Microstructural analysis of edible plants : the possibility of designing low glycaemic biomimetic plant foods : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, Manawatū, New Zealand

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In the past decades, there has been an increasing interest in quantifying the relationship between structures inherent in plant-based foods and health-benefiting functionality, in particular the dynamic release of nutrients and bioactive compounds during food assimilation in the human gastrointestinal tract. This structure-function relationship can inspire the design of novel biomimetic food structures for nutrient delivery. The major objectives of research studies presented in this dissertation were (1) to investigate the role of plant-based food structures in slowing down in vitro starch digestion and (2) to contemplate the possibility of designing biomimetic plant foods for reduced glycaemic impact. Starch granules are physically trapped in plant cell walls. In order to gain a clear understanding of the cell wall encapsulation of starch, raw intact cotyledon cells and starch granules were isolated from various legumes. The gelatinisation and digestion properties of intracellular starch were quantified and compared with those of isolated starch. Cotyledon cells (mean diameter, D50~98–118µm) contain numerous starch granules that are tightly embedded in the cytoplasmic protein matrix and enclosed within cell walls. Results showed that starch inside cells exhibited restricted swelling and delay in gelatinisation as well as a substantial reduction in the rate and extent of α-amylase hydrolysis compared with isolated starch. Scanning electron microscopy of cells revealed that the cell walls remained intact throughout cooking and digestion. In another study, raw intact parenchyma cells and starch granules were isolated from Agria and Sunlite potato cultivars. These cells (D50~223–227µm) contain numerous starch granules of varying sizes and shapes that are trapped in cell walls. This entrapment of starch resulted in higher gelatinisation temperatures in Agria cultivar as well as lower peak and breakdown paste viscosities observed in both cultivars. However, no measurable differences in in vitro amylolysis kinetics were found between parenchyma cells and isolated starch. The results from these two studies showed that the entrapment of starch within the robust thick cotyledon cell wall and protein matrix restricts water and space inside the cell for gelatinisation and limits enzyme access, therefore slowing down starch digestion in legumes. However, the entrapment of starch within the thin parenchyma cell wall doesn’t inhibit starch gelatinisation and digestion in potatoes. It was suggested that the presence of the compact protein matrix encapsulating starch granules in the legume cell, which is absent in the potato cell, could explain the observed difference in digestive behaviours of entrapped starch. To investigate this, isolated navy bean cells were treated with pepsin for 1, 4, or 24 h to degrade the protein matrix to different degrees prior to in vitro digestion. It was found that increase in the treatment time generally resulted in lower protein content of cells and higher initial rate and extent of amylolysis. It was speculated that the protein matrix, aside from the cell wall, could act as an additional physical barrier limiting starch-amylase interactions. Consequently, the pepsin cleavage of intracellular proteins may promote access/binding of α-amylase to starch. To delve into how starch inside plant cells is digested, a novel apparatus was developed for time-lapse optical microscopy of a cohort of individual navy bean cells through each stage of simulated cooking followed by in vitro gastric and small intestinal digestion. The apparatus enabled direct observations of cell wall intactness and small intestinal digestion of starch that progressed inwardly from the periphery towards the centre of each cell. The new technique also allowed quantitative characterisation of the kinetics of amylolysis at the single-cell scale. The knowledge gained from previous studies enabled the biomimetic creation of novel food structures. Calcium-induced gelation of pectin in the presence of corn starch led to the formation of starch-entrapped particles. Entrapped starch exhibited a marked reduction in the rate and extent of α-amylase hydrolysis compared with free starch. It was suggested that the pectin matrix hinders α-amylase access to starch in a similar manner to the legume cotyledon cell wall. The final study was conducted to explore the effect of sorghum protein in liming in vitro starch digestion in two flour systems: (i) natural whole grain sorghum flour and (ii) binary blends of sorghum starch and kafirin protein isolate (biomimetic flour). Proteins in both systems greatly decreased the rate and extent of starch hydrolysis, possibly due to the formation upon wet cooking of disulphide-bonded kafirin network impeding α-amylase access to starch. In conclusion, the relationship between natural plant-based food structures and functionality can help to guide rational design and engineering of novel biomimetic foods. The present work demonstrated that starch-entrapped particles can be fabricated from isolated food-grade ingredients using processing technologies for the delivery of nature-like functionality in food systems (i.e. modulation of starch digestion for slow glucose release).
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Food, Analysis, Plants, Edible, Biomimetics, Glycemic index, Nutrition, 400405 Food engineering