The Growth Process of Brewing Yeast and the Biotechnological Challenge

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.