Effect of pH and seaweed (Undaria pinnatifida) meal level on rheological and antioxidant properties of model aqueous systems

The use of whole seaweed flour would be in line with current consumption trends that allow incorporating its bioactive compounds and conferring structure to different food matrices through their hydrocolloids. The knowledge and modeling of the viscoelastic behavior of model systems of Undaria pinnatifida flour could be a useful tool to understand their structural changes and predict their behavior. The aim of this study was to investigate how the food matrix environmental conditions, especially pH, and flour level could alter the rheological, structural, and antioxidant properties of model aqueous systems. Undaria pinnatifida blade and midrib flour between 5 and 9 g/100 g and pH in the range 2.5-7.5 were studied. The results showed that the linear viscoelastic range of the suspensions was mainly controlled by flour level with critical stress between 1 and 200 Pa for 5 and 9 g/100 g, respectively. However, the mechanical spectra were significantly affected by pH, resulting in viscoelastic solids at low pH (2.5) and more gelled systems at high pH (7.5), which could be related to the precipitation of solubilized alginic acid. The power-law model satisfactorily fitted the flow behavior of the suspensions. Model parameters showed that flow index was only affected by concentration above pH = 5; the more concentrated the suspension, the more shear-thinning their behavior, which could be attributed to the higher dissolved sodium alginate. Scanning electron microscopy and FTIR results agreed with the rheological analysis performed, exhibiting a more cross-linked, uniform matrix with a lower mannuronic/guluronic ratio at high pH values. Therefore, the wide range of rheological characteristics obtained allows predicting the behavior of the seaweed meal in various food matrices without losing its antioxidant capacity.