Edible bird's nests—How do the red ones get red?

Red edible bird's nests are regarded as of higher beneficial value for health and hence fetch a higher price than the white ones. Their red colour remains a myth. To determine if white edible bird's nests can turn red by vapours generated from sodium nitrite in acidic conditions and by vapours from ‘bird soil’. White edible bird's nests were exposed to vapours from sodium nitrite dissolved in 2% HCl or from ‘bird soil’ in hot and humid conditions. Vapours from sodium nitrite dissolved in 2% HCl or from ‘bird soil’ containing guano droppings from swiftlet houses were able to turn white edible bird's nests red. The reddening agent in ‘bird soil’ was water-soluble and heat-stable. The red colour of edible bird's nests is likely caused by the environmental factors in cave interiors and swiftlet houses. [Display omitted]