In situ observation of the freezing process in wheat dough by confocal laser scanning microscopy (CLSM): Formation of ice and changes in the gluten network

Abstract

Freezing of bread dough is widely applied in food industry. However, freezing impairs the baking performance of dough, which is largely attributed to structural changes as induced by ice formation. The aim of the present investigation was to image ice formation during freezing of dough and to assess the structural changes in the gluten network. A confocal laser scanning microscope (CLSM) equipped with a freezing stage was used to follow ice formation in the reflection and transmission (bright field) mode. Wheat dough with air inclusions served as a model for fermented dough. The gas pores and the ice crystals could be imaged by confocal laser reflection. Ice formation was initiated at the gas pore interface, where large ice crystals were formed during a freezing time of 4 h at −15 °C. The freezing of gluten samples stained with rhodamin was followed in the fluorescence mode. The cryoconcentration of gluten could be observed, but no irreversible changes in the microstructure of gluten were detected upon thawing. It is concluded that the gas pore interfaces in dough are preferential sites for ice nucleation, favouring the growth of ice crystals in these regions and by this a freeze induced redistribution of water in dough.