Comparison of the Baker's yeast process performance in laboratory and production scale S. George, G. Larsson, K. Olsson, S.-O. Enfors Abstract 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 concen- tration 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 ex- posed 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 reduc- tion 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 max- imum 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 de®ned 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. 1 Introduction Baker's yeast processes are mostly performed in a fedbatch mode in bioreactors of more than 100 m 3 . Molasses commonly is used as the fed carbon/energy source, with sucrose as the main sugar constituent. This is rapidly hydrolysed by the yeast invertase, giving a mixture of mainly glucose and fructose in the medium [1]. Also ammonia is fed during the process. Typical process feed and concentration pro®les are shown in Fig. 1. The feed rate increase during the ®rst part of the process yields an exponential biomass growth at a speci®c growth rate below the maximum. To avoid oxygen transfer and cooling limitations the feed rate is later set constant. The feed pro®le can be used to control the extent of aerobic ethanol formation. To achieve high productivity the feed pro®le may be set to initially yield some ethanol. When the de- creasing sugar concentration becomes low ethanol pro- duction is replaced by consumption without a diauxic lag phase of growth [1]. During the ®nal stage of the process the feed of ammonia and later the feed of molasses are gradually reduced to zero. This ensures yeast maturation and creation of a suf®cient supply of storage carbohy- drates (glycogen and trehalose). The total process time is about 14 h, after which cells are harvested, dewatered and packed as the Baker's yeast product. The quality of the harvested yeast is measured in a number of ways (e.g. colour, consistency, smelling etc.) but most important is the gassing power, i.e. the ability of the cells to raise a dough, by means of CO 2 production. The fedbatch technique is often used to avoid engi- neering limitations, but is in this process also used to exert a metabolic control. The sugar limitation is utilized to avoid extensive over¯ow metabolism, which otherwise would result in too high ethanol production and an ac- companying reduction of biomass yield. Even if the etha- nol is subsequently consumed, the total biomass yield from glucose is reduced when the combustion to carbon dioxide passes via ethanol, as can be concluded from yield calculations in literature [2]. This decrease of the yield has also been observed when comparing processes in reactor set-ups with zones with ethanol production due to high sugar concentration [3, 4]. This over¯ow metabolism of sugar to ethanol is due to a restriction in the carbon metabolism, somewhere after the pyruvate. When subjected to sugar concentrations above a certain critical value, S crit , the glycolysis rate exceeds a critical value and the cells can not fully oxidize all the available sugar to CO 2 . Some of the sugar uptake is con- Bioprocess Engineering 18 (1998) 135±142 Ó Springer-Verlag 1998 135 Received: 24 March 1997 S. George, G. Larsson, S.-O. Enfors Department of Biochemistry and Biotechnology, Royal Institute of Technology, S-100 44 Stockholm, Sweden K. Olsson Ja Èstbolaget AB, Box 7003, S-192 07 Sollentuna, Sweden Correspondence to: G. Larsson This work was supported by a grant from the Nordic Programme on Bioprocecss Engineering under the auspices of NI, the Nordic Fund for Technology and Industrial Development, and by NUTEK, the Swedish National Board for Technical Development. We also want to thank Ja Èstbolaget AB for performing part of the cultivations and placing laboratory facilities and the production scale Baker's yeast process at our disposal.