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Organic acids are organic compounds (meaning they are carbon-based) which are also acids (meaning they release hydrogen ions in solution, usually lowering the pH). Unlike mineral acids which are generally odourless, they usually have quite strong smells, usually identifiably sour ones. What makes them acidic is almost always a carboxyl group - that is, a carbon with two oxygens bonded to it, one of which has a hydrogen attached... until that hydrogen comes off, which is what makes it an acid. That only happens a fraction of the time, which technically makes them weak acids, but they can still mess you up if you're not careful.

1. Acetic acid

Vinegar, aka ethanoic acid

This is the best acid to put on chips. It is also great on crisps.

I like ethanoic acid because the smell pleases me. It is probably the most mouth-watering of all smells, which is funny because it would likely be aversive if I hadn't grown up with vinegar put on delicious things. I used to drink vinegar out of the bottle sometimes to enjoy the intense sensation of sourness. It is also used in condiments like ketchup, mayonnaise and mustard, where it acts as a preservative as well as a flavouring.

Vinegar is widely used in pickling; like most organic acids, it has pretty strong anti-microbial properties. It is also notably easy to make - if you leave alcoholic liquids open to the air for a few days, airborne bacteria will colonise them and digest the ethanol to produce ethanoic acid. If you don't have any alcoholic liquids, you can start with a sugary liquid, give yeast a chance to act on it (again, plenty of this is available just floating around in normal air) and then let the bacteria do their work.

In its pure form, ethanoic acid is sometimes called 'glacial acetic acid' because it freezes at 16°C - the temperature of a room that is slightly too cool to be comfortable. I'd always wondered why they called it 'glacial' until the first time I came into my lab's store room, to find that the ethanoic acid had frozen solid, glistening in the light like a cylindrical ice sculpture. Interestingly, in this pure form the acid is flammable as well as corrosive.

Vinegar is famously useful as a household cleaner, and it is especially effective on glass, but you might like to know that it is useful for this purely because it's an acid. Less smelly alternatives are available, if you don't want your windows to smell like they would be delicious on chips.

2. Lactic acid

aka 2-hydroxypropanoic acid

In some ways I like lactic acid even better than ethanoic acid, but it loses marks because it is the acid that builds up in our muscles when we over-exert them (it is produced by anaerobic respiration) and it is also the main acid responsible for tooth decay.

Still, without lactic acid we would have no yoghurt - it is called 'lactic' after milk because of this, but when you see 'lactic acid' as an ingredient in food (which you will, often, if you look) it is almost always from a vegan source. The acid curdles the milk, denaturing its proteins so that they form a semi-solid matrix.

Lactic acid is also the other main acid used in pickling, and it is what makes sourdough sour: as with ethanoic acid, lactic acid producing bacteria are everywhere. They work best under anaerobic conditions though, so to make sauerkraut or kimchi, for example, you need to submerge the cabbage and any other vegetables and spices under water after shredding it to help the bacteria penetrate. The water is salted to make it a hostile environment for less human-friendly bacteria, and the mixture is likely to need a little 'burping' over the next week or so, to vent any gases produced. Sugar in the source material is converted into lactic acid, releasing energy that the bacteria can use. The whole process really requires very little human input, and it allows vegetables to be preserved for months, if not longer, providing a crucial source of vitamin C over the winter when refrigeration is not available.

One other notable thing about lactic acid is that it can be polymerised to make one of the most popular biodegradable plastics, polylactic acid or PLA. It is able to do this because it is not just an acid but also an alcohol: it has a carboxyl group and a hydroxyl group, and these can join together through a condensation reaction to make long chains. Long chains (polymers) are just what you need to produce plastic.

3. Citric acid

aka 2-hydroxypropane-1,2,3-tricarboxylic acid

Citric acid is famous as a component of citrus fruits; it is the main thing that makes them sour. It is unusual in this list for two reasons: it is solid at room temperature, and it is a polyprotic acid (meaning that each molecule has more than one hydrogen atom that can become an ion in solution - in this case, three of them; a hydrogen ion is just a proton, hence polyprotic). Both of these contribute to its widespread use as a flavouring, especially in candies and other sweets. If you want a food to be solid, it is convenient for most of its ingredients to be solid; and because citric acid can release three hydrogen ions per molecule, it is that much sourer than other organic acids. It is also odourless, which is related to its being solid at room temperature; that's handy if you want things to taste sour, but not to smell sour. You can buy bags of citric acid crystals to add to food, or to use as a household cleaner - it is handy for removing rust and limescale.

Citric acid is also notable for its vital role in biochemistry: the citric acid cycle (or Krebs cycle) is part of respiration, so every living thing that respires makes use of it.

4. Ascorbic acid

aka vitamin C

Ascorbic acid is another one that's solid at room temperature, that you can buy as powdered crystals. I used to use it as a condiment sometimes, just sprinkling the powder on food, but to be honest lemon juice is tastier, and there is very little evidence for the whole idea that it's good to get far more vitamin C than the recommended daily allowance.

We do need vitamin C in our diets, though, because our distant ancestors got hit with a mutation that knocked out a single step in the biochemical pathway that would have allowed us to produce it, and because they ate plenty of fresh fruit and vegetables all year round, they didn't miss it. That makes humans and other great apes unusual among animals; most species produce their own vitamin C in abundance.

There are two things about vitamin C that are particularly notable from a chemistry standpoint. First, it is an antioxidant and reducing agent: for this reason, it is an even more powerful preservative than the rest of the acids in this list. In the body, it is thought to act as a 'free radical scavenger', mopping up highly reactive fragments of molecules that can cause chain reactions of molecular damage. The other chemically notable thing about vitamin C is that it is the only acid in this list that is not a carboxylic acid; the hydrogen ions it releases come from carbons which are not double bonded with oxygen atoms, and that's weird, but I know that if you're not a chemist you probably don't care. That's okay.

5. Butyric acid

aka butanoic acid

This stuff is horrible. Like, really awful. One of the foulest-smelling liquids. I enjoy that about it.

Butanoic acid is the main chemical contributing to the characteristic aroma of vomit. It originally got its name 'butyric acid' from butter, because butter reeks of this stuff when it goes rancid. It also contributes significantly to the smell of parmesan, and much American chocolate. As previously mentioned, organic acids tend to have strong anti-microbial properties, and Milton S. Hershey of The Hershey Company discovered, in an age before widespread refrigeration, that if he let the milk in his chocolate spoil just right, his chocolate wouldn't get any more rancid after that. Apparently Americans are just used to their chocolate smelling faintly like parmesan.

I have a whole bottle of butanoic acid in my lab store, which I keep inside a sealed bag to keep the smell in. In theory the cap should be sufficient for that, but I have found I can't completely rely on it. I once made the mistake of passing around the whole bottle for a class to smell. The room was unusable for several hours afterwards - people kept gagging. Now I keep a tiny bottle with just a couple of drops of the stuff at the bottom for people to smell, and as it turns out, that is quite enough.

6. Formic acid

aka methanoic acid

Formic acid is named after ants (Formica) because it was originally distilled from ants. I don't like to think too much about what was involved in that process. If you get enough ants in one place there is often a faint smell of formic acid, which is almost like the smell of vinegar but somehow even sharper. Certain species of ants use formic acid as a weapon, shooting it at their enemies from specially adapted glands. It is also found in the needles of stinging nettles, along with various other irritating compounds.

My other favourite thing about formic acid is that it is the simplest possible organic acid: literally just a carboxyl group plus a hydrogen, CHOOH.

7. Glycine

aka 2-aminoethanoic acid 

Glycine is the simplest stable amino acid. Remember how carboxylic acids all have a carboxyl functional group? Well, amino acids have those and amine groups, which are usually a nitrogen and two hydrogens. Amine groups are basic, meaning that they can take up a hydrogen ion, which has the mildly disorienting effect of meaning that amino acids are also alkalis: they are amphoteric, and which one they act as depends mainly on the pH of the solution they are in.

I appreciate glycine for its relative simplicity, and for the fact that it's the only amino acid found in nature that is not chiral. That is to say that glycine is an exact mirror image of itself, unlike all the other amino acids used in making proteins. Chirality is also known as 'handedness' - like our hands, two molecules can be exactly the same except that one is reversed. If you somehow passed through four-dimensional space and came back as a mirror image of yourself, you wouldn't be able to eat Earth food because all of our proteins and carbohydrates are chiral. You could still digest glycine, though, and fat.

8. Glutamic acid

aka glutamate

My other favourite amino acid has a propanoic acid side-chain, so it's another polyprotic acid. That's not why I like it, though. I like it mainly because it's umami: it tastes kind of savoury. It's famous in the form of its sodium salt, monosodium glutamate, which is added to foods to make them tastier. It is widely believed to be bad for you, and to set off unpleasant reactions in a lot of people, but this seems to be almost entirely untrue; it is probably no worse for you than table salt, although some people may have bad reactions.

We probably evolved to taste and enjoy glutamic acid and its salts because it is associated with protein in food. Cooked meat is often rich in it, and so are many products made by fermenting protein-rich food: soy sauce, Marmite and so on. Other umami foods include celery, mushrooms and especially seaweed. 

Interestingly, glutamate is also a neurotransmitter - in fact, it is the main excitatory neurotransmitter in vertebrates. This means that when one neuron releases it, it encourages other neurons to fire.

9. Goat acids: capric, caprylic and caproic acid

aka decanoic, octanoic and hexanoic acid

I just love that there are three different carboxylic acids all named after goats.

You know how goat's milk and goat's cheese smell distinctly different from cow's milk? Well, that's mostly down to the abundance of these carboxylic acids. They all smell goaty in different ways.

I don't really drink milk any more, but I used to occasionally buy some goat's milk, because I like that goaty flavour, and because in my experience goats refuse to take any crap from anyone.

 There are many other acids I could have talked about here, but I suppose I had better stop somewhere.

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