Comparison of the Baker's yeast process performance in laboratory and production scale

A 215 m(3) industrial bubble column reactor for fedbatch production of Baker's yeast was sampled for sugar, to investigate the extent of concentration gradients. The results verify that such gradients exist: the concentration is higher closer to the feeding point. Effects of sugar heterogeneities at different scales were studied by 1) performing a volumetric scale-down of the industrial process in a laboratory stirred tank reactor (STR); 2) performing the same scaled down process in a Scale-Down Reactor (SDR) with repeated short term exposure of the cells to high sugar concentrations. In this reactor about 10% of the Baker's yeast culture was intermittently exposed to high (0.45-1.9 g l(-1)) concentrations of sugar, for periods of 60 seconds. It was found that physiological parameters of glycolysis and respiration were affected by environmental heterogeneities: 1) A biomass yield reduction of about 6-7% was found, with both the production reactor and the SDR, as compared to the homogeneous reactor. The loss of yield is interpreted in terms of a metabolic by-pass via ethanol, where cells are consuming and producing ethanol with different yields. 2) The maximum respiration rate was higher in cells produced in the production unit and in the SDR. 3) The product quality, expressed as gassing power of the yeast in a dough, was increased for sweet and non-sugar doughs in the SDR, and for sweet doughs in the production reactor. Thus, the SDR, when run with defined glucose gradients, in some aspects resembles the large reactor. It could be regarded as a tool for scale-down and scale-up studies and may be useful in investigations on the scale-up sensitivity of a process.