Mechanistic investigation via QCM-D into the color stability imparted to betacyanins by the presence of food grade anionic polysaccharides

Abstract

The known ability of select anionic polysaccharides (gum arabic, beet pectin, xanthan gum, and sodium alginate) to improve shelf and thermal color stabilities of beet betalains at mild (pH 5) and typical beverage acidity (pH 3.2) is highlighted and studied herein. Mechanisms imparting stability are investigated by in-solution particle size, zeta-potential, and auto-fluorescence measurements coupled with quartz crystal microbalance with dissipation (QCM-D) binding studies. Sodium alginate and xanthan gum best preserved red hues in aqueous solutions through accelerated shelf storage (40 °C) at both pH. However, while alginate complexes improved thermal stability at pH 3.2, xanthan gum-pigment mixtures had the poorest of all polysaccharides at 55 °C. Beet pectin showed stabilizing effects at pH 5.0 only. The highly negative zeta potentials of alginate and xanthan pigment mixtures at both pH and beet pectin mixtures att pH 5.0 suggests a negative charge environment created by polysaccharide uronic acid substituents may aid color stabilization. Particle size data further suggest sodium alginate forms soluble complexes with beet extracts. QCM-D showed purified betacyanin binds irreversibly and most significantly to surfaces coated with the more negative and linear polysaccharides, alginate or xanthan gum. Pigment binding to heavily branched polysaccharides (gum arabic and beet pectin) was not significant and was reversible. Furthermore, dissipation changes plotted against frequency change suggest betacyanin binding to alginate and xanthan gum is more rigid at pH 3.2 than at pH 5.0, and suggests betalain stability at low pH may be imparted by strong irreversible association between these polysaccharides and the pigment.