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

Changes in avocado (Persea americana) fruit texture during ripeningwere evaluated by stress-relaxation analysis. A conical probewas imposed into the mesocarp tissue to a depth of 0.6 mm andthe initial stress and the stress relaxation over 60 s weredetermined. The initial stress, an elastic parameter, was substantiallyreduced within one day when ripening was initiated by transferringthe fruit from 15 to 25°C. The minimum and maximum relaxationtime, parameters which reflect viscosity, were also reducedwithin 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 cellulosedid not change during ripening. Rhamnose, arabinose and uronicacids in the WS fraction increased during the initial day ofripening; those same components decreased in the EDTA fraction.A molecular weight downshift in the WS acidic polysaccharideswas detected within one day, while only slight changes wereobserved in the molecular weights of the EDTA fraction. Thequantities of individual sugar components of major hemicellulosefraction were unchanged, but there was a prominent molecularweight downshift in the xyloglucan components within one day.These results clearly revealed that both elastic and viscousproperties of avocado mesocarp tissues were substantially alteredduring ripening, and that the solubility changes in acidic polysaccharidesand decreases in the average molecular weight of cell wall xyloglucancomponents were associated with significant changes in fruittexture. (Received December 13, 1996; Accepted March 5, 1997)