In her splendid book English Bread and Yeast Cookery, Elizabeth David has this to say. 'In Chaucer's England one of the names for yeast or barm was goddisgoode, “bicause it cometh of the grete grace of God”. These words simply imply a blessing. To me that is just what it is. It is also mysterious, magical.

No matter how familiar its action may become nor how successful the attempts to explain it in terms of chemistry and to manufacture it by the ton, yeast still to a certain extent retains its mystery. 'Perhaps one of the reasons yeast seems so mysterious is that it is in fact alive. When we buy it, it is alive (one hopes) but inactive: under the right conditions, that is with a little warmth and the addition of some water, it is activated and releases the gas that will raise the dough. All this activity ceases only when the dough is placed in the oven and the extreme temperature kills off the yeast.Yeast is available in three forms: fresh from some bakers or chemists, and dried (granular) or powdered in tins or packets.

Fresh yeast: This should look firm and moist, cream coloured and cool to touch. If it's crumbly, dryish and dark in places it is stale and might not do its work. To use it simply mix the required amount into the liquid and mix into the dough straight away. Fresh yeast can be bought in bulk and stored in a deep freezer in, say, wrapped 1 oz (25 g) portions. It will keep in a freezer for three months, or in a refrigerator for three days.

Dried yeast: As supplies of fresh yeast are sometimes unreliable I either use dried yeast, which is reconstituted with warm water and sugar, or easy-blend powdered yeast. Ah, but it's not like the real thing, some people will say. But first of all it is the real thing, because dried or powdered it's still yeast. Secondly, I have tasted fresh yeast and dried yeast bread side by side, and really couldn't tell the difference. One word of warning though: dried yeast, just like fresh, does become stale. So dried granular yeast must be stored in an airtight container and used according to the date stamp, and if it doesn't produce a `frothy head' (see above) it is not fresh and won't raise the dough.

Powdered, easy-blend (or easy bake) yeast: This kind is the easiest of all to use – no measuring out, no hand-hot water, no waiting for frothy heads. All you do is sprinkle the required amount in with the flour and add the water separately; 2 level teaspoons of powdered easy-blend – exactly the same as 2 teaspoons dried yeast – is the correct amount for each 1 lb (450 g) of flour.

But yet another warning! Do inspect the date stamp carefully, as yeast will not do its work once it becomes stale.

DID YOU KNOW? Yeast is gluten free and suitable for vegetarians

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