Naturally leavened (sourdough) bread is made using a levain, also known as a 'starter'. This cosy little microbial ecosystem is made up of wild yeasts and lactic acid bacteria, found in the air around us, which colonise a mixture of water and flour to begin a process of fermentation. These microbes munch on the proteins and starches in the flour producing carbon dioxide, which puts the air into your loaf to make it rise, and lactic acid which breaks down the structure of the flour to make it more digestible for our bodies and gives the bread that yoghurty tang.
You can make your own levain very simply. All you need is a glass jar with a lid, a piece of cloth (like a clean handkerchief or muslin square), a couple of elastic bands, flour, water and a snuggly corner. A thermometer is always helpful to get a more accurate idea of what is going on.
Day 1 : Use organic, high quality, fresh flour, preferably stoneground. This is important because you need your flour to be 'alive' with nutrients and natural microbes. The cheaper and whiter it is, the more has been sifted and bleached out of it, and artificially put into it, and you will struggle to achieve a good fermentation. Rye and wholemeal work well, or a blend of these with strong white flour. The flours you use will influence the speed that your starter will develop (Rye is quickest) and also the flavour of your starter and therefore your bread. Try to be consistent with the flour that you use and make any changes gradually.
Day 1: Mix 40g of flour with 40g of lukewarm water. Cover with the cloth, secured with an elastic band, and put it somewhere warm (around 23-25c).
Day 2: Stir your flour mix, put the cloth back on and return to its warm place.
Day 3: You may see that your flour mix is beginning look bubbly, but don't worry if you don't. Measure out 20g of the flour mix into a bowl and mix with 20g of lukewarm water and 20g of flour. Stir well to combine. Rinse out your jar, give it a dry and then pop the new mix back in. Mark the height of the mix with your second rubber band (or a Sharpie pen). Do this every time you feed your starter from now on to make it easier to gauge its progress. Put it back in its warm place.
Day 4: Discard all but 20g of your starter. Mix with 20g flour and 20g water. Mark the height with the band and return it to its warm place.
You may find that your starter is beginning to smell fruity or cheesy. This is a good sign. Don't worry, any pungent smells will gradually mellow over the coming few days. However, if at any point your starter smells putrid or develops mould, something has gone wrong. Throw it away, clean everything thoroughly and start again.
Repeat this feeding for at least a further 10 days to two weeks. The consistency should change from very liquid to more like a thick pancake batter. The smell should become a pleasant yoghurty or fruity smell. It should taste like a tangy yoghurt (yes, you can and should taste it). Importantly, the volume should reliably increase to around triple between feeds before you begin using your starter to bake with. If you want to accelerate the process, try feeding your starter twice a day, once in the morning and once before bed, using the same regime set out above.
Once you are happy your starter has nicely fermented, you can begin refreshing your starter daily on a ration of 1:5:5 (starter:flour:water) and fasten the jar loosely with a lid instead of a cloth. Make sure that air can still get in and out, your starter is a living thing and needs to breath. If you want to hit pause, wait a few hours after a feed and the pop your starter in the fridge until you're ready to use it.
Now you're ready to plan your first bake! You should also give your starter a name - it is like having a pet after all. Mine is called Betty. It may shock you to hear that she's quite bossy.