Influence of level and oxidation state of iron on colors of edible bird’s nest

Abstract

Edible bird's nests (EBNs) are known as a medical food with abundant mineral and nutrition. Their colors vary from white, yellow and red according to their production sites. White EBNs are produced by swiftlets living in both natural caves and man-made bird's houses. Red EBNs, on the other hand, can be only found in a certain cave. Due to the higher price of red EBNs in comparison with white EBNs, artificial red EBNs are made by exposing white EBNs with nitrous acid vapor generated from a solution of sodium nitrite and hydrochloric acid. According to ICP-OES, red cave EBNs displayed 3–35-fold higher amount of iron (4.18 ± 1.00 mg/100g), compared to white EBNs (0.57 ± 0.40 - 0.66 ± 0.13 mg/100g) and artificial red EBNs (0.12 ± 0.06 - 1.45 ± 0.03 mg/100g). Fe K-edge XAS demonstrated the presence of iron (II) and iron (III) species in all EBNs. Both red cave EBNs and artificial red EBNs from both cave and house showed significantly higher amount of iron (III) (96.69–99.08%) than white EBNs from both cave and house (68.81–74.19%). This study suggests that the red color in EBNs of both authentic red cave and artificial red EBNs are associated with their iron (III) content.