Abstract
Consumer popularity of plant-based dairy alternatives is surging driven by dietary and environmental factors, however current products lack in nutritional and functional protein qualities compared to their dairy counterparts. The main objective of this PhD research was to evaluate the potential of pulsed electric field (PEF) technology to improve the nutritional and functional quality of plant protein when applied as a commercially relevant pasteurisation process in plant-based dairy (milk and yoghurt) applications. Oat milk (Avena sativa) was prepared using α-amylase for saccharification followed by filtration and blended with a commercial pea protein isolate (Pisum sativum). The resulting oat-pea blended milk alternative (OPMA) was applied as model food system in this research and is classified as a good source of protein with a complete amino acid profile. Following a validation of PEF pasteurisation parameters, impacts of PEF were assessed on relevant nutritional and functional qualities of the OPMA and the corresponding fermented oat-pea blended yoghurt alternative (OPYA).
Application of a continuous PEF treatment at 10 kV/cm combined with mild preheating was effective at microorganism inactivation in OPMA based on the applied preheating temperature and specific energy input. Microbial challenge testing was conducted and showed a minimum 5-log reduction in relevant surrogates (Escherichia coli and Listeria innocua) alongside a complete inactivation of naturally occurring microbial contaminants after preheating to a minimum of 35 oC followed by PEF at 192 kJ/L. A 90 % reduction in exogenous α-amylase activity was also achieved at specific energy of 205 kJ/L with preheating to 40 oC, related to a complementary presence of electric field and thermal effects during PEF. This finding suggested that PEF processing could be used to ensure food safety in plant-based dairy applications whilst also terminating enzymatic treatments, used during plant milk preparation.
Examining the effects of PEF at the same process parameter on OPMA protein contents, a beneficial influence could be detected on in vitro protein digestibility with increasing specific energy and preheating temperatures. This resulted in a maximum 8 % increase in the degree of proteolysis after intestinal digestion at the highest PEF treatment intensity using preheating to 45 °C and PEF at 213 kJ/L. As a result, nutritional protein qualities of PEF-pasteurised OPMA improved by up to 6 % compared to OPMA without PEF treatment, as reflected by in vitro protein digestibility corrected amino acid scores (ivPDCAAS). This was accompanied by a 15 % decrease in trypsin inhibitor activity. Significant impacts on ζ-potential, particle size, surface hydrophobicity and intrinsic fluorescence further demonstrated modifications of protein and non-protein compounds during PEF. This resulted in an enhanced solubilisation of globular protein (legumin, vicilin and convicilin) with increasing process intensities, as observed in the results of SDS-PAGE and soluble nitrogen contents.
The subsequently fermented OPYA showed a further 16 % increase in protein digestibility alongside 17 - 18 % decreases in trypsin inhibitor and phytic acid contents which improved ivPDCAAS by 14 % when applying PEF (40 °C and 205 kJ/L) prior to lactic acid fermentation (35 °C, 10h) compared to the control (OPMA without PEF and fermentation). The benefits of applying PEF pasteurisation prior to fermentation exceeded the additive results of PEF and fermentation individually which indicated for the first time a potential synergistic effect between these two processes. Structural modifications of globular protein during PEF further induced acid gelation properties. This could be applied to form yoghurt-like textures by applying PEF prior to fermentation in which the resulting textural attributes can be controlled by adjusting PEF treatment intensities. While temperature increases during PEF were identified as a driving force for the observed results, significant impacts on protein structure, solubility and gelation could be related to electric fields applied during PEF as secondary (non-thermal) mechanism.
This thesis provides evidence that PEF can be used to pasteurise plant-based milk alternatives and inactivate enzymes whilst achieving beneficial modifications of plant protein structure that results in enhanced nutritional (digestibility) and functional (solubility, gelation) protein properties. This will be of value to address challenges of current commercial plant milk and yoghurt products that consistently fall short in their nutritional and functional equivalence to dairy. The developed oat-pea model dairy alternative presents a suitable option to provide required protein contents, while processing using PEF can be used to optimise its functional and nutritional qualities. Improvements to protein solubility are of particular interest to mitigate sedimentation related quality defects in plant milks, whilst a formation of yoghurt like textures by acid protein gelation will be essential during the development of clean label plant-based yoghurt alternatives. Overall, this presents PEF as a multifunctional tool with promising potential in the emerging product category of nutritionally enhanced and functionally desirable plant-based dairy alternatives.