Properties of protein isolates extracted by ultrasonication from soybean residue (okara)
Introduction
Plant-based sources of protein are considered more sustainable for the increasing global population compared to animal protein. Among all vegetable sources of protein, soybeans have gained much popularity due to their rich content in all eight essential amino acids, as well as exhibiting similar digestibility to that of cow’s milk, meat and egg proteins (Soderberg, 2013). Soybean seeds are principally used to produce soymilk and tofu. Soymilk is increasingly becoming more popular by consumers, although, it is mostly consumed by lactose intolerant individuals, and those conscious of cholesterol. Currently, the demand for soymilk is high with the global soymilk market value amounted to about 15.33 billion US dollars in 2018 and estimated to be 23.2 US dollars in 2025 (Statista, 2021a). In the UK, the sale volume of soybean milk rose to 85.7 million litres in 2016 (AHDB, 2017) and to about 92.6 million litres in 2018 (Statista, 2021b). The main market drive for soymilk production and consumption is its association with health benefits, especially following the FDA health claim approval on soy protein effectiveness in reduction of coronary heart disease risk. Consequently, the accumulation of soymilk by-product (okara) is expected to increase; it is estimated that for every 1000 L of soymilk produced via commercial process (the soya technology systems process) or traditional process (cold extracted and no treatment to remove off-flavour), 250 kg or 398 kg of okara are generated, respectively (Gavin & Wettstein, 1990). Okara contains notable amounts of protein (26.8 %-37.5 % w/w) (Ma et al., 1997, Vishwanathan et al., 2011), since a significant proportion is left in the residue following soymilk production (O'Toole, 1999) due to the complexity of soybean structure. However, this residue is scarcely utilised and as such, it currently has little market value. Recently, okara has caught the interest of some researchers for its potential application in the food industry, as a raw material for soy protein isolate extraction, with potential applications in beef burger production, cookies, and sausage formulations. Soybean-based protein isolates are reported to demonstrate useful functional properties, that would enable their application in a variety of food systems (Singh et al., 2008). Therefore, okara could be converted into a valuable starting for commercial production of protein isolates and this approach could, in turn, minimise waste, in-line with the cradle-to-cradle concept for sustainability (Eze, 2019).
Soybean protein isolate has been produced commercially, using soybean meal or soybean flakes as feedstocks and previous studies have evaluated the extraction of proteins in soybean flakes or soybean meal in aqueous-alkaline (NaOH) conditions (conventional method), via enzymatic routes or through the use of ultrasonication-assisted methods (Ma et al., 1997, Vishwanathan et al., 2011). According to Ma et al. (1997), okara protein extraction in aqueous-alkaline (NaOH) conditions at 25 °C resulted in low protein recovery (14.1%, w/w); when temperature was elevated 80 °C, 53.4 % (w/w) of okara proteins were extracted. However, the latter extraction conditions may cause protein denaturation and aggregation as the denaturation temperatures of the two major soybean proteins, (glycinin and β-conglycinin) are approximately 82 °C and 68 °C, respectively (Riblett et al., 2001). It is also expected that most of the soluble proteins are removed during soymilk production, leaving the residue (okara) with mostly water-insoluble proteins. Heating at elevated temperature of about 80 °C is an indispensable process step during soymilk production to get a final product with desirable quality, but leaves the residue (okara) with aggregated proteins in the intact cotyledon cells that are not easily extracted (Preece et al 2017). Moreover, protein fractions that have been extracted under high temperature conditions could have low solubility and decreased thermal stability (Ma et al., 1997).
Ultrasonication technology has recently attracted much research interest as a technique to assist protein extraction processes from a variety of raw materials (Zhang et al., 2018). Ultrasonication allows the development of sustainable extraction processes by increasing extraction efficiency and at the same time reducing solvent and energy utilisation (Chemat et al., 2017). These advantages seem to be more prominent on lab-scale studies, whereas for industrial scale applications, further aspects of the process still need to be optimised (e.g. reactor design, energy consumption reduction, solid–liquid separation post-extraction) to establish ultrasonication’s industrial prospects (Chemat et al., 2020, Preece et al., 2017, Vernès et al., 2019). The mechanism of extraction by ultrasonication is based on the cavitation phenomenon which leads to particle or cell disintegration (Khanal et al., 2007). The disintegration of cell walls by cavitation exposes hidden compounds in the cells to the extracting medium, hence promoting higher extraction yields at shorter times (Mason et al., 1996). However, this effect can alter the native conformational structure of proteins with resultant changes in their functional properties (McClements, 1995). Various studies have reported the exposure of hydrophobic groups and redistribution of the secondary structure (Li et al., 2016), as well as promoting the unfolding and dissociation of protein isolates extracted via ultrasonication (Huang et al., 2017). As such, ultrasonication can employed not only for extraction processes but also for the enhancement of functional properties of proteins.
The aim of this study was to develop a protein extraction process via ultrasonication from soybean residue (okara) under alkaline conditions. It is hypothesised that ultrasonication could enhance the release of proteins located in the protein bodies of palisade-like cells in soybean cotyledon within a short time and could possibly preserve the functional properties of the protein isolates. To this end, detailed chemical analysis of okara protein isolates was carried out, together with the assessment of their structural properties.
Section snippets
Raw material and chemical reagents
Yellow soybean seeds (Glycine max) sourced from a local shop in Nigeria (year of harvest 2017) was used to produce okara (soymilk residue). All chemicals used in this research were of analytical grade and were purchased from Sigma-Aldrich (UK) and Fisher Scientific (UK).
Preparation of defatted okara flour
Yellow soybean seeds (Glycine max) were soaked in water (1:5 w/v) for 8 h and hulls were removed manually by washing. Dehulled, washed beans were ground with a hammer mill. Milk was separated from ground soybean slurry using a
Protein extraction from okara
Initially, aqueous extractions at varying pH values were carried out to investigate the influence of pH on protein recovery from okara. A range of alkaline pH values was studied (from 9 to 12) using 0.1 M phosphate buffer. The protein recovery during conventional aqueous alkaline extractions of defatted okara at different pH values is shown in Fig. 1. Protein recovery in phosphate buffer ranged from 4.3 % (w/w) at pH 9 to 35.9 % (w/w) at pH 12 (p < 0.05). The low protein recovery at pH values
Conclusion
Ultrasonication was proven an efficient tool for protein extraction from soybean residues. The cavitation process did not affect the macronutrient content of the OPI nor the amino acid profile of the proteins but caused alterations in their secondary structure and size. Structural changes in OPI samples were linked to cavitation effects and the duration of the extraction, rather than the intensity of ultrasonication process. The combination of increased β-sheet content, improved zeta-potential
CRediT authorship contribution statement
Ogemdi F. Eze: Methodology, Formal analysis, Investigation, Data curation, Writing – original draft, Funding acquisition. Afroditi Chatzifragkou: Conceptualization, Resources, Writing - review & editing, Supervision. Dimitris Charalampopoulos: Conceptualization, Resources, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors wish to thank Commonwealth Scholarship Commission Award for the financial support provided to Dr Ogemdi Eze.
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