A product is gluten-free when it contains less than 20 parts per million (ppm) of gluten, the threshold set by the FDA for any food carrying a “gluten-free” label in the United States. That 20 ppm standard is also used by the European Union and the international Codex Alimentarius, making it effectively the global benchmark. But reaching that number involves much more than just leaving wheat out of a recipe. It depends on what ingredients go into the product, how the facility handles cross-contact, and how the final product is tested.
Which Grains Contain Gluten
Gluten is a family of storage proteins found in wheat, barley, and rye. Each grain has its own version: wheat contains gliadins and glutenins, rye contains secalins, and barley contains hordeins. These proteins make up roughly 75 to 80% of the total protein in each grain, which is why even small amounts of contamination can push a product over the 20 ppm limit. Oats have a related protein called avenin, which most people with celiac disease can tolerate, but oats carry their own set of complications (more on that below).
For a product to qualify as gluten-free, it must either be made from ingredients that inherently contain no gluten, like rice, corn, or quinoa, or be made from gluten-containing grains that have been processed to remove gluten to below 20 ppm. Wheat starch is the most common example of that second category. When properly processed, it can appear in a product that still meets the gluten-free standard.
The 20 PPM Threshold
Twenty parts per million translates to 20 milligrams of gluten per kilogram of food. That’s a tiny amount, but it’s not zero. The FDA chose this number because it represents the lowest level that can be reliably detected and quantified across different types of food, from raw ingredients to baked goods. It also reflects the scientific consensus on what most people with celiac disease can safely consume without triggering intestinal damage.
The FDA doesn’t mandate a specific testing method but requires that any method used be scientifically valid and capable of detecting 20 ppm across a variety of food types. In practice, most manufacturers use a type of lab test called an ELISA, which detects gluten protein fragments. Several commercial versions are available with detection limits as low as 2 to 5 ppm for gluten, and one kit can detect wheat protein down to 0.3 ppm. This means manufacturers can verify their products well below the legal threshold.
How Cross-Contact Happens in Manufacturing
Even when a product’s recipe is entirely gluten-free, it can pick up gluten during production. This is called cross-contact, and it’s one of the biggest challenges in making a truly gluten-free product. If a facility also processes wheat-based foods on the same equipment or in the same space, gluten residues can transfer through shared surfaces, airborne flour dust, or even employees moving between production lines.
Preventing cross-contact requires a layered approach. Facilities that produce both gluten-containing and gluten-free products typically use dedicated equipment, color-coded utensils, and separate zones with controlled airflow to keep allergen-containing dust from drifting into gluten-free areas. Some use positive air pressure in packaging rooms to push contaminated air out rather than letting it in. Employee movement between zones is restricted, and dedicated aprons and gloves are standard.
Production scheduling matters too. Many facilities run gluten-free products first, then move to products with increasing levels of allergens, with thorough cleaning between changeovers. Closed processing lines use valve systems to clear allergenic residues. Dust control is another focus: adding liquids to mixers before or alongside powders, using vacuum systems, and covering equipment all reduce the risk of airborne gluten settling onto clean products.
Hidden Sources of Gluten in Ingredients
Some ingredients carry gluten that isn’t obvious from the name on the label. Malt extract is one of the most common culprits. It’s derived from barley, which makes it a direct source of gluten. Malt flavoring, malt syrup, and malt vinegar all fall into this category.
Yeast extract is trickier. It can be grown on sugar beets, which are gluten-free, or it can come from spent brewer’s yeast, a byproduct of beer brewing that may be contaminated with barley and malt. The British spread Marmite, for instance, contains barley-derived yeast extract and has tested at around 30 ppm, above the gluten-free cutoff. There’s currently no easy way to tell from a label whether yeast extract in a given product comes from a gluten-free source or a brewery byproduct. If a product isn’t labeled gluten-free, yeast extract and autolyzed yeast extract are worth questioning.
Soy sauce is another frequent surprise. Traditional soy sauce is brewed with wheat. Tamari, a Japanese-style soy sauce, is often made without wheat, but not always. Checking the label or looking for a gluten-free certification is the only reliable way to know.
Distilled Products Are Generally Safe
Distillation separates liquids by boiling point, and gluten proteins are too large and heavy to carry over into the distilled liquid. This is why distilled vinegar and distilled spirits made from wheat, barley, or rye are considered gluten-free. The proteins stay behind in the still.
The exception is malt vinegar, which is not distilled. Because it’s made from barley malt without a distillation step, the gluten proteins remain intact. If you see vinegar on a label without further specification, it’s almost always distilled white vinegar or a naturally gluten-free variety like apple cider or wine vinegar.
The Problem With Oats
Oats don’t naturally contain the same gluten proteins found in wheat, barley, and rye. Their storage protein, avenin, is structurally different and tolerated by most people with celiac disease. The problem is contamination. Oats are routinely grown near wheat fields, transported in shared trucks, and processed in facilities that also handle wheat or barley.
To address this, some producers follow what’s called a purity protocol: strict controls from the farm through transportation and processing to keep oats separated from gluten-containing grains. Even so, large studies of gluten-free labeled oatmeal have found that roughly one in every few dozen servings contains a detectable gluten-containing grain, including some purity protocol oats. Research across 12 clinical studies found a statistically significant link between less pure oats and higher rates of adverse reactions in celiac patients. If you’re highly sensitive, oats remain a food to approach carefully even when labeled gluten-free.
Certification vs. the Gluten-Free Label
Any manufacturer can put “gluten-free” on a label as long as the product meets the FDA’s 20 ppm standard. The FDA does not require pre-approval or third-party testing before the claim goes on the package. Enforcement is complaint-driven and based on periodic sampling.
Third-party certification programs go further. The Gluten-Free Certification Organization (GFCO), one of the most widely recognized programs, sets its threshold at 10 ppm, half the FDA limit. Products bearing the GFCO certification mark undergo annual facility audits and ongoing verification. In Australia and New Zealand, the GFCO standard drops even lower, requiring no detectable gluten at all to meet those countries’ stricter regulations.
A GFCO seal or similar certification mark on a package tells you the product has been independently verified, not just self-declared. For people managing celiac disease or severe gluten sensitivity, the tighter threshold and audit process offer a meaningful extra layer of confidence. Products with only the “gluten-free” text on the label may be perfectly safe, but the standard they’re held to is less rigorous and less frequently checked.
How to Evaluate a Product Yourself
Start with the ingredient list. Look for wheat, barley, rye, malt, and brewer’s yeast. Check for less obvious terms like “malt extract,” “malt flavoring,” or “hydrolyzed wheat protein.” Under U.S. law, wheat must be declared on the label as a major allergen, but barley and rye do not have the same requirement, so they can hide inside vague terms like “natural flavors” or “yeast extract.”
Next, look for a gluten-free label or certification mark. A certified product has been tested to 10 ppm and audited annually. A self-labeled gluten-free product meets the legal 20 ppm standard on the manufacturer’s word. An unlabeled product that appears to be naturally gluten-free (like plain rice or canned vegetables) may still have been processed on shared equipment.
“May contain wheat” or “processed in a facility that also processes wheat” are voluntary advisory statements. They’re not regulated by the FDA, and their absence doesn’t guarantee a cross-contact-free product. Some manufacturers use them conservatively, others not at all. These statements can appear on products that also carry a gluten-free label, which can be confusing. In that case, the gluten-free claim carries more legal weight: it means the product has been determined to contain less than 20 ppm regardless of the shared facility.