Linalool, Castoreum, and MSG: What Hides in Natural Flavors?

What does the FDA actually mean by 'natural flavors'?

The FDA's definition appears in Title 21 of the Code of Federal Regulations, section 101.22. It covers any flavoring substance derived from a plant, animal, seafood, dairy product, fermentation product, or similar biological source — provided the primary function is flavoring rather than nutrition.

That definition is deliberately broad. It covers roasted extracts, cold-pressed oils, enzymatic digests, and fermentation broths, as long as the starting material is biological. What it does not require is that a company list each individual compound contributing to the flavor. A finished natural flavor blend can contain dozens of active molecules under a single two-word declaration.

The practical result is that 'natural flavors' sits third or fourth on many ingredient lists by weight, yet tells the consumer almost nothing about what ingredients are actually present. Understanding the definition is the first step to reading those labels more clearly.

What is linalool, and why does it appear so often?

Linalool is a naturally occurring terpene alcohol found in lavender, coriander, basil, and several citrus peels. It has a light floral-woody scent and a mildly sweet taste, which makes it useful in drinks, baked goods, dairy alternatives, and fruit-flavored snacks.

Flavor houses frequently include linalool as part of a broader citrus or berry natural flavor blend because it rounds out sharper notes and survives moderate heat. The FDA's FEMA GRAS (Generally Recognized as Safe) program, administered by the Flavor and Extract Manufacturers Association, lists linalool as a permitted flavoring substance. The substance appears in USDA FoodData Central records for several flavored beverages.

Although linalool is considered safe at typical food-use levels by current regulatory assessments, it is also a recognized skin sensitizer in fragrance safety literature. Those two contexts — food flavoring and cosmetic fragrance — operate under entirely separate regulatory frameworks, so the food-grade safety assessment does not automatically extend to topical products, and vice versa.

What is castoreum, and is this ingredient still used?

Castoreum is a waxy secretion produced in the castor sacs — scent glands located near the tail — of North American and Eurasian beavers. It has a warm, vanilla-adjacent aroma with subtle leather and tobacco undertones. Historically it appeared in vanilla, raspberry, and strawberry imitation flavors.

Its use today is genuinely rare. The FEMA GRAS list permits it, and it technically qualifies under the FDA natural flavors definition because it is an animal-derived biological secretion. But production is costly, supply is limited, and synthetic or plant-based alternatives are cheaper and more consistent. Industry production data cited in FEMA usage surveys suggests annual consumption in the United States is measured in pounds rather than tons.

Castoreum is the ingredient that circulates most in online discussions about hidden natural flavor sources, often exaggerated into claims of widespread use. The evidence from flavor industry trade records does not support those claims for modern mainstream products. Vegans and people with certain sensitivities still have legitimate reason to ask manufacturers directly, because the label will not tell them.

How do yeast autolysate and hydrolyzed proteins add glutamates?

Yeast autolysate is produced by breaking down yeast cells with enzymes, releasing their internal contents including free amino acids. Glutamic acid — the same amino acid that defines monosodium glutamate — is released in significant quantities during this process. When yeast autolysate is used as a natural flavor, it can elevate the free glutamate level in a food without any separate MSG declaration.

Hydrolyzed plant proteins work similarly. Hydrolysis breaks peptide bonds and liberates individual amino acids, glutamic acid among them. Both ingredients are legitimate natural flavor sources under 21 CFR 101.22. The FDA requires separate labeling of monosodium glutamate as an additive but does not require disclosure of free glutamates introduced through natural flavor components.

This matters most for people who are trying to reduce their glutamate intake for personal dietary reasons. It is not a safety concern at normal serving levels according to current FDA and European Food Safety Authority positions, but it is a transparency gap that a careful food ingredient scanner user will want to be aware of.

What other compounds can natural flavors legally contain?

Beyond linalool, castoreum, and glutamate contributors, the natural flavors umbrella also permits smoke condensates (used to add a smoked note without actual smoking), enzymatic hydrolysates of milk proteins, fish-derived smoke extracts, and fermentation-derived esters and aldehydes. Each of these has an identifiable flavor function and a biological source.

The FEMA GRAS program maintains a list of more than 2,700 flavor substances. Not all of them appear behind the natural flavors declaration, but a substantial share do. Many are structurally identical to synthetic counterparts — the regulatory difference is the source, not the molecule.

One emerging area involves fermentation-derived compounds produced by precision fermentation, where microorganisms are engineered to produce a specific flavor molecule. As long as the starting microorganism and feedstock qualify as biological, the output can still meet the FDA natural flavors definition. This was a developing regulatory discussion in 2025 and remains unsettled heading into 2026.

• Smoke condensates from hardwoods
• Enzymatic milk protein hydrolysates
• Fish sauce extracts standardized for flavor
• Citrus terpenes including d-limonene and linalool
• Fermentation-derived esters such as ethyl butyrate (pineapple note)
• Vanilla-related phenolics extracted from sources other than vanilla bean

Is this ingredient safe — what do regulators actually say?

For most people eating normal amounts, the compounds found in natural flavor blends sit within safe intake levels established through the FEMA GRAS program and reviewed periodically against current toxicological data. The FDA does not pre-approve individual natural flavors but can act if evidence of harm emerges.

The meaningful exceptions involve allergens. Milk, fish, shellfish, tree nuts, peanuts, wheat, eggs, sesame, and soybeans are the major allergens that require disclosure under the Food Allergen Labeling and Consumer Protection Act even when they appear as flavor components. A natural flavor derived from milk still requires a milk allergen statement. The same rule applies to the other major allergens listed in U.S. law.

Allergens not covered by that statutory list — including certain seed proteins and some legumes — do not carry the same mandatory disclosure requirement. That is where the transparency gap is most consequential. If you have a confirmed allergy to a less common ingredient, contacting the manufacturer is the only reliable path to a clear answer.

How can you find out what ingredients are in a specific product's natural flavor?

Manufacturers are not legally required to disclose natural flavor formulations, which are considered trade secrets. However, many brands will share allergen-specific information by phone or email if asked directly. Some list 'contains: milk' or 'contains: fish' sub-disclosures in the ingredient panel even when those allergens appear only in the natural flavor component.

Using an ingredient analyzer like InZoRAH surfaces the allergen disclosures, any linked openFDA enforcement actions, and Open Food Facts data for the specific product. That does not reveal the full flavor formula, but it consolidates the public information that does exist in one place.

Certified kosher and certified vegan products offer an indirect signal: kosher certification typically excludes non-kosher animal sources including castoreum from non-ritually slaughtered animals, and vegan certification excludes all animal derivatives. Neither certification addresses every compound in a flavor blend, but they narrow the field considerably.

What changed in 2025-2026 that makes this worth revisiting now?

Two regulatory developments are relevant. First, the FDA issued updated guidance in late 2025 on precision fermentation-derived flavor ingredients, acknowledging that the existing natural flavors definition was written before engineered microorganisms became a routine production tool. The guidance did not change the definition but asked for industry comment on whether additional disclosure is warranted.

Second, the European Food Safety Authority published a re-evaluation of several terpene-based flavoring substances — including linalool — under its systematic program to reassess approved flavorings using current methodology. Preliminary findings maintained existing safe-use levels, but the process highlighted how much of the underlying safety data is decades old.

Neither development results in immediate label changes for U.S. consumers, but both point toward a period of increased regulatory scrutiny of what the natural flavors category actually contains. Staying current on these developments is exactly the kind of context a well-maintained ingredient checker should surface as guidance evolves.