One of the first steps in developing production processes based on microorganisms is their kinetic characterization. Typically, this involves determining the kinetics of growth, substrate consumption and product production. Saccharomyces cerevisiae is considered a valuable yeast in biotechnology due to its fermentative capacity and ethanol production, as well as for other emerging applications. Notwithstanding, ethanol is toxic for this microorganism and, at high concentrations, has an inhibitory effect on its growth kinetics. The rigorous experimental determination and analysis of microbial kinetics under inhibition is a relevant topic in bioprocess engineering and is included in biotechnology curricula.

In this context, the present contribution aims to illustrate the growth kinetics of S. cerevisiae from a practical point of view in biotechnology courses, as well as its subsequent mathematical analysis. To achieve this, the practical session had two main objectives: #1, to determine the maximum specific growth rate of this yeast and #2, to characterize and mathematically describe the inhibition of S. cerevisiae growth due to ethanol.

To perform the experiment, students cultivated an overnight inoculum of S. cerevisiae T73 strain in YPD (Yeast Peptone Dextrose) medium with varying ethanol concentrations (ranging from 0 to 90 g/L), using an orbital shaker at 33 ᵒC. Every 30 minutes for a total duration of 3 hours, students took sterile samples from the culture and measured the absorbance at 600 nm using a spectrophotometer. Biomass concentration in the medium was then calculated for each time point and ethanol concentration based on absorbance readings. Subsequently, the experimental specific growth rate of S. cerevisiae was estimated by calculating the slope of the regression line between the natural logarithm of biomass and time. From the experiments with different ethanol concentrations, the apparent specific growth rate was determined. The Han-Levenspiel model was then fitted to the experimental data to estimate its kinetic parameters.

This experiment allowed students to simulate and manipulate a batch reactor and to become familiar with spectrophotometer operation. After data collection, each group of students shared their results with the rest of the class, encouraging collaboration and increasing students' motivation to perform the experiment properly. Students then submitted lab reports in the format of a scientific paper, gaining experience with the structure and terminology of scientific writing. Trainees also compared their experimental results with findings from the literature, developing the habit of seeking reliable and relevant information. Additionally, students evaluated the ability of the Han-Levenspiel model to fit the experimental data in comparison with alternative models, fostering critical discussion.

The practical session enhanced students' critical thinking by guiding them through a comprehensive study, from the experimental phase to the development of an academic manuscript.

Keywords: Practical biotechnology lesson, Saccharomyces cerevisiae, ethanol, simulation, scientific paper, modelling.