The tale of the shear-thickening mamaku polysaccharide, from forest to gut : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Food Technology), Massey University, Palmerston North, New Zealand. EMBARGOED to 26 April 2025.

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Massey University
Listed in 2023 Dean's List of Exceptional Theses
Embargoed to 26 April 2025
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The New Zealand black tree fern (Cyathea medullaris, ‘mamaku’ in the Māori language) is grown across the Pacific Islands and has a long history of use for therapeutic benefits or as food by Māori people. The water-soluble gum extract from mamaku fern contains a novel glucuronomannan biomacromolecule, called mamaku polysaccharide (MP), which has been shown to exhibit a unique shear-thickening (i.e., increase in viscosity on shearing) behaviour at a similar shear rate as that found in the human stomach. Herein, the objective was to gain a better technical and physiological understanding of MP for designing a novel shear-thickening ingredient for the industry with proven effects in humans. The shear-thickening behaviour of MP was sensitive to the harvesting age of mamaku fronds and industrial operations such as high temperature and shear. With the increase in harvesting age, the molecular weight of MP reduced, which consequently reduced the shear viscosity. The shear-thickening behaviour was lost in MP from old fronds. Furthermore, the temperature treatment disintegrated the backbone of MP into smaller fragments which caused a reduction in viscosity and extent of shear-thickening. Similar rheological trends were observed post-shear treatment, however, there was no evidence of depolymerisation. A combination of in vitro models revealed that mamaku gum extract could improve host gut functioning by reducing the activity of digestive enzymes (α-amylase, pepsin and lipase) and binding bile acids. Mamaku gum can act as a substrate for colonic fermentation, promote the production of short-chain fatty acids and alter the colonic microbial composition. Upon ingesting mamaku gum, the shear-thickening behaviour may develop in the oesophagus causing a possible choking hazard. Therefore, the potential of using the whole pith—natural entrapment of MP in the tissue of pith—as an alternative to gum extract was studied. Freeze-dried pith was ground to powder. The powder particles swelled upon rehydration with water and released the water-soluble MP into the continuous phase in a time-dependent manner. The presence of enough MP in the continuous phase to form polymer-polymer interactions resulted in a shear-thickening behaviour of the pith powder suspension similar to the MP extract solution. Moreover, the co-consumption of 1 h pre-hydrated mamaku pith powder with a carbohydrate-rich meal significantly reduced the postprandial glycaemic response (blood glucose peak height) in human participants. Additionally, the consumption of mamaku pith powder in rats could alter colonic microbiota. Interestingly, more than half of the MP (uronic acid) consumed by rats survived the gut transition and was obtained in faeces, suggesting that MP could potentially be used as a laxative. Thus, mamaku pith could be used as an alternative to gum extract to develop a natural shear-thickening ingredient which may potentially help to manage diabetes and improve colon health.
Cyatheaceae, New Zealand, Biotechnology, Polysaccharides, Physiological effect, Therapeutic use, Dean's List of Exceptional Theses