International Produce Training

Onions- Sunburn

Although you may come across some onions with a green color, caused by sunburn, the scoring guidelines will most likely prevent you from scoring sunburn as a defect.

We will discuss northern grown onions (onions grown in Idaho, Oregon Northern California, New York, etc) as the scoring guideline for Bermuda Granex Grao type onions is decidedly different.  Sunburn is a greenish discoloration of the outer scales caused by exposure to the sun.  There is no killing of the tissues, so you will not see sunken areas or sunken pitted areas associated with sunburn.

If you come across sunburn, the USDA has established a scoring guideline to follow:  Without removing the scales, if more than 33% of the surface of the onion has a medium green or darker color, then the onion is scored as a defect, sunburn.  Because the green color does not change, progress, or become more green , this defect is considered a quality defect.  But remember, you are not allowed to peel back the papery scales to look for sunburn, if you pick up an onion, and you see green color, then you would begin to determine if the color is dark enough green and is affecting at least 33% of the surface of the onion.

You may wondering what is “medium green” color?  The USDA has developed a visual aid, a color chip depicting what is meant by “medium green” color.


This color chip is to be used only to determine the green color of sunburn for northern grown onions. For BGG onions affected by sunburn, the USDA has developed an entirely different color chip to use.

Seems simple enough; if you find an onion with green color, and without peeling back the outer scales, if the green color is at least as dark or darker green than the color chip, and the affected area is more 33% of the surface of the onion, then the onion would be scored as a defect.

Although the U.S. Standard allows a total defect tolerance of only 5% for northern grown onions, sunburn has its own special tolerance.  If you find more than 33% of the onions being damaged by sunburn, then the lot would be out of grade.  In other words, you would have to find more than 33% of the onions with more than 1/3 of the surface affected by at least a medium green color to have the lot fail to grade U.S. No. 1 account of sunburn. 

I am sure you will find an occasional onion here or there that may be damaged by sunburn, but to find more than 33% of the onions with enough sunburn to be scored as damage is highly unlikely.

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