NOVELTY - Fermentation of liquid medium with a yeast slurry from a previous fermentation involves passing a portion of yeast slurry through a membrane contactor comprising hydrophobic, microporous membrane having liquid side and a gas side, where contactor is connected to oxygen source, flowing yeast slurry over liquid side of the membrane, and delivering oxygen to the gas side of the membrane.    USE - For fermenting liquid medium with yeast slurry (claimed) useful as biocatalyst to promote fermentation of carbohydrate to ethanol in production of beer and other fermented beverage and in wine industry to manipulate flavor.    ADVANTAGE - The method economically and efficiently oxygenates yeast slurry prior to pitching. The method oxygenates slurry without foaming of the slurry. This allows the yeast cells to obtain sufficient oxygen to synthesize sterols and fatty acids, which are needed for cell growth during fermentation process. By oxygenating or aerating yeast, slurry prior to pitching that does not cause foaming. Oxygenating yeast prior to pitching allows the yeast cells to obtain sufficient oxygen to synthesize sterols and fatty acids, which are needed for cell growth during the fermentation process. By oxygenating the yeast slurry rather than the wort, greater consistency of the final product is attained with reduced levels of staling precursors. By the method, the pressure on the liquid side is kept higher than the pressure on the gas side to prevent the gas from passing through the membrane as bubbles, and reduces bubble formation and foam formation. The membrane provides a high surface area contact between the liquid yeast slurry without direct mixing. The high surface area to volume ratio provides a high specific surface area for mass transfer. By minimizing foam, the method allows operation at higher mass transfer efficiency values. Because the membrane is hydrophobic, the liquid does not penetrate the pores and the gas-liquid interface is maintained on the liquid side of the membrane. High higher mass transfer efficiency values are achieved by renewing the liquid surface continuously through the application of a high cross flow velocity. The method obtains high fermentation rates by using oxygenated yeast to pitch aerated wort, and fermentation is completed in a shorter time using yeast aerated by the method than by conventional fermentation methods, in which aerated wort is pitched with non-aerated yeast slurry.    DETAILED DESCRIPTION - Fermentation of a liquid medium with a yeast slurry from a previous fermentation involves providing an undiluted yeast slurry from a previous fermentation having 40 - 80 g/1 yeast on a dry weight basis, where the yeast experienced anaerobic conditions in the previous fermentation; passing a portion of the yeast slurry through a membrane contactor comprising at least one hydrophobic, microporous membrane (30) having a liquid side (40) and a gas side (50), where the contactor is connected to an oxygen source, and portion of the yeast slurry is in proximity to the membrane on the liquid side; delivering oxygen from the oxygen source to the gas side of the membrane under conditions that cause at least a portion of the oxygen to transfer from the gas side of the membrane to the yeast slurry such that the mass transfer efficiency is at least 0.005/seconds and transfer of oxygen to the yeast slurry is bubble-free; determining an oxygen uptake rate of the yeast slurry; increasing an oxygen delivery pressure on the gas side of the membrane and increasing a liquid pressure on the liquid side of the membrane in dependence on the oxygen uptake rate wherein the liquid pressure on the liquid side of the membrane is kept higher than the oxygen delivery pressure on the gas side of the membrane; and pitching a liquid medium with the yeast slurry. The medium is wort that is optionally aerated prior to pitching.    DESCRIPTION OF DRAWING(S) - The figure shows the flow of yeast slurry from a tank to the oxygenate ion membrane and the return of the oxygenated slurry to the tank.    tank (10)    membrane oxygenator (20)    microporous hydrophobic membrane (30)    liquid side (40)    gas side (50)    oxygen tank. (60)