Degradation of cell wall polysaccharides and change of related enzyme activities with fruit softening in Annona squamosa during storage
Introduction
Annona squamosa (custard apple), belonging to family Annonaceae, is a kind of tropical fruit with tender sweet flesh and abundant nutrition components (Gong et al., 2007). Similar to some other fruit, Annona squamosa becomes ripe and soft within a few days after harvest when stored at room temperature (Chen and Zhang, 2000). As it matures, its sensory quality also decreases. Coupled with pathogen attack, it finally leads to the reduction of shelf-life of Annona squamosa. To solve this problem, some species of fruit, such as peach, are refrigerated to slow down ripening and extend their market life (Chang et al., 2017). But it is not suitable for the tropical fruit Annona squamosa, which may make it become susceptible to chilling injury, even browning on the surface. Undoubtedly, the short ripening stage and poor storability of Annona squamosa have brought about great economic loss in the postharvest period and severely constrain its potential commercial and economic development.
Fruit softening is mostly caused by changes in cell-wall structure and composition, which would influence appearance, texture, flavour and aroma of fruit (Goulao and Oliveira, 2008). Cell walls are usually composed of xyloglucan molecules bonded to constrained areas of cellulose (Cosgrove, 2001). It is reported that structural changes of cell wall during ripening mainly occur in pectin, hemicellulose, cellulose and involve the concerted and synergistic action of cell wall-modifying enzymes (Rose and Bennett, 1999), which mainly include polygalacturonase, pectin methylesterase, cellulase, xylanase, β-galactosidase, α-arabinofuranosidase and protease (Barka et al., 2000). To explore the internal relationship between textural properties and fruit softening, extensive research have been developed on action patterns of cell wall modifying enzymes in different varieties of fruit, such as zucchini fruit (Carvajal et al., 2015), apples (Wei et al., 2010), pears (Dong et al., 2018), tomato (Bu et al., 2013) and so on. However, there was seldom reports on fruit softening of Annona squamosa so far and it is quite essential to help solve this problem. Therefore, the objective of this paper was to investigate the cell wall metabolism in Annona squamosa during storage including firmness, moisture distribution, molecular weight distribution and composition of cell wall polysaccharides, as well as the activity of enzymes related to fruit softening, which would provide theoretical basis to help control fruit softening, extend the shelf life and solve fruit quality decline of postharvest Annona squamosa in further studies.
Section snippets
Experimental materials
Custard apple (Annona squamosa L.) was obtained from a local orchard in Guangdong province of China and transported to our laboratory within several hours. Fruit with uniform size, shape, color, no mechanical damage and no diseases were selected. The weight of the random selected fruit ranged from 390 g to 410 g and their length from 13 cm to 15 cm. In addition, to prevent rapid softening, fruit were picked firm, unripe but mature and yellow green. Then they were placed in a sterile incubator
Change of appearance and firmness
Generally, acceptability of fruit was closely related to its surface color. Fig. 1a indicated that the color of Annona squamosa gradually changed from green to yellow, then to black with the extension of storage time (1 d–11 d). It also could be seen that the transverse section of fruit was smooth and flat at 1 d, but became sticky at 5 d - 7 d. At the end of storage, the section became soft and even out of shape, which suggested excessive softening of Annona squamosa during storage.
Firmness is
Discussion
Fruit softening is a major factor responsible for limitations of shelf life, transportability and storage. Extensive research have been carried out on fruit softening of various fruits and decrease in firmness is one of their most direct characteristic changes. In our study, obvious changes in appearance as well as transverse section of Annona squamosa during storage were recorded (Fig. 1a) and a significant decline (p < 0.05) was observed in its firmness (Fig. 1b). This phenomenon was
Conclusion
During postharvest storage of Annona squamosa, fruit softening was accompanied by sharply declined firmness and moisture with high freedom degree migrating to low freedom degree in pericarp. This phenomenon was mainly related to the change of cell wall polysaccharides including WSF, CSF, SSF, HC1, HC2 in content, molecular weight distribution as well as monosaccharide composition. That was to say, the degradation of cell wall polysaccharides especially pectin polysaccharides destroyed the
Author contribution statement
Zhu Z.Y. conceived and designed research. Ren Y.Y conducted experiments and wrote the manuscript. Sun P.P. contributed new reagents or analytical tools. Wang X.X analyzed data. All authors read and approved the manuscript.
Declaration of Competing Interest
The authors declare that they have no conflicts of interest.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (31871791), the key program of the Natural Science Foundation of Tianjin (16JCZDJC34100) and the key program of the Foundation of Tianjin Educational Committee (2018ZD06).
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