Capsaicin

Whether it is adding chili flakes to top off your pizza, Tabasco to your omelet, chili oil to your ramen, there’s no doubt adding these condiments can add flavor intensity to all our dishes. Interestingly, the burning sensation is actually not a taste, since the sensation does not arise from taste buds. Capsaicin stimulates nerves that respond only to mild increases in temperature, the ones that give the sensation of moderate warmth [1]. Capsaicin sends two messages to the brain – intense stimulus and warmth. The burning sensation you feel when eating spicy food is due to the combination of these two messages.

What is the science behind all this magic? The answer to that is a supernatural compound capsaicin (or 8-methyl-N-vanillyl-6-nonenamide). Capsaicin is a common pungent molecule. It is found in capsicum fruits that are used in a variety of cuisines.

Figure 1: Chemical structure of capsaicin. Photo Credit: Essential Oil

Pure capsaicin is colorless, odorless, and crystalline-to-waxy solid at room temperature. The Scoville Heat Scale for pure capsaicin is about 16,000,000 SHU (Scoville Heat Units). SHU is a measurement of pungency. Ghost pepper is approximately 1,000,000 SHU; cayenne is approximately 40,000 SHU. Capsaicin is a hydrophobic molecule, meaning that it preferentially partitions into fatty environments. When consumed, capsaicin binds with pain receptors in the mouth and throat, which are normally responsible for sensing heat [2]. The taste buds on our tongue contain taste receptors. Taste buds sense tastants (taste molecules) and send the information from the tastants to the brain, where the molecule is processed as a certain taste. There are five main tastes: bitter, salty, sweet, sour and umami (savory) [3]. “Spicy” or “hot” is not sensed by our taste buds; instead, they are sensed by pain receptors, which are also found in the tongue. These receptors send pain signals via our nerve fibers to the brain, where it is perceived as a sensation of pain and heat.

But how does capsaicin give us the sensation of “tongue on fire”? Capsaicin is the active component of chili peppers that produces a burning sensation in any tissue it comes in contact with. How does this signal get conveyed? There are three classes of nerve fiber in our central and peripheral nervous system – the ‘C’ type of nerve fiber are the ones that are stimulated by capsaicin – specifically the molecule binds to the vanilloid receptors (VR-1, TRPV1) on the nerve endings of the C-fibers. These receptors are ligand-gated ion channels that are closed in the absence of capsaicin. When they are stimulated by capsaicin, they open and allow an influx of sodium and calcium ions, which initiate an action potential across the fibers. This action potential is what allows us to feel the burn. Normally, physical heat stimulates these receptors. However, capsaicin can also interact with these receptors and activate proteins that cause the same signal to be transmitted to the brain into thinking that it is being burned.

How often we consume spicy food can affect the sensitivity of these receptors. If you consume them too frequently you can essentially “kill” your receptors. This is one of the factors that contribute to different people having a different spice tolerance level. The other factor is genetics. Our vanilloid receptors can be mutated such that they are less susceptible to capsaicin. Thus, it can be implied that we can inherit our tolerance for spices and also why different cultures and genetic populations can have different spice tolerances.

Figure 2: TRPV1 receptor. Photo Credit: Wikipedia

Although chili can add some flavor and magic to our dishes, it can also be unpleasant and painful for some people who are not used to it. When chili becomes too hot or too painful to handle, dairy products can come to your rescue. Understanding the physical properties of capsaicin can help to explain why milk can help rescue you from the fire on your tongue.

Capsaicin has a long hydrophobic tail, which allows it bind with high affinity to protein receptors on the tongue, which have hydrocarbon side chains of their own. This fatty tail of capsaicin also allows the molecule to diffuse through cell membranes, making the burn more pervasive and persistent. While water may offer a temporary relief, it is not entirely effective because capsaicin oil and water do not mix. In fact, water will actually spread the capsaicin oil instead of soothing the burn. By contrast, milk contains proteins and fat globules that the capsaicin can partition into. For example, casein is a milk protein that has a higher affinity to capsaicin and can compete with our lipoprotein receptors, surrounding capsaicin molecules and relieving the burn.

Pretty cool.

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