Relationship between Fruit Softening and Wall Polysaccharides in Avocado (Persea americana Mill) Mesocarp Tissues

Abstract

Changes in avocado (Persea americana) fruit texture during ripening were evaluated by stress-relaxation analysis. A conical probe was imposed into the mesocarp tissue to a depth of 0.6 mm and the initial stress and the stress relaxation over 60 s were determined. The initial stress, an elastic parameter, was substantially reduced within one day when ripening was initiated by transferring the fruit from 15 to 25°C. The minimum and maximum relaxation time, parameters which reflect viscosity, were also reduced within one day. Mesocarp cell walls were fractionated into water-soluble (WS), hot EDTA-soluble (EDTA), alkaline soluble (hemicellulose) and the residual (cellulose) fractions. The amount of cellulose did not change during ripening. Rhamnose, arabinose and uronic acids in the WS fraction increased during the initial day of ripening; those same components decreased in the EDTA fraction. A molecular weight downshift in the WS acidic polysaccharides was detected within one day, while only slight changes were observed in the molecular weights of the EDTA fraction. The quantities of individual sugar components of major hemicellulose fraction were unchanged, but there was a prominent molecular weight downshift in the xyloglucan components within one day. These results clearly revealed that both elastic and viscous properties of avocado mesocarp tissues were substantially altered during ripening, and that the solubility changes in acidic polysaccharides and decreases in the average molecular weight of cell wall xyloglucan components were associated with significant changes in fruit texture.