Storage stability of laccase induced arabinoxylan gels
Introduction
Arabinoxylans are important cereal non-starch polysaccharides constituted of a linear backbone of β-(1→4)-linked d-xylopyranosyl units to which α-l-arabinofuranosyl substituents are attached through O-2 and/or O-3 (Izydorczyk & Biliaderis, 1995). Some of the arabinose residues are ester-linked on (O)-5 to ferulic acid (FA) (3-methoxy, 4 hydroxy cinnamic acid) (Smith & Hartley, 1983). These polysaccharides have been classified as water extractable (WEAX) or water-unextractable (WUAX). WEAX form highly viscous solutions and can gel through ferulic acid covalent cross-linking upon oxidation by some chemical or enzymatic free radicals-generating agents (Figueroa-Espinoza and Rouau, 1998, Geissman and Neukom, 1973, Hoseney and Faubion, 1981, Izydorczyk et al., 1990). Laccase (p-diphenol oxygen oxidoreductase, EC 1.10.3.2), blue multi-copper enzyme of white rot fungi (Bollag and Leonowicz, 1984, Holwerda et al., 1976) can oxidize FA beared on WEAX resulting in the formation of five diferulic acids (di-FA) (5-5′, 8-5′ benzo, 8-O-4′, 8-5′ and 8-8′ di-FA), the 8-5′ and the 8-O-4′ forms being always preponderant (Figueroa-Espinoza et al., 1998, Vansteenkiste et al., 2004). These di-FA structures were first identified in grass cell walls by Ralph, Quideau, Grabber, and Hatfield (1994). Di-FA covalent cross-linking have been commonly considered as firstly responsible of the WEAX gel development (Figueroa-Espinoza and Rouau, 1998, Geissman and Neukom, 1973, Izydorczyk and Biliaderis, 1995) The implication of higher coupling products of FA in WEAX cross-linking has been suggested by Vansteenkiste et al. (2004) studying the cross-linking of wheat WEAX by laccase. However, although two trimers of FA (from the addition of a phenoxy radical to the decarboxylated 8-5′or 8-O-4′-di-FA) have been reported in FA polymerized by a lignin peroxidase (Ward, Hadar, Bilkis, Konstantinovsky, & Dosoretz, 2001) and the 4-O-8′, 5′-5″-dehydrotriferulic acid has been identified in maize bran (Bunzel et al., 2003, Rouau et al., 2003), evidence of such cross-links products in enzymatically induced WEAX gels has not yet been reported.
WEAX gels possess interesting functional properties for food applications thanks to their neutral taste and odour, high water absorption capacity (up to 100 g of water per gram of dry polymer) and absence of pH or electrolyte susceptibility (Izydorczyk & Biliaderis, 1995). Unlike most of the polysaccharide gels currently used as texturizing and stabilizing agents in food systems, WEAX gels are mostly stabilized by covalent linkages. However, both covalent and non-covalent linkages are thought to participate in the gel structure (Izydorczyk and Biliaderis, 1995, Rattan et al., 1994, Vansteenkiste et al., 2004). Covalently cross-linked gels are generally strong, form quickly, are stable upon heating and exhibit no syneresis after long time storage.
The stability of hydrogels used in food applications is a major problem, as final products must keep their properties throughout their shelf life. Up to now the literature devoted to WEAX gels (Dervilly-Pinel et al., 2001, Figueroa-Espinoza and Rouau, 1999, Izydorczyk et al., 1990, Vansteenkiste et al., 2004, Vinkx et al., 1991) only reported on cross-linking kinetics of WEAX or on properties of freshly cured gels. In this work, we have followed the evolution of rheological, chemical and physicochemical properties of laccase-induced WEAX gels during a 6-day period.
Section snippets
Materials
Wheat water extractable arabinoxylans (WEAX) were obtained and characterized as reported by Vansteenkiste et al. (2004). Laccase (benzenediol:oxygen oxidoreductase, E.C.1.10.3.2) from Pycnoporus cinnabarinus was supplied by the Unité de Biotechnologie des Champignons Filamenteux-(UMR 1163 INRA-ESIL, Luminy, France). Citric acid, sodium phosphate dibasic, syringaldazine, ferulic acid and TMCA (3,4,5-trimethoxy-trans-cinnamic acid) were purchased from Sigma Chemical Co. (St Louis, MO, USA).
Characterization of WEAX
WEAX were obtained from a mixture of Soissons, Thésée and Apollo wheat cultivars as described by Vansteenkiste et al. (2004). Their composition is presented in Table 1. Galactose residues were detected due to the coextraction of arabinogalactan-proteins (Fincher & Stone, 1974). Nevertheless, after correction of arabinose from arabinogalactan-proteins (Ara/Gal=0.7; Loosveld, Maes, Van Casteren, Schols, Grobet, & Delcour, 1998), arabinoxylan was the main polysaccharide in the sample (62% db). WEAX
Conclusion
WEAX gel degradation observed after 6 days of storage appeared to proceed through a free radical mechanism initiated by laccase. The phenoxy radicals produced underwent secondary reactions resulting in a loss of WEAX cross-linking bonds and a partial depolymerization of the WEAX chains, which led to a decrease in gel hardness during storage. Laccase thermal inactivation just after gel formation, in spite of diminishing the network hardness initial values, stopped the free radical production and
Acknowledgements
Authors are very thankful to Anne Surget for her excellent technical assistance. M.Sc. Carvajal-Millan thank the Consejo Nacional de Ciencia y Tecnologia (CONACyT, Mexico) for providing a doctoral scholarship in a cooperation program with the Société Française d'Exportation des Ressources Éducatives (SFERE, France).
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