Analysis of resistant starch degradation in postharvest ripening of two banana cultivars: Focus on starch structure and amylases

• Higher RS content and lower RS/total starch reducing rate in Plantain than Cavendish.
• Starch granules of Cavendish were bigger and rounded than in Plantain.
• β-Amylases play a vital role in faster starch degradation in Plantain.
• Other amylases were also up-regulated specifically in Plantain.

Bananas are well known as good sources of dietary energy, with high levels of sugar and starch. To further analyze the starch-degradation mechanism in bananas, two species, Cavendish and Plantain, were used to investigate the influences of hydrolases and granule structure on starch degradation. Determination of the levels of resistant starch (RS), non-resistant starch (non-RS), total starch, and amylose content showed that each starch component content decreased gradually during the fruit-ripening process in both Cavendish and Plantain. Compared to Cavendish, Plantain had a higher content of total starch and RS, a faster starch-degradation rate, and a lower decrease in the ratio of RS/total starch. Scanning electron microscopy (SEM) images revealed that the starch granules of Cavendish were larger and more rounded than the smaller and ellipsoidal starch granules found in Plantain. Also, the analysis of gene expression suggested that β-amylases make a central contribution to starch degradation in both species and highly up-regulated β-amylases were correlated with the faster starch-degradation rate in Plantain. Two α-amylases, one starch phosphorylase, and one starch debranching enzyme were specifically up-regulated in Plantain, which might hydrolyse more non-RS compared with Cavendish.