Thermal transitions and microstructure of starch-apple pomace-based systems

Apple pomace (AP) is the by-product obtained after juice extraction. It is a potential ingredient for the fibre enrichment of foods. In the present work, AP was combined in gluten free-mixtures with cassava starch (CS), rice flour (RF), egg white (EW) and sucrose (SR) to analyse its effect on thermal transitions of starch and the batter and bread crumb microstructures. Model systems with all or part of the main ingredients (CS:RF:AP:EW:SR in 4:4:1:1:1 proportions) and a batter formulation with AP were prepared by dry mixing and then dispersing the solids in water. CLSM allowed observing the distribution of egg proteins, AP and RF particles and CS granules. The batter enriched with AP exhibited higher apparent viscosities and dilatant behaviour. Thermal transitions were analysed by DSC. Endotherms ranged between 57 and 97 degrees C. Gluten-free bread microstructure was analysed by SEM. A more irregular and compact crumb structure was observed when AP was used, with the air cell walls constituted by gelatinized starch granules and AP particles interpenetrating them. By measuring the firmness of crumb along storage, it could be concluded that amylopectin retrogradation was partially inhibited by the presence of AP in model systems but in bread, this inhibiting effect was not evident.