Effects of high sugar concentrations in production scale bioreactor : a scale-down study of s. cervisiae

Sammanfattning: Substrate concentration gradients exist in large-scalebioreactors, due to a combination of insufficient mass transferand mixing, and microbial activities. Zones with high substrateconcentration may arise close to a feed point. It is largelyunknown to what extent these gradients influence themicroorganisms and what the microbial response might be. Inthis work a scale-down method was developed, for studies ofsuch responses. A coupled reactor system was constructed, inwhich effects of repeated short term exposures to concentrationgradients can be studied. Zones of different kinds, welldefined with respect to substrate concentration and residencetime, can be created by applying different feeding and aerationtechniques.The microbial test system used was the aerobic ethanolproduction ofSaccharomyces cerevisiae,due to high sugarconcentrations. Effects of repeated exposures to suchconcentrations were demonstrated. Influences of the culturewere observed at many different levels: a decrease in biomassyield and an increased ethanol production, an increase in themaximum respiration rate, and enhancement of the quality of theBaker's yeast, expressed as the gassing power.The dynamics of the actual response to high sugarconcentrations were studied in the reactor system: Transientsin specific rates of glucose consumption and ethanol productionwere demonstrated during the first minute of such an exposure,explained by a short term regulation of the glucose uptake. Theresponse was also shown to be influenced by the history of thecells. The response for cells formerly not exposed to highsugar concentrations differed considerably from that of cellsrepeatedly exposed to such conditions.Methods for examining a large scale bioreactor environmentwere developed. A study of a215m3 bubble column reactor was performed, with respectto concentration gradients (sugar, oxygen and intracellularadenosine nucleotides) and hydrodynamic flow pattern (gashold-up and liquid velocities). The liquid velocity profileexhibited the expected gulf-stream pattern. Themicroenvironment for the cell was however probablycharacterized by environmental fluctuations, since axialgradients of oxygen, sugar and adenosine nucleotides weredemonstrated.Keywords:scale-down, plug flow reactor, S. cerevisiae,Crabtree effect, Baker's yeast, large scale study,concentration gradients, mixing, overflow metabolism.