Weigh all your ingredients with a kitchen scale. Check the label on your raisins to make sure they're free of sulfur dioxide (a preservative) and any type of oil coating. These can inhibit yeast growth. If possible, use organic raisins.
Add all the ingredients to a clear jar with an airtight lid. Shake it until the sugar dissolves and place it in a warm place out of direct sunlight. The raisins should lay at the bottom of the container.
Vigorously shake the mixture for 3 seconds morning and evening for the next 3 to 8 days or until you start to see the raisins float in the water. The length of time will depend on yeast activity and temperature.It's very important to open and close the lid before AND after every shake. This ensures you don't have excessive gas build up in your container which can cause the lid to pop off! Keep the lid sealed at all other times.
After a few days, you should start to notice signs of fermentation in the yeast water. The water should be cloudy and there should be tiny bubbles around the raisins and along the sides of the container. The raisins will be suspended in the water instead of laying at the bottom of the container.
The yeast water is ready to use once the raisins float to the top. (Note: Yeast consumes sugar as food and then produces carbon dioxide as a byproduct. When the raisins float, it's a sign that there's enough yeast activity to generate gas build up. This means it's ready to strain and use in a wild yeast starter.)
Strain the raisins in a mesh strainer and gently press out any remaining liquid. You can either discard the raisins or use them for baking.
Store the yeast water in an airtight container and keep it in the fridge for up to two months. You can use this liquid to create a natural yeast starter or add it to bread recipes. Be sure to mix it well before adding it to any recipe as there will be settlement.