Heat water to boiling and let it cool down to approximately 30-35°C. Then, mix sugar and raisins in a sterilized glass jar with an airtight lid, and shake until the sugar has dissolved conpletely. Place the jar in a warm place.

Pour everything through a clean strainer and take the yeast liquid. This liquid is full of yeast.

You can also keep the starter dough for 10 days in the fridge or keep feeding it with the same amount of water and flour (50% and 50%) at least once a week. When the yeast becomes weak, not expands twice quick enough, you could boost it up by feeding with raisin yeast water. But when it becomes strange or smells funny, please dispose the dough starter and restart from fresh raisin water from the beginning.

The followings are the calculations of the amounts of strong flour and water you need for your recipe. This yeast starter dough is 100% hydration, i.e., the ratio of flour and water is 1:1.

The amount of flour in the recipe - (starter dough/2) = strong flour
The amount of water in the recipe - (starter dough/2) = water

If your recipe uses milk rather than water, please calculate as water in milk is 87% and 13% as skim milk powder.

milk (A) = the amount of milk in the recipe x 0.87 - (starter dough/2)
skim milk powder = the amount of milk in the recipe x 0.13 - amount of milk (A) x 0.13

For instance, if you need 92g of milk for your recipe and you added 60g of the starter. That means 30g water in the starter. So, you will deduct 30g from 92x0.87 (80g, total water needed). Then, add skim milk powder. 92x0.13 (11.96g) is you need in total and you add 50g of milk (6.5g), so you need 5.5g of skim powder.

92 x 0.87-(60÷2) = 50 g of milk
92 x 0.13-50x0.13 = 11.96 - 6.5 = 5.5 g of skim milk to add

which will become equivalent to the 92 g of milk (80g water and 12g=5.5g +6.5g milk)