Brown rice syrup is the go-to for many gluten-free bakers. Gluten-free recipes everywhere call for the stuff and indeed, it lends a perfectly sweet flavor to cakes, cookies, muffins, and scones. But the syrup is controversial in the world of gluten intolerance and Celiac Disease because in some cases, it contains gluten.
Some, though not all, brown rice syrup is actually fermented in a process that employs barley. Barley, of course, contains gluten (as do spelt, wheat, triticale, some oats due to cross-contamination concerns, and others). In order to ferment the syrup, manufacturers add barley enzymes. Though it may result in seemingly insignificant amounts of barley in the final product, sensitive individuals would be best to avoid the syrup. It's also important for these sensitive people to be aware when purchasing pre-made baked goods that are labeled "gluten-free", if brown rice syrup was used because the item may be contaminated.
However, this is not true in all cases because some companies employ a fungal enzyme to aide in the fermentation, and therefore the product is considered gluten-free. In order to determine if this is the case, be sure to call the company and investigate the origins of the sweetener.