Characterisation of de-structured starch and its interactions in whey protein isolate gels : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand

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Starch serves as an important additive to enhance the physico-chemical properties of many food products. With the increased pursuit of natural products, there is an increasing demand for “clean-label” starches. In this study, waxy potato starch was physically-modified at elevated temperatures of 120–150 °C for 30 min at 300 rpm, in a pressurised reactor. The treatment converted native starch granules into their macromolecular chains (denoted as de-structured waxy potato starch, DWPS). This doctoral thesis presents the: (i) method of modifying starch (i.e., the de-structuring process), (ii) the mechanism of starch de-structuring, (iii) the rheological changes in DWPS samples and the shear-thickening mechanism, and (iv) the interactions of these DWPS ingredients with whey protein isolate (WPI) in a protein-based gel system, at different pH and ionic strength. The molar mass (Mᵥᵥ), particle size, rheological properties, degree of branching (DB) and side-chain length distribution of DWPS samples were characterised to elucidate the starch de-structuring mechanism. DWPS treated at 120 °C DWPS showed similar Mᵥᵥ (~3.6 × 10⁸ Da) as its native form (~3.7 × 10⁸ Da) indicating that the treatment at 120 °C resulted in the disassembly of starch granules into their macromolecular chains. Reduction in viscosity, Mᵥᵥ and particle size was observed with an increase in temperature from 120 to 150 °C, suggesting a cleavage in amylopectin chains. The DB and side-chain distribution data suggest that the reduction in Mᵥᵥ is likely due to the cleavage at α-1,4 linkages near the middle of the main amylopectin backbone. Particle size analysis by laser diffraction measurements revealed the presence of large fragment particles (> 1 µm) in DWPS samples, indicating that the starch de-structuring process into its macromolecules was incomplete even at 150 °C for 30 min. The DWPS (5% w/w) samples were found to exhibit a wide range of rheological properties—Newtonian, shear-thinning, shear-thickening and anti-thixotropy behaviours—depending on their treatment temperature (120–150 °C). In particular, 120 °C DWPS exhibited interesting shear-thickening, anti-thixotropy and shear-induced gelation. These rheological properties are different from the shear-thinning and thixotropy behaviours observed in most conventionally gelatinised waxy potato starches treated at 95 °C. The complex shear-induced structures of 120 °C DWPS were attributed to a two-step process: (i) upon shear at the critical shear rate (~10–20 s⁻¹), the shear stress caused a size reduction in the starch fragments and (ii) the increased number of small fragments together with the amylopectin chains in very close proximity could lead to the formation of a complex network probably consisting of amylopectin chains and a large number of fragments (2–20 μm). Shear thickening properties were attributed largely to these soft fragment particles colliding and sliding past each other during shear. The data from this study has also shown that the hydrogen bonding, electrostatic, hydrophobic interactions, or the combination of these interactions did not cause the shear-thickening behaviour. The influence of 4% w/w DWPS on 13% w/w WPI gels was studied by characterising the phase stability of the liquid mixtures, and mechanical properties, microstructure, and water-immersion stability of fine-stranded polymeric and coarse-stranded particulate protein gels at pH 7 and pH 5, respectively. At neutral pH, synergistic gel hardness of WPI was obtained with the incorporation of 140 °C DWPS. The increased gel strength was attributed to the enhanced density of a very fine-stranded gel network. The ability of the gel to retain its shape when immersed in water for 40 h was most noticeable for the composite gels containing either gelatinised starch or DWPS samples (swollen gels but with intact shape). In contrast, pure WPI gel and composite gel containing maltodextrin turned into very weak fluid-like and disintegrated gels, respectively. At pH 5, WPI formed particulate gels. The addition of gelatinised starch or DWPS weakened the particulate protein gels, likely due to phase separation and interrupted protein network with starch polymers acting as inactive fillers. The effects of NaCl and CaCl₂ (i.e., type of salts and ionic strength) on the mechanical and microstructural properties of composite gels containing 13% w/w WPI and 4% w/w 140 °C DWPS were also evaluated. Thermodynamic incompatibility between WPI and 140 °C DWPS was observed upon the addition of NaCl (~75 mM) or CaCl₂ (10–75 mM). The combined effects of such thermodynamic incompatibility with the changes in protein connectivity induced by varied ionic strength led to the formation of distinctive gel structures (inhomogeneous self-supporting gels with a liquid centre and weak gels with paste-like consistency) that were different from thermodynamic compatible homogeneous self-supporting gels (pure WPI and WPI + maltodextrin gels). At ≥ 250 mM NaCl, instead of a paste-like texture, a recovered soft self-supporting gel structure was observed when using 140 °C DWPS. The ability to generate a range of textures in WPI gelation-based foods by using 140 °C DWPS under different ionic conditions, is a feasible strategy for structuring high-protein foods for dysphagia—aimed to be either thickened fluids or soft solids. Additionally, this acquired knowledge is also relevant when formulating food gels for 3-D printing. The desirable rheological properties of DWPS samples and their ability to alter WPI gel structure signify the potential of DWPS as a clean-label ingredient to structure foods of specific needs (e.g., whipping cream for enhanced structure upon shear and high-protein foods for dysphagia sufferers).
Permission from Elsevier was granted for the re-use of four articles published in Food Hydrocolloids. Figures 2-2, 2-4, 2-5, 2-6, 2-8, 2-9, 2-12, 2-14, 2-16, 3-7, 3-8, 3-9, 3-19, 3-20 & 3-22 are also re-used with permission. Figures 2-3, 2-7 & 2.10 are re-used under a Creative Commons CC-BY license.
Starch, Structure, Whey products, Milk proteins, Colloids