This year we had a bumper crop of habanero peppers in our garden, and I decided to pickle them (see recipe here). So last weekend I had the Big Pickling Event and I spent about an hour slicing and dicing habaneros and a variety of vegetables and putting them in jars. After I was done I washed all the dishes and my hands, etc. I was very careful not to touch my face or eyes during the whole process and even afterwards. Nevertheless I could still feel some pepper fumes while I was cutting them and my eyes and mouth maybe felt a bit scratchy, but nothing too bad. Later that evening I was giving my kids a bath and when the hot water hit my hands they really began to burn, which makes sense, since the capsaicin receptor in our mouths and skin is the same receptor that senses noxious heat (+43°C), and maybe there was some residual capsaicin on my hands which I felt when the receptors were co-activated by heat. But still nothing too bad.
OK, here is where it gets weird. The next morning I'm having breakfast and I ate a piece of cut-up fruit and… it tastes spicy. First I thought that I didn't wash the knife or cutting board properly after habanero fest. But both my wife and kids said that the fruit did not taste spicy to them at all.
So there were a couple of possibilities, one being that prolonged exposure to pepper fumes for a good hour or so the previous day somehow sensitized my capsaicin receptors in my tongue such that I could taste trace amounts in the cut up fruit. But it wasn't just the fruit, I had some toast with jam and the jam tasted unquestionably spicy. Interesting. So that would mean that somehow the receptors may have been altered such that they could also respond to sweet flavors. This makes no sense, since the receptors for sweet flavors and for capsaicin work in very different ways. Sweetness is detected by what is known as a G-protein coupled receptor which resides in sensory neurons in the tastebuds. Activation of this receptor sets off a biochemical reaction which ultimately culminates in activation of a sensory nerve sending the signal to the brain. In contrast, capsaicin receptors belong to a class of proteins known as ion channels. And contact with capsaicin molecules, or exposure to burning heat, causes these channels to open and directly activate pain fibers going to the brain. But the effect was definitely there, and it lasted the rest of the day.
So I did a little experiment. I looked up some papers on spice perception and found a couple of studies showing that repeated exposures, about a minute apart, to very low doses of capsaicin caused sensitization of the capsaicin receptor. Meaning that each time capsaicin was given to a subject it was spicier than before. If they then waited a longer time interval then there was desensitization – which means that capsaicin became less spicy. Maybe, the hour-long exposure to pepper fumes the previous day somehow sensitized the receptors and altered the perception of sweetness in some yet unexplained way. Sort of like those African berries that make sour things taste sweet. Could I replicate this? Since I didn't feel like making more pickles I designed a carefully controlled study. I tried to recruit my wife to take part in the study, so we could have n=2, but somehow she refused to participate in my little shenanigans. I cut up a leftover habanero into tiny squares, about 1 millimeter across. I also used a different knife to cut up some strawberries, took out a loaf of bread and poured a glass of milk (bear with me). So for 8 minutes I chewed once a minute, for a few seconds, on one little pepper square, letting the pepper juice come in contact with the left side of my tongue. Consistent with the prior study, each pepper square tasted a bit spicier than the previous one, until the end when it tasted VERY, VERY spicy. I don't know if this was sensitization or just accumulated capsaicin on my tongue. Let's go with sensitization. Then I rinsed my mouth well with water, ate half a slice of bread and drank the milk. All of these things supposedly help get rid of capsaicin and sort of work if you've eaten something too spicy, like that crazy Korean food from the other day. Then I waited 5 minutes and ate a strawberry. It tasted spicy! And only on the side of my tongue which came in contact with the pepper. On the naive side it still tasted sweet. Groovy. Then came the desensitization part. I waited another 5 minutes and tasted a slightly bigger pepper square and let it get all over my tongue. Now I felt that in the previously exposed left side the spicy taste was less spicy than on the naive right side. Meaning that the left side had become desensitized. If I tasted a strawberry, it no longer tasted spicy on the left side and, ok, not on the right side either.
So, does sensitization of capsaicin receptors make them sensitive to sweet flavors? Who the hell knows. There were a few problems with my little experiment. One is that there was no good way to control for the pepper juice only touching one side of my tongue, so the whole left vs. right thing could have been imagined. Which is the other problem, since I knew what the expected results were I could have biased the perception. In addition, I only tried fruit. So it may be something else in the fruit that may have been activating the capsaicin receptors, like acidity. Sugar would have been a better test, but strawberries are delicious. Maybe I can recruit my kids to repeat this, but I can just picture the screaming after the eighth little habanero square. One thing is for sure – I'm glad I don't study human psychophysics for a living, there's too many variables.
So there you have it. Do try this at home and let me know the results!