Cooking chemistry: American biscuits

American biscuitsIt’s come up before of course, but there’s a lot of chemistry in cooking. I do like tinkering with recipes: all that lovely weighing things on digital scales, measuring liquids, working out ratios and tweaking the exact sequence of steps – what more could a chemist want? I spent ages working on my chocolate brownie recipe when I should have been writing up my PhD thesis (it does produce excellent chocolate brownies, so I maintain it was a entirely valid use of my time).

Last week fate transpired to drop more than one reference to ‘biscuits’ in my lap. Now, these were American sources, so I was aware that they weren’t talking about what we call biscuits (and Americans call cookies), not least because in one of them there was talk of making a ‘biscuit sandwich’ that included sausage. Now, I like a chocolate digestive as much as the next person, but I wouldn’t slap a chunk of grilled pork product in between two of them and call it breakfast.

So I decided to try and find a recipe. And, after a bit of faffing around converting Fahrenheit to Celsius and cups to grams (honestly, I do understand the principle of cups and baking by ratio, but is it really easier to measure out a cup of butter than just use scales?) I finally came up with a workable recipe.

Turns out American biscuits are basically sugar-less scones. Who knew.

What’s the chemistry connection? Well, just like scones, the raising agent in American biscuits is baking soda, or sodium hydrogen carbonate. It causes the mixture to rise because it does this when it’s heated:

2NaHCO3 –> CO2 + H2O + Na2CO3

This type of reaction is called thermal decomposition, because the heat is causing the sodium hydrogen carbonate (NaHCO3) to break apart. The carbon dioxide (CO2) is a gas and produces lots of lovely bubbles that make your finished product nice and light.  Water (H2O) is also a product, which helps to keep everything nice and moist.

This clever bit of cookery chemistry starts to happen slowly at 50 oC, but once you get over 200 oC (a more typical baking temperature) it’s pretty fast. So much so that you can bake your biscuits for just 12 minutes or so and they’ll be perfectly risen. Contrary to common belief, there’s no need to add some kind of acid to the mixture (buttermilk is often mentioned). Acids do react with carbonates to produce carbon dioxide, but there’s no need – heat will do the job for you.

Cheese and ham biscuit sandwichSo without further ado, here’s my tinkered recipe. It’s really great this, it literally only takes 10 minutes plus baking time, and you probably have all these ingredients already:

Ingredients

  • 360 g plain white flour
  • 4 tsp of baking soda (sodium hydrogen carbonate)
  • 1 tsp sugar
  • ½ tsp salt
  • 75 g cold (straight from the fridge) unsalted butter, cut into cubes
  • 230 g* milk

(*If you prefer to use a jug this is as close to 230 ml as makes no difference, since milk is mostly water and water has a density of 1 g/ml, but it saves washing up to just stick the bowl on the scales and weigh it.)

Method

  1. Heat the oven to 230 oC.  It needs to be nice and hot, so turn it on in good time.
  2. Measure the dry ingredients in a large bowl and mix them.
  3. Using clean, cold hands rub the butter into the dry ingredients until the mixture resembles fine breadcrumbs and there are no lumps of butter.
  4. Pour in the milk and mix with the flat of a knife until the dough comes together.
  5. Take the dough out of the bowl and place it on a lightly floured surface. Knead it gently a few times until it forms an even ball and has an elastic, ever so slightly sticky, texture.
  6. Press into a rough oblong, about 2 cm thick.  Cut the dough into six roughly equal pieces (you can use cookie cutters, but again, why create unnecessary washing up).  Place these on a greased baking tray.
  7. Bake for about 12 minutes, until the biscuits are golden brown (the colour is, of course, courtesy of another bit of chemistry: the Maillard reaction).
  8. Transfer them to a rack to cool, but no need to leave them too long – they’re best eaten warm!

Split the biscuits in half and fill them with anything you like, savoury or sweet. They’re delicious served plain with lashings of butter. As a more substantial lunchtime snack, try cheese and ham. Lemon curd has also proved a favourite. If you have leftovers they will keep until the next day if wrapped up, and are especially nice toasted and buttered.

And there you are, a metric version of the classic American biscuit recipe, with a bit of chemistry thrown in. I think this might be a first. Heston eat your heart out.

Advertisements

Are you a chemist and you didn’t know it?

When I tell people that I’m a chemist, I often get an “oooh, I was really bad at that at school” type response. It’s surprising the number of people that think chemistry has nothing whatsoever to do with their daily lives. Memorably, one acquaintance of an acquaintance (I wouldn’t go so far as to say friend of a friend) once even proclaimed, quite proudly, that the whole of science had nothing to do with her, and she lived her life entirely without it. I was so gobsmacked I didn’t really know where to start, and trust me, that doesn’t happen often.

washing-hands--soap-jpgSo with that in mind, here are five bits of chemistry you do every day. Or at least regularly. You’re a chemist and you didn’t know it!

1. Wash your hands.
Well, we all hope you do this one every day anyway. Soap is very clever stuff. It’s one of the oldest bits of chemistry there is, going back thousands of years, when people first discovered that if they washed their pots with the ashes of cooking fires they got a better result. Soap is made by a process called saponification, where fats are mixed with strong alkalis (traditionally lye: sodium or potassium hydroxide). The fats break apart and form fatty acid salts. What’s clever about those, is that they have a water-loving end (the salt bit) and a water-hating end (the fatty acid bit). So they can grab onto both, and hold the water and oil together. That’s what you do every time you use soap: the dirt ingrained in oil on your skin (nice) can, with the help of those lovely soap molecules, mix with water and so be washed away. Brilliant!

2. Drink a pH indicator.
‘What’ I hear you cry, ‘I do no such thing!’ Ah but do you drink tea (the black kind)? If so, then you do, even if you’ve never noticed. Have you ever put lemon in your tea instead of milk? If not, and you have tea and lemon juice (bottled is fine) in your house, go and try it now. The colour change is really quite lovely to watch. Lemon juice is a source of ascorbic and citric acids, and has a pH of roughly 2-3. You’ll see the same effect with vinegar too, although that mixture wouldn’t be quite so nice to drink. (If you’re feeling adventurous, try some common alkalis such as baking soda or bleach, but DEFINITELY don’t drink those concoctions afterwards…)

3. Carry out combustion.
Ever lit a match? Or a lighter? Started your gas cooker? Turned on your gas boiler? Started your petrol or diesel car? Of course you have. Every single time you do any of those things, the carbon atoms in their molecules are reacting with oxygen to produce carbon dioxide and water. And even if you live under a damp and fireless rock, you’re still doing it – respiration, the process by which all your cells obtain energy – is a form of combustion.

4. Watch some ice float.
Ice floats. Stop press!
We take that for granted, but it’s amazing really. This is a brilliant bit of chemistry that has its tendrils in physics and biology too. Solids don’t generally float on their liquids. Solids are usually more dense than their liquid form, so they sink. But if water behaved like that we wouldn’t have life on this planet, because every time any body of water got really cold it would freeze from the bottom up, taking out all the life swimming in its depths in the process. Since we’re fairly sure that life began in the oceans, evolution would have come to a full stop. But water doesn’t behave like that; water expands when it freezes. Why? Because water has something called hydrogen bonds between its molecules, and as it solidifies these bonds increasingly force the crystalline structure to be very ‘open’. As a result, ice is actually less dense than water, so it floats. This is also why ice is so brilliant at cooling liquids; the warm stuff rises, hits the cold ice and sinks again, creating a sort of cycle called a convection current. Who knew there was so much sciency stuff in your spritzer?

5. Bake a cake.
Food is a rich source of chemistry, just ask Heston. In this case, I’m thinking of baking soda, otherwise known as sodium hydrogencarbonate, or sodium bicarbonate (NaHCO3). When it’s heated above about 70 oC it undergoes a chemical reaction called decomposition. In other words, its molecules break apart without actually needing to react with any other substance. When you put baking soda into your recipe, or use ‘self-raising’ flour (which has it already added), you’re setting it up for this chemical reaction. As the cake cooks, the mixture heats up, and the baking soda does this:
2NaHCO3 –> CO2 + H2O + Na2CO3
The carbon dioxide, CO2, is a gas and it pushes your mixture up and out, causing it to rise. No baking soda chemistry, no lovely, fluffy cake.

So, next time someone tells you they’re rubbish at chemistry, you can point out that they’re doing it every day!