Microstructural, techno-functional, and in vitro starch digestion characterization of New Zealand pea varieties : a template to design sustainable low glycaemic 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. EMBARGOED until 22 May 2025.
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2023-12-16
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Massey University
Embargoed until 22 May 2025
Embargoed until 22 May 2025
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The results presented in this dissertation provided a systematic investigation into the microstructural components (starch granules, protein matrix, and encapsulating cell wall) in the different forms of pea seed microstructure from four (4) varieties and how the interaction between these structural components during processing influence its starch gelatinization and hydrolysis properties. Based on the fundamental information provided above, the thesis explored a sustainable processing technique to transform pea seeds into a novel pea ingredient with low glycaemic features.
This first study examined how the microstructure of New Zealand pea varieties: White/yellow (WP), Marrowfat (MFP), Blue (BP), and Maple (MP) respond to pre-and-post starch gelatinization conditions. The microstructural characteristics of raw pea seeds were evaluated via scanning electron microscopy and image analysis before studying their hydration kinetics at 30, 40, 50, or 60 °C (pre-starch gelatinization conditions) while in-vitro oral-gastro small intestinal digestion was performed on the cooked pea seeds (post-starch gelatinization condition). For the raw sample, the cell wall thickness for the pea varieties differed significantly from each other and followed a decreasing order of MP > MFP > BP > WP. The shortest time (139 min) for the soaked pea to reach its saturation point was exhibited by BP at 60 °C while the lowest moisture content of soaked peas at saturation point was found in MP at 60 °C. The starch hydrolysis (%) of the cooked pea varieties during oral-gastro-intestinal digestion in vitro followed a decreasing order of WP > MP > MFP > BP. The discernible irregular particles (protein bodies, fibre fragments) attached to or between the starch granules observed in both hydrated and cooked pea seed microstructure seemed to modulate the inflow of water and starch-degrading enzymes.
The next study investigated the role of cell wall permeability in the microstructure and rate of starch digestibility in intact cotyledon cells from different varieties of pea seeds. PFG-NMR coupled with light and confocal microscopy were employed to evaluate the cotyledon cells' diffusion coefficients and cell wall permeability. The diffusion coefficients and cell wall permeability of the cotyledon cells followed a decreasing trend; WP>MFP>MP>BP. The varying size of internal cavities in the microstructure in the cotyledon cells as observed by the light and confocal micrographs may be responsible for this trend. The extent of starch hydrolysis recorded from the cotyledon cells somewhat followed the same trend of the cell wall permeability. Thus, indicating that the more permeable the cotyledon cell to the starch-degrading enzymes, the higher the extent of intracellular starch hydrolysis. The microstructure changes in the cotyledon cells during digestion also confirmed this observation.
Based on the fundamental insights provided by the previous studies, the next study compared the microstructural, nutritional, and starch digestibility properties of a novel cotyledon flour prepared via micronization techniques (colloid milling) with a blended flour from the same botanical sources. The SEM characterization of both flours showed a distinct difference in their microstructural arrangement. The protein and fibre contents of cotyledon flour were higher than those of the blended flour from the same plant sources. The starch hydrolysis and glycaemic response of cotyledon flour were almost 10 % lower than that of the blended flour. This could result in the cell wall of cotyledon cells acting as a primary barrier that regulates the inflow of starch-degrading enzymes to the intracellular starch granules. Also, the high-quality protein/cellular matrix found in the cotyledon flour may reduce the exposure of the extracellular starch granules to degrading enzymes. This study provided fundamental insights into how to sustainably process whole pulse seeds.
Finally, wheat flour for making bread was replaced with 25 and 50 % of cotyledon flour and the effect of this on the microstructure, physical-functional properties, starch digestion in vitro, and the glycaemic response were investigated. The micrographs of these three bread samples showed a distinct microstructural organization between the cotyledon flour-formulated bread and the control bread samples. Intact cotyledon cells and high levels of cellular materials were observed in the cotyledon flour-formulated bread samples. The protein, fibre, and resistant starch in the cotyledon flour-formulated bread were significantly higher than the control bread. The bake loss, volume, and specific volume decreased with an increased percentage of cotyledon flour used in the bread formulation. The colour of the crumb and crust of the cotyledon flour-formulated bread was significantly different from the control bread while the textural profile showed that the crumb hardness and cohesiveness of the bread samples increased with an increase in the percentage of the cotyledon flour added to the formulation of the bread. The starch hydrolysis for this study showed bread made with 25 and 50 % cotyledon flour was significantly lower than the control bread sample. The intact cotyledon cells with high cellular integrity observed in the microstructures of the bread samples confirmed this trend.
In conclusion, this thesis provided fundamental insights into forms of microstructure (cotyledon cells and pulse flour) that can be generated from whole pulse seed via size-reducing techniques structural components (starch granules, protein matrix, and cell wall) in each form and how the interaction between these components influences the starch hydrolysis in each form. One of the significant fundamental knowledge areas provided by this dissertation was that achieving a sort of equilibrium between “applicable particle size” and “intactness of microstructure” in processing a pulse seed could be a suitable template for designing a “wholesome” pulse food ingredient with medium glycaemic features.