Microstructural study of air-drying kinetics of Ca(II)-alginate beads to preserve cowpea extracts

Abstract

The great potential of Ca(II)-alginate beads in preserving biomolecules and their use as a carrier of bioactive compounds has been demonstrated. However, several food applications require long-term storage and thus, numerous preservation techniques such as drying have been applied. Still, research of stability of bioactive compounds in these treatments focus on the morphology of the beads and the microstructure of the network but do not consider the microstructural changes that occur during the drying process.Ca(II)-alginate beads were prepared to preserve phenolic compounds extracted from cowpea (Vigna unguiculata) by-products by using high-intensity ultrasound (VCX500, Sonics, US). Also, the influence of the addition of cowpea isolated proteins as an excipient of the beads has been analysed. The microstructural changes of the network during air-drying treatment have been studied by SAXS at the LNLS SAXS1 beamline in Campinas, Brazil at λ = 0.1488 nm. The wave vector (q) was selected in the range 0.142 nm−1 < q < 5.035 nm−1. All the Ca(II)-alginate beads analysed showed isotropic scattering and were modelled as a fractal system composed of rod-like structures.Significant differences were observed between the alginate beads containing the whole extract with respect to those containing cowpea isolate protein, revealing that cowpea protein is embedded within the rods, obtaining thicker but less dense rods disposed in a more disordered structure. These differences remained constant until 60 min of the air-drying treatment at 25 °C (even with moisture losses up to 30%). From this time, the microstructure of the dried beads at the final stage if drying (xw = 0.5 kgw/kgT) showed the loss of the typical scattering of rod-like structures and even displayed new signals at high q in the presence of cowpea proteins.