Stability of exo-inulase production on lactose in batch and continuous culture of a Kluyveromyces fragilis hyperproducing mutant

The maintenance of hyperproduction of an exo-inulase (β- d-fructofuranoside fructohydrolase, EC 3.2.1.26) by a mutant of the yeast Kluyveromyces fragilis was examined in continuous culture on lactose. After initial phenotypic adaptation with activities reaching up to ∼35 U mg −1 dry cell weight (i.e., ∼4%, w/w), cultures lacking detectable residual lactose exhibited a rapid loss of enzyme yield. The original hyperproducing strain was displaced by a wild-type cell population with twice the affinity for lactose, resulting from increased lactase (β- d-galactoside galactohydrolase, EC 3.2.1.23) activity. The displacement could be prevented by shifting from carbon to nitrogen limitation, using a large excess of lactose .