Solid-state 13C NMR study of the mobility of polysaccharides in the cell walls of two apple cultivars of different firmness

• NMR was used to study the mobility of polysaccharides in the cell walls of apples.
• Two Pulse Phase Modulation (TPPM) decoupling significantly improved the resolution.
• The texture of ‘Scifresh’ apples was markedly firmer than that of ‘Royal Gala’.
• The arabinans in ‘Scifresh’ were more mobile than those in ‘Royal Gala’.
• The arabinans were more mobile than the galactans in both cultivars.

Solid-state 13C nuclear magnetic resonance (NMR) was used to compare differences in mobility of the cell wall polysaccharides of ‘Scifresh’ and ‘Royal Gala’ apples after 20 weeks of storage. The texture of ‘Scifresh’ apples was markedly firmer than that of ‘Royal Gala’ at the end of storage. In a novel approach Two Pulse Phase Modulation (TPPM) decoupling was combined with cross polarisation (CP) and single pulse excitation (SPE) experiments. The resulting high resolution solid-state SPE spectra, unprecedented for apple cell walls, allowed a detailed insight into the physical and chemical properties of very mobile polysaccharides such as the arabinan and galactan side chains of the pectic polysaccharide rhamnogalacturonan I (RG-I). NMR showed that the cellulose rigidity was the same in the two cultivars, while arabinans were more mobile than galactans in both. Unexpectedly, arabinans in ‘Scifresh’ cell walls were more mobile than those in ‘Royal Gala’ which was unforeseen considering the greater firmness of the ‘Scifresh’ cultivar.

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