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United States Department of Agriculture

Agricultural Research Service

Title: The Green Oat Story: Possible Mechanisms of Green Color Formation in Oat Products During Cooking Color Changes in Cooked Oat Products

Authors
item Doehlert, Douglas
item Simsek, S -
item WISE, MITCHELL

Submitted to: Journal of Food Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 11, 2009
Publication Date: May 9, 2009
Citation: Doehlert, D.C., Simsek, S., Wise, M.L. 2009. The Green Oat Story: Possible Mechanisms of Green Color Formation in Oat Products During Cooking Color Changes in Cooked Oat Products. Journal of Food Science. 74(6):S226-S231

Interpretive Summary: Consumers occasionally complain to oat companies that their steel cut oat groats turn a greenish color when they are cooked in tap water. Because no scientific explanation for this color change is known, we have tested the idea that the color change is due to iron in the water supply. Some well water will contain ferrous iron, which is iron with two positive charges. However, ferrous iron will react with oxygen once exposed to the air to form ferric iron ion, which is iron with three positive charges. Ferric iron is relatively insoluble in water, so well water with high iron content will usually turn cloudy shortly after being pumped, as the ferric iron precipitates out of solution. We have found that if you cook oats with water containing ferrous iron, before it has had a chance to react with oxygen, a green gray color will appear on oats, primarily on seed coat. Thus, the color is more noticeable on steel cut groats than on oat flakes because the seed coat layer is less disrupted in the steel cut groats. If you let the water sit for a few hours and let the cloudy precipitate settle out, the water will not affect the color of the oats. We have also found that oats will turn a greenish color under alkaline conditions, although it seems unlikely that the consumer would encounter these conditions.

Technical Abstract: Consumers occasionally report greenish colors generated in their oat products when cooking in tap water. Here we have investigated pH and ferrous (Fe2+) ion as possible mechanisms for this color change. Steel-cut oat groats can turn brown-green color when cooked in alkaline conditions (pH 9-12). Extraction of this color with methanol, and high pressure liquid chromatography indicated a direct association of this color with the phenolic acid or avenanthramide content of the oat. The presence of 50 mM NaHCO3 in water will cause oat/water mixtures to turn alkaline when cooked as CO2 is driven off, generating OH- ion. Although tap water rarely, if ever, contains so much bicarbonate, bicarbonate is used as a leavening agent in baking applications. Industrial interests using baking soda or alkaline conditions during oat processing should be aware of possible off color generation. We have also found that as little as 10 ppm Fe2+ will turn oat products gray-green when cooked. The aleurone stained darker than the starchy endosperm. Other divalent cations, such as Ca2+ or Mg2+ had no effect on cooked oat color. As much as 50 ppm Fe2+ may be found in freshly pumped well water, but Fe2+ reacts quickly with oxygen and precipitates as Fe(OH)3. Thus, some freshly pumped well water may turn oats green when cooked, but if the water is left under atmospheric conditions for several hours, no discoloration will appear in the cooked oats.

Last Modified: 8/19/2014
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