Microscopic and chemical changes occurring during the ripening of two forms of jackfruit ( Artocarpus heterophyllus L.)

Abstract

Trees known to produce two distinct textural forms of jackfruit, in which the fruit either remained firm when ripe or became soft and pulpy, were sampled when fruit was immature (10–11 weeks after anthesis) and when judged ripe (15–16 weeks after anthesis). The dry matter content of the edible perianth increased with maturity from 125 to 215 and 140 to 240 g kg −1 wet weight perianth in the firm and soft fruits respectively. Perianth from immature fruits had a high water-insoluble content (840–890 g kg −1 dry matter) consisting largely of cell wall material (450–530 g kg −1 dry matter) and starch (approximately 330 g kg −1 dry matter). Microscopic examination of fruit at this stage showed the perianth to contain thin-walled cells packed with starch granules, some organised into distinct clusters. In ripe fruits the starch (20–110 g kg −1 dry matter) and cell wall (170–200 g kg −1 dry matter) contents were substantially reduced, the extent of hydrolysis being greatest in the soft form. Cell maceration and starch dissolution were evident in both forms of the fruit when examined by light microscopy, but were more pronounced in the soft form. Concomitant with the decrease in water-insoluble dry matter was a substantial increase of water-soluble material (660–790 g kg −1 dry matter) which included fructose (76–113 g kg −1 dry matter) and sucrose (approximately 95 g kg −1 dry matter). Mannitol (22–68 g kg −1 dry matter) was also found in ripe but not immature fruits. Concentration of low molecular weight carbohydrate was greatest in the soft form. Polygalacturonase and pectin esterase activities were 12-fold and 40-fold higher in ripe fruits of the soft form compared to those of the firm form. This was reflected in the greater extent of tissue maceration and loss of homogalacturonan in the soft form. Since both forms of fruit demonstrated a common pattern of ripening, textural differences evidently related to the extent of change which was greatest in the soft form. The firm form of jackfruit may represent fruit in which cell wall degradation is arrested or delayed during ripening and possibly this was related to a reduced capacity to produce pectic and other cell wall degrading enzymes.