I asked a question about what I could add to homemade bread to stop it going off so fast and someone suggested trying "a poolish starter". I've never heard of this before, so what is it?
Poolish is a type of sponge. You prepare it before mixing your dough. As in 16 hours before. Then, you mix it with the rest of the ingredients.
What I usually do is mix 150g of flour and 150g of water with a pinch of yeast (less than 1g) and no salt at all the night before the day I'm going to use it. The idea is to use the same amount, in weight, of flour and water, and the least amount possible of yeast and let it work as slow as possible.
This will depend on many factors. I'm giving you my quantities, but it changes from winter to summer, you will have to experiment. That's half of the fun. For the mixing I use a fork and a 1 litter/quarter container and 30 seconds of work, it shouldn't take you more. The poolish usually gets almost to the top in 14-18 hours. Once I put 5g of yeast and got to the top in 1 hour. This is not bad but defies the purpose of the poolish taking a lot of time. Some people even put in the refrigerator to make it take longer.
Why would you want it to take time? This is what I was told and makes sense to me: when you mix water, flour and yeast two things happen: the yeast transform the sugars and some good bacteria starts creating acids that will determine flavour. The yeast work depends on the temperature and amount of yeast (among other things, like water, salt, fat, etc). The bacteria doesn't. Also, the bacteria and the acids help preserve the bread.
Once it has reached the top you have 1 hour approx to use it. If not, it will collapse and you better start with a new one. If you think you are not going to use it in 1 hour, use a fork to degass it before it collapses and let it rise again. It will rise faster this time, but that will buy you some time. You can see what it looks like.
If you have a recipe you like, try replacing 150g flour and 150g water with the poolish and see what happens. You may replace between 10% and 50% of the final dough. You'll see changes in color and flavor.
I'm not sure what you mean by "stop it from going off so fast."
A poolish is a pre-ferment that is made of equal parts flour and water. If you are familiar with a biga or sponge, it is a close relative.
You can learn more about starters in general from The Bread Baker's Apprentice, one of the best books for learning about the science behind bread out there. He covers pre-ferments, bigas, and poolish as well as sourdough starters. Poolish is a more wet starter than many of the other kinds.
A poolish is somewhat similar to a sourdough starter that isn't strong enough to provide the yeast all on its own.