SCIENTISTS in Switzerland are reporting that bacteria in the human mouth play a role in creating the distinctive flavours of certain foods. They found that these bacteria actually produce food odours from odourless components of food, allowing people to fully savour fruits and vegetables. Their study is scheduled for the November 12 edition of the ACS bi-weekly Journal of Agricultural and Food Chemistry. In the study, Christian Starkenmann and colleagues point out that some fruits and vegetables release characteristic odours only after being swallowed. While scientists have previously reported that volatile compounds produced from precursors found in these foods are responsible for this 'retroaromatic' effect, the details of this transformation were not understood. To fill that knowledge gap, the scientists performed sensory tests on 30 trained panellists to evaluate the odour intensity of volatile compounds - known as thiols - that are released from odourless sulphur compounds found naturally in grapes, onions, and bell peppers. When given samples of the odourless compounds, it took participants 20 to 30 seconds to perceive the aroma of the thiols - and this perception persisted for three minutes. The researchers also determined that the odourless compounds are transformed into the thiols by anaerobic bacteria residing in the mouth - causing the characteristic 'retroaromatic' effect. "The mouth acts as a reactor, adding another dimension to odour perceptions," they explain. However, the authors conclude, it is saliva's ability to trap these free thiols that helps modulate the long-lasting flavours.           " SD