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Polysaccharides play a role in firming up apples

Polysaccharides play a role in firming up apples
The interactions between cellulose, pectins and water in plant cell walls are responsible for the mechanical properties of apples.
The Nova2Cidre joint technology unit and the IFPC (French Institute for Cidermaking) have been working to understand the root cause of variability in cider-apple pressability.
The juice yield extracted by pressing depends on the firmness of the fruit at grating and at filtration when it goes through the press. These mechanical properties stem from the assembly and arrangement of the apple cell-wall polysaccharides but also from tissue-scale fluid flow in the gratings and from the pattern of cell-cell deformations occurring through the process.
To understand the variability in apple batches, we need to first unravel the complex factors associated with the mechanical properties at work at these different scales.
The firmness of six apple varieties was measured on fresh crops and after destructuring by freeze-thaw cycles, and the results showed distinct patterns of rheological behaviors between the varieties.
Analysis of cell-wall polysaccharides chemistry revealed fine structures of the pectins that add or subtract firmness.
Structural and dynamic parameters measured by nuclear magnetic resonance (degree of crystallinity in the cellulose, relaxation and diffusion in solid media) were also linked to the firmness and showed that the crystalline organization of the cellulose plays no role in the firmness of the fruit.
Furthermore, when the cellulose fibres were more loosely organized in the cell walls (faster relaxation), the pectins were more hydrated (slower diffusion) and the fruits proved firmness.
These results clearly indicate that mechanical properties of the fruit are governed not only by fine-grained polysaccharide structure and polysaccharide group-polysaccharide group interactions, but also by how well these different polysaccharide groups (especially pectins and cellulose) are organized and hydrated.

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