Fermentation Symposium—Part I The Growth Process of Brewing Yeast and the Biotechnological Challenge.
Ryder, D.S. and Masschelein, C.A.
Immobilized brewing yeast cell technology provides
the opportunity to improve productivity and volumetric efficiency
compared with traditional, free cell, and batch fermentations.
Successful commercial application depends on optimizing the interrelated
factors of cell physiology, mass transfer, and immobilization procedures
to ensure sustained cellular activity without or with very limited yeast
growth. Fermentations, using wort as substrate, were undertaken with Saccharomyces uvarum cells immobilized in calcium alginate gel
beads. Packed bed and fluidized bed reactor designs were used. In
continuous mode, the packed bed fermentations demonstrated severe
limitations at the levels of fermentative power per unit yeast, amino
acid uptake, and formation of higher alcohols and esters, compared with
a free cell system. These aspects were more favorable when the fluidized
bed reactor was used in discontinuous mode. The results may be explained
to a degree by mass transfer limitation by substrate concentration
effects, oxygen tension, gel diffusion kinetics, and reactor design.
Interpretation of fundamental physiological data, however, indicates
that the biotechnological challenge lies not only with these factors. To
completely understand and overcome control exerted by growth-regulated
activities would prove attractive when brewing yeast cells operate at
low turnover rates. In this respect, cellular intermediate levels under
nonproliferating conditions are highlighted for key enzyme activity of
glycolytic, gluconeogenic, glycogenic, and glycogenolytic pathways.
Keywords: Amino acids, Brewer’s yeast,
Esters, Glycolysis, Higher alcohols, Immobilization