This is the yeast used by bakers to make bread, brioches, etc. It is basically the same kind that brewers use to make beer, so it is sometimes also known as brewer's yeast.
Yeast is a living organism, a microscopic fungus, called saccaromycès cerevisae, which reacts with the sugars in flour to form carbon dioxide. It is this fermentation process that makes the dough rise. Yeast is available in 2 forms:
This is the classic baker's yeast which is normally sold as a small, beige-coloured block. It should be kept in the fridge and not for too long.
This is exactly the same yeast, but completely dehydrated. It comes as a beige powder and keeps very well.
This white powder is a mixture of bicarbonate of soda and cream of tartar. It reacts on contact with the water in the dough or batter to form the carbon dioxide which makes it rise. There is no fermentation.
Baking powder is used mostly for cakes
Leaven is a natural raising agent, like yeast. It is made from a mixture of water and flour that begins to ferment when exposed to the naturel yeasts present in the air.
It is a living substance and reacts with the sugars in flour to form carbon dioxide by fermentation, which makes the dough rise. Two different forms can be made:
This is a leaven made with equal parts of water and flour. As the name suggests, it is liquid, rather like pancake batter.
This is a leaven made with one part water to two parts flour. It has a consistency similar to bread dough.
Whether the leaven is liquid or stiff it makes little difference in its use, but a lot in the taste of the bread obtained.
It is worth noting in passing that when making leavened bread
, it is usual to combine a large amount of leaven with a little yeast. This improves the bread's appearance especially the crust.
We can say that yeast and leaven work by fermentation, whereas baking powder uses a chemical reaction.
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