Water migration and molecular mobility in baked cakes during storage: an NMR investigation

Consumers can notice a recent diversification in commercial pastries, notably products with filling in. The quality changes and consequently the shelf-lives of these products depend on dynamic properties of water inside food. The staling process and molecular mobility of these heterogeneous systems are still unclear. Because consumers are more and more demanding on preserving foodstuff, it is a challenge to understand molecular mobility and chemical reactions during storage of pastries. Several works, using Nuclear Magnetic Resonance relaxation analysis, also called NMR relaxometry, have studied the retrogradation of starch and the water behavior in simple matrices: bread, starch-sugar(s)-water solutions. However, few works have described the mobility of molecules in complex systems, containing fat. The aim of this work was to investigate the water migration inside shell-shaped sponge cakes, called "madeleines", and the consequences on molecular mobility from NMR relaxation measurement during storage. After cooking, a large water gradient between crust and center was observed. The results showed that this gradient was influenced by the composition of the recipe. From a careful analyze of the NMR data we observed a fat gradient, superimposed with a gradient in molecular mobility. During storing, the system tended towards equilibrium of liquid distribution between center and crust, but molecular mobility's were still different. The results showed that center and crust did not have the same structure, neither the same behavior during staling. Diffusion coefficient of water was also measured by pulsed field gradient NMR during storing, independently for crust and center of cakes. This work is thus a new step into molecular dynamics understanding of amylaceous foodstuff, especially those with low humidity and high fat content. Because samples are real food products, this study will find a direct application in Industry.