Physiology and mathematical modelling of immobilised saccharomyces spp. in brewing.

There is an ever-increasing demand for reduction of unit operations employed in industrial alcohol fermentation for beverage production, and a growing interest in the physiology of yeasts used in the process. In this context, cell immobilisation is an interesting alternative, since it reduces the number of steps to separate biomass from the fermented broth. There is a need to study the physiological effects caused by immobilisation on cells used in beer fermentation, so that the mechanisms that govern such metabolic alterations can be investigated in more detail and, eventually, fully described. Thus, the main objective of this work was to study the physiology of three brewer\'s yeast strains (SY025, SY067 and SY001) immobilised on a porous cellulose-based support. The immobilisation methodology was defined as entrapment in a porous structure by biomass adsorption on the internal walls of the support, along with in situ formation of a calcium alginate gel, followed by a second gelation step, external to the matrix. Batch fermentations in malt extract 12 °P were carried out with the three strains in free and immobilised form, to compare kinetic parameters of both process conditions. Mathematical modelling of fermentation kinetics was performed, using experimental data to estimate physiological parameters under the studied conditions. A mathematical reaction-diffusion model was also used to estimate the total substrate concentration gradient inside the immobilisation support, in order to improve the understanding of how the immobilisation microenvironment influenced the physiology of yeast cells. Fermentations with different initial concentrations of substrate and biomass were conducted, aiming to analyse the influence of these variables on the formation of flavour compounds, using statistical models. Compared to free cells, immobilised yeasts showed higher glycerol production (SY025 - 40 %; SY067 - 53 %; and SY001 - 19 %) and total biomass in the system (SY025 - 67 %; SY067 - 78 %; and SY001 - 56 %). Free cells produced more ethanol than immobilised ones (SY025 - 9 %; SY067 - 9 %; and SY001 - 13 %), which indicates that somehow the support stimulated microbial growth. There were also physiological changes in the formation of flavour compounds in immobilised yeasts. In addition, the maximum specific rate of cell growth (µmax) was increased in immobilised SY025 yeasts (0.73 h-1) compared to free yeasts (0.13 h-1 ). The same occurred with the SY001 strain; µmax values were 0.08 h-1 and 0.47 h-1 for free and immobilised ones, respectively. There was a reduction of µmax in the case of SY067: 0.14 h-1 for free and 0.093 h-1 for immobilised cells. It wasestimated that a gradient of substrate concentration occurred inside the support. Even so, the estimated values of the effectiveness factor (?) of the immobilisation matrix indicate that the process was not severely impacted by limitations in mass transfer. This was verified by micrographs of the interior of the support, which revealed that yeast cells were able to grow throughout the entire length of the particle. (AU)