Why can we drink ethanol but not methanol?
FERNANDO SAPIÑA answers:
In our everyday life, we have contact mainly with one type of alcohol: ethanol. It is the liquid we use as disinfectant for our wounds, due to its antiseptic properties. In addition, we consume drinks with this substance, as is the case with some recreational drugs.
In alcoholic drinks, ethanol appears by the fermentation of sugar thanks to yeast. However, yeast cannot survive when the concentration of ethanol is high, so fermentation produces solutions with up to 16% alcohol content: drinks with higher alcohol content are obtained distilling the liquid obtained after fermentation.
Our ability to metabolise alcohol was developed millions of years ago by some mammalian ancestors in order to deal with ethanol and with much lower amounts of methanol present naturally in food (basically in ripe fruit) or produced by bacteria in our digestive tract. And this ability made it possible for us to consume ethanol in alcoholic beverages such as wine or brandy.
Another alcohol we have contact with is methanol, in methylated alcohol. It is known as «burning alcohol» in Spanish (“alcohol de quemar”) because it works quite well in non-pressurised burners; in fact, it is often used by people who spend a lot of time in nature. Our metabolism can cope with small amounts of consumed or generated methanol with no effects on our body. The problem appears when a person drinks significant amounts of methanol: this product ends up turning into formic acid, which is metabolised very slowly. Formic acid accumulates in the organism and finally reaches toxic levels.
Ethanol and methanol are very similar in appearance and colour, so when someone drinks an alcoholic beverage, they cannot know whether it contains dangerous amounts of methanol, whether it is because it was adulterated or because the distillation process was incorrect. In addition, the initial effects are similar. However, between 10 and 30 hours after methanol ingestion, symptoms appear, due to toxic levels of formic acid and formates: blurred vision, complete loss of vision, acidosis and, if the dose was too high, death by respiratory failure.
Fernando Sapiña. Professor and researcher at theDepartment of Inorganic Chemistry and the Institute of Material Science of the University of Valencia.