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Targeted processing of lemon peels to unlock the endogenous texturizing properties of its cell wall fibers

Book - Dissertation

In the last decades, the increase of consumer interest in health and environmental issues has boosted the demand for overall natural sustainable food products and ingredients with health-related properties. Consequently, clean label products are gaining increasing market value, including products that are produced with more organic and natural formulations under sustainable conditions. In this context, the integrated use of fruit and vegetable by-products can provide an interesting solution to reduce food waste, enrich food products with dietary fibers, and generate clean label natural texturizers. However, despite the abundance of fruit and vegetable by-products, their exploitation as natural food ingredients is still limited. This under-exploitation is partly caused by the dense intertwined network formation of the cell wall fibers, limiting their hydration and texturizing potential. Within this PhD research it was aimed to functionalize the dense cell wall network by performing targeted processing, allowing for better hydration and swelling of the fibers, thereby improving the texturizing properties of the cell wall fibers. To achieve this objective, (non-)selective extraction of cell wall polymers, mechanical processing and adjustment of the intrinsic system properties of the cell wall fiber suspensions, being pH and salt concentration, were considered as targeted processing methods. In addition, lemon peels were selected as fiber rich plant material due to its abundant production as by-product from the juicing industry. The effect of targeted processing was evaluated by determining the physico-chemical, microstructural and viscoelastic properties of the processed lemon peel fiber suspensions. Throughout this research, physico-chemical characterization included the determination of the galacturonic acid content, neutral sugar content, degree of methoxylation and molar mass, microstructural characterization the determination of the particle size distribution and performance of light and epifluorescence microscopy using acridine orange staining, while viscoelastic characterization included the performance of rheological dynamic oscillatory measurements. Furthermore, a new measuring geometry for the latter analysis was specifically designed and validated, adapted to the specific nature of lemon peel cell wall suspensions studied here. In the first part of this research, the role of different polymers in determining the functionality of lemon peel fiber suspensions was investigated by performance of a (non-)selective pectin and/or hemicellulose extraction, resulting in insoluble residual fiber material with different physico-chemical characteristics and composition. However, extensive pectin removal, without changing the hemicellulose content, was found to be of utmost importance towards the functionalization of lemon peel fibers. No or only partial pectin removal resulted in a stiff intertwined network of cell wall polymers, that fragmented upon mechanical processing without improving its viscoelastic properties. Furthermore, extensive pectin extraction under acid or alkaline conditions improved the viscoelastic properties of the lemon peel fibers upon suspension, probably due to an enhanced flexibility of the fiber network by the creation of interfibrillar spaces between the cellulose/hemicellulose network. Moreover, targeted mechanical processing by high pressure homogenization (HPH) of the pectin depleted cell wall fiber suspensions was found to be crucial to open up the remaining cell wall fiber network. In fact, investigation of the effect of shearing intensity showed that optimal viscoelastic properties are obtained by blending and subsequent HPH at a minimal pressure of 20 MPa. Increasing the HPH pressure above 20 MPa up to 80 MPa did not further enhance the fiber suspension functional properties. Furthermore, targeted processing by adjustment of the pH and salt concentration prior to HPH greatly influenced the functional properties of the pectin depleted lemon peel fibers. Specifically, pH adjustment to a pH ranging between 4.5 and 7 prior to HPH enhanced the viscoelastic properties of the fiber suspensions. Within this pH range, it is suggested that the residual pectin polymers are negatively charged, favoring the fiber separation upon HPH due to repulsion. By contrast a high salt concentration, obtained after pH adjustment to pH 10 or after excessive addition of NaCl or CaCl2, decreased the fiber functionality probably due to shielding of the repulsive fiber interactions. Lastly, small molecule diffusion was used as method to determine the volume occupancy of differently functionalized lemon peel fibers, to gain insight in the mechanism underlying the rheological properties of the lemon peel fibers upon suspension. Interestingly, from this work it is concluded that the viscoelastic properties of lemon peel fiber suspensions are determined by the fiber network opening induced by HPH. Nevertheless, the intrinsic system properties affect both the extent of network formation by HPH as also the hydration properties of the fibers. In conclusion, targeted processing under the specified processing conditions resulted in lemon peel fiber suspensions with viscoelastic properties befitting their exploitation as natural clean label texturizer.
Number of pages: 166
Publication year:2018
Accessibility:Open