Postharvest fruit density as an indicator of dry matter and ripened soluble solids of kiwifruit

The density of unripe kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson cv. Hayward) early in storage was investigated as a means of determining the current fruit dry matter (DM) and total sugar-plus-starch concentrations, and of predicting DM and soluble solid concentrations later when the fruit had ripened. To investigate the robustness of the density relationships with DM and ripe fruit soluble solids, measurements were made on fruit collected in a survey involving 208 New Zealand orchards (similar to 10% of the total) across the normal harvest season (early May-early June). Density measured on unripe fruit early in storage correlated with both DM measured at the same time (r(2) = 82.6% S.E. = 0.68% fresh weight (FW)), and with SSC measured after ripening (r(2) = 85.1%; S.E. = 0.57% FW). As fruit taste is related to sugar concentration, and sugars make up the bulk of the soluble solids in fruit, this suggests potential for automated bulk taste sorting of kiwifruit at harvest by floating off low DM fruit in prepared salt solutions. To investigate the reasons for the relationship between density and other fruit measurements, fruit composition was analysed. Fruit taken from storage 2 weeks after harvest were graded non-destructively by density into four distinct dry matter groups, each containing eight fruit (four pairs matched by density). Paired fruit were separated and analysed for DM, soluble solids, starch and sugars (sucrose, glucose and fructose), one fruit immediately and the other after ripening. Equations relating density to both DM and ripe fruit soluble solids in the composition trial had similar parameter values to those of the survey trial and gave S.E. of prediction of about 0.3% FW. DM levels were about 3.2% FW above the sum of soluble solids and starch concentrations in both ripe and unripe fruit, a difference largely independent of DM concentration. Starch lost during ripening was accounted for by the increase in the glucose and fructose sugar pools, and these two sugars had near equal concentrations at each DM level. Sucrose and minor sugar levels were independent of DM and ripeness. (C) 2000 Elsevier Science B.V. All rights reserved.