Abstract: A bioreaction is affected by oxygenating a liquid/solids mixture containing a micro-organism to enable the bioreaction to proceed, wherein the said mixture is confined in a reaction vessel and circulated around a loop, the loop including a column in which gas transfer is effected, at least the liquid being introduced into the top of the column in the form of at least one stream, the velocity of which stream is sufficient to generate and maintain, at least in an upper region of the column, a substantially continuous foam formed of close-packed bubbles of the gas in the liquid extending across the entire cross-section of the column, driven to violent agitation and backmixing by the incoming stream of liquid, and of relatively uniform bubble size, the velocity of the inlet stream of liquid and the rate of introduction of the gas being sufficient to prevent gas accumulating at the top of the column.