Every year, 39 million tonnes of brewers' spent grains (BSG) are generated around the world as a side stream of the brewing industry (Macias-Garbett, Serna-Hernández, Sosa-Hernández, & Parra-Saldívar, 2021). BSG makes up 85% of the total byproducts from the brewing industry and is produced after the malting and mashing process, during which starch in barley grains is converted into soluble sugars. This leaves behind the original barley grain coverings, such as the pericarp, husk and aleurone layers which are rich in fibres and proteins. As a largely water-insoluble material with a high moisture content, BSG is highly susceptible to microbial contamination and most BSG is currently discarded or used as animal feed. BSG fibres comprise of 17% cellulose, 28% hemicellulose and 28% lignin, while the proteins, which make up 20% of the material, are mostly alcohol-soluble prolamins and alkali-soluble glutelins that remain in the barley grains (Mussatto, Dragone, & Roberto, 2006). These proteins have a good balance of hydrophobic and hydrophilic amino acids, which should allow them to exhibit good emulsifying properties (Wang et al., 2010).

Various methods to put BSG to valuable use have been explored, including the extraction of BSG fibres and proteins under alkaline conditions (Connolly, Piggott, & FitzGerald, 2013; Vieira et al., 2014). In particular, the enzymatic hydrolysis of BSG has been widely investigated to generate protein hydrolysates that have better technofunctional and bioactive properties (Celus, Brijs, & Delcour, 2007; Connolly et al., 2019; Niemi, Martins, Buchert, & Faulds, 2013). However, these fractionation processes often involve intensive physicochemical and thermal steps, the use of harsh solvents or end in large mass losses. In addition, previous studies have demonstrated that mildly purified mixtures can also be functional, depending on their intended application (Karefyllakis, Octaviana, van der Goot, & Nikiforidis, 2019; Möller, van der Padt, & van der Goot, 2022; Sridharan, Meinders, Bitter, & Nikiforidis, 2020), thus complex purification steps may in fact not be necessary. A straightforward and more sustainable approach is to utilise BSG as such. One example is the direct incorporation of BSG ‘as is’ in food products such as baked goods, pasta or yogurt, with the purpose of improving the dietary fibre content in the final application (Mussatto, 2014). Being a nutrient-rich material, BSG was also used as a substrate for growing microorganisms to produce industrially relevant compounds (Lynch, Steffen, & Arendt, 2016), or by employing microbes to break down the lignocellulosic structure and produce other useful components (Chin, Chai, & Chen, 2022; Cooray & Chen, 2018; Tan, Mok, & Chen, 2020).

However, until now, little attention has been paid towards physical modifications that could impart new or better functional properties to the whole BSG material. A recent study by Ibbett, White, Tucker, and Foster (2019) showed that colloid milling of BSG produced a protein-rich, fine dispersion with the ability to stabilise oil-in-water emulsions, but the interfacial properties and emulsifying behaviour were not investigated in further detail. In addition, only the proteins in BSG were considered for its potential to stabilise emulsions, neglecting the possibility that fibres could also have an effect on emulsification and emulsion stability. Previous studies have shown that insoluble fibres that are pretreated via a homogenisation step can possess emulsifying properties and stabilise emulsions through a combination of Pickering stabilisation and the formation of a fibre-based network (Bao et al., 2021; Wallecan, McCrae, Debon, Dong, & Mazoyer, 2015; Yang, Liu, Li, & Tang, 2019). The homogenisation process disrupts the plant cell wall matrix and can be carried out by high pressure homogenisation, ultrasonication or wet media milling. Emulsions stabilised by these fibres have demonstrated high stability against pH and ionic strength variations, with long-term storage stability. Similarly, water-insoluble proteins from corn and pea have also demonstrated to be effective particle stabilisers of oil-in-water emulsions (de Folter, van Ruijven, & Velikov, 2012; Hinderink, Schröder, Sagis, Schroën, & Berton-Carabin, 2021).

In recent years, substantial interest in clean-label and sustainable products has led to extensive research on particle-stabilised emulsions, i.e. Pickering emulsions, mainly due to their high physical stability (Schröder, Laguerre, Tenon, Schroën, & Berton-Carabin, 2021). For these reasons, it is relevant to consider exploiting the insolubility of BSG for Pickering stabilisation. We hypothesise that as a mixture containing amphiphilic proteins and a large amount of fibres, BSG can be used to stabilise emulsions without isolation of pure components. Therefore, the aim of this study is to characterise colloid mill-treated BSG and investigate the emulsifying ability of BSG as a whole and more specifically of its insoluble fraction. In this work, BSG was first treated with a colloid mill and separated by centrifugation to obtain an insoluble pellet fraction (insoluble BSG). The composition, physical behaviour, particle size and morphology of full and insoluble BSG dispersions were evaluated while the interfacial properties of the soluble component from centrifugation was determined. Emulsions were prepared with full and insoluble BSG and assessed based on their droplet sizes, charges, protein surface load, microstructure and creaming stability.