106 Influence of an antistaling enzyme on the mechanical, microstructural and molecular properties of bread and model systems.

S. HUG-ITEN, B. Conde-Petit, and F. Escher. Swiss Federal Institute of Technology (ETH), Institute of Food Science, CH-8092 Zurich, Switzerland.

The influence of a starch degrading enzyme, a thermostable alpha-amylase, on bread crumb and a model system consisting of wheat starch was investigated taking into account different structural levels. Bread was prepared by a conventional baking procedure. Starch gels were produced by heating a concentrated starch suspension (40 g/100 g wb) in closed moulds. Bread and starch gels were characterized by compression tests, light microscopy (LM) and differential scanning calorimetry (DSC). The amylase enhanced the initial firmness of both systems and reduced their firming rate on ageing. DSC showed that the enzyme hindered the recrystallization of amylopectin (retrogradation). Polarized LM revealed that starch granules of freshly baked control bread and starch gels showed a weak birefringence which became more intense on ageing. Amylase containing bread and starch gels exhibited strong birefringence after baking which did not significantly increase on ageing. By combining bright-field and polarized LM the birefringent structures were identified as amylose-rich regions within the starch granules. The accumulation of intragranular amylose is due to its phase-separation from amylopectin during baking. It is hypothesized that the antistaling effect of the amylase is based on the capacity to partially degrade amylopectin and by this hindering its recrystallization. On the other hand, the enzyme slightly degrades amylose by an endo- mechanism which, in turn, promotes the gelation of amylose and prevents its rearrangements on ageing.