December 3rd, 2014
Why do we choose to drink the beverages that we do? When we reach for a cup of coffee, perhaps it’s the caffeine we’re after; in a glass of wine, the social lubricating effects of alcohol may have lured us. But for carbonated drinks, the subtle zing of effervescence often combined with sugary sweetness, creates a sensory delight that can be irresistible.
It’s often thought that the tingling bite we experience when drinking a carbonated drink is due to the feeling of bubbles bursting in our mouth. But research over recent years has shown that carbonation bite is mostly caused by acidity.
When we un-cap a bottle of soft drink – call it soda or pop or seltzer if you prefer – some of the carbon dioxide dissolved in the pressurised liquid is released into tiny bubbles of effervescence. Carbonic anhydrase, an enzyme secreted in our saliva, converts the carbon dioxide into carbonic acid. This acid triggers receptors on nerve cells in our mouth to detect a sharp, slightly painful, biting sensation. Block the enzyme, and soft drinks quickly lose their bite.
But does this mean that bubbles play no role whatsoever in how we experience carbonation?
According to researchers at the Monell Chemical Senses Center in Philadelphia and the University of Pennsylvania, the answer is no. The way that the bubbles tickle our tongue also plays a part in making carbonated drinks such a sensory delight.
To investigate what role the bubbles might be playing in the way we experience fizzy drinks, the researchers conducted two simple yet telling experiments.
In the first experiment, the researchers tested whether the bite of carbonated water disappears when the bubbles are removed. To do this, they had made use of the fact that carbon dioxide doesn’t form bubbles at high atmospheric pressure – the dissolved carbon dioxide instead remains dissolved in the water.
Twelve volunteers were seated in a hyperbaric chamber and asked to sip and then rate the level of bite in water with varying levels of carbonation. The ratings were collected with the chamber set to normal atmospheric pressure — with bubbles — and a pressure of 2 atmospheres, twice what we experience at sea level, and roughly equivalent to diving at a depth of 10 meters. At this higher pressure, no bubbles formed in any of the carbonated water samples rated.
The intensity of the carbonation bite was identical at both pressure conditions, indicating that the bubbles were not necessary for the bite.
In a second experiment, the researchers tested whether, although not necessary for bite, the bubbles might still alter the bite sensation.
It is well known that cold and light touch can suppress sensations of pain. This is one of the reasons that cooling menthol is used in creams to alleviate muscle pain, and simply touching an itch can ease the urge to scratch.
So expectations were that the bubbles might lessen the biting pain of carbonation. But the researchers found that the bubbles had exactly the opposite effect — they enhanced the bite rather than dampening it.
In the second experiment, 11 volunteers dipped their tongues into mildly carbonated water. A stream of air bubbles was somultaneously directed onto their tongues to supplement the carbonation bubbles. The flow of air bubbles enhanced the intensity of bite felt by the volunteers.
Whether the bubbles have this enhancing effect through chemical oe mechanical means is still unclear. The extra bubbles could help to stir the carbonated liquid, helping to maintain high concentrations of carbon dioxide at the surface of the tougue. But the mechanism could also have something to do with pain perception. Tissue acidification by carbon dioxide (and carbonic acid) could be enhancing perception of otherwise innocuous tactile stimuli.
Better understanding the mechanisms involved could have some important implications for people who suffer from allodynia, a condition where harmless touch can be excruciating. It could also lead to a better understanding of what’s going on in cancers where tissue acidification occurs.
Wise PM, Wolf M, Thom SR & Bryant B (2013). The influence of bubbles on the perception carbonation bite. PLoS ONE DOI: 10.1371/journal.pone.0071488
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