The incorporation of Curcuminoids in oat fibre extruded products

© 2016 Dr Sara SayanjaliCurcuminoids are polyphenolic bioactive ingredients found in the roots of the plant Curcuma Longa. These compounds have health benefits and can be used to develop health promoting functional foods but the low water solubility of these compounds limits their bioavailability. One of the main approaches to overcome the low solubility and stability of curcuminoids in an aqueous environment is to make use of the interaction between curcuminoids and other food ingredients. Oat dietary fibre is one such food ingredient that could potentially be used as a carrier for curcuminoids. Oat fibre is known to have health benefits for humans, including lowering cholesterol serum, the risk of coronary heart disease and blood pressure. Dietary fibre has a potential to interact with polyphenolic compounds, such as the curcuminoids, by a number of mechanisms but the possible interactions between curcuminoids and oat fibre ingredients have not been studied to date. Extrusion cooking is one of the main technologies known to have a great potential for the manufacture of snack products. Bioactive components can be added to extruded snacks in order to improve their health benefits but these components can degrade during extrusion processing. The possible effect of bioactive addition on the physical properties of extruded products should be also considered. This thesis aims to investigate the potential of oat fibre ingredients as a carrier for curcuminoids. It also aims to examine the feasibility of producing a curcuminoid-enriched oat fibre-corn based extruded product, with a focus on curcuminoid stability during extrusion processing. It also aims to evaluate the effect of curcuminoid addition on the physical properties of extruded products. The thesis is divided into three main sections: 1. The first section focuses on the use of oat fibre as a potential carrier material to increase the solubility and stability of curcuminoids. Studies are carried out to understand the interaction between curcuminoids and the oat fibre ingredients. The stability of the curcuminoids in the presence of oat fibre materials during storage was also determined. 2. In section two, the effect of extrusion technology on the physico-chemical properties of oat fibre containing 28 % β-glucan are examined. The physico-chemical characteristics of oat fibre including molecular weight, soluble solids content, water absorption index, dynamic vapour sorption, thermal and pasting properties were measured after extrusion using two feed or barrel moisture contents (50 % - 60 %) and two screw speeds (200 rpm – 300 rpm) and results were compared with the properties of non-extruded oat fibre. 3. In the third section, extrusion technology was used as a delivery method for the production of a curcuminoid enriched oat fibre-corn based extrudates. This section focuses on the stability of curcuminoids, as affected by extrusion conditions, including the two levels of feed moisture content and two levels of screw speed. In addition, the effect of curcuminoids on the physical characteristics of extrudates including bulk density, expansion, hardness and colour were investigated. The spectroscopic experiments showed that both protein and β-glucan components of oat fibre are able to interact with curcuminoids. Solubility experiments indicate the curcuminoids in the supernatant fraction of a 1 % w/w oat fibre dispersion in 2 % v/v EtOH (88 μg/mL) increased by a factor of 21 compared to only 2 % v/v EtOH (4.1 μg/mL). This concentration of curcuminoids in the supernatant is also much higher than that of the reported for the solubility of curcuminoids in aqueous media (11 ng/mL, pH 5). In the presence of oat fibre materials, curcuminoids were converted from a crystalline to an amorphous state, as observed by Wide-angle X-ray powder diffraction. The amorphous state of the curcuminoids in the precipitate of (25.8 μg/mL) curcuminoids−oat fibre (1% w/w TS) dispersion with 2 % v/v EtOH resulted in higher stability for curcuminoids in the precipitate rather than supernatant of the oat fibre dispersion. These findings show the potential of oat fibre as a carrier for curcuminoids in functional foods. Extrusion of oat fibre with high levels of β-glucan under conditions of high moisture did not significantly change the molecular weight of the soluble fraction, the total soluble solids, the water absorption index and thermal properties of oat fibre. A higher specific mechanical energy was found to result in an increased specific surface area and absorption of water vapour as a surface monolayer. The viscoelastic properties of oat fibre were also maintained after extrusion. This study indicates that extrusion processing is a promising technology to produce extruded products based on oat fibre where the functional properties and potential health benefits of oat fibre are preserved. The physical properties of oat fibre based extrudates containing curcuminoids were significantly affected by feed moisture content, whereas the effect of screw speed and curcuminoid addition was not significant. Higher feed moisture resulted in darker extruded snacks with higher bulk density, hardness, 90 % retention of curcuminoids after extrusion and drying but lower expansion. Curcuminoids were also stable in dried extruded products during 80 days of storage at 25 °C. These studies provided information for the selection of process variables for extrusion. The supporting compositional evidence will help with the introduction of curcuminoid-enriched extruded snacks as a new product category in the functional food market. In conclusion, this study showed that both protein and β- glucan components of oat fibre are able to interact with curcuminoids and increase the solubility of curcuminoids in an aqueous solution of 2 % v/v EtOH. It is possible that curcuminoids also interact with proteins and dietary fibres in the precipitated fraction. These findings illustrate the potential for the curcumin carrying capacity of oat fibre to be capitalized upon in the fortification of food with curcuminoids. The application of extrusion processing to modify the functional properties of oat fibre, showed that current extrusion conditions can be used to produce extruded products from a commercially available oat fibre preparation with high β-glucan content where the properties of the extrudates were largely preserved and the health benefits are expected to be similar with non-extruded oat fibre. The ability to achieve high retention of curcuminoids (~ 90 %) at feed moisture content of 35% and screw speed of 200 rpm or 300 rpm during extrusion combined with the stability of the curcuminoids in the dried extruded snack shows the potential to improve the health benefits of oat fibre extruded products by the incorporation of curcuminoids