Scientists solve mystery of ‘green oats’
Consumers sometimes report that cooking of oat products in tap water results in generation of greenish colours, “providing a distinctly unpalatable appearance, and [this] is thus undesirable”, explained researchers led by Douglas Doehlert from the USDA’s Agricultural Research Service’s Wheat Quality Laboratory.
Oats do not contain any pigments, and replication of these observations has remained beyond the grasp of many laboratory-based scientists. But now, according to scientists from the ARS and North Dakota State University, the problem may have been solved.
Writing in the Journal of Food Science, the researchers report that baking soda (sodium bicarbonate), a leavening agent used extensively in baked products, and other oat processing protocols that require alkaline conditions, may be behind the phenomena.
Cooking of oats and oat products in hard water, which contain higher levels of bicarbonate than soft water, may also induce the formation of green pigments.
“Although bicarbonate concentrations required to generate colour are generally not present in tap water, sodium bicarbonate (baking soda) is used as a leavening agent in many baked products and because other oat processing protocols may require alkaline conditions, it is important to point out possible off colours from this mechanism,” wrote Doehlert and his co-workers.
The mechanism centres on the presence of compounds in the oats called avenanthramides. When sodium bicarbonate is boiled in water, carbon dioxide is produced (and escapes as a gas), while hydroxide ions (OH-) remain in the water. This increases the pH into the alkaline range. As with many compounds, changes in pH are associated with colour changes, and under alkaline conditions the avenanthramides take on a yellow-green colour, said the researchers.
“Industrial interests using baking soda or alkaline conditions during oat processing should be aware of possible off-colour generation,” wrote the researchers.
Other issues to iron out
There is also the indication that iron ions present in water may react with oat aleurone and produce off-colours. “This problem would tend to be observed in steel-cut groats more readily than in oat flakes, because of the intact aleurone on the exterior of the groat would exhibit the staining more obviously than flakes,” explained the researchers. .
As little as 10 ppm of divalent iron (Fe2+) can induce the formation of a grey-green colour when cooked, they said. On the other hand, other divalent ions, such as calcium and magnesium, did not have any effect on the colour of cooked oats.
Source: Journal of Food Science
Volume 74, Number 6, Pages S226-S231
"The Green Oat Story: Possible Mechanisms of Green Color Formation in Oat Products during Cooking"
Authors: D.C. Doehlert, S. Simsek, M.L.Wise
