Abstract: A nutrient solution of a microorganism to be supplied to a fermentor or a culture broth in the fermentor is aseptically filtered by a porous filter medium made of polytetrafluoroethylene polymer and having properties impervious to microorganism, resistant to heat and chemicals, and hydrophilic, wherein said hydrophilic property was given to the surface of said filter medium with a treatment for hydrophilicity. This treatment comprises treating polytetrafluoroethylene porous polymer as a raw material for the filter medium with surface treating agent comprising an alkali metal naphthalene and diamine and immersing the treated polymer into concentrated sulfuric acid containing an aliphatic or aromatic aldehyde dissolved therein in a concentration exceeding half saturation.

Abstract: A broth containing yeast cells is continuously or intermittently removed from a fermentor. Then, yeast cells are separated from the filtrate using a cell separator, or further washed with water. Then, the yeast cells so obtained are recycled to the fermentor, whereby yeasts are cultivated at a high cell concentration of from 6% to about 20% based on dry weight. As discussed herein, by removing the filtrate from the cultivation system, there is no accumulation of metabolites and salts prohibiting the cultivation of yeasts, and further the growth of miscellaneous microorganisms which interfere with yeast cell growth, is surprisingly suppressed.

Abstract: Apparatus for cultivating aerobic microorganisms is provided which comprises cell multiplying compartment and cell ripening compartment. The invention also provides a cultivation process using the same apparatus. The invention improves oxygen transfer rate and utilization of oxygen, thus enabling cultivation at a higher concentration of microorganisms as compared with any conventional process.