Cell immobilisation is the physical restriction of cells in a delimited region by means of physical and chemical approaches. It usually comprises a solid support containing cell biomass. In brewing fermentations, yeast cell immobilisation was widely explored during the 1970s to the 90s, with the expectation that immobilised systems would revolutionise the brewing industry. The most studied immobilisation method has been the attachment to a surface and entrapment within a porous solid. Some industrial applications were developed, but the flavour profile of the product rarely matched that produced by batch fermentation. Numerous factors are important in immobilised yeast systems and its successful industrial implementation. Although cell immobilisation results in many advantages, such as high biomass loading and ease of cell reuse, there are drawbacks including physiological changes and mass transfer limitations. Therefore, in order to design a feasible brewing fermentation process using immobilised yeast cells, the solid support, immobilisation method and the bioreactor system require to be properly developed. In this review, yeast cell immobilisation technology in brewing is considered together with methods of immobilisation with the associated advantages and drawbacks. Physiological and metabolic alterations in yeast are also explored and industrial applications are highlighted. It is suggested that immobilisation technology has new opportunities as the market is increasingly open to novel flavours and styles.
There is an ever-increasing demand for reduction of unit operations and a growing interest in the physiology of yeasts used in beer fermentation. In this context, cell immobilization is an interesting alternative, since it reduces steps to separate biomass from fermented broth. Yet, physiological alterations in yeast metabolism caused by immobilization are still to be fully described. Thus, the main objective of this work was to evaluate the physiology of three brewer's S. cerevisiae yeast strains (SY025, SY067 and SY001) immobilized on a porous cellulose-based support. Batch fermentations in malt extract 12 degree P were carried out for all strains both in free and immobilized forms in order to compare kinetic parameters obtained from distinct process conditions. Mathematical modeling was performed following two viewpoints: modeling of fermentation kinetics by parameter estimation from experimental data and application of a reaction-diffusion model for estimation of substrate concentration gradient inside the immobilization support. Moreover, fermentations with different initial substrate and biomass concentrations were carried out using strain SY025, aiming to evaluate their influence over flavor compounds, using statistical models. Compared to free cells, immobilized yeasts showed both higher glycerol yield (SY025, 40%; SY067, 53%; SY001, 19%) and biomass yield in the system (SY025, 67%; SY067, 78%; SY001, 56%). On the other hand, free cells presented higher ethanol yields when compared to immobilized ones (SY025, 9%; SY067, 9%; and SY001, 13%). According to the model developed, a substrate gradient inside the support was predicted, but with low mass transfer limitations.
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 cellulosebased 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 was estimated that a gradient of substrate concentration occurred inside the support. Even so, the estimated values of the effectiveness factor (η) 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 thro...
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