Why do we grimace when we eat sour or bitter food?
鈥� The body has stereotyped sequences of action for avoiding and responding to noxious stimuli such as pungency and acidity. Some resemble involuntary defences against physical attack and are similar through most of the animal kingdom, so they are almost certainly primitive in origin.
In humans, a faceful of ammonia or acetic acid fumes causes retreat, closed eyes and arms thrown across the face, among other responses. A noxious mouthful of a salty, bitter, acidic or otherwise vile chemical that our species instinctively avoids, such as one鈥檚 own ordure, causes another range of reactions, related to spitting or vomiting. Typical responses include: drawing down the corners of the mouth or gagging in preparation to vomit; salivating to clear the mouth and dilute harmful substances; puckering to avoid more intake; closing the eyes for protection; and performing convulsions that would help free oneself from assault.
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鈥淚n humans, a faceful of ammonia or acetic acid fumes causes retreat, closed eyes and arms thrown across the face, among other responses鈥�
More trivial stimulation 鈥� for instance, from piquant foods like pickles or mustard 鈥� provoke milder incipient reactions such as grimaces and shuddering. Perhaps the reason these different levels of reaction have survived and become entrenched is that such behaviour has evolved into a warning to offspring and associates: 鈥淏ad stuff! Beware!鈥� These less vigorous communication signals evolved more recently than the primitive reactions, and accordingly vary more widely between species, but they serve the same functions: warning of danger or nastiness and indicating good feeding.
Jon Richfield, Somerset West, South Africa