Kinetics of substrate inhibition of exponential yeast growth

Abstract

SummaryThis work concerns mathematical modeling of the rate of microbial growth on inhibitory levels of nutrients as affected by pH, concentration of the nutrients, temperature, cultivation method, and method of data analysis. Candida uiilis (ATCC 9226) was grown with sodium acetate as growth-limiting carbon and energy source in mineral salts medium in shake flask and continuous cultures to study inhibition by excess acetate. Differential shake flask cultures were grown at low yeast concentrations at temperatures (T) of 25 and 30°C, pH's between 5.5 and 7.0, and acetate concentrations (S) between 0.25 and 3.0% (w/v). Growth data were exponential with correlation coefficients greater than 0.995 in 49 of 56 experiments; the lowest correlation coefficient was 0.986. Specific growth rates ( p ) determined by graphical methods showed only fair correlation with those determined by regression analysis. Both sets of specific growth rate data were grouped at constant T and pH and fitted to the three parameter equation,The improvement in use of the fitted equation instead of the mean value was significant with greater than 70% confidence in all (14 groups) and 90% confidence in only half of the data groups; the correlation does not improve with the increaaing acetate inhibition a t lower pH. Both defects in the model and insufficient data a t each pH are responsible. A modified six parameter model was developed to include pH effects by varying five of six parameters with hydrogen ion concentration (H+) aa follows: JACKSON AND EDWARDS99% confidence. The six parameter model adequately represents effects of acetate and hydrogen ion concentrations under constant or slowly changing environmental conditions and balanced growth; although better models probably exist. Thus steady-stste and transient continuous culture experiments agreed with many published growth yields, but specific growth rates could only be predicted qualitatively from the model fit to the shake flask data. The data and present models could be incorporated into published models for transient growth at low nutrient concentrations to correlate and perhaps predict microbial growth kinetics over a much wider range of growth conditions than now possible.

Ethanol inhibition of continuous anaerobic yeast growth

“…The data of Cysewski [16] showed a higher cell mass concentration and followed a pattern similar to that for the 20 gl~l feed concentration data found here, namely, that the cell mass concentration did not decrease at lower dilution rates.…”

Substrate inhibition kinetics of Saccharomyces cerevisiae in fed-batch cultures operated at constant glucose and maltose concentration levels

“…Batch fermentations carried out as reference for fed-batch cultivation in this work, show that yeast growth follows Monod kinetics. The observed sugar inhibition effect in glucostat cultures was taken into account by using the model equation proposed by Jackson and Edwards [14] in their study on substrate inhibition kinetics in yeast. K i is the inhibition coefficient expressed in g l -1 .…”

“…The modeling software ModelMaker 4 was used for determination of kinetic parameters values from experimental data. The Monod equation and the above equation by Jackson and Edwards [14] were used for the simulating model to fit experimental data and optimize these parameters.…”

“…The specific glucose uptake rate q S is the quotient of growth rate/yield (in millimolar sugar per g CDW per g S). The inhibition coefficient K i was determined by using the equation l ¼ l max S K S þSþS 2 =K i according to Jackson and Edwards [14]. Definitions of kinetic parameters and calculations were according to Sinclair and Cantero [38], and Sinclair [37].…”

“…Edwards [14] 0,00 60,00 120,00 0,00 20,00 40,00 t S, x S x Fig. 6 The kinetic model fitted to glucostat culture operated at 100 g l -1 glucose concentration.…”