People sensitive to gluten may have a condition known as celiac disease. In people with this disorder, gluten in wheat products and baked goods irritates the lining of the gut, resulting in uncomfortable symptoms ranging from diarrhea to nausea. In general, coffee doesn't contain gluten so won't trigger the effects of celiac disease. However, some types of substitute coffee or coffee packaged in certain environments may contain gluten.
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Fresh roasted coffee grounds contain no gluten. Coffee plants produce coffee "beans" as their seeds. The process of grinding coffee doesn't add any gluten to the beans. Though gluten and similar substances do come from plant sources, including wheat, rye and barley, proteins inside coffee seeds don't have gluten. The Ohio State University Medical Center lists coffee and decaffeinated coffee as an acceptable food choice for those with celiac disease.
Some flavored coffee contains additives made with wheat flour or other forms of grain. Wheat flour contains gluten. Syrup flavorings designed to be added to hot coffee may contain gluten, depending on the type of syrup. Glucose or pure corn syrup should be gluten-free. Some instant coffee -- the freeze-dried granules you get in glass containers -- contains wheat flour as an ingredient. Another potential source of gluten in a cup of coffee is from dried creamer or powdered milk.
Some freeze-dried coffee may include filler ingredients containing gluten, particularly if the coffee comes in an unmarked package or from a country that doesn't require product information on the label. Those with celiac disease should always be careful of products that may be packed in an environment where flour or grain could contaminate the main product. All coffee packages sold in U.S. stores should tell you if the product contains gluten.
Check the storage jars in which you keep your coffee. If a jar previously contained flour, then it may contaminate your coffee and trigger some discomfort in the gut. Some health stores sell coffee alternatives made with barley or similar grains. These may contain gluten, though it should clearly state on the package if they do. Ingredients in coffee substitutes to look out for include gelatinized starch, modified food starch and vegetable starch. These can all indicate the possibility of gluten in the product.