Does harvesting age matter? Changes in structure and rheology of a shear-thickening polysaccharide from Cyathea medullaris as a function of age

A shear-thickening polysaccharide from the New Zealand Black tree fern (Cyathea medullaris, commonly known as mamaku) extracted from different age fronds (stage 1: young, stage 2: fully grown and stage 3: old) was characterised in terms of structure and rheological properties. Constituent sugar analysis and H-1 and C-13 NMR revealed a repeating backbone of -4)-beta-D-GlcpA-(1 -> 2)-alpha-D-Manp-(1 ->, for all mamaku polysaccharide (MP) samples from different age fronds without any alterations in molecular structure. However, the molecular weight (M-w) was reduced with increasing age, from similar to 4.1 x 10(6) to similar to 2.1 x 10(6) Da from stage 1 to stage 3, respectively. This decrease in M-w (and size) consequently reduced the shear viscosity (eta(s-Stage 1) > eta(s-Stage 2) > eta(s-Stage 3)). However, the extent of shear-thickening and uniaxial extensional viscosity of MP stage 2 was greater than MP stage 1, which was attributed to a greater intermolecular interaction occurring in the former. Shear-thickening behaviour was not observed in MP stage 3.