Effects of non-thermal food processing technologies on physicochemical properties of quinoa protein isolate dispersions : a thesis presented in partial fulfilment of the requirement for the degree of Master of Food Technology at Massey University, Auckland, New Zealand
The effects of high-pressure homogenisation (HPH) (0-50 MPa for 1 pass), sonication (14.4 W, 10 mL for 0-15 min), high hydrostatic pressure (HHP) (0-600 MPa for 15 min) and heat treatment (81 ± 2 °C for 30 min) on structural and physicochemical properties changes of quinoa protein isolate (QPI) dispersions were investigated in this thesis. Firstly, the impacts of HPH on the physicochemical, microstructural, and rheological properties of QPI suspensions were studied. Individual protein bands were kept unchanged with HPH treatment up to 50 MPa, as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). After HPH treatment at 50 MPa, large protein aggregates were disrupted, and particle size was substantially reduced from ~8.2–0.5 µm. Concurrently, water solubility, emulsifying, and foaming capability were significantly enhanced. QPI molecules after HPH treatment became more flexible with increased surface hydrophobicity and adsorbed at the air-water and oil-water interfaces faster, resulting in a more rapid decrease of surface tension and interfacial tension. The secondary structure of QPI proteins was not significantly altered after HPH treatment as probed by Fourier transform infrared spectroscopy (FTIR). All QPI suspensions formed weak gels after thermal treatment (85 °C, 30 min), and HPH-treated QPI suspensions resulted in a stronger gel strength with a more compact and homogeneous protein network microstructure. Secondly, 1% w/w QPI was exposed to sonication or HHP treatments at pH 5, pH 7, and pH 9. As revealed by SDS-PAGE, substantial amounts of 11S globulin participated in the formation of aggregates via the disulfide (S-S) bond under HHP, particularly at pH 7 and pH 9. Free sulfhydryl (SH) groups and surface hydrophobicity were increased after the sonication treatment, while an opposite trend was observed in HHP-treated samples. Finally, the sonication treatment induced a significant improvement in the solubility and a reduction in particle sizes particularly at pH 7 and pH 9. Lastly, the gelation of 10 w/w % QPI dispersions induced by HHP at three pHs (5, 7, and 9) was investigated. HHP was conducted at 20 °C for 15 min at 100, 250, 400, and 600 MPa. Thermally induced gelation (81 ± 2 °C, 30 min) was also conducted as a comparison to HHP. Both thermal and HHP (above 250 MPa) treatments can induce protein gelation, and the viscosity and elasticity of the gels increased with the increase in pressure levels. The gels formed either by heat or HHP treatment at pH 5 exhibited a stronger gel strength and a compact network structure than at pH 7 and 9. Confocal microscopy showed that HHP treatment formed a heterogenous and well-spanned network with large protein aggregates, while heat treatment resulted in a more homogeneous structure. SDS–PAGE revealed that QPI gels were formed by the denaturation and aggregation of 11S globulin. These results showed that HPH and sonication had similar changes to the physicochemical, techno- functional, and microstructural properties of QPI suspensions, while HHP mainly induced protein gelation. Impacts of treatments are pH and concentration dependent. Based on the findings of this thesis, tailored properties of QPI could be achieved by selecting appropriate processing technologies.
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