Changes in Starch Microstructure on Baking and Staling of Wheat Bread

Abstract

The microstructure of starch in dough and in fresh and aged bread crumb was studied by light microscopy. The samples were cryosectioned and stained with Light Green and iodine to localize protein and starch, respectively. In dough a partial segregation of starch from the protein phase is observed. On baking, starch was gelatinized and led to the formation of a continuous starch network. The starch fraction itself was inhomogeneous and consisted of swollen and interconnected starch granules. The two starch polymers, amylose and amylopectin, were found to phase separate and amylose was accumulated in the centre of starch granules. Polarized-light microscopy of fresh bread crumb showed that starch gelatinization was accompanied by the loss of birefringence. On ageing the bread crumb regained birefringence. The combination of light microscopy in the bright-field and polarized mode allowed identification of the birefringent structures. The most intense birefringence was observed in the amylose rich centre of starch granules, whereas the outer amylopectin rich zones showed slight birefringence. It is concluded that the ordered structures result from the reordering of amylose and amylopectin. It is hypothesized that the reorganization of the intra-granular amylose fraction enhances the rigidity of starch granules on bread staling.