Compound Interest: What Compounds Cause Garlic Breath?

The Chemistry of Garlic 2016 — Click to enlarge

The latest food chemistry graphic looks at garlic and a couple of its well known effects. Garlic is frequently used in cooking, but its use comes with the unwanted accompaniment of ‘garlic breath’. On the more beneficial side of things, it can also have antibacterial properties. This post examines the chemical compounds behind these two phenomena.

Much as with onions, the chemicals that lead to ‘garlic breath’ aren’t actually present in unchopped garlic. They are formed when the garlic clove is mechanically damaged; this causes enzymes to break down the compound alliin, found in the cloves, to form allicin. Allicin is the major compound that contributes to chopped garlic’s aroma. It too is broken down into a range of sulfur-containing organic compounds, several of which contribute to the ‘garlic breath’ effect.

Research has identified four major compounds that contribute: diallyl disulfide, allyl methyl sulfide, allyl mercaptan, and allyl methyl disulfide. Of these, allyl methyl sulfide is the compound that takes longest for the body to break down. It is absorbed in the gastrointestinal tract and passes into the bloodstream, then passes on to other organs in the body for excretion, specifically the skin, kidneys and lungs. It is excreted through the skin via sweating, in the urine – and through your breath. This effect can last up to 24 hours, until all of the compound is excreted from the body, causing a faint, lingering, garlicky aroma.

So, what can you do to mitigate this effect? Some research has been carried out in the area, and a number of foods have been discovered to mildly reduce garlic breath. These include parsley, milk, apple, spinach and mint. The mechanism for their reduction of the effect is still unclear, with some research suggesting chlorophylls in vegetables have the potential to freshen breath, whilst more recent research provides evidence that this is not the case. Instead, it’s suggested that the deodourising effect is due to enzymatic action of some compounds in the deodourising substances, which ultimately assist in breaking down the organosulfur compounds.

Sulfur-containing compounds are again involved in the antibacterial properties of garlic. Researchers tested these compounds on a type of bacteria found in animal faeces, one of the most common bacterial causes of gastroenteritis, and found that the anti-microbial activity of the compounds increased with the number of sulfur atoms present; diallyl trisulfide being the most effective, followed by diallyl disulfide, then diallyl sulfide. These compounds are effective as they can penetrate the cell membranes of bacteria cells, and cause changes in structure in thiol (-SH) containing enzymes and proteins, injuring the cell.

Another suggested potential effect of garlic is as an insecticide against ticks and mosquitos. However, the research suggesting this effect was subsequently criticised as to the design of the study, particularly the lack of comparison to known insect repellents, and as such cannot be considered conclusive. A subsequent study, examining repellent effects of a variety of substances on mosquitos, found that garlic exhibited no noticeable effect, but also pointed out that longer term consumption of garlic might be required to achieve any repellent effects in humans.

An alternate version of this graphic features in the Compound Interest book on the chemical explanations for various odd food and drink phenomena, available here.