What can we use yeast for? Well, for starters, (get it?) you can use it for fermentation, but did you know you could use it for adding different flavors or aromas? Sometimes you have a Sake and realize how did they get that hint of apple in it? Yeast.
You can get yeast from the wild. Wild yeast can be on an apple sitting in a tree, or a banana you get from the store, perhaps even the orange sitting in your fruit bowl, even the air around you. Collecting yeast from the wild can be a rewarding, educational experience in microbiology.
To collect yeast is easy, to cultivate it can be hard if not done correctly. Yeast can be captured using a sanitized cotton swab, (the kind you see at the doctor's office or if you prefer, crime shows) and a petri dish. There are specific tasks you have to complete to grow the yeast and designate a streak that is healthy inside the petri dish.
Your environment needs to be like a lab. Sterile. You don't want to risk an infection within the yeast colonies. After you have grown a healthy colony you then move it to a second petri dish using a sterilized paperclip or loop of wire to grow more of it.
Scrape some of the colony into a small Starter, I'm talking like a drinking glass size. The Starter should consist of the white fluid left behind from the steamed rice and Koji that was sitting in 70-degree water for a few days as it broke down the Polysaccharides into Monosaccharides. To prevent unwanted yeast from getting into the Starter place a cheesecloth over the top of the glass.
As the yeast grows and consumes the simple sugars, it will eventually fall to the bottom of the glass. Filter out the yeast and gradually step it up to a larger starter amount by repeating the process. Once you get the amount of yeast you are looking for, pitch it into your normal Sake Starter that has normal yeast, Koji, and steamed rice.
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