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Instant emulsions stabilised by cellulose derivatives with specific digestion profiles

Final Report Summary - CELGESTANT (Instant emulsions stabilised by cellulose derivatives with specific digestion profiles)

The development of emulsion-based food products through optimisation of ingredients and energy-input during processing and minimisation of redistribution costs, while fulfilling healthy attributes, is a major objective within the food industry. Accordingly, the two major proposed goals of this project are the rational design of instant emulsions mainly stabilised by cellulose derivatives and controlling their digestion profiles through microstructure manipulation in order to reduce/improve fat absorption. The derivatised celluloses are stabilisers belonging to the category of dietary fibres, which are often incorporated in food to improve nutritional properties. Dietary fibres are also linked to health benefits because of their physiological and metabolic effects.
The stabilisation of emulsions formed by the dispersion of oil droplets in water requires the presence of emulsifier molecules located at the surface of the oil droplets (oil-water interface). This occurs due to the amphiphilic character of the emulsifier, which means that the molecule comprises one hydrophobic part and one hydrophilic part. In this regard, hydrophobically modified celluloses are amphiphilic molecules, hence able to adsorb at the oil-water interface of emulsions. Therefore, the first step carried out towards the first objective was the fundamental characterisation of derivatised celluloses at the oil-water interface. Commercially available cellulose ethers and olive oil were used for this purpose, and the rest of the project. Namely, an interfacial tension and dilatational rheology study was performed to explore the interfacial properties concerning adsorption dynamics and structure formed by these cellulose ethers at the oil-water interface. As a result, it was shown that they all efficiently adsorb at the oil-water interface reducing the interfacial tension to a steady state in the timeframe of a few seconds for medium-high bulk concentrations, relevant to emulsification of oil in water to prepare instant emulsions within this timescale. The reduction of interfacial tension between oil and water is crucial to promote the droplet break-up upon emulsion formation. Moreover, the adsorbed layers of cellulose ethers provide certain elasticity to the interface which is of fundamental importance towards the stability of emulsions against droplet aggregation (flocculation) or merging (coalescence). Specifically, it seems that cellulose ethers at medium-high concentrations form interfacial structures of the type loops-trains-tails and even multilayers with certain thickness that would bestow steric stabilisation on emulsions. This means that a physical barrier is provided by the cellulose ether coating which may prevent droplets from aggregating. These results have been published in Carbohydrate Polymers, 2014, 113, 53-61.
Later on, the concept of the patented instant emulsions has been applied to investigate the effect of the initial formulation on the final microstructure and texture of the product. The instant emulsion is formed after the addition, under mild stirring, of the required amount of water to the oil phase already containing a mixture of the emulsifier Tween 20, conventionally used in the food industry, swelling particles of cross-linked dextran and two thickeners: hydroxypropylmethylcellulose (HPMC), one of the cellulose ethers studied above, and guar gum (GG), which is also a dietary fibre. The emulsion and texture creation was monitored by viscosity analysis and the microstructure visualised by microscopy. The technological advantage of this approach is that the emulsion is created within a reduced time frame and stabilised in the presence of, and solely by, the constituent ingredients, with a minimum energy input, such as moderate shear generated by hand stirring, as compared to conventional emulsification processes. Indeed, results showed that complete emulsification is achieved within less than 1.5-3 min in general. Several parameters have been evaluated on the final texture/microstructure: concentration and molecular weight (Mw) of the thickeners, swelling capacity of the viscosifying particles and the presence of the emulsifier Tween 20. Increasing the concentration and Mw of GG leads to smaller emulsion droplets: the average size decreases from 40 to 13 μm (see attachment named Fig.1) which are within the size range found in food products (1-100 μm), and thicker emulsions, that is more viscous. However, this modulation in the droplet size and texture (in terms of viscosity) is magnified by increasing the Mw of HPMC, which was used as the main thickener. Hence, using high-Mw cellulose derivatives allows optimisation of the thickener concentration to achieve similar final properties of the instant emulsion. On the other hand, increasing the swelling capacity of cross-linked dextran particles also provides finer emulsions due to higher bulk viscosity. This also contributes to optimise the total amount of swelling particles and thickeners used in the formulation. Moreover, highly stable emulsions are created by increasing the emulsion viscosity when reducing the oil droplet size. These are stable against creaming (migration of oil droplets to the top due to difference in density) for weeks. Finally, although the emulsifier Tween 20 plays an important role decreasing the interfacial tension, the sole use of a high-Mw HPMC in the absence of Tween 20 has been shown to be effective in emulsion formation due to combined increased bulk viscosity and a reduction of interfacial tension. This is also relevant towards ingredient minimisation. These outcomes are going to be considered for commercial exploitation and submitted for publication.
Next, in order to make healthier products, a controlled oil digestibility was targeted as the second main objective of the project. To this end, it is necessary to account for the physicochemical and/or enzymatic changes occurring through the gastrointestinal tract. It must be stressed the interactions with biological fluids and their exposure to complex flow profiles and mechanical forces. The majority of lipid (oil) digestion occurs in the upper small intestine, c.a. 70-90%, for that reason the in vitro digestion model was focused on duodenal conditions. The first study considered here was on the interactions of bile salts with cellulose ethers. Bile salts are physiological compounds that play an important role on lipid digestion, therefore any way of interfering with their role may also affect the rate of lipid digestion. First the interactions were evaluated at the oil-water interface. Again, interfacial tension and dilatational rheology were combined in this study. It was shown that cellulose ethers compete for the oil-water interface with the bile salts (results published in Carbohydrate Polymers, 2014, 113, 53-61), and also resist displacement by the bile salts from the oil-water interface (results to be submitted for publication). This is of great importance since the extent to which adsorbed molecules and formed interfacial structures resist bile salt adsorption in the duodenum is considered to be vital to control enzyme adsorption and hence regulate the rate of lipid digestion. The interactions were then evaluated in the aqueous phase. This involved calorimetric and shear rheology techniques to account for the structures formed in solution. Cellulose ethers were demonstrated to effectively bind to bile salts, as reflected in the inhibition of the thermogelation properties of derivatised cellulose. These interactions seem to be driven by the hydrophobicity of the cellulose ether and the bile salt and are important to interfere with the role played by the bile salts in the digestion of the emulsions. These results have been published in Carbohydrate Polymers, 2014, 113, 53-61 and Food & Function, 2015, 6, 730-739.
The next step carried out was evaluating the effect of formulation and microstructure of instant emulsions on their digestibility. To this end, measurements of the lipid digestion profiles of the instant emulsions were performed by means of chemical analysis (pH-stat method). Namely, the release of free fatty acids (FFA) was quantified by titration, which is linked with the enzyme activity and hence lipid digestion. A combination of techniques such as measurement of oil droplet size and microscopy was also used to identify the duodenal conditions on emulsions. The initial oil droplet size seemed crucial on the lipid digestion. Emulsions with larger droplet size (> 17 μm) display lower rate and extent of digestion, while finer emulsions (droplet size ≤ 17 μm), achieved by increasing the concentration or Mw of the thickener, exhibit maximum rate and extent profiles (see attachment named Fig. 1). The results provided by the combination of techniques suggest that cellulose ethers may promote droplet flocculation under duodenal conditions, affecting to a larger extent emulsions with larger droplets. This destabilisation reduces the oil-water interface available for the enzymatic reaction to take place. In addition, the adsorption of dietary fibre onto the oil–water interface, if used directly as emulsifier, may form a protective layer against the action of physiological components, hindering the lipid digestion. Therefore a controlled digestibility of the instant emulsions has been achieved through microstructure manipulation. These results are going to be submitted for publication.
The knowledge generated in this project will contribute to the rational design of functional instant food emulsions minimising ingredient, process and supply costs, and providing healthier, affordable, and ready-to eat options. This scope is consistent with the aims of Horizon 2020 in the European Union and the European Technology Platform Food for Life. The technology allows the manufacture of food emulsions with minimal energy input processing, which minimises production costs while completely removing water from the distribution and supply chain, which can additionally reduce associated costs, as well as eliminating the need for thermal processing treatments during manufacture. The development of these products will meet the growing needs of re-distributed manufacture to cope with the changes in urbanisation demographics and avoiding excessive use of additives currently used for extended shelf life of products. The technology from this project is also promising for future applications in both the food and pharmaceutical industry to deliver bioactive compounds to specific locations through specific digestion profiles. This approach can then be applied to a wide range of novel food products and formulations, impacting on the area of nutraceuticals and cosmetics. This work could potentially be applied in flexible, small-scale manufacturing, bringing about significant economic benefits for businesses, meeting the changing consumer behaviours. In addition, it could contribute to UK and European cooperation with developing countries by providing the technology for new product launches, reaching low income consumers as well as those in the A-C classes. The possibility of ambient distribution also opens up new channels within developing and emerging countries for point-of-sale preparation of numerous food emulsion products, especially when designed for optimal nutritional content.

Contact details:
Amelia Torcello-Gómez (amelia.torcello_gomez@nottingham.ac.uk)
Timothy J. Foster (tim.foster@nottingham.ac.uk)
Division of Food Sciences, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, LE12 5RD, United Kingdom.