The self-assembled network and physiological degradation of pectins in carrot cell walls

The pectin structure determines cell wall properties, such as their stiffness, diffusivity and intercellular integrity, and this, in turn, determines the macro-properties that are important for consumers and industry. However, due to complex and dynamic physiological changes, the pectin molecular structure and their assemblies in cell walls are still not well described. In this study we characterise the pectin molecular structures of cell walls at different ripening stages using an atomic force microscope. The pectins used in this study were isolated from carrot cell walls as water soluble (WSP), chelator soluble (CSP) and diluted alkali soluble (DASP) pectin fractions. New structural features of pectins in the WSP and DASP fractions were obtained. WSP contained a high proportion of small pectic polymers, whereas DASP molecules on mica were organized into a regular network in early matured tissue. The CSP fraction contained a mixture of both chains and also contained short polymeric structures. The pectin network in the DASP fraction on mica formed 50–70 nm spaces, which contained cellulose and hemicellulose fibrils. The length of the molecules in CSP and DASP decreased and the network structure observed in DASP was lost during postharvest storage. This provides evidence that pectin enzymatic degradation, which occurs in the network and to individual molecules, is one of the mechanisms responsible for softening fruits and vegetables.