Arranroots,
As it happens, my husband recently did an environmental audit of a yeast factory. He says,
"There are many different strains of cultivated yeasts, for production of bread, beer, wine, etc. Each producer carefully protects their basic yeast strain by laboratory cultivation under extremely sterile conditions to block the intrusion of wild yeasts (which have spores present in the air in any part of the world). In production, a sterile sample of the protected yeast is placed in large vats to which are introduced water, sugars (molasses, honey, or such sources), and mineral nutrients (e.g., phosphorus). Vat temperatures are carefully controlled and the contents are constantly stirred. Waste products (such as alcohol) generated by the growing yeast eventually inhibit further growth in the vat. The contents of the vat are "harvested" as a liquid slurry. The slurry is centrifuged to reduce the water, and the resulting yeast paste is either sold as is to bakeries or breweries, or is compressed to reduce the water and then sold as semi-dry blocks or powder."
