Rethinking plant protein extraction : interfacial and foaming properties of mildly derived plant protein extracts

A major trend in the food industry is the protein transition from animal- to plant-derived proteins. Prior to the utilisation of these ingredients, plant proteins must be extracted from the plant or crop. The most common extraction processes are extensive processes with many processing steps, requiring copious amounts of water and energy, and generating substantial amounts of waste streams. Also, this processing might alter plant protein functionality, leading to protein aggregation and lower solubility. A solution is a milder and more sustainable process, but less processing leads to protein extracts with lower protein purity due to the presence of more non-proteinaceous components, such as lipids and phenols. The aim of this thesis was to investigate the contribution of these non-proteinaceous components to the interfacial and foaming properties of mildly derived plant protein extracts. The system of interest is rapeseed, as it is high in both phenols and lipids.We performed these studies by producing a mildly derived plant protein extract and systematically mixing purified proteins with phenols or lipids. The mildly derived rapeseed protein extract was able to form stiff and viscoelastic solid-like interfacial layers, as shown by Lissajous plots in non-linear surface rheology. However, the stiffness of the interfacial films reduced at higher bulk concentrations, indicating an impact of the non-proteinaceous components. The presence of non-proteinaceous components was also shown by performing atomic force microscopy (AFM) on Langmuir-Blodgett films. The main rapeseed phenol sinapic acid was able to interact non-covalently and covalently with proteins. Covalent interaction even led to the formation of large aggregates. The protein-phenol interaction reduced the ability of the proteins to form stiff interfacial layers, leading to less stable foams.An advantage of mild purification is the extraction of intact oleosomes, which are normally disrupted in the defatting step in the conventional protein extraction process. The lipids in the protein extract are present in structures called oleosomes (also known as oil bodies or lipid droplets), natural storage lipid storage organelles in plants. These natural lipid droplets have a triacylglycerol (TAG) core, surrounded by an interfacial layer with phospholipids and membrane proteins. An advantage of mild purification is the extraction of intact oleosomes, which are normally disrupted in the defatting step in the conventional protein extraction process. Oleosomes were found to rupture after adsorption at the air-water interface. Subsequently, the TAG core, phospholipids and membrane proteins started spreading and rearranging at the interface. The impact of oleosomes on the protein interfaces was dependent on the oleosome-to-protein ratio. The proteins could not form a stiff protein layer at low protein concentrations, leading to co-adsorption of both oleosomes and proteins. As a result, the oleosomes dominated the interfacial properties, leading to weak and easily stretchable interfacial films. On the other hand, proteins could outcompete the oleosomes, if sufficient protein is present. The proteins were able to form stiff and solid-like layers. Oleosomes were unable to adsorb when such a stiff protein layer is present. This behaviour was also reflected in foam stability, as the detrimental effect of oleosomes on protein foams was lower at higher protein concentrations.Finally, we also studied the protein properties, as extensive processing can negatively impact the protein structure. We aggregated whey proteins, and studied their interfacial properties. Large aggregated proteins were unable to adsorb at the interface. Additionally, the aggregated proteins coexist with non-aggregated proteins. This smaller non-aggregated material was highly surface active and, thus, dominated the interfacial properties. Here, we show that aggregation of plant proteins negatively impacts the interface and foam stabilising properties. Also, we studied the protein composition, as mild purification results in the co-extraction of both globulins and albumins. Albumin is a protein class that is normally considered a side-stream. In this thesis, the albumins exhibited excellent foaming properties, similar to dairy proteins. The globulins, which are the majorly studied plant protein class, were unable to stabilise foams. Albumins can be incorporated in plant protein extracts by co-extracting globulins and albumins using a mild extraction method.In summary, the non-proteinaceous components in mildly derived protein extracts can be present without altering the protein functionality. It is key to avoid covalent protein-phenol interactions during the extraction process. Also, lipids can be present if the natural structure remained intact. Protein aggregation should be avoided as much as possible, which is possible in mild protein extraction. With this thesis, we hope to provide crucial design rules in obtaining plant protein extracts with good functional properties and contribute to the protein transition.

Bibliographic Details

Main Author:	Yang, Jack
Other Authors:	Sagis, L.M.C.
Format:	Doctoral thesis biblioteca
Language:	English
Published:	Wageningen University
Subjects:	Life Science,
Online Access:	https://research.wur.nl/en/publications/rethinking-plant-protein-extraction-interfacial-and-foaming-prope
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