Onerous ovens: why is cleaning the cooker such a chore?

As I write Thanksgiving was a few days ago, when most Americans traditionally cook a very large meal based around roasted turkey. Most Brits – and other countries of course – have the same thing coming up soon in the form of Christmas, and there are lots of other celebrations around this time of year that seem to feature cooking and food quite heavily.

Whatever your traditions, then, it’s a time when many of us frown critically at the dark, sticky depths of our oven and wonder if, perhaps, we should attempt to give it a clean. Or at least pay someone else to come and clean it.

Why is oven cleaning such a difficult and unpleasant job, anyway? It’s not that hard to clean other surfaces, is it? Why are ovens so particularly awful?

Well, to explain this, we first need to understand fats.

Fats vaporise during cooking.

Most of the grime in your oven is fat, combined with the carbonised remains of… something or other. The sorts of fats that are common in animal and plant products have boiling points around the 300 oC mark (animal fats typically having higher values than plant oils), but they start to form vapours at much lower temperatures, and certainly at typical cooking temperatures there’s plenty vaporised oil around. Besides, under typical conditions most oils will “smoke” – i.e. start to burn – long before they get close to boiling.

We’re all familiar with the idea that fats don’t mix well with water, and herein lies the problem: all that fatty gloop that’s stuck to the inside of your oven just doesn’t want to come off with standard cleaning methods, particularly when it’s built up over time.

Can chemistry help us here? What are fats, chemically? Well, they’re esters. Which may or may not mean anything to you, depending on how much chemistry you can remember from school. But even if you don’t remember the name, trust me, you know the smell. In particular, nail polishes and nail polish removers contain the simple ester known as ethyl acetate, otherwise known as ethyl ethanoate. (Some people say this chemical smells like pear drops which… only really helps if you know what pear drops smell like. Look, it smells of nail polish, okay?)

Fats are esters (image source)

Anyway, the point is that esters have a particular sequence of atoms that has a carbon bonded to an oxygen, which is bonded to another carbon, which is in turn double-bonded to oxygen. This is a bit of a mouthful, so chemists often write it as COOC. In the diagram here, oxygen atoms are red while carbon atoms are black.

There are actually three ester groups in fat molecules – which explains why fats are also known as triglycerides.

In terms of general chemistry, esters form when a carboxylic acid (a molecule which contains a COOH group) reacts with an alcohol (a molecule that contains an OH group). And this is where it all starts to come together – honest – because you’ve probably heard of fatty acids, right? If nothing else, the words turn up in certain food additive names, in particular E471 mono- and diglycerides of fatty acids, which is really common in lots of foods, from ice cream to bread rolls.

Glycerol is a polyol — a molecule that contains several alcohol groups (image source)

Well, this reaction is reversible, and as a result fats (which are esters, remember) break up into fatty acids and glycerol – which is a polyol, that is, a molecule with several alcohol groups. Or, to look at it the other way around: fats are made by combining fatty acids with glycerol.

And the reason it’s useful to understand all this is that the way you break up esters, and therefore fat, is with alkalis. (Well, you can do it with acid, too, but let’s not worry about that for now.)

Strong alkalis break up fats in a chemical reaction called hydrolysis — the word comes from the Greek for water (hydro) and unbind (lysis) and so literally means “split up with water”. Humans have known about this particular bit of chemistry for a long time, because it’s fundamental to making soap. As I said a few months ago when I was banging on about hand-washing, the ancient Babylonians were making soap some 4800 years ago, by boiling fats with ashes – which works because alkaline compounds of calcium and potassium form when wood is burnt at high temperatures.

The grime in ovens is mostly fat.

The really clever thing about all this is that two things are happening when we mix alkali with fat: not only are we breaking up the fat molecules, but also the substances they break up into are water-soluble (whereas fats, as I said at the start, aren’t). Which makes them much easier to clean away with water. Obviously this is the very point of soap, but it’s also handy when trying to get all that baked-on gunk off your oven walls.

Now, in theory, this means you could get some lye (aka sodium hydroxide, probably), smear it all over your oven and voilà. But I don’t recommend it, for a few reasons. Firstly, it’s going to be difficult to apply, since sodium hydroxide is mostly sold as pellets or flakes (it’s pretty easy to buy, because people use it to make soap).

Sodium hydroxide, sometimes called lye, is often sold in the form of pellets.

But, you say, couldn’t I just dissolve it in water and spray or spread it on? Yes, yes you could. But it gets really, really hot when you mix it with water. So you need to be incredibly careful. Because, and this is my next point, chemically your skin is basically fat and protein, and this reaction we’re trying to do on oven sludge works equally well on your skin. Only, you know, more painfully, and with scarring and stuff. In short, if you’re handing lye, wear good nitrile on vinyl gloves and eye protection.

Actually, regardless of how you’re cleaning your oven you should wear gloves and eye protection, because the chemicals are still designed to break down fats and so… all of the above applies. It’s just that specially-designed oven cleaners tend to come with easier (and safer) ways to apply them. For example, they might come as a gel which you can paint on, and/or with bags that you can put the racks into, and may also be sold with gloves and arm protectors (but rarely goggles – get some separately). They might also have an extra surfactant, such as sodium laureth sulfate, added to help with breaking down grease. The main ingredient is still either potassium hydroxide or sodium hydroxide, though.

Well, possibly, but also not really, if you’re sensible.

As an aside, it makes me smile when I come across an article like this which talks about the “serious” chemicals in oven cleaners and more “natural” ways to clean your oven. The “natural” ways are invariably weak acids or alkalis such as lemon juice or baking soda, respectively. They’re essentially ineffective ways of trying to do exactly the same chemistry.

And okay, sure, the gel and the bag and so on in the modern kits are newer tech, but the strong alkali? Nothing more natural than that. As I said at the start, humans have literally been using it for thousands of years.

A point which really cannot be repeated enough: natural does not mean safe.

Fumes can be irritating to skin, eyes and lungs.

Speaking of which, you will get fumes during oven cleaning. Depending on the exact cleaning mixture involved, these will probably be an alkaline vapour, basically (haha) forming as everything gets hot. Such vapour is potentially irritating to skin, eyes and lungs, but not actually deadly toxic. Not that I recommend you stick your head in your freshly-scrubbed oven and inhale deeply, but you take my point. It might give food a soapy, possibly bitter (contrary to what’s stated in some text books, not all alkalis taste bitter, but do not experiment with this) taste if you really over-do it.

In short, if you’re cleaning your oven yourself: follow the manufacturer’s instructions, make sure your kitchen is well-ventilated, leave the oven door open for a while after you’ve finished and, to be really sure, give all the surfaces an extra wash down with plenty of water.

Put the cleaning off until January – after all, the oven’s only going to get dirty again.

And that’s… it, really. Whether you’re cleaning your own oven or getting someone else to do it for you, the chemistry involved is really, really old. And yes, the chemicals involved are hazardous, but not because they’re not “natural”. Quite the opposite.

Or you could just leave it. I mean, it’s only going to get dirty again when you cook Christmas dinner, right?

If you’re studying chemistry, have you got your Pocket Chemist yet? Why not grab one? It’s a hugely useful tool, and by buying one you’ll be supporting this site – it’s win-win! If you happen to know a chemist, it would make a brilliant stocking-filler! As would a set of chemistry word magnets!

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Want something non-sciency to distract you from, well, everything? Why not check out my fiction blog: the fiction phial.

Cleaning chemistry – the awesome power of soap

Well, times are interesting at the moment, aren’t they? I’m not going to talk (much) about The Virus (there’s gonna be a movie, mark my words), because everyone else is, and I’m not an epidemiologist, virologist or an immunologist or, in fact, in any way remotely qualified. I am personally of the opinion that it’s not even especially helpful to talk about possibly-relevant drugs at the moment, given that we don’t know enough about possible negative interactions, and we don’t have reliable data about the older medicines being touted.

In short, I think it’s best I shut up and leave the medical side to the experts. But! I DO know about something relevant. What’s that, I hear you ask? Well, it’s… soap! But wait, before you start yawning, soap is amazing. It is fascinating. It both literally and figuratively links loads of bits of cool chemistry with loads of other bits of cool chemistry. Stay with me, and I’ll explain.

First up, some history (also not a historian, but that crowd is cool, they’ll forgive me) soap is old. Really, really, old. Archaeological evidence suggests ancient Babylonians were making soap around 4800 years ago – probably not for personal hygiene, but rather, mainly, to clean cooking pots. It was originally made from fats boiled with ashes, and the theory generally goes that the discovery was a happy accident: ashes left from cooking fires made it much easier to clean pots and, some experimenting later, we arrived at something we might cautiously recognise today as soap.

Soap was first used to clean pots.

The reason this works is that ashes are alkaline. In fact, the very word “alkali” is derived from the Arabic al qalīy, meaning calcined ashes. This is because plants, and especially wood, aren’t just made up of carbon and hydrogen. Potassium and calcium play important roles in tree and plant metabolism, and as a result both are found in moderately significant quantities in wood. When that wood is burnt at high temperatures, alkaline compounds of potassium and calcium form. If the temperature gets high enough, calcium oxide (lime) forms, which is even more alkaline.

You may, in fact, have heard the term potash. This usually refers to salts that contain potassium in a water-soluble form. Potash was first made by taking plant ashes and soaking them in water in a pot, hence, “pot ash”. And, guess where we get the word potassium from? Yep. The pure element, being very reactive, wasn’t discovered until 1870, thousands of years after people first discovered how useful its compounds could be. And, AND, why does the element potassium have the symbol K? It comes from kali, the root of the word alkali.

See what I mean about connections?

butyl ethanoate butyl ethanoate

Why is the fact that the ashes are alkaline relevant? Well, to answer that we need to think about fats. Chemically, fats are esters. Esters are chains of hydrogen and carbon that have, somewhere within them, a cheeky pair of oxygen atoms. Like this (oxygen atoms are shown red):

Now, this is a picture of butyl ethanoate (aka butyl acetate – smells of apples, by the way) and is a short-ish example of an ester. Fats generally contain much longer chains, and there are three of those chains, and the oxygen bit is stuck to a glycerol backbone.

Thus, the thick, oily, greasy stuff that you think of as fat is a triglyceride: an ester made up of three fatty acid molecules and glycerol (aka glycerine, yup, same stuff in baking). But it’s the ester bit we want to focus on for now, because esters react with alkalis (and acids, for that matter) in a process called hydrolysis.

Fats are esters. Three fatty acid chains are attached to a glycerol “backbone”.

The clue here is in the name – “hydro” suggesting water – because what happens is that the ester splits where those (red) oxygens are. On one side of that split, the COO group of atoms gains a metal ion (or a hydrogen, if the reaction was carried out under acidic conditions), while the other chunk of the molecule ends up with an OH on the end. We now have a carboxylate salt (or a carboxylic acid) and an alcohol. Effectively, we’ve split the molecule into two pieces and tidied up the ends with atoms from water.

Still with me? This is where it gets clever. Having mixed our fat with alkali and split our fat molecules up, we have two things: fatty acid salts (hydrocarbon chains with, e.g. COONa+ on the end) and glycerol. Glycerol is extremely useful stuff (and, funnily enough, antiviral) but we’ll put that aside for the moment, because it’s the other part that’s really interesting.

What we’ve done here is produce a molecule that has a polar end (the charged bit, e.g. COONa+) and a non-polar end (the long chain of Cs and Hs). Here’s the thing: polar substances tend to only mix with other polar substances, while non-polar substances only mix with other non-polar substances.

You may be thinking this is getting technical, but honestly, it’s not. I guarantee you’ve experienced this: think, for example, what happens if you make a salad dressing with oil and vinegar (which is mostly water). The non-polar oil floats on top of the polar water and the two won’t stay mixed. Even if you give them a really good shake, they separate out after a few minutes.

The dark blue oily layer in this makeup remover doesn’t mix with the watery colourless layer.

There are even toiletries based around this principle. This is an eye and lip makeup remover designed to remove water-resistant mascara and long-stay lipstick. It has an oily layer and a water-based layer. To use it, you give the container a good shake and use it immediately. The oil in the mixture removes any oil-based makeup, while the water part removes anything water-based. If you leave the bottle for a minute or two, it settles back into two layers.

But when we broke up our fat molecules, we formed a molecule which can combine with both types of substance. One end will mix with oily substances, and the other end mixes with water. Imagine it as a sort of bridge, joining two things that otherwise would never be connected (see, literal connections!)

There are a few different names for this type of molecule. When we’re talking about food, we usually use “emulsifier” – a term you’ll have seen on food ingredients lists. The best-known example is probably lecithin, which is found in egg yolks. Lecithin is the reason mayonnaise is the way it is – it allows oil and water to combine to give a nice, creamy product that stays mixed, even if it’s left on a shelf for months.

When we’re talking about soaps and detergents, we call these joiny-up molecules “surfactants“. You’re less likely to have seen that exact term on cosmetic ingredients lists, but you will (if you’ve looked) almost certainly have seen one of the most common examples, which is sodium laureth sulfate (or sodium lauryl sulfate), because it turns up everywhere: in liquid soap, bubble bath, shampoo and even toothpaste.

I won’t get into the chemical makeup of sodium laureth sulfate, as it’s a bit different. I’m going to stick to good old soap bars. A common surfactant molecule that you’ll find in those is sodium stearate, which is just like the examples I was talking about earlier: a long hydrocarbon chain with COONa+ stuck on the end. The hydrocarbon end, or “tail”, is hydrophobic (“water-hating”), and only mixes with oily substances. The COONa+ end, or “head”, is hydrophilic (“water loving”) and only mixes with watery substances.

Bars of soap contain sodium stearate.

This is perfect because dirt is usually oily, or is trapped in oil. Soap allows that oil to mix with the water you’re using to wash, so that both the oil, and anything else it might be harbouring, can be washed away.

Which brings us back to the wretched virus. Sars-CoV-2 has a lipid bilayer, that is, a membrane made of two layers of lipid (fatty) molecules. Virus particles stick to our skin and, because of that membrane, water alone does a really bad job of removing them. However, the water-hating tail ends of surfacant molecules are attracted to the virus’s outer fatty surface, while the water-loving head ends are attracted to the water that’s, say, falling out of your tap. Basically, soap causes the virus’s membrane to dissolve, and it falls apart and is destroyed. Victory is ours – hurrah!

Hand sanitisers also destroy viruses. Check out this excellent Compound Interest graphic (click the image for more).

Who knew a nearly-5000 year-old weapon would be effective against such a modern scourge? (Well, yes, virologists, obviously.) The more modern alcohol hand gels do much the same thing, but not quite as effectively – if you have access to soap and water, use them!

Of course, all this only works if you wash your hands thoroughly. I highly recommend watching this video, which uses black ink to demonstrate what needs to happen with the soap. I thought I was washing my hands properly until I watched it, and now I’m actually washing my hands properly.

You may be thinking at this point (if you’ve made it this far), “hang on, if the ancient Babylonians were making soap nearly 5000 years ago, it must be quite easy to make… ooh, could I make soap?!” And yes, yes it is and yes you can. Believe me, if the apocolypse comes I shall be doing just that. People rarely think about soap in disaster movies, which is a problem, because without a bit of basic hygine it won’t be long before the hero is either puking his guts up or dying from a minor wound infection.

Here’s the thing though, it’s potentially dangerous to make soap, because most of the recipes you’ll find (I won’t link to any, but a quick YouTube search will turn up several – try looking for “saponification“) involve lye. Lye is actually a broad term that covers a couple of different chemicals, but most of the time when people say lye these days, they mean pure sodium hydroxide.

Pure sodium hydroxide is usually supplied as pellets.

Pure sodium hydroxide comes in the form of pellets. It’s dangerous for two reasons. Firstly, precisely because it’s so good at breaking down fats and proteins, i.e. the stuff that humans are made of, it’s really, really corrosive and will give you an extremely nasty burn. Remember that scene in the movie Fight Club? Yes, that scene? Well, that. (Follow that link with extreme caution.)

And secondly, when sodium hydroxide pellets are mixed with water, the solution gets really, really hot.

It doesn’t take a lot of imagination to realise that a really hot, highly corrosive, solution is potentially a huge disaster waiting to happen. So, and I cannot stress this enough, DO NOT attempt to make your own soap unless you have done a lot of research AND you have ALL the appropriate safety equipment, especially good eye protection.

And there we are. Soap is ancient and awesome, and full of interesting chemistry. Make sure you appreciate it every time you wash your hands, which ought to be frequently!

Stay safe, everyone. Take care, and look after yourselves.

Want something non-sciency to distract you? Why not check out my fiction blog: the fiction phial. There are loads of short stories, and even (recently) a poem. Enjoy!

If you’re studying from home, have you got your Pocket Chemist yet? Why not grab one? It’s a hugely useful tool, and by buying one you’ll be supporting this site – it’s win-win!

Like the Chronicle Flask’s Facebook page for regular updates, or follow @chronicleflask on Twitter. Content is © Kat Day 2020. You may share or link to anything here, but you must reference this site if you do. If you enjoy reading my blog, please consider buying me a coffee through Ko-fi using the button below.
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