How to convert fresh yeast to dry yeast, and back

Dry yeast is fresh yeast with the water taken out, so it's stronger: 1 g of instant dry yeast does the work of 3 g of fresh yeast. To convert fresh yeast to instant, divide by three; the other way round, multiply by three.

In Germany, fresh yeast comes in 42 g cubes. One cube matches 14 g of instant yeast, exactly two 7 g sachets, so one sachet replaces half a cube.

Many American recipes call for active dry yeast. It has coarser granules, is dissolved in warm water before use and is a little weaker than instant yeast: 1 g replaces 2.5 g of fresh yeast. A US packet weighs 1/4 oz, about 7 g, the same as a German sachet.

Bake half the recipe and you need half the yeast: the amount changes in step with the flour. If the recipe uses a cube for 1 kg of flour and you use 500 g, half a cube is enough, or one sachet of instant yeast.

It works with sourdough too: for a 7 g sachet of instant yeast, use about 100 to 140 g of ripe starter. A starter is half flour and half water, so take half its weight off the recipe's flour and half off its water. The dough stays just as firm.

Allow much more time, though: sourdough raises the dough more slowly than yeast, and one hour of rising easily becomes several. Exactly how long depends on the temperature of the dough.

Fresh yeast only keeps for a limited time in the fridge, while unopened dry yeast lasts much longer. Dry yeast is mixed straight into the flour; fresh yeast is usually crumbled into a little lukewarm water first.