AsianScientist (Apr. 22, 2016) – We are all familiar with that strange feeling in the mouth after a sip of red wine or tea, or a bite of unripe fruit. It has been described as dry, leathery or even furry. This astringent effect is caused by tannins or polyphenolic compounds that bind to mucins, lubricating proteins in the mucus membranes of the mouth. Now, in the journal Angewandte Chemie, a Chinese and Korean research team has described the relationship between astringency and this disrupted oral lubrication.
Mucins consist of a central protein chain with side chains made of sugar compounds that can bind a large amount of water. Mucins form a barrier and protect sensitive mucus membranes from drying out and from chemical and mechanical interactions. They provide adequate lubrication and correspondingly low friction. This lubricating film in the oral cavity fails when tannins come in: a sip of wine causes the tongue to feel less slippery.
The research team, from the Chinese Academy of Sciences and Korea Advanced Institute of Science and Technology, decided to study this friction aspect. In their experiments, they used a dumbbell-shaped mucin extracted from oral mucus membranes and tannic acid, a star-shaped polyphenol found in wine and unripe fruit, as their tannin.
They found that when the tannic acid binds to the mucin, their interactions reduce the solubility of the protein in water. The mucins consequently aggregate and may precipitate, leading to a failure of the mucin lubrication film.
Under a miscroscope, a substrate coated in mucin showed a flat, dense, film. After addition of tannic acid, many “defects” could be seen in the film and the surface was significantly rougher.
In comparison to a glass surface coated only with water, mucin-coated glass had much lower friction when coming into contact with a soft plastic ball. Addition of tannic acid caused the friction to rise substantially. An extract of coffee beans, which also contain tannins, had a similar effect.
Finally, in order to mimic a tongue, the scientists produced a mucin-containing plastic hydrogel. When wet, this elastic but barely tear-resistant material had very low friction, slipping easily through the fingers. A weight placed on an inclined surface of this hydrogel slides right off.
Addition of a tannic acid solution makes the gel sticky and it begins to shrink as a result of losing water. The mechanical strength increases significantly and the elasticity decreases; the weight no longer slides off.
“This finding may guide people to change their eating habits. For example, protein-rich and polyphenol-rich foods can’t be eaten together,” first author, PhD student Mr. Ma Shuanhong from the Chinese Academy of Sciences, told Asian Scientist Magazine.
“Further, this finding enables us to develop tannic acid-rich materials used for capturing animals whose surfaces are always covered with a thick protein layer, such as fish.”
Bioactive proteins that maintain the slippery state of fish skin also react to tannins, and as such, the researchers made gloves that release tannic acid when touched. These gloves made it easy to grasp and hold fish.
According to study lead Professor Feng Zhou from the Chinese Academy of Sciences, in the future, the researchers plan to further explore friction-enhanced mouth feel.
“I do believe that many tastes can be enhanced by friction between the tongue and oral cavity,” he said.
The article can be found at: Ma et al. (2016) Astringent Mouthfeel as a Consequence of Lubrication Failure.
Source: Angewandte Chemie; Photo: Shutterstock.
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