Dishonest cooks have used herbs and spices to mask the odor of spoiling food for thousands of years. Now scientists studying how odors interact in the brain may have discovered the reason certain spices are especially effective at covering up the stench of rotting meat.
As foods decay, they release chemicals into the air, where they are inhaled and detected by receptor cells in the nose. The information is sent to the olfactory bulb, then to other brain regions that eventually identify the odors as foul. To study how the brain copes with competing odors, researchers led by Kensaku Mori of the University of Tokyo exposed rats to a fatty, fishy odor released by rotting meat and to two spices known to mask it.
The researchers identified clusters of cells in the olfactory bulb that were particularly responsive to the fishy odor, which is a combination of alkylamines, fatty acids, and aliphatic aldehydes. The cells were activated by each of the three chemicals but not by other molecules (such as their amino acid precursors), strongly suggesting that the cells are "tuned" to this odor. The scientists then exposed the rats to cloves and fennel oil, which mask the fishy odor. Each odor lit up clusters of cells adjacent to the clusters of cells sensitive to the fishy odor and suppressed their response to that odor, the researchers report in the 6 October issue of the Journal of Neuroscience. "It was surprising that the interaction between spoiled food smells and herb spice odors occurred at the level of the olfactory blub," says Mori. "Such interactions were thought to occur at higher centers."
The finding could make it easier to identify odors that will be effective at masking another odor, for use as perfumes or to cover up unpleasant odors in the environment, says neuroscientist Charles Greer of Yale University School of Medicine. But Charles Wysocki, a neuroscientist at the Monell Chemical Senses Center in Philadelphia, Pennsylvania, cautions that the brain mechanisms described in the study may not be applicable to humans. "Imaging studies in humans would make a much stronger argument for the case," he says.