If you are using whole grains there is already natural yeast on it. Yeast is also pretty much everywhere. Nutrient rich solution left open will ferment. It j

If you are using whole grains there is already natural yeast on it. Yeast is also pretty much everywhere. Nutrient rich solution left open will ferment. It just might not be a super powered brewing yeast. Scandinavian brewers had a "magic stick" used to capture and store yeast until the next batch.