Of the five basic tastes, I find bitterness to be the most interesting—not only from an ingredient perspective but also looking at our behavior in response. In most recipes, we usually try to avoid tasting bitterness, and, as cooks, we’ve developed various ways in the kitchen to make the taste more palatable by trying to reduce it, cover it up, or remove it entirely.
There are, however, some bitter foods that we’ve learned to like, such as alcohol, tea, cocoa, and, of course, coffee, one of the most popular bitter beverages in the world. There’s a common theme throughout these four ingredients: They’re stimulants. We appreciate the bitter taste in these foods because the stimulant activity—working on our brain and nervous system to make us more alert—acts as a reward. Our brains learn that drinking a cup of coffee provides positive reinforcement in the form of its stimulant activity.
Coffee contains a variety of different taste and aroma molecules. Some are naturally present in the raw green beans, and others develop after roasting. Of these substances, caffeine is one of the most well known that not only tastes bitter, but also acts as a stimulant. In addition, caffeine is both a water- and fat-soluble molecule. Because of this, it can easily travel through the body (in the blood and through the membranes of cells), acting on the central nervous system and keeping us alert.
Genetics also plays a role in our perception of bitterness. Those of us with certain genetic sequence variations are more sensitive to bitter foods and will avoid them whenever possible.
However, with coffee, things get even more interesting. Based on a 2018 study, people who were more sensitive to caffeine, due to those genetic variations, actually consumed more coffee but lower amounts of tea. There could be a few reasons for this, one of which might be the strong reward stimulus elicited by caffeine, a learned positive effect that gets reinforced with every sip of coffee. While some teas contain caffeine (present in a smaller amount than in coffee), another stimulant called theophylline is also present; in comparison, theophylline is weaker at inhibiting the enzyme cyclic AMP phosphodiesterase, which caffeine acts on. (Note: There are other stimulants in tea, like theobromine, and the concentration of these stimulants varies by the type of tea. Tea additionally contains L-theanine, an amino acid with a relaxing effect and umami taste.)
There are various ways people try to reduce the bitterness in coffee. Think: stirring in milk or creamer; a sweetener, such as sugar; even butter. These mix-ins dial down coffee’s bitterness, to whatever levels we are comfortable with. (I drink mine with a big splash of milk.) You can also temper the bitterness in coffee by using cold-brew methods, since caffeine’s solubility in water decreases with temperature.
How does salt influence bitterness in coffee? One way is by decreasing caffeine’s solubility. Hence, adding salt during brewing potentially turns down the bitterness. Another way this effect can occur is by activating the salt receptors on the tongue; salty tastes at any concentration are known to suppress bitter tastes.
To test how salt affects coffee, I brewed four different batches. For the first one, I used a cold-brew method: salted ground coffee, steeped in cold water for two days (about ¼ teaspoon of salt for every ¼ cup of ground coffee). For the second batch, I brewed salted ground coffee with hot water (with the same ratio of salt to coffee). Then I compared the results to a cold-brew batch without any salt and a hot-water batch without any salt.
The cold brew, both with and without salt, had the least bitter taste, while the ones brewed with hot water tasted totally bitter. (Disclaimer: I don’t like bitter foods that much, though I’ve never been tested for any genetic predispositions around bitterness.) I did not find the coffee to be noticeably salty.
When I asked a few friends to repeat my experiment, they came back with mixed results. Two of them found that salt minimized bitterness in both cold and hot brews, while another found no difference.
So, what’s the final verdict? The effect of salt on bitterness really depends on a lot of variables: the type and concentration of coffee, the temperature of water used, the amount of salt added, and also genetics. Will I add salt to my coffee down the line? In all likelihood, yes, especially if I can’t get my hands on any milk or creamer (caffeine can bind the proteins present in milk).
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