Plant protein microparticles as beverage clouding agents Associative phase separation between potato protein and anionic polysaccharides

Research output: Book/ReportPh.D. thesisResearch

  • Lykke Christina Stounbjerg
Clouding agents (CA) are dispersions added to beverages to impart turbidity. They are often emulsions but may be biopolymer (BP) particles, composed of proteins and polysaccharides (PS). Proteins and PS can form particles by electrostatic complexation but may dissociate when changed solution conditions alter their electrical properties, which limits their applicability. Improving particle integrity in food products is often achieved by heating. Plant proteins are receiving growing interest from both academia and industry, as they provide a sustainable, vegan, and often less expensive alternative to animal proteins. However, industrial plant proteins and PS are heterogenous materials, often sparsely characterised. The aim of this thesis was to create microparticles from commercial plant proteins and PSs commonly used in beverages, through thermal treatment of electrostatic complexes. Potato protein (PP) was selected based on a number of criteria. Also, only a few studies concern the interactions between PP and anionic PS, and none was found describing the thermal behaviour of such complexes. Three carboxylated PSs varying in structure, size, and charge density were selected, namely alginate (ALG), carboxymethylcellulose (CMC), and gum arabic (GA). Firstly, the properties of PP and PSs were characterised and the conditions leading to associative phase separation (APS) assessed. Through acidification and polymeric titration, the effect of lowering pH (9 to 3) and increasing mixing ratio (rw) (0.1:1 to 5:1) was assessed by turbidimetry, isothermal calorimetry, electrophoretic light scattering, and compositional analysis of particles. Secondly, PP and CMC and GA were aggregated into particles, and the effect of processing sequence on particle properties was investigated. Particles produced by homogenising (200/50 bar) and heating (90 °C, 10 min) complexes at pH 3.5, were characterised by laser diffraction, confocal microscopy, compositional analysis, and turbidity, before and after thermal treatment. Their stability towards aggregation, sedimentation, salts, and pH increment was determined. Finally, the clouding strength and stability of the particles in beverage model systems were evaluated. During acidification of PP—PS mixtures, soluble complexes were formed which subsequently aggregated, causing APS and precipitation. Complexes with ALG persisted at low pH, where the other dissolved. Increasing rw shifted the onset of APS towards higher pH, widening the pH interval where insoluble complexes could form, while decreasing rw diminished APS. The weight-based PP binding capacities of the PSs were 3.7:1, 2.9:1, and 1:1 for ALG, CMC and GA, respectively. While precipitation of PP—CMC was driven by charge neutralisation, complexes with ALG precipitated while highly charged, and GA facilitated formation of coacervates or microdispersions. These differences strongly suggest that the electrostatic and steric properties of the PS are decisive for supramolecular association of BPs. Due to the tendency of ALG to cause extensive precipitation, it was excluded from further studies. Particles produced by successive acidification, homogenisation, and heating, yielded particles in the micron-size range for both PP—CMC and PP—GA. PP—CMC were polydisperse, fractal aggregates, that were heat stable. Most of the PP was bound in the particles, while the major fraction of CMC remained soluble. Contrarily, spherical, monodispersed particles formed between PP and GA, consisting of equal amounts of PP and PS. Heating caused a massive increase in turbidity, assumed to arise primarily from the change in particle size, but also due to incorporation of more PP and increasing density. Particle sizes remained unchanged during 4 weeks of storage. In this time, PP—GA particles sedimented fast, while the PP—CMC largely remained suspended. The PP—CMC particles dissolved at increasing pH but mostly remained intact in high salt concentrations, and this was unaffected by heating. Conversely, the resistance to both pH change and salt improved radically for PP—GA particles, hypothesised to be caused by coagulation of PP in the complex, reinforcing the particles by non-electrostatic forces. The particles imparted high turbidity in model beverages in dosages usually applied in the beverage industry. In the dilute beverage, the PP—GA suspensions had a superior clouding effect, while in the concentrate the clouding efficiencies were similar. The particlessedimented fast in the dilute beverage but were highly stable in the concentrate. As a r esult, microparticulated PP—PS complexes have potential for application as CA in concentrates,where an ordinary cloud emulsion loses its clouding effect, and the beverage is diluted shortl y before consumption.Overall, novel microparticles from commercial plant protein and fibres were created byconsecutive electrostatic complexation and thermal aggregation and were found to function asCA in model beverages. This Ph.D. thesis thus enhances the understanding of the colloidalphenomena and mechanisms underlying the function of PS as stabilisers in CA. It contributesnew knowledge about the nature of interactions and conditions affecting the APS behaviourbetween PP and anionic PS by relating molecular interactions to macrostructural changes.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Number of pages146
Publication statusPublished - 2019

ID: 248937267