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Understanding the regulation of texture degradation during apple softening - a kinetic modelling approach

Gwanpua, S. G., Verlinden, B. E., Hertog, M. L. A. T. M., Nicolai, B. M., Hendrickx, M., Geeraerd, A. H.
Acta horticulturae 2018 no.1194 pp. 1399-1406
alpha-N-arabinofuranosidase, apples, arabinose, beta-galactosidase, cell walls, depolymerization, differential equation, fruit quality, galactose, gene expression, genes, mathematical models, pectinesterase, pectins, polygalacturonase, porosity, solubilization, texture
Despite being a subject of intense research over the past four to five decades, a comprehensive mechanism of fruit softening is yet to be deciphered. In the current study, mathematical modelling was used to investigate different hypotheses of regulation of apple softening. First, a conceptual model of pectin degradation during apple softening was developed. It was assumed that the first modifications in pectin are the loss of side chain neutral sugars, galactose and arabinose, by the action of beta-galactosidase and alpha-L-arabinofuranosidase, respectively. This results into increased porosity of the cell wall. Pectin polysaccharides with low degree of branching are demethoxylated by pectin methyl esterase. Finally, demethoxylated pectin chains are attacked by polygalacturonase, resulting into depolymerisation, and later solubilisation, of cell wall pectin. As a consequence of these modifications in the pectin chains, there is a partial loss in the adhesive properties of pectin, resulting in softening. This conceptual model was translated into a mathematical model, representing the different reactions by ordinary differential equations. The model was calibrated using data on cell wall-related gene expression, enzyme activities of key pectin degrading enzymes, and the associated modifications in cell wall pectin during softening in 'Jonagold' apple. Using the model, in silico investigations of the impact of downregulation of different key cell wall-related genes on softening were carried out, and the results compared with observations in literature.