How chemistry shapes the taste of alcohol

Animals have been consuming alcohol for millions of years, including primates and humans who have digested alcohol for around 7 to 21 million years. Over the course of human history, alcohol consumption and production has been a part of many different cultures. Experts in human societies, or anthropologists, and native people know that fermenting rice wine (mijiu) and beer (lao li) has been part of ancient cultures in China for 7,000 to 13,000 years. Similarly, people from the Andean region of South America have been brewing a beer made of corn called chicha for around 5,000 years. 
Despite these ancient ways of alcohol production being distributed around the globe, people all brew beverages with the same amounts of alcohol in them, based on a measurement known as alcohol by volume or ABV. While beverages can be brewed to different ABVs, beers are preferred to be brewed to around 4% alcohol, wines between 11% and 16%, and stronger spirits to around 43%, 52%, 68%, and 75%. However, scientists haven’t found the reasoning behind these universal ranges of ABV.
A research team in China studied why people have chosen specific ABVs for different alcoholic beverages by determining how water and ethanol molecules interact at different ABVs. Though alcoholic beverages contain many different molecules that add flavor, color, and smell, the main molecules they contain are water and ethanol. These molecules are made of atoms such as hydrogen and oxygen. While the atoms of a molecule are bound together by electric forces like two magnets, the atoms between two molecules can also be attracted to one another. Water and ethanol molecules are attracted to one another through their hydrogen and oxygen atoms, in a process called hydrogen bonding.
The research team investigated how hydrogen bonding caused water and ethanol to come together in different orientations or interaction angles. They used an instrument that determines the structure of molecules, called the hydrogen nuclear magnetic resonance spectrometer or H NMR. The H NMR machine can detect hydrogen atoms and tell what they are connected to and what angles they are forming.
The research team made mixtures of water and ethanol with an ABV of 0% to 100% and used H NMR to detect changes in the interaction angle between the two molecules. They found that as the ABV became larger, the interaction angle became smaller. It dropped from a 90° angle at 1% ABV to a 10° angle at 99% ABV. They noticed that this change was not smooth, and the interaction angle dropped in a stepwise manner. For example, between 11% and 13% ABV the interaction angle was about 70°, but it suddenly dropped to 60° when the ABV reached 14%. The team noticed that these sharp changes occurred at the preferred ABV ranges of alcoholic beverages around the world, as described above.
A common type of hydrogen bonding that occurs between hydrogen atoms and oxygen atoms is known as a hydroxyl. Using H NMR, the team saw that these hydroxyl interactions created an even 3D network of water molecules at 90° interaction angles, forming a tetrahedral geometry. However, the hydroxyl interactions between ethanol molecules were nearly straight lines, with 0° interaction angles that formed long chains. As the ABV of a beverage increased, the molecules in a tetrahedral geometry and long chains competed with one another. 
The research team found that the number of hydroxyl interactions also decreased as the ABV increased, in the same stepwise manner as the interaction angles did. The team concluded that alcoholic beverages at different ABVs form distinct mixtures of chains and tetrahedral interactions. When they increased the amount of ethanol molecules, the number of chain interactions increased as the molecules found new preferred orientations. 
Finally, the researchers examined whether the amount of these chain and tetrahedral interactions might change the flavor of an alcoholic beverage when it is cooled or heated. When they cooled down 11% ABV beverages to 42°F (5°C), more hydroxyl interactions occurred. This cooling increased the number of chain interactions between water and ethanol molecules. 
Next, the researchers hired professional and amateur beer tasters to test the flavor of cold and warm alcoholic beverages at 11% ABV. These tasters noticed a stronger difference in the alcohol flavor of low and high ABV beers when they were cooled, because of the greater number of chain interactions within these beverages. 
On the other hand, when the researchers heated their beverages to 104°F (40°C), the number of hydroxyl interactions stayed consistently between 38% and 52% ABV. When the professional and amateur beer tasters tasted the warmed alcoholic beverages at 38% and 52% ABV, they could not tell the difference. The research team concluded that warming these drinks caused them to have the same amount of chain interactions, so the flavor was unaffected by the change in ABVs. They suggested this taste difference could explain why people like to drink warm sake and other alcoholic beverages at 38% ABV. 
The research team concluded that over the course of human history, brewers and drinkers have followed the taste in their tongues to find the right ABVs and temperatures necessary to make drinks with high molecular chain interactions between water and ethanol. By learning about the importance of hydrogen bonding and molecular interactions, the research team hopes that future brewers and scientists will experiment with different ways to control these molecular interactions to create even more refined and interesting flavors.

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