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Reactivation of Exhausted Yeast Physiological Activity

Mario Novak1, Filip Cingesar1, Zvonko Petrović2, Philippe Cario3, Božidar Šantek1

1Department of Biochemical Engineering, Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6/IV, HR-10000 Zagreb, Croatia
2Ireks Aroma d.o.o., Radnička cesta 37, HR-10000 Zagreb, Croatia
3AEB SPA, Via V. Arici 104 - S.Polo - 25134 BRESCIA, Italy

Aims:

For lager beer production selected strains of yeast Saccaharomyces cerevisiae were used. During fermentation they have to be able to convert wort nutrients into ethanol, carbon dioxide and appropriate level of other metabolites as well as to reduce diacetyl. Furthermore, they should also be able to flocculate and precipitate after fermentation as well as to be physiologically stable for multiple re-pitching processes with guarantee that always the same beer flavour will be obtained. The aim of this research was to prolong the yeast recirculation (re-pitching) process without loosing beer flavour and stability. Therefore, different nutrients mixtures were used for exhausted yeast physiological reactivation.

Methods:

In this investigation lager yeast S. cerevisiae (5th generation that was stored for 5 days at + 4 oC) was used for wort fermentation obtained by infusion process. Fermentation experiments were carried out in 5 L tank bioreactor and they were monitored by withdrawing samples in which yeast characteristics (e.g. viability, morphology, physiological activity and concentration) and wort metabolites concentrations were determined. Changes of wort metabolites concentration during fermentations were detected by HPLC. In order to reactivate yeast physiological activity different nutrients mixtures (consisting of amino-acids, peptides, proteins, vitamins and minerals) were prepared and used to insure that wort fermentation would be successfully performed.

Results:

Results of this research clearly show that the use of selected inorganic and organic nutrients mixtures has considerable impact on the exhausted yeast physiological activity. Addition of both nutrients mixtures insures that fermentation process will be successfully completed compared to the wort fermentation without nutrients mixture. Furthermore, the use of organic nutrients mixture resulted in the additional increase of yeast concentration (cca 5 • 106 CFU/mL) and fermentation activity compared to the wort fermentation with inorganic nutrients mixture. The use of organic nutrients mixture in the declined fermentation process resulted in its reactivation (in only a few hours) and successful fermentation finish without significant impact on the beer flavour. Reactivated yeast was also capable to carry out a few more fermentations without considerable impact on the beer flavour and stability.

Conclusions:

On the basis of these results it is clear that the use of organic nutrients mixture can significantly improve exhausted yeast physiological activity as well as to reactivate declined fermentation process. Furthermore, obtained beer flavour and stability was not considerably affected by yeast reactivation process.