Soy β-Conglycinin−Curcumin Nanocomplexes for Enrichment of Clear Beverages

Delivery of sensitive health-promoting compounds in foods and beverages is an important and timely challenge. Curcumin, a natural polyphenol and the major pigment of the turmeric root, is a potent antioxidant with numerous attributed health benefits. However the major hurdle for applicability of curcumin as a food ingredient is its extremely low solubility in aqueous solutions at both acidic and neutral pH, and its consequent poor bioavailability. Here we have studied the possibility of using the soybean protein β-conglycinin (β-Cg) to form co-assembled nanovehicles for delivery of curcumin in clear beverages, where dietary and religious constraints preclude the use of milk proteins. Using visible light spectrophotometry we found high affinity between β-Cg and curcumin with a binding constant of about 1.27 · 10⁶ M⁻¹. The curcumin − β-Cg co-assemblies were much smaller than the curcumin aggregates without β-Cg. While in the absence of β-Cg, curcumin aggregates were several microns large and visible by eye, in the presence of β-Cg, particles were much smaller and showed a bimodal size distribution, with modal-average diameters of 27 nm (~90 % of the particles on a volume basis) and 110 nm (~10 %), and average of 46 nm, as measured by dynamic light scattering, and supported by cryoTEM images. Therefore, transparency was maintained in the β-Cg-curcumin co-assembly systems, enabling application in clear beverages. The curcumin – β-Cg system was characterized at increasing molar ratios. The binding sites saturation point of β-Cg was found to be around 15:1 curcumin:β-Cg molar ratio, or about 3 g curcumin/100 g β-Cg. Based on light microscopy observations, the presence of β-Cg both suppressed curcumin crystallization and modified crystal morphology. Moreover, β-Cg − curcumin co-assemblies were found to confer considerable protection to curcumin against light and oxidation induced degradation.