Fibrous structure formation in meat analogues based on plant and microalgae protein
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2024
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Doctoral Thesis
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Abstract
The rationale behind this research lies in the global challenge to sustainably nourish the growing world population. In this context, plant-based meat analogues (PBMA) produced through high moisture extrusion cooking (HMEC) present a promising alternative protein-rich food source to meat, whose consumption must be decreased. This research aimed to elucidate the mechanism behind the fibrous structure formation during HMEC, extending its application to a broader range of plant and single cell ingredients. Although PBMA exhibit a fibrous meat-like structure macroscopically, their microscopic structure only reveals inhomogeneities that may display some orientation and observations at nanoscale are scarce. In this work, in situ observation of the solidification of soy-based extrudates using small-angle neutron scattering (SANS) disclosed that the nano-structural constituents are globular-shaped proteins with a diameter of 9 nm, forming densely packed nano-aggregates. The lack of anisotropy in the scattering patterns, indicated that neither the individual proteins nor the aggregates, nor any other components, were elongated and oriented. Thus, previously proposed underlying mechanisms of protein unfolding, elongation and orientation or spinodal phase separation are unlikely. Instead, three novel hypotheses regarding the fibrous structure formation were suggested consistent with the SANS observations. Namely, chain-like arrangement of protein nano-aggregates, fractures of the viscoelastic mass in the flow field and sharp temperature-dependent solidification were proposed. Supplementary rheological tests were conducted in a high-pressure shear cell (HPSC) under extrusion-like conditions at 140 to 160°C, 20 bar, with shear rates of up to 40 s-1 for up to 600 s to study viscoelastic properties during structure formation. In the HPSC, PBMA-like fibrous structures could be successfully replicated based on soy protein concentrate, indicating the mechanistic sequence of melting, polymerisation and fracturing as underlying mechanism. In contrast to HMEC, shear was only applied during the hot-holding time in the shear cell. Hence, polymerisation at elevated temperature appears more relevant than solidification, as often suggested for HMEC. Besides utilizing HPSC for in situ viscosity tracking, its potential as a rapid screening tool for structurability of raw materials was assessed. While the process-structure response for HMEC and HPSC processing of soy protein concentrate was similar, pea protein isolate could not be reproducibly structured in HPSC. Thus, the potential exists, but advancements in setup, such as higher torque or implementation of a sealing, are required to enhance its applicability range and to enable its potentially reliable use as screening tool.
Microalgae biomass has been repeatedly proposed as ingredient for meat analogues due to its high nutrient density, fast biomass productivity and ability to be cultivated on non-arable land. Nonetheless, its capability to form fibrous structures has been reported to be limited. This research screened commercially available microalgae powders for their suitability as raw materials for HMEC. According to the above-mentioned mechanistic insight, highlighting the relevance of polymerisation, particular attention was given to interaction potential of proteins. In a first study, the amino acid profiles of eight commercially available microalgae powders were determined. The quantification of true protein content encountered challenges due to substantial variability of up to 17 % in total amino acid content across determinations from two different research facilities, reflecting the difficulty of achieving complete hydrolysis without degradation. Nevertheless, the relative amino acid profiles were consistent for both determinations. Multiplying total nitrogen by a conversion factor was also not an effective method due to high non-protein nitrogen levels, which reached up to 15.4 % as well as variations in amino acid profiles among different biomasses, resulting in conversion factor deviations of up to 20 %. Ultimately, true protein content was determined as the product of proteinaceous nitrogen and a biomass-specific nitrogen-to-protein conversion factor kA. The protein analysis revealed a balanced amino acid profile for all tested Chlorella biomasses, whereas Auxenochlorella contained higher protein content but an imbalanced amino acid profile due to limited sulfuric sidechains. Subsequent extrusion trials were executed with heterotrophic, yellow Auxenochlorella biomass because of the high true protein content of 41.7 to 44.3 g / 100 g. Microalgae-containing extrudates exhibited weaker structural integrity compared to conventional ones, even at equal protein and fat content and after cell disruption. This observation underscored the knowledge gap regarding required raw material properties for HMEC. A comparison of protein properties of conventional ingredients and microalgae biomass disclosed that microalgae contained proteins that were on average 20 % smaller, with higher solubility and less hydrophobic sidechains. Additionally, microalgae biomass suspensions had lower pH and a high concentration of kosmotropic ions, which may hinder protein interactions. Measures such as denaturation, pH adjustment, addition of polyvalent cations, transglutaminase or cysteine were proposed to enhance the structurablity of Auxenochlorella biomass. Furthermore, the importance of understanding the influence of species selection, cultivation and processing on composition and techno-functional properties was discussed.
Overall, novel insights into the formation of meat analogues were gained and hypotheses regarding the underlying mechanism in HMEC were formulated. The understanding of compositional differences between microalgae biomass and conventional raw materials was expanded. These findings pave the way for advancing microalgae as staple food ingredient and for making HMEC applicable to novel raw materials, thereby optimising the sustainability of meat analogues.
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ETH Zurich
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EXTRUSION AND TEXTURATION OF FOOD (FOOD INDUSTRY); microalgae protein; Plant protein-based meat alternatives; Meat analogues; Fibrous structure formation
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09571 - Mathys, Alexander / Mathys, Alexander