A method of producing an organic acid by staining a yeast population with a stain capable of internal pH (pH.sub.i)-dependent fluorescence, to yield a stained yeast population; determining a gate pH and a corresponding fluorescence parameter of the stained yeast population; and sorting the cells of the stained yeast population such that the cells having a pH.sub.i above the gate pH are retained and the cells having a pH.sub.i below the gate pH are discarded, to yield a yeast population for the production of the organic acid. Also, a method of producing an organic acid by performing the above steps, followed by isolating individual cells of the yeast population, to yield individual yeast cells for the production of an organic acid; culturing an individual yeast cell, to yield a cloned yeast population for the production of an organic acid; and incubating the cloned yeast population for the production of an organic acid in a medium containing an organic acid precursor, to produce the organic acid.