A growth-associated model was applied to the production of recombinant ovine interferon-tau (rOvIFN-tau) with Pichia pastoris for the purpose of manufacturing preclinical and clinical active material. This model predicts that product yields will be the greatest when the specific growth of the culture is maintained at a steady and optimal rate. However, rOvIFN-tau yields did not meet the expected linear model but most closely corresponded to a polynomial relationship. After transitioning from glycerol to methanol, product accumulated for 31-45 h, and then the yield decreased. This production shift, which has been termed decoupling, was clearly related to time on methanol and not culture density. It was determined that a correlation exists between the decoupling point and a drop in energy state of the cell when expressing beta-galactosidase. By assigning decoupling as a constraint that limits productivity and by reformulating the growth medium, the time prior to decoupling increased to 46.8 +/- 2.4 h, product yield improved for rOvIFN-tau from 203 to 337 mg l(-1), and the coefficient of variation for yield decreased from 67.9 to 23.3%. A robust and stable fermentation process was realized, resulting in a 210% improvement in total yield from 557 +/- 357 to 1,172 +/- 388 mg.