results show that the enzymic-gravimetric method underestimated the actual soluble fiber content of 'Rocha' pear and does not account for the ripening-related changes in pectin solubility.