Cross Contamination of Gluten-Free Grains

A small, preliminary study of cross contamination has raised questions about how much gluten there might really be in some naturally gluten-free grains and flours. The pilot study, published in the June issue of the Journal of the American Dietetic Association, found that seven of 22 samples of gluten-free grains, seeds and flours contained more gluten than allowed in proposed rules for gluten-free labeling in the US.

The seven samples, which included millet, buckwheat, sorghum and soy flours and millet grain, were tested and found to have gluten contamination that ranged from 2,925 to 25 parts per million of gluten. The US Food and Drug Administration has proposed allowing gluten-free food to contain up to 20 ppm of gluten.

Specific amounts (reported in mean ppm) were:

  • Soy flour, one brand 2,925 ppm and another 92 ppm
  • Millet flour, one brand 327 ppm and another 305 ppm
  • Sorghum flour, 234 ppm
  • Buckwheat flour, 65 ppm
  • Millet grain, 25 ppm

These results might have you wondering if many of the gluten-free foods you consider to be safe really are. While the numbers in the study are unnerving, there are a few things you should keep in mind.

First, the study was so small it is impossible to draw any definitive conclusions from it. In fact, Tricia Thompson, a dietitican who is one of the authors of the study, pointed this out in a blog she wrote announcing the findings.


“Sampling was not large enough to make any assessment on the overall percentage of contaminated product,” Thompson wrote. It was also too small to make any inferences on the specific grains, flours and seed likely to be cross contaminated, according to Thompson.

Thompson said in an email that study was designed specially to find out if gluten-free grains that are not labeled gluten free are likely to be cross contaminated. The intention is to use the study results to get the FDA to take a second look at a provision in its gluten-free labeling proposal that says when a naturally gluten-free food has a gluten-free label it must also say all foods of that type are gluten free. For example soy flour labeled gluten free would have to say all soy flour is gluten free. (The study suggests that’s not the case.)

Thompson said she hopes a larger study with statistically significant results will be conducted in the near future, but noted it would require testing of multiple samples of multiple brands. That kind of study would be exceedingly expensive, she said.

But an expanded study pretty cleary now needs to be done. It would be unfortunate if those who follow the gluten-free diet started to question all naturally gluten-free grains on the basis of results from a limited number of companies. And now that the question of just how cross contaminated naturally gluten-free grains really are has been raised, a definitive answer is needed.


A second thing to keep in mind is that since the pilot study looked only at grains not labeled gluten free, its conclusion that a certain percentage of inherently gluten-free grains contain gluten does not apply to sorghum, millet, soy and buckwheat flours that are labeled gluten free. Multi-ingredient gluten-free products, like baking mixes, were also not part of the study.

A number of companies that label their flours, grains and other products gluten free do test to be certain they contain less than 20 ppm of gluten. This information is often available on the labels themselves, on company websites and sometimes by calling the company directly.

Finally, nearly two thirds of the samples in the study did meet the FDA’s proposed standard for gluten-free labeling even though they did not have a gluten-free label. This was true even for three products that had allergen warning statements that said they may contain wheat.


Samples that tested below 20 ppm included basmati rice, long grain brown rice, enriched corn meal, instant polenta, rice flour, hulled buckwheat, amaranth flour, flax seed and amaranth seed. One sample of millet grain met the standard by testing to 14 ppm, but another exceeded it slightly at 25 ppm.

While you have to keep the results of the study in perspective, there is no denying it has uncovered some issues that have to be dealt with both by those who consume gluten-free grains and the FDA, which is supposed to regulate the companies that produce them.

We have known for a long time that all grains are cross contaminated to some extent by other grains because of the way they are grown, transported and milled. Still, the levels of cross contamination reported in the study are news to the gluten-free community. It’s well known that cross contamination of oats prevents them from being considered gluten free unless they are specially grown and processed. But the grains tested in the study have never before been singled out as potentially unsafe because of cross contamination.

While the study might have been intended mainly to provide information to the FDA about one labeling provision, it has farther reaching consequences. Alison St. Sure in her Sure Foods Living blog summed up the dilemma this way: “What are we to do now? Trace every grain, seed and flour, including those used as ingredients in gluten-free products, back to its origin to ensure it has not been contaminated with wheat?


I don’t think the situation is that dire, though I would like to know exactly which soy flour had more than 100 times the amount of gluten considered safe.

For now, it seems common sense and a bit more diligence regarding gluten-free flours in particular are in order. Gluten-free companies could help by making their testing information readily available to consumers, with the label being the best place to put it. Otherwise, stick with products you know and trust. Those who follow the gluten-free diet have managed to eat and maintain good health for many years relying on naturally gluten-free grains.

The study results might convince the FDA to revisit the labeling provision for naturally gluten-free grains and flours so we have more accurate information about them. And if the study of cross contamination is expanded, as it should be, it could provide us with more information that improves our gluten-free lives.

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