Changes in anatomical structure and cell wall metabolism of 'Hom Thong' Banana peel following hot water treatment

Abstract

Fruit softening is an important change during the ripening process that affects fruit quality and postharvest shelf life. Cell wall degrading enzymes are considered to be the main factor responsible for cell wall disassembly and texture changes in fruit. To elucidate the effect of hot water treatment (HWT, 50 oC, 10 min) on the softening of ‘Hom Thong’ banana (Musa acuminata, AAA group), firmness, cell wall composition, the activities of cell wall degrading enzymes, and the expression of related genes were investigated in the peel. The decrease in firmness was delayed in hot water treated fruit as compare to control fruit during storage at 25 oC. Application of hot water could delay the increase of water-soluble pectin correlated to fruit firmness during storage. A higher level of HCl-soluble pectin was detected in treated fruits, indicating the reduction of cell wall polymer solubilization. Loss of hemicellulose was also slowed down in treated fruits. HWT did not affect the amount of EDTA-soluble pectin and cellulose. For enzyme activity and gene expression, treatment of intact fruit with hot water reduced activities of pectate lyase (PL) and β-galactosidase (β-Gal) in the peel, and also decreased the accumulation of their mRNAs (MaPL1 and MaGAL, respectively). Polygalacturonase (PG) enzyme activity was inhibited by heat treatment, and the increase in MaPG1 expression was delayed. HWT had little effect on pectin methylesterase (PME) or β-1,3 glucanase (Glu). After treatment, reduced PME activity was measured, but activity returned to levels similar to that of control fruits. PME activity varied during storage and did not correlate well with MaPME1 expression. There was no significant difference in Glu enzyme activity or MaGLU expression between treated and control fruit. Further investigation in cell wall structural changes following HWT through microscopic observation showed that structural alterations in peel of hot water treated banana fruits being slower as compared to untreated fruits. The reduction of peel thickness and number of cell layer occurred during storage was retarded by exposure to hot water. The formation of air space in ground tissue of hot water treated fruit was slower than that of control fruits. These results suggest that postharvest HWT was effective in preventing cell wall degradation of ‘Hom Thong’ banana fruit and reduced activities and gene expression of some enzymes associated with cell wall modification leading to maintainence of firmness in ‘Hom Thong’ banana.