Analysis of the effect of inoculum characteristics on the first stages of a growing yeast population in beer fermentations by means of an individual-based model

Abstract

The yeast Saccharomyces cerevisiae has a limited replicative lifespan. The cell mass at division is partitioned unequally between a larger, old parent cell and a smaller, new daughter cell. Industrial beer fermentations maintain and reuse yeast. At the end of fermentation a portion of the yeast is 'cropped' from the vessel for 'serial repitching'. Harvesting yeast may select a population with an imbalance of young and aged individuals, but the output of any bioprocess is dependent on the physiology of each single cell in the population. Unlike continuous models, individual-based modelling is an approach that considers each microbe as an individual, a unique and discrete entity, with characteristics that change throughout its life. The aim of this contribution is to explore, by means of individual-based simulations, the effects of inoculum size and cell genealogical age on the dynamics of virtual yeast fermentation, focussing on: (1) the first stages of population growth, (2) the mean biomass evolution of the population, (3) the rate of glucose uptake and ethanol production, and (4) the biomass and genealogical age distributions. The ultimate goal is to integrate these results in order to make progress in the understanding of the composition of yeast populations and their temporal evolution in beer fermentations. Simulation results show that there is a clear influence of these initial features of the inocula on the subsequent growth dynamics. By contrasting both the individual and global properties of yeast cells and populations, we gain insight into the interrelation between these two types of data, which helps us to deal with the macroscopic behaviour observed in experimental research.

The effect of pitching rate on fermentation, maturation and flavour compounds of beer produced on an industrial scale

“…Experiments conducted by Ginovart et al . presented a stimulating effect of young yeast cells on biochemical changes in fermenting wort and maturing beer.…”

Bioconversion of degraded husked sorghum grains to ethanol

“…This was because, while S. cerevisiae cells are only able to efficiently ferment glucose, P. stipitis can ferment both glucose and xylose. However, S. cerevisiae cells exhibit tolerance for high alcohol concentration during fermentation relative to P. stipitis [14].…”

“…However, Pichia (Scheffersomyces) stipitis are able to efficiently ferment both xylose and glucose [14]. Nevertheless, while S. cerevisiae cells exhibit tolerance for high alcohol concentration during fermentation, P. stipitis growth is restricted at alcohol levels beyond 33 g/L [14,24]. Thomas and Ingledew [25] suggested a minimum of 150 mg/L of FAN is required for efficient yeast metabolism during fermentation.…”

Sensitivity Analysis And Individual-Based Models In The Study Of Yeast Populations

“…The mission of modelling the behaviour of a single yeast cell is one of the cores of this approach. Some interesting qualitative results have already been achieved with its use in the study of fermentation profiles, small inocula dynamics and the lag phase, among others (Prats et al 2010, Ginovart et al 2011. Nevertheless, in order to improve its predictive capabilities and further its development, a deeper understanding of how the variation of the output of the model can be apportioned, qualitatively or quantitatively, to different sources of variation must be investigated.…”