Interactions between wheat starch and Mesona chinensis polysaccharide : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand
This thesis studies the interaction between wheat starch and Mesona chinensis polysaccharide (MCP) and its impact on the properties of the composite gel. Composite MCP and wheat starch gels were studied using solid-state NMR, whereby close proximity (5 Å) between MCP and glucan polymers was established, indicating that both polymers interacted at molecular level. MCP was found to increase the molecular mobility of the carbon 6 fraction found in starch glucan polymers, suggesting that it is likely to be the site of interaction.
The granular swelling, amylose leaching, and gelatinisation properties of 2 % w/w wheat starch were studied in the presence of increasing concentrations of MCP. At room temperature, the presence of MCP (0.1 – 5 % w/w) induced shear-reversible starch aggregation. Heating of starch-MCP suspension resulted in an earlier onset of viscosity increase that was characterised by a peak “M”. This was also accompanied by delayed granular swelling and a 40 % reduction in amylose leaching when 7 % w/w MCP suspension was present. Interaction was thought to occur when amylose leaches out of wheat starch granules forming an MCP-amylose barrier, increasing the apparent size of the granules. The final G’ of starch gel (1.1 Pa) increased during cooling in the presence of increasing concentrations of
MCP up to 20 Pa when 5 % w/w MCP was present, indicative of a 3D network comprising of amylose,
MCP and starch granules. The final gel properties were dependent on the concentration of MCP
present whereby an extensive MCP-amylose network was required to stabilise the starch granule aggregates despite a reduction in the amount of amylose leached. The elastic modulus (G’) and hardness of gels containing high starch concentration (≥ 8 % w/w) was decreased at low (< 3 % w/w) MCP level and subsequently increased by more than 35 % at 5 % w/w MCP. In contrast, when starch concentration was lowered (5 % w/w), the viscoelasticity gradually increased in the presence of MCP. Microscopy, DSC and syneresis data showed that a heterogeneous gel network microstructure was formed when there was insufficient MCP and/or amylose, which reduced the final G’ and hardness of the gels, and accelerated their retrogradation.
Factors such as starch type, starch to MCP ratio, pH and salt were found to affect the rheological
properties of wheat starch-MCP gels. All of the starches (wheat, maize, potato and waxy maize)
studied were found to gel at high MCP concentrations (3 – 5 % w/w). At low MCP concentration (1 % w/w), starch gelation was only observed for non-waxy starches, with the largest increase (1800 %) in gel strength being observed for 5 % w/w wheat starch. This was reduced when the ratio of wheat starch to MCP was decreased and this was thought to be due to a reduction in the total amylose present to form a 3D network and insufficient granules to reinforce the network. The pH of the system was found to influence the strength of the composite gel, whereby the G’ of the gel was decreased at pH 4 and increased when the pH was higher than 5. This is likely to be due to a change in the conformation of MCP (compact at low pH and open random coil at high pH), which allows more/less sites for interaction with amylose. The addition of monovalent salts was found to increase the strength of the gel, while the opposite was found for divalent salts.
The digestibility of wheat starch gels in the presence of MCP was also investigated. MCP was found to be the most effective polysaccharide in reducing wheat starch digestion in comparison to starch gels of similar hardness containing xanthan, guar, locust bean gum (LBG) or agar. A 33 % reduction in the digestibility of intact starch gel containing 5 % w/w MCP (after 120 minutes of digestion) was observed and this was attributed to the strengthening of the gels in the presence of high concentration of the polysaccharide. In contrast, despite a reduction in the firmness of the gel when 2 % w/w MCP was present, there was a 7 % reduction in starch digestibility and hence, firmness was deduced to be not solely responsible for the digestibility of the gels. When these gels were macerated, starch digestibility was reduced regardless of the MCP concentration. Starch digestion in the macerated samples seemed
to cease after 10 minutes with about 30 % more starch remaining when 5 % w/w MCP was present.
This suggests that the amount of starch available for digestion was reduced when MCP was present.
The reduced availability of starch for digestion was hypothesised to be due to starch-MCP interaction, which formed amylose-MCP complexes that would be resistant to enzymatic digestion.
Additionally, MCP itself was found to inhibit α-glucosidase uncompetitively. Upon calcium removal from the extract, the extent of this inhibition was reduced but MCP exhibited an additional inhibition towards α-amylase in the same manner (uncompetitive inhibition). The removal of calcium increased the zeta potential of MCP, indicating that the conformation of MCP may be less compact due to the presence of negatively charged groups on the polysaccharide. This was thought to expose sites that could bind onto α-amylase, hence inhibiting the enzyme. Due to its enzyme inhibitory activities, MCP may be used as an ingredient to reduce postprandial glycaemia by inhibiting α-amylase and α-glucosidase.