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Always avoid wheat flours or flours labeled as being derived from barley, rye, or triticale.  Other flours to avoid include those listed as bulgur, durum, farina, graham, kamut, semolina, and spelt flours. Avoid flours that list ingredients likely to be contaminated with gluten unless the product is labeled as gluten-free. Frequently contaminated grain flours may include corn, soy, rice, potato, and beans unless otherwise stated. Flours that include flax, millet, quinoa, sorghum, tapioca, teff, buckwheat, arrowroot, or amaranth as ingredients may also be contaminated. Most flour products that are truly gluten-free will be labeled as such. Always check ingredient lists to verify that the flour could not be cross-contaminated or hiding a gluten-based ingredient if the label does not specify. Not all non-wheat flours will truly be gluten-free. If you really want to reduce the risk of purchasing baking flour that contains gluten, your best option is probably focusing your search efforts in a special gluten-free food aisle or specialty food store that focuses on providing reliable and safe gluten-free identification for its food products. Letting the vendor do the work for you provides extra reassurance that you have selected a gluten-free flour.
Memorize the different names for gluten. Buy flours specifically labeled as gluten-free. Consider purchasing flour from a specialty aisle or store only.