Interactions between hemp globulins and dairy proteins : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Riddet Institute, Massey University, Palmerston North, New Zealand
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2021
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Massey University
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Abstract
Industrial Hemp (Cannabis sativa L.) is a sustainable protein source and is easily digested. However, the hemp globulins (HG), which constitute around 70% of total hemp seed protein, have low solubility in water at neutral pH. The insolubility of HG limits its usage in many food systems.
This work explored the interactions between hemp globulins (HG) and dairy proteins, and aimed to increase the functionality of HG. HG was extracted with a mild salt-extraction and heat treatment was avoided, so the native structure of HG was preserved. The composition of HG was studied and a phase diagram of HG solubilisation was obtained regarding pH and ionic strength.
Two methods were used to increase the solubility/colloidal stability of HG by introducing interactions between HG and sodium caseinate (SC). The first method is by heating HG and SC together at 90 °C and the ionic strength of 0.5 M. SC exhibited the chaperone-like activity and inhibited the formation of large aggregates of HG. The addition of SC did not change the denaturation kinetics of HG, but rather changed its aggregation pattern. The second method is by pH-cycling. HG and SC formed colloidally stable nanoparticles (Z-average diameter ≈ 130 nm) after adjusting the pH to 12, reacted for 1 hour and neutralised back to pH 7. The solubility of HG increased from ~20% to > 80% after the pH-cycling.
The mechanisms and molecular interactions of both processing methods (heating and pH-cycling) were proposed. During heating, SC interacted with HG via hydrophobic interactions and the aggregation regime of HG changed from diffusion-limited cluster aggregation to reaction-limited cluster aggregation, while the kinetics of HG denaturation was unaffected. During the pH-cycling, hydrogen bond was one of the driving forces for assembly of HG|SC nanoparticles. HG partially unfolded at pH 12 and interacted with caseins during the neutralisation and the stable HG–SC nanoparticles were formed.
The pH-cycled HG-SC nanoparticles can be used to make Pickering emulsions. Concentrated emulsions were prepared, and the rheological properties of emulsions during storage can be tuned by controlling HG:SC ratio in the HG|SC nanoparticles, i.e. emulsions became more solid-like when there was more HG in the HG-SC nanoparticles. The internal structure and interactions within the emulsions were evaluated by fitting frequency sweep test data according to a co-operative theory of flow. The results suggested that the solid-like emulsion resulted from stronger short-range interactions between flocs of oil droplets, which developed during storage when there was more HG in the HG–SC nanoparticles.
In conclusion, the findings in this study advanced our understanding of the interactions between plant seed globulins and caseins during processing. Such knowledge will help to increase the functionalities of plant proteins by mixing plant and dairy proteins.
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Hemp, Globulins, Seed proteins, Solubility, Casein, Stabilizing agents