For those of you who haven’t done this in class, here’s what happens. You eat the berry, and then try a slice of lemon. It tastes sweet! But the berry didn’t taste sweet when you ate it. Try a sour patch kid candy – it tastes sweet too! The effect lasts about an hour and it feels weird; your brain expects one thing yet experiences another – it’s like an optical illusion for your mouth. Biologically, this is a lot of chemistry just for taste. You get the sugar, protein, fat, or salt from what you eat whether you taste them or not, so is it important to taste things?
It must be important to taste things, or else we wouldn’t do it. Gustatory sensation is more than just a little complicated at the cellular and molecular levels, so it must play an important role in the survival and evolution of many species, otherwise it wouldn't be worth the costs.
We see things to find food, avoid predators, or find mates. We hear things to localize predators/prey or to find our kin when we can’t see them. Smell is central for communication amongst species (pheromones) and for sensing danger (like smoke). But these examples describe gaining information at a distance, and are important for communication and safety. Does gustation fit into any of these categories?
Tasting something can’t be done from a distance – humans have to stick the target in their mouths – so what’s the big deal? It’s important because your brain is asking the question, “Should I swallow what’s in my mouth?" "Is this O.K., or is it going to kill me?”
Evolution has honed our brains to crave those things we need and spit out those that will do us harm. Sweet foods translate as energy – your brain says, “Eat this, it has carbohydrates – you need those.” What would be the best way for your brain to convince you to eat what is good for you? It bribes you with a pleasant payoff; we perceive it as tasting good, and we want more.
Rotting foods are acidic; acids are often the by-products of contaminating bacteria and fungi. Therefore, our old brain tells us to stay away from sour (acidic) foods. The gustatory sense is definitely protective. As humans, we can use our large brains to evaluate other cues as to food safety, so we can learn to like bitter and sour tastes; most animals just go with what Mother Nature tells them.
Gustation is a direct chemosensory process. Molecules to be tasted must come into direct contact with the sensors (receptors) in the mouth. This is similar to your sense of smell, but with one distinction; the chemicals you smell are volatilized in the air. For example, you don’t smell a rose by sticking it up your nose, the rose scent molecules traveling in the air from the flower to your olfactory receptors high in your nasal cavity. Smell is distance chemo-sensing.
For taste, the target molecules to be sensed are carried in liquid, not air. You take a bite of something, chew it up to release some of the molecules, and they mix with your saliva. Saliva is more than 99% water, and it is the water they delivers the dissolved (water-soluble) molecules to your taste receptors. Our taste receptors respond to things that dissolve in water or fat. Things like vanilla, cinnamon and spices are not soluble in water, but they are in fat. Hurrah for fat!
Papillae are basically mounds of epithelial tissue that stick up from the surface of the tongue (see picture to left). Those with taste buds tend to be round or mushroom shaped, while filiform papillae are cone shaped and tend to point toward the back of the mouth.
Filiform papillae are the most numerous, but are not directly involved in taste; they increase the tongue’s friction to help move foods toward the throat and to help break up food to release the taste molecules. In different animals they can have different shapes; in cats they are long and spindly, and though they feel like sandpaper, they are useful for grooming.
Taste buds are found only on the sides of the papillae; the food molecules must dissolve and move into the crevices between papillae. The taste buds themselves are small packages of gustatory receptor and supporting cells.
Each receptor corresponds to one taste sensation, so each receptor cell responds to just a single taste. It is the combination of all the receptors cells activated and the intensity of their activation that leads to complex tastes. We use to think that specific taste receptors were limited to certain areas on the tongue. But now we know that specific taste receptors are more concentrated in certain areas, but are present everywhere. For instance, you can sense sweet everywhere on the tongue, but sweet receptors are most concentrated at the tip.
When the particular tastant (the molecule that activates the receptor cell), fits into the taste receptor on the microvilli, it sends a single to a nerve which is embedded at the base of the cell. The more receptors that are engaged by tastant, the bigger the signal. This signal leads to a neural action potential that travels along the taste neurons to the brain, where they are converted to our sense of taste.
The receptors fit with their ligands (the tastant molecule) in a lock and key arrangement, although often more than one ligand will fit into a receptor. For example, the sweet receptor is a heterodimer (made from two different parts, called T1R2 and T1R3), and sucrose fits well into the receptors and is sensed as sweet. However, lactose (the sugar in milk) doesn’t fit as well, so it is sensed as less sweet.
Fructose is a great fit, so it is sensed as more sweet than sucrose. This is probably why high fructose corn syrup is added to everything today, it satisfies our craving for sweet better than regular sugar does. Artificial sweeteners are thousands of times sweeter than sucrose because they bind to the sweet receptor more tightly. Therefore, you can use a lot less of the sweetener than you would use of the sugar – and no (or few) calories.So we have two parameters that regulate our sense of taste; 1) how many receptors are activated at one time, and 2) how good a fit the molecule makes with the receptor. So now that we know about taste receptors and action potentials, how might miraculin make sour things taste sweet?
A 2011 paper from the University of Tokyo has started to let us in on the secret. Remember that miraculin doesn’t make bitter things taste sweet, or salty things taste sweet, only sour – and sour things are acidic. It seems that it’s the acid that makes the difference. By manipulating the pH of the mouth, the researchers showed that miraculin has no flavor at neutral pH, but as the pH of the mouth decreases, the sweet taste increases.
So you eat something sour (acidic) after the miracle berry, and the pH of your mouth drops. Now the acid tastes sweet. The hypothesis is that miraculin binds to the sweet receptor in the lock/key fashion, but the shape of the protein doesn’t activate the receptor. But when the pH drops, the shape of the miraculin protein changes (protein folding is very much affected pH), and it activates the sweet receptor. This sends an action potential to the brain and we perceive sweetness. Anything that lowers the pH of the mouth is perceived as sweet. Sweet!
We have more to say about our sense of taste, like what a supertaster is, and how we keep adding new tastes – no longer are we confined to just sweet, salt, sour, and bitter!
For more information and classroom activities, see:
Taste sensation –