Denatonium

Denatonium, usually available as denatonium benzoate (under trade names such as BITTER+PLUS, Bitrex or Aversion) and as denatonium saccharide, is the most bitter chemical compound known, with bitterness thresholds of 0.05 ppm for the benzoate and 0.01 ppm for the saccharide. It was discovered in 1958 during research on local anesthetics by MacFarlan Smith of Edinburgh, Scotland, and registered under the trademark Bitrex. Dilutions of as little as 10 ppm are unbearably bitter to most humans. Denatonium salts are usually colorless and odorless solids but are often traded as solutions. They are used as aversive agents (bitterants) to prevent accidental ingestion. Denatonium is used in denatured alcohol, antifreeze, nail biting preventions, respirator mask fit-testing, animal repellents, liquid soaps, and shampoos. It is not known to pose any long-term health risks.

Structure and physical properties
Denatonium is a quaternary ammonium cation. It is a compound of a salt with an inert anion like benzoate or saccharide. The structure of denatonium is related to the local anesthetic lidocaine, differing only by the addition of a benzyl group to the amino nitrogen.

Biochemistry
Denatonium in humans is recognized by eight distinct bitter taste receptors: TAS2R4, TAS2R8, TAS2R10, TAS2R39, TAS2R43, TAS2R16, TAS2R46, TAS2R47, with TAS2R47 being by far the most sensitive to the compound.

Denatonium can act as a bronchodilator by activating bitter taste receptors in the airway smooth muscle.

Applications
The bitterness of the compound guides most applications of denatonium. Denatonium benzoate is used to denature ethanol so that it is not treated as an alcoholic beverage with respect to taxation and sales restrictions. One designation in particular, SD-40B, indicates that ethanol has been denatured using denatonium benzoate. The common name for this chemical, denatonium, alludes to this application.

Denatonium is commonly included in placebo pills used in clinical trials to match the bitter taste of certain medications.

Denatonium also discourages consumption of harmful alcohols like methyl alcohol, and additives like ethylene glycol. Denatonium is used in rubbing alcohol as an inactive ingredient. It is also added to many kinds of harmful liquids including solvents (such as nail polish remover), paints, varnishes, toiletries and other personal care items, special nail polish for preventing nail biting, and various other household products. It is also added to less hazardous aerosol products (such as Gas dusters) to discourage inhalant abuse of the volatile vapors.

Since 1995, when the U.S. State of Oregon required that denatonium benzoate be added to antifreeze and windshield washer fluid, the compound has been increasingly found in these substances throughout the world. The addition is credited with saving children and animals who might otherwise drink sweet antifreeze or wiper fluid and get ethylene glycol or methanol poisoning respectively. As of December 2012, major marketers of antifreeze are adding a bittering agent such as denatonium benzoate to antifreeze in all 50 US states.

Animals are known to have different sensitivities to the effects of denatonium. It is used in some animal repellents (especially for such large mammals as deer). It has been used to safeguard rat poisons from human consumption, as humans are able to detect denatonium at much lower concentrations than rodents. However, some cats have been known to be tempted by them — it may not be as effective a deterrent for cats as it is for humans, or perhaps some cats are not deterred due to a genetic factor similar to that affecting human perceptions of the taste of phenylthiocarbamide.