Interfacial properties of green leaf cellulosic particles
Graphical abstract
Introduction
The use of solid particles as emulsion stabilisers is highly promising in many applications (e.g. food, pharma, cosmetics) because of their outstanding stabilisation against coalescence compared to low molecular weight emulsifiers (Tcholakova, Denkov, & Lips, 2008). The stabilising mechanism of solid particles in Pickering emulsions is due to the accumulation of particles at the oil-water interface as a densely packed layer, which is determined by the particle properties and emulsification conditions (Binks, 2002, Tzoumaki et al., 2011, Wu and Ma, 2016). The particles are irreversibly adsorbed at the interface, and the mechanical properties of the adsorbed layer contribute to the long-term stability of emulsions (Dickinson, 2012). The particles may be synthetic or obtained from different types of biobased feedstocks (Rayner et al., 2014), which can be extended to leaves and leaf by-products.
Leaves and leaf by-products are available on massive scale from industrialized crops. Using leaves that are now discarded, may at least partly provide a sustainable alternative to producing more food from existing resources (Dijkstra, Linnemann, & van Boekel, 2003). For this, leaves are processed to extract valuable components such as proteins. The extraction involves a pressing/grinding step that separates the intracellular fluids from the leaf fibrous pulp. So far, most scientific interest is focussed on further juice purification, thereby neglecting the fact that the pulp represents nearly 25% w/w of the starting biomass (Tamayo Tenorio, Gieteling, Nikiforidis, Boom, & van der Goot, 2016), and giving added value to this large side stream would contribute to a more complete use of the green leaves.
The leaf fibrous pulp is rich in dietary fibres like cellulose, hemicellulose and lignin, with the amounts present varying between crops (Siqueira, Bras, & Dufresne, 2010). A simplified scheme of a plant cell wall is depicted in Fig. 1, identifying the major components and structural arrangements. The plant cell wall constitutes an ordered network of cellulose microfibrils coated with hemicellulosic polysaccharides, a gel-like matrix made of pectic polymers, and a network consisting of structural proteins that hold the polymers in place (Held, Jiang, Basu, Showalter, & Faik, 2015). Dietary, fibre rich materials have multiple uses, including clouding of beverages, thickening and gelling, and also low calorie bulking (Galanakis, 2012). Moreover, fibre rich biomass is a potential source of Pickering emulsion stabilisers. Such materials have been produced from other agro-industrial waste like orange peel (Wallecan, McCrae, Debon, Dong, & Mazoyer, 2015), mango peel (Serna-Cock, García-Gonzales, & Torres-León, 2016), celery and spinach (Göksel Saraç & Dogan, 2016) and cocoa fibres (Gould, Vieira, & Wolf, 2013).
The type of particles produced is related to the cell wall structure and to the processing conditions that disrupt the cell wall. In general, plant cell walls are networks of interconnected biopolymers such as polysaccharides (cellulose, hemicelluloses, pectin), glycoproteins (i.e. extensins), and lignin (Beck, 2005). With harsh processing conditions (i.e. high pressure, high temperature, chemical hydrolysis), pure crystalline cellulose can be obtained, which has been used in particle stabilised emulsions (Pickering) and composites (Siqueira et al., 2010). With milder mechanical processing, the resulting particles are rich in cellulose but still contain other cell wall components, like polysaccharide-protein complexes (Harris & Smith, 2006), which can provide additional Pickering stabilisation (Wallecan et al., 2015).
In this study, cellulosic particles were extracted from sugar beet leaves (SBL), using mild aqueous extraction. Our objective was to characterise these particles and to study their interfacial behaviour. Composition and size characterisation were followed by interfacial tension and interfacial rheology analysis, and the particles were finally added to oil-water emulsions as stabilisers. We concluded that fibrous leaf pulp is a potential source of cellulosic particles that can be used as Pickering emulsifiers in food applications.
Section snippets
Purification of leaf fibres
Sugar beet leaves from mature plants were harvested from a sugar beet production field in Wageningen, The Netherlands. The leaves were pressed with a screw press (Angelia juicer II 7500 from Angel Juicers, Queensland, Australia) to separate the leaf juice from the fibrous pulp. The pulp was then processed until a fine powder as depicted in Fig. 2. The fibrous pulp was washed 5 times for 1 h with alkaline deionised water (pH 9.0) in a pulp-to-water weight ratio of 1:5. NaOH 1M was used for
Composition and particle size
Fresh sugar beet leaves (SBL) were mechanically pressed to release their soluble intracellular fluid into a leaf juice while the cell wall material was retained as insoluble pulp or leaf fibres. The leaf pulp was used as raw material to obtain cellulose-rich particles, by washing at mild alkaline conditions (pH 9.0), drying, milling and air classification. The alkaline washing was applied to solubilize and remove components such as proteins (Walstra, 2003), while retaining most polysaccharide
Conclusions
Leaf pulp is an abundant, food grade resource that is currently unexploited. Mild fractionation resulted in sheet-like, deformable, rough particles that could stabilise emulsions through Pickering stabilisation. The protein impurities contribute to the surface charge of the particles and provide surface activity, leading to spontaneous diffusion of the particles during the interfacial tension analysis; whereas the solid nature of the particles explains the slow adsorption kinetics and small
Acknowledgment
The authors thank Thirea Peters for her contribution to the practical work; Jun Qiu for his contribution to the data fitting in MATLAB; and the Technology Foundation STW 12627 (The Netherlands) and Royal Cosun (The Netherlands) for their financial and project support.
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