I remember when I was interviewed about my wild yeast project, Martha told me that it might be nice if I can share the method. I asked Taro if that is okay with him, and he kindly agreed. I cannot thank Taro enough for letting me share his technique of making the yeast water. To me, there is no better. I respect other methods by other bakers; this is simply a different approach in baking.
Although I've been calling "yeast water" for my natural leavening, I am used to calling it, "Kobo" which means yeast or saccharomyces cerevisiae in Japanese. But in the US, I hear "yeast water" among scientists and bakers. So here I am, I am calling my natural leavening as yeast water. (I might call it "Kobo" instead of yeast water in the future, though) After making various types of yeast water for my naturally leavened bread, I have found apple and raisins are fairly easy to work with. Making yeast water from fruits is simple. It is a similar concept as alcoholic fermentation.
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Making a Yeast Water from Raisin
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What You Need:
glass jar with solid lid
organic raisins (without wax coating is preferrable)
filtered water
Method:
1. sterilize the glass jar and the lid with hot water
2. remove dust and gently rinse raisins
3. fill raisins for little less than 1/3 of the jar
4. fill the rest of the jar (2/3) with filtered water until all the way to the top
5. seal the jar very tightly with the lid, leaving no air
6. let the raisins ferment the inside the jar for approximately five days in room temperature
Note:
Depending on the condition (seasons, room temperature, sugar contents of the raisins), fermentation time varies. One way to determine whether the yeast water is ready is when the yeast population (saccharomyces cerevisiae) reaches to its maximum, and you will see the active bubbling inside the jars. It takes approximately five to six days. Again, it depends on the circumstances.
Once it is ready, discard the raisins, and use the remaining juice as leavening agent. Using the yeast water for making starter, levain, or bread instead of plain water in bread recipe helps leavening . Unlike the sourdough starter, It is best to use up all the starter and yeast water after first or second batch of bread making for a better result.
Caution:
If you are not going to use the raisin water when it is almost ready, keep the jar inside the fridge to slow the fermentation process. If you leave the jar for too long at room temperature, carbonation will build up, and the bottles is going to be exploded.
Special thanks to Taro Hashiguchi for sharing the technique of making the yeast water.