Panagiota Mouraka, Vincenzo di Bari, and Gleb Yakubov
University of Nottingham, UK
The incorporation of dietary fibres (DFs) in the food systems provides structuring and nutritional benefits. The last decade saw the rise in the use of minimally processed plant cell-wall materials in the food industry to produce healthier products by replacing sugar and starch with non-digestible fibre ingredients (Anderson et al., 2009; Foster, 2011; Yu, Yakubov, Martínez-Sanz, Gilbert, & Stokes, 2018). Since natural DF have different mechanical properties, water holding capacity and surface functionalities, careful consideration of the most appropriate source(s) is necessary to fit the specific food application (Elleuch et al., 2011). The objective of this study is to elucidate the effect of particle properties on the micromechanical behaviour of concentrated DF suspensions. In this work, we use fibre particles of different botanical origin, which enables varying suspension microstructure and packing volume, as well as particle modulus and particle size distribution. The DFs were chosen from wheat, oat, citrus, apple, and pea fibres and were dispersed in sunflower oil to make the suspensions. The icing sugar suspensions were used as control.
The Angell’s-like plot representation, where suspension’s G’ is plotted against particle concentration, enabled probing consolidation properties of concentrated suspensions. We show that most cellulose/starch rich fibres show ‘fragile’ behaviour, whereby G’ remains low up until the jamming point, beyond which a rapid increase in G’ is observed. By contrast icing sugar suspensions show ‘strong’ glass behaviour, with G’ increasing linearly up until the jamming point. For concentrations above jamming we observe G’ to plateau. However, contrary to the expectations, the modulus of suspensions in the jammed state is found to be independent of the particle modulus, as would be predicted by the Evans and Lips model (Evans & Lips, 1990). We propose that the result indicates that the interaction potential between DF particles deviates from the Hertzian hard wall mechanical contact. Instead, the modulus of the consolidate granulated material is governed by the inter-particle adhesion energy.
These insights allow an improved control of suspension rheological and micromechanical behaviour when plant fibre is used in food formulations. By optimising particle size, shape, and hydrophobicity to is possible to achieve the targeted flow and jamming behaviour.