Yum! How am I able to taste this?

When I was learning about enantiomers in Year 13 (or whenever it was; sixth form is kind of a blur to me now, my memory is horrid), I remember learning about limonene. One enantiomer of limonene smells like fresh oranges (D-Limonene), and the other smells like pine (L-Limonene); for those of you who didn't do A-Level chemistry, when you look at your hands, they're identical (assuming you aren't an amputee or one hand has more fingers than the other), but no matter how you rotate them, you can never get them to fully match up. That's because they're mirror images, and enantiomers are exactly that. It requires one carbon atom to have 4 different groups bonded to it (e.g. a methyl group, a single hydrogen, an ethyl group, and an alcohol group), so it can have two versions of the exact same molecule, like our hands. Our noses are chiral, and so they can tell the difference between each molecule, which is why pine and fresh oranges smell different despite being, in the physical and chemical sense, the same molecule. I wanted to explain this because, after learning about this, I assumed taste buds worked in an identical or very similar way, and I was partially right.

Everyone knows that our mouths contain taste buds (the name "bud" coming from the fact that they are shaped like flower buds), but what people might not know is that they are found on three different types of papillae (small nipple-shaped bumps on the surface of the tongue):

• Fungiform: this is the most common type of papilla, with people having around 200-400 of them, and they are mostly found around the tip of the tongue and the edges of the tongue. Each papilla contains around 3-5 taste buds, but these papillae are also highly sensitive to things such as temperature and texture (which also influence the taste of things).
• Circumvallate: whilst there are only 7-8 types of these papillae, they each contain thousands of taste buds, and are found in a V-shape at the back of the mouth. They each have 'trenches' around them with glands that push the taste-producing substances towards the sensory cells.
• Foliate: we all have around 20 of this kind of papillae, and they're visible to the naked eye (they sort of look like folds within the tongue?), with each one containing hundreds of taste buds.
• There's also technically a fourth type of papillae called filiform, but they contain no taste buds and are only able to detect temperature and texture.

Humans have 2000-4000 taste buds, with them renewing about every two weeks, and each taste bud contains 10- 50 sensory cells. The number of papillae someone has can determine how you taste things. For example, some people are supertasters, for whom flavours can be very overwhelming, so they prefer milder foods, and other people can be subtasters, so they aren't as sensitive to strong flavours, and so may prefer more pronounced flavours and spicier foods. When I talk about 'flavours', there are five main types that taste buds can detect:

• Sweet: this flavour comes from sugars and sugar derivatives (e.g. fructose, lactose, glucose, and sucrose) and is detected by G-protein-coupled receptors T1R2 and T1R3.
• Sour: this flavour is not detected by proteins, but from the fact its acidic. The sour flavour comes from acidic solutions, which, when they come into contact with water, begin to release their H+ (hydrogen) ions.
• Salty: this is also not detected by proteins, but also due to electrical signals to the brain. The most obvious salty flavour is table salt, which contains both Na+ (sodium ions) and Cl- (chloride ions), so K+ (potassium ions) and Mg2+ (magnesium ions) also produce a salty taste.
• Bitter: within sensory cells, there are around 35 proteins (G-protein-coupled receptors T2Rs) which can react to bitter tastes. Being able to taste bitterness was an evolutionary advantage, as a lot of poisonous plants were bitter; the back of our tongue is the most sensitive to bitterness, likely to help us spit out any foods which could be poisonous.
• Umami: when I first saw this word, I was like, "What the heck is that?" But it just means savoury. Apparently it's so it specifies the specific chemical, whereas savoury in a culinary sense just means any non-sweet food. This flavour comes from glutamate and is detected by G-protein-coupled receptors T1R1 and T1R3.
• Spiciness is not a flavour; it's the pain response to capsaicin. 

You might think that every single taste bud can taste every single flavour, but you'd be wrong! Don't worry too much; I thought the same thing until I was doing research for this blog post. Essentially, half of our taste buds can detect just one flavour, so their focus is on the concentration of the flavour (like very salty or not salty, I assume), whereas the other half can detect several and apparently have an order of flavour priorities. You might also believe that certain areas of the tongue can only detect certain types of flavours. You'd be partially correct. Do worry because I also knew this was false. As I mentioned before, the back of the tongue can detect bitterness much more strongly, but this doesn't mean it's the only part of the tongue that can detect bitterness; in general, the sides of the tongue taste much better than anything else. I'll attach a photo to show what studies show to be the strongest areas for different flavours.

source: https://www.ncbi.nlm.nih.gov/books/NBK279408/

So there isn't really as much difference as some people make out there to be. I hope that sentence makes sense because I'm not clever enough to think of any other way to word it.

Anyways, what I thought was really weird is that it seems like the tongue can't actually taste. They can sense the general flavours of sweet, umami, etc, but they can't get distinct flavours. This is because it's the smell of these foods that determines the flavour. Have you ever noticed that when you're sick and have a stuffy nose, you can't taste as well, or you have no sense of taste at all? That's because when you chew (or masticate) your food, the smell of the food goes up the nasopharynx to the olfactory receptors, which are responsible for your sense of taste and are chiral (so they can detect the differences in enantiomers). So, the sense of taste is kind of due to enantiomers, but it has nothing to do with your tongue; it's your nose! So apparently, if you plug your nose and eat something, you should be able to detect the general type of flavour, but not the specific flavour until you unplug your nose. Which I'm going to go and try now.