Monitoring changes in crust and crumb of cakes after baking: relation to their microstructures

Cakes, as the majority of bakery products, are characterized by two different structures: crumb (internal part) and crust (external part). The physicochemical properties of crumb and crust in cakes have not been yet studied. This work focused on starch and protein characteristics in cake crumb and crust after baking (2h) using Micro-Visco-Amylograph (MVAG), Fourier Transform Infrared (FTIR) spectroscopy, Fluorescence spectroscopy, and Light Microscopy (LM). MVAG profiles showed that crumb has higher pasting temperature and lower peak viscosity than crust. Setback value, which is related to the ability of the amylose chains to reassociate and form a gel, was higher in crust than in crumb (600 and 500 mPa*s, respectively) giving a firmer gel in crust than in crumb. FTIR spectra showed that the starch crystallinity ratio, the intensity ratio of crystalline (1047 cm-1) and amorphous (1022 cm-1) bands, was higher in crust than in crumb (1.07±0.01 and 1.05±0.01, respectively) suggesting different retrogradation extents of starch polymers during cooling. Regarding proteins, FTIR spectra showed that beta-sheet structures were higher in crumb than in crust (10% and 4%, respectively) which could be explained by the higher water content in crumb than in crust. Fluorescence spectroscopy showed that crumb has higher tryptophan emission than crust (250 and 75 AU, respectively) probably related to the difference in water redistribution between crust and crumb. LM showed that starch granules were not gelatinized in crust due to its fast baking rate. This study showed that the changes taking place in cakes after baking could come from the difference between crumb and crust properties (starch gelatinization and retrogradation and water distribution) which are not clear enough by textural analysis. The role of crust and crumb during storage on cake quality needs to be investigated.