Despite a long history of use of the partially purified pigment as a natural food colour, the actual identity of the compound responsible for the yellow colour from Allium cepa had remained elusive, claim scientists from Japan’s National Institute of Health Sciences.
As part of their wider work into natural food additives, the researchers sought to find out the exact nature of pigment, which is obtained from the dried outer scales and has a tone they describe as “warm and quiet”.
Writing in Tetrahedron Letters, Yusai Ito and co-workers from Japan’s National Institute of Health Sciences report that the yellow pigment is 9-carboxy-1,3,6,8-tetrahydroxyxanthylium, or cepaic acid.
The pigment is produced by oxidation of quercetin, a well-known flavonol in onion scales, said Ito and co-workers.
“To our knowledge, this is the first report of such a pigment in yellow onion,” they state.
The fall and rise of natural colours
Natural colours lost their appeal when synthetic colours arrived on the scene, as they provide less consistency, heat stability and colour range than their chemical alternatives. Moreover, natural colours are more expensive.
However, as consumer awareness increases over the link between diet and health and trends move towards more clean-label products, natural colourings are back in fashion.
Yellow options
The key natural compounds responsible for yellow colours available to food scientists include annatto, beta-carotene, lutein, mixed carotenoids, riboflavin, and curcumin, said LFI.
Companies such as Wild and Chr Hansen offer a range of yellow and orange pigments based on carotenoids, for example.
Yellow pigments may also be sourced from cactus pear, according to the Hohenheim University researchers.
In addition, researchers from France's Institute for Agronomy Research (INRA) have been studying the potential of the natural yellow pigment, POP (phloridzine oxidation product), which is a by-product of cider production.
Source: Tetrahedron Letters
Volume 50, Issue 28, Pages 4084-4086
“Cepaic acid, a novel yellow xanthylium pigment from the dried outer scales of the yellow onion Allium cepa”
Authors: Y. Ito, N. Sugimoto, T. Akiyama, T. Yamazaki, K. Tanamoto