Lots of people add sugar to coffee, and it’s not necessarily because they have a major sweet tooth. While adding a pound of sugar to one cup of coffee will turn the beverage into a liquid candy bar, putting just a little sugar in your coffee can have a profound effect that goes beyond sugar’s simple sweetness. According to a paper from the University of York’s Structural Biology Lab, sugar causes a molecular change in brewed coffee that makes coffee less bitter.

Sugar Affects Coffee at a Molecular Level

Through statistical thermodynamics, which is a branch of theoretical physical chemistry, Dr. Seishi Shimizu found that sugar doesn’t just mask coffee’s bitterness with sweetness, but it actually changes the molecular structure of a brewed cup of coffee. According to his lab’s paper, Food and Function, sugar affects the dimerization of caffeine molecules in coffee and caffeinated tea, which ultimately has an impact on how bitter the beverages are. Here’s an explanation of the paper’s findings in lay terms.

Caffeine is a molecular compound that’s a naturally occurring pesticide. While it’s known for its stimulating effects, it’s also largely responsible for the bitterness in coffee and caffeinated teas. Coffee and tea plants produce caffeine to deter insects and other predators from eating the plants’ fruits and leaves. The predators don’t like the bitter flavor of the caffeine molecules.

In a brewed cup of black coffee or unsweetened tea, caffeine molecules bind to water molecules. The distribution of caffeine molecules among the water molecules is roughly equal.

When sugar is added to brewed coffee or tea, however, the distribution of the caffeine molecules change. The sugar molecules bind to the water molecules. The caffeine molecules react by clustering together, likely trying to avoid the sugar molecules. The net result is that there are clumps of caffeine molecules in the brewed coffee or tea, rather than an even distribution of them.

As the caffeine clumps, the coffee or tea becomes less bitter. The paper’s abstract didn’t go into detail as to why the beverage becomes less bitter. It’s likely because not as many caffeine molecules are hitting the taste buds when the coffee or tea is drunk. Instead, they’re clumped together so that only a portion directly contact the taste buds at any one time, which reduces the bitterness.

Artificial Sweeteners’ Effects Weren’t Stated in the Abstract

Dr. Shimizu’s abstract also didn’t go into detail about whether artificial sweeteners would have the same effect, or a reduced or amplified effect. How synthetically made sweeteners affected the bitterness of coffee and tea would depend on their solubility in water and affinity with caffeine. To reduce bitterness, and not simply mask it, a sweetener needs to dissolve and bind with water, and also drive caffeine molecules into clusters.

Why Do You Sweeten Coffee?

Does this fit with why you sweeten your coffee? Do you add just a little sugar not for the sweetness, but to reduce your brew’s bitterness? Let us know by tweeting at us.



Scott is a professional writer for Driftaway Coffee. He worked as a barista for eight years, but today prefers to enjoy his beverages from the other side of the counter. When not drinking Driftaway Coffee, Scott usually has a mug of his own roasted coffee nearby.
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