Doron asked: People say it is best to add sugar to coffee after you have added water, then the grains will not float on the surface. However, as I recall from 10th grade biology, sugar is water-soluble, so it should not matter when it is added. What do you think?

Hello Doron,

As you have mentioned grains, I gather you are referring to the preparation of what is called “mud coffee”, in which ground coffee grains are mixed together with water and sugar in the same cup. If this is the case, I would say there is a scientific basis to the original claim.

As for the floating grains, the fact that they float when first added, but then precipitate when the coffee is mixed strongly enough, indicates they do not have a lower density than water. It is likely they are actually just floating on the surface-tension of water, and that the addition of the sugar from on top breaks the surface-tension and causes them to precipitate. Examples for this phenomenon can be found in our experiments the floating clothespin and top, pepper and soap. You can find more information in our full article about surface tension.

Roasted coffee beans | Photograph by: Robert Knapp, Wikipedia

There is another sensible reason to add the sugar after the water: the substances that give coffee its flavor transfer from the coffee grains to the water by extraction in a process called diffusion. If the water contains additional solutes such as sugar, they will interfere with this diffusion process - similar to what happens during osmosis. Therefore, it is better to begin the coffee preparation without solutes, and only add the solutes, like sugar, after the coffee has been extracted to the liquid.

Nevertheless, it is important to keep in mind that our explanation here is only a theoretical analysis of the situation. In practice, if we want to be sure of the answer, we will have to perform a controlled experiment in which we measure the amount of floating grains and contents of flavoring substances. Most importantly, we have to perform a randomized study on a group of experimenters that will taste (with no prior knowledge!) coffee that was prepared via each method and they decide which actually tastes better. There you have a study people would pay to be part of!



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Boomerang – Question Back at You
Does the same apply to instant coffee?

Dr. Avi Saig
Davidson Institute of Science Education
Weizmann Institute of Science

Article translated from Hebrew by Elee Shimshoni, PhD student at the Weizmann Institute of Science.
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