In a 1936 magazine article about Kentucky’s top distillers, their full job description was usually given as "distiller and yeast maker." Over this past weekend I asked David Beam, who was a distiller at Jim Beam Clermont for many years, if it is difficult to make yeast from scratch. He said it is very difficult. They never had to do it at Jim Beam because they never lost their yeast. The yeast they are using is descended (yeast being a living thing) from the yeast Jim Beam himself made right after Prohibition. David told me about a couple of distillers he knew who had done it just to prove to themselves that they could and others who tried and never were successful. This is "practical distilling," as opposed to scientific distilling, where you mix up a medium and try to capture a suitable yeast out of the air. This is one of those old fashioned, traditional things. The same end can be accomplished by going to the yeast store and buying a suitable yeast cooked up by scientists in a lab, but then you have a yeast anyone else can buy too. If you make your own that's a proprietary yeast and only you have it. Do Jim Beam and Heaven Hill use the same yeast. Let's just say yes without going into it more than that.
The point of this tale is that yeast making, at least the traditional method, is another dying art. There probably are only a handful of people, all men in their golden years, who know how to do it. Even that may be a stretch, it may be that really there is no one who can do it. Another lost art.