Monosaccharide composition and degree of acetylation of non-cellulosic cell wall polysaccharides and their relationship to apple firmness

The firmness of the two apple varieties: Idared and Pinova was similar during ripening, while it decreased significantly during 3-month storage only for Idared. Pectin-rich fractions were isolated from apple flesh tissue: water-soluble pectin (WSP), imidazole-soluble pectin (ISP), and hemicellulose-rich fractions: natively acetylated hemicelluloses (LiCl-DMSO), deacetylated hemicelluloses (KOH). It was shown that the degree of acetylation (DAc) of the hemicelluloses fraction (LiCl-DMSO) increased during apple ripening and storage, with higher values for Idared. Furthermore, the DAc of the hemicellulose fraction (LiCl-DMSO) was shown to be negatively correlated with apple firmness, and thus, among other factors, the effect of the degree of acetylation of hemicelluloses on fruit softening during storage. In the WSP and ISP, galacturonic acid content increased during ripening and storage of apples, which also showed a correlation with firmness. A higher content of linear pectin was recorded for Idared, while the contribution of rhamnogalacturonans was higher for Pinova.