Lakkaasin ja ksylanaasin vaikutus vehnäjauho- ja gluteenitaikinan rakenteen muodostumiseen: Diplomityö

In the literature survey, properties of wheat flour and dough as well as the structure formation of wheat bread were discussed, with special emphasis on rheology. Also the theory and a few measurement methods of the rheology were shortly introduced. Considering the influence of enzymes on breadmaking, a structure formation by an oxidative mechanism was discussed. The enzymes that were viewed more precisely on their effect on the structure formation of wheat bread included lipoxygenase, glucose oxidase, peroxidase, laccase and transglutaminase. In the experimental part of the study, the effect of Aspergillus oryzae and Bacillus subtilis xylanases and Trametes hirsuta and Melanocarpus albomyces laccases, separately and together, on the structure of wheat flour dough and gluten dough were examined. The experiments of added free ferulic acid (FA) were done to clarify the role of FA in dough structure formation. The rheological experiments were performed with Kieffer dough and gluten extensibility rig, when the dough extensibility Ex and the resistance to stretching Rmax, were determined. In a rheology point of view, laccases increased the maximum resistance Rmax of dough and decreased the dough extensibility Ex at Rmax, whereas xylanases decreased the Rmax of dough and increased the Ex at Rmax in flour and gluten doughs. Considering the protein-AX fraction, hardening by laccases and softening by xylanases were weaker in gluten doughs, which was presumably due to the lower AX content in gluten. Like xylanases, the added free FA softened the flour dough structure, as well. As a function of dough resting time, the structure of laccase treated doughs was observed to soften. The softening effect was strengthened as a function of laccase activity. The reason for the softening phenomenon was probably the laccase-mediated depolymerization of crosslinked arabinoxylan network, resulting from mobile FA radicals. Combined laccase and xylanase experiments led to doughs of higher Rmax, with no remarkable change in Ex. The effect of laccase seemed to be predominant, especially at low xylanase dosages, but when xylanase was added to flour dough at high concentration, the hardening effect of laccase on dough was decreased. Presumably, when the AX fraction was hydrolyzed effectively by xylanase, laccase was not able to create a strong AX network. Similar decrease in laccase-mediated hardening in doughs was not seen, when performing the combined laccase and xylanase test with gluten, which was presumably due to low AX content of gluten. The results indicated the critical role of feruloylated arabinoxylan fraction in laccase-catalyzed structure formation both in flour and gluten, although in gluten doughs, protein fraction might have also been affected.