Variations in Cell Wall-Modifying Enzyme Activities and Physicochemical Properties of Pectin in Kiwifruit during Postharvest Softening

In this study, we monitored changes in fruit hardness, cell wall-modifying enzyme activities such as polygalacturonase (PG), pectin methylesterase (PME), pectin lyases (PL) and β-galactosidase (β-Gal), and physiochemical properties of three cell wall polysaccharides including water soluble polysaccharide (WSP), trans-1,2-cyclohexanediaminetetraacetic acid (CDTA)-soluble polysaccharide (CSP) and Na2CO3-soluble polysaccharide (NSP) in ‘Yate’ kiwifruits during postharvest cold storage. Results showed that postharvest kiwifruits went through a ‘fast-slow’ softening process. PG activity showed a maximum value of 670.76 U/mg after 10 days of storage. PL and PME activities showed a maximum increase over the first five days of storage, reaching up to 3.41 and 9.17 U/mg, respectively. Particularly, β-Gal activity continuously increased during storage and showed a negative correlation with fruit hardness (r = −0.888). The pectin contents of WSP, CSP and NSP increased significantly with storage time. Moreover, the results of high performance gel permeation chromatography (GPC) showed that the weight average molecular mass of WSP increased, that of CSP initially decreased and then increased, and that of NSP gradually decreased during storage. Furthermore, a decrease in the area and an increase in the lamellarity of WSP were observed by scanning electron microscopy (SEM), whereas CSP was found to be soft and muslin-like after 15 days of storage, and the cluster-like accumulation of NSP was gradually dispersed with more debris being observed. These results may provide a solid theoretical basis for elucidating the exact mechanism of kiwifruit softening.