Rheological and structural properties of gels cross-linked with fungal laccases

Abstract

Arabinoxylans (AXs) and sugar beet fibre (Fibrex) are substances which have the potential of forming cross-linked hard gels. AXs, which are classified as hemicelluloses, are the major non-starch polysaccharides in wheat bran. Sugar beet pectin is the main soluble fibre in Fibrex. The cross-linking potential of AX and Fibrex refers to the ferulic acid units, which are ester linked to the molecules´ main chain. The more ferulic acid moieties are present on the polysaccharide backbone, the more covalent cross-links may be generated. A modified AX extraction method, in which the incubation time and concentration of alkaline extracts were optimised, has been applied to extract AX from wheat bran. In addition, a modified hydrothermal extraction method has been used to extract Fibrex. The extractable AXs from wheat bran were cross-linked by the commercial laccase C (LccC) and self-produced laccases from Funalia trogii (LccFtr) and Pleurotus pulmonarius (LccPpu). Dynamic oscillation measurements on AX gels presented mechanical spectra, including storage modulus, loss modulus and loss factor of the gels. Gel samples demonstrated viscoelastic properties, which remained constant for four weeks in the gels formed with LccFtr and LccPpu. Arabinoxylan gel characteristics, including high water holding capacity, swelling ratio in saliva, and heat resistance indicated a covalently cross-linked network. Mediators (caffeic acid and ABTS) and citrus pectin did not enhance the elastic properties of the gels. Fibrex was successfully cross-linked with LccFtr to an edible gel. Analysing rheological and structural characteristics of the Fibrex gels including high water holding capacity, swelling ratio in saliva, and heat resistance) showed the viscoelastic behaviour of the gels which was attributed to the covalently cross-linked network. However, vanillin, as a mediator, and citrus pectin did not enhance covalent cross-links and elastic properties of the Fibrex gels. Rheological properties and storage stability of sweetened Fibrex gels indicated a constant storage modulus during four weeks storage. Rheological behaviour and structural properties of the Fibrex gels and AX gels were compared with gelatin gels as a reference. Using laccases as an oxidant provided gels with a solid and stable texture, comparable in firmness to traditional gelatin gels. Cross-linked gels may find uses in pharmaceutical and other industrial applications, which require a heat resistant gel that forms easily at room temperature. They also represent an ethical alternative for manufacturing vegan, halal, and kosher food.