Abstract: An improved bioreactor and process for continuously propagating microorganisms, such as yeast, wherein the culture medium is purified in contact with a spirally-wound ultrafiltration membrane and then passed through the outer surfaces of a tubular membrane material for further purification before contact with microorganisms flowing in the interior of the tubular membrane material.

Abstract: Disclosed are an improved process for preparing yeast from molasses. In its preferred aspects, final molasses is adjusted to a density of from about 20.degree. to about 40.degree. brix, desludged, passed through an ultrafiltration device effective to reject molecules having molecular weights greater than about 30,000 daltons to produce a first permeate, and passed through at least one additional filtration device having an average pore diameter within the range of from about 0.2 to about 0.5 microns to produce a yeast culture medium. The filtration steps are effective in combination to reduce the microorganism count to a level effective to produce yeast suitable for food use; preferably, less than 1 organism per gram of medium. Bakers' yeast are propagated in the medium in increased yield of yeast per unit weight of molasses solids.