Elkwil, Omran
(2019)
Design of complex emulsion interfaces for sugar reduction application.
PhD thesis, University of Nottingham.
Abstract
A low carbohydrate and free sugar diet is generally recommended by nutritionists and clinicians for losing weight, reducing risk of diabetes, cardiovascular diseases and metabolic syndrome. The aim of this research was to design complex emulsion interfaces for sugar reduction in emulsion-based food. A strategy was applied to evaluate whether O/W emulsions can be stabilised at a lower emulsifying starch content by utilising protein as co-emulsifier. It was hypothesised that oil-in-water (O/W) emulsions stabilised using octenyl succinic anhydride (OSA) starch do not require a closely packed starch layer at the interface for breakdown in the oral cavity by saliva α-amylase, also the interfacial starch can be reduced by utilising whey protein (WP) or sodium caseinate (NaCAS) as co-emulsifier a co-emulsifier when compared to using starch alone. If this emulsion was successfully stabilised with starch/protein mixed emulsifier, it can be developed into W1/O/W2 emulsion in which a highly concentrated sucrose solution can be encapsulated as a requirement of the taste enhancement approach. When the W1/O/W2 emulsion breaks down in the oral cavity and releases the sucrose, this will give consumer satisfaction in terms of sweet taste, without being a high carbohydrate-digestion product. By using this strategy, emulsion-based food products can be produced with an overall low carbohydrate content and whilst maintaining a satisfying level of sweet taste.
Three O/W emulsions comprised of 20% w/w oil and 80% w/w aqueous phase were formed, which were stabilised independently with three emulsifiers; 0.8% octenyl succinic anhydride (OSA)-starch, 1% whey protein (WP) and 1% sodium caseinate (NaCAS) w/w. Stability against coalescence of the three emulsions was observed by measuring their droplet size distributions. Next, a mixed emulsifier strategy was used - the quantity of OSA starch was halved and combined with half of the quantity of either WP or NaCAS to make two mixed emulsifier systems: 0.4% OSA/0.5% WP and 0.4% OSA/0.5% NaCAS. Both mixed emulsifiers showed a comparative ability to reduce the interfacial tension at the oil/water interface compared with using OSA starch alone. The mixed emulsifiers used to produce two O/W emulsions were characterised by droplet size measurements, Zeta potential, the microstructure and interfacial composition analysis. These all demonstrated that the emulsions showed high stability against coalescence. There were no significant changes found in the volume based mean droplet size (p > 0.05). Zeta potential was (39.0± 2.6) and (57.3 ± 3.9) mV for emulsions stabilised with OSA/WP and OSA/NaCAS, respectively. The microstructure and continuous phase analysis before and post-emulsification revealed the presence of both starch and protein at the oil/water interface.
The two O/W emulsion stabilised with the mixed emulsifiers were developed to create W1/O/W2 emulsions, where 50% and 5% w/w sucrose solution was used as the inner and the outer aqueous solution, respectively. The multiple emulsions were designed to breakdown in the oral cavity and release high concentrations of encapsulated sugar close to the taste receptors during consumption, allowing the overall levels of sugar used in the bulk of the food to be reduced without compromising the sweet taste. These emulsions were stabilised with 0.57% PGPR as the inner emulsifier and with the mixed emulsifiers in the outer phase. They were characterised by measuring the stability against coalescence, the encapsulated efficiency (EE) of the dissolve sugar in the internal aqueous phase and the release of the encapsulated sugar during an enzymatic digestion before and post fortification with different mineral salts. Both emulsions were stable against coalescence over a two-week storage period at ambient temperature with an average droplet size of (47.8 ± 0.8) µm and (46.0 ± 2.4) µm. The EE of these emulsions, which was defined as the mass of the sugar entrapped in the internal aqueous phase immediately after the emulsification process divided by the total mass of sugar added to this phase, was between (78.6 ± 2.7) % and (76.7 ± 1.2) %, respectively, at day one. Amylase mediated sucrose release from the internal aqueous phase of both W1/O/W2 emulsions was measured in-vitro at day 1 of the storage period, showing the release of (66.92 ± 6.5) % and (58.8 ± 4.6) % of the encapsulated sugar, respectively. The emulsions’ behaviour at the typical levels of mineral salts in a close-to-market approach and the release of sugar, after fortification with these minerals was investigated. W1/O/W2 emulsions were created using a solution of 0.111% NaCl, 0.146% CaCl2, 0.224% KCl and 0.016% MgCl2 w/w in both aqueous phases. The emulsions containing OSA/WP were stable and there was no change observed in the sugar release behaviour after mineral fortification. On the contrary, emulsions containing OSA/NaCAS did exhibit stability and maintained the release behaviour but only when fortified with NaCl and KCl. However, these emulsions were unstable against coalescence when they were fortified with CaCl2 and MgCl2, mostly because caseinate can strongly bind divalent cations like Ca2+ and Mg2+.
Double emulsions stabilised with OSA/WP showed the best EE, sugar release and stability against coalescence after mineral salts fortification comparing with the same emulsions when stabilised with OSA/NaCAS. This research shows that the interfacial starch of W1/O/W2 emulsions can be partially replaced by milk proteins for sugar reduction as final product of starch digestion with keeping stability against coalescence. Depending on the outcomes of emulsion digestion (in-vitro) by α-amylase enzyme at 37.0 °C, sugar can be encapsulated in these emulsions to release in the mouth cavity keeping a level of sweet taste. The benefit of replacement of the starch by protein was that the amount of starch in both O/W and W/O/W emulsions has been reduced up to 50%. Therefore, carbohydrate and then glucose as final product of digestion is be reduced without effect on consumer satisfaction in terms of sweet taste.
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