How to make your first sourdough starter at home. Beginner's guide to sourdough starter from scratch.
Combine 70 g of flour and 70 g of water (lukewarm, neither cold nor hot) in a plastic or glass container. Stir with a spoon until a thick paste is formed. Cover the container with a lid, but do not screw it on to allow gases to escape. Leave at room temperature in a warm room (I leave it in the kitchen).
Add 70 g of flour and 70 g of water to the mixture in the jar and stir in well. Make sure to have a jar that is large enough for this quantity to double in size. Put lid on, unscrewed, and let sit at room temperature.
At this point, some fermentation should have started to take place and the mixture could already show some bubbles. Today we start discarding and feeding. Remove 100 g of the mixture, then add 70 g of flour and 70 g of water. Stir to combine, put lid over and let sit at room temperature.
From day 4 on, you will repeat the step from day 3. About 24 hours after the previous feeding, discard 100 g of mixture and stir in 70 g of flour and 70 g of water. It is important to always leave the lid unscrewed to allow fermentation gas to escape. Now is when proper fermentation will happen: the starter will get very bubbly and the smell will become proper acidic, resembling that of beer.
Depending on the temperature in your room, your sourdough starter may already be good to use on day 7 or it may take a few days more. To check if your starter is ready to use, drop a small dollop of starter into a glass with water. If it floats, it's good to use and you can start baking bread. If it sinks, you will need to keep feeding it a little longer.
When your sourdough starter is done, you can choose to keep it at room temperature and feed it daily, or let it rest in the fridge and feed it weekly. If you keep it at room temperature, you can use the part you discard daily to make bread right away, as a starter kept at room temperature and fed daily is active. When you keep your sourdough starter at room temperature, the lid should always be unscrewed. Unless you plan on making bread every day, it is not necessary to keep your starter at room temperature and you can store it in the fridge.
If you keep it in the fridge, you will need to have your starter in an airtight container, so screw on the lid. Have it as a routine to take it out once a week (it helps to stick with the same day) for a feeding. To feed the starter, follow the steps from day 3. If you're planning to use your starter, allow it to sit at room temperature for one day before you use it, in order to let it re-activate. It is not recommended to use the part you discard on the feeding day, as a starter straight from the fridge is not active. If you're not planning on using it, then you can just proceed with the feeding and then place the starter back in the fridge. Repeat this process weekly.
You can use several types of flour in your starter, but the easiest way to success as a beginner is to use all purpose flour. If you plan on adding a different type of flour - I added barley flour to one batch, and durum to another - do it as a feeding after day 3, in order to have all purpose flour in the early stages and adding a different flour once the fermentation process has kicked in.