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The Chemistry of Bad Odour
A number of different types of bacteria reside on the surface of the skin. These include species such as Corynebacterium, Propionibacterium and Staphylococcus epidermidis (click links for pronunciations). These bacteria transform many of the components in our secretions (especially those from the apocrine glands) into smaller compounds . Many of these are responsible for the characteristic odour of smelly feet.
For example, Corynebacterium will release an enzyme called lipase. In turn, lipase breaks down lipids or fatty substances released by the apocrine glands. This results in the formation of butyric acid which has a ‘cheesy’ odour. Similarly, leucine (an essential amino acid in the biosynthesis of proteins) is transformed into one of the 'smelliest' culprits called iso-valeric acid. Other odour causing compounds such as propionic acid are also synthesized in a similar fashion. These compounds are collectively part of a chemical class known as carboxylic acids or short-chain fatty acids.
Fortunately, these short-chain fatty acids can be neutralized thanks to the odour eating qualities found in agents such as zinc oxide and sodium bicarbonate. Another way to fight malodour is attacking the source itself. In other words, antiseptic or anti-microbial agents can be used to neutralize the bacteria which in turn will reduce the manufacturing of the odour causing compounds.
Zinc oxide is one of the more effective odour eating agents. It actually neutralizes the odour causing carboxylic acids by binding to them. The chemical reaction below shows how zinc forms a complex by binding and eliminating the odour causing fatty acids.
Zinc oxide has also been shown to be one of the more effective odour absorbing agents. The chart that follows demonstrates its ability to 'eat' iso-valeric acid compared to other topically applied substances.
Look for zinc oxide and sodium bicarbonate as ingredients when you purchase an antiperspirant odour-eating powder. Both of these agents have very good water-absorption and odour-eating capacities. Powders containing these ingredients are substantially more effective than talc based powders.
* Adapted from Fujikiro et al, J Soc Cosmet Chem 40; 335-43; 1989