Journal of the american society of brewing chemists vol:61 issue:4 pages:175-181
Yeast cell immobilization causes changes in several cellular physiological properties, resulting in severely unbalanced flavor profiles of beers produced in immobilized-cell systems. To explore the reasons for the altered production of flavor-active metabolites, such as volatile esters, the transcription rates of several genes were measured in free and immobilized cells. Expression profiles of the stress-related genes HSP12 and SSA3 revealed that cell immobilization exerted certain stresses on the yeast cells, possibly nutrient starvation and osmotic stress. However, yeast cells may also benefit from the protecting effect of the immobilization matrix, since without a protecting matrix, the free cells in the standard free-cell system showed a stronger induction of HSP12, a general stress marker, compared with the immobilized cells. In accordance with the lower stress gene expression levels, the immobilized cells accumulated trehalose as low as 26% of that of the standard free cells. The expression level of ATF1, encoding alcohol acetyl transferase 1, was significantly raised in the immobilized cells, resulting in a twofold increase of iso-amyl acetate formation. The activity of ATF1 is regulated by the protein kinases Sch9p and PKA, involved in stress responses and nutrient-sensing signaling pathways. The particular microenvironment created by cell immobilization thus possibly activates the cAMP/PKA/Sch9 pathway, resulting in an induction of ATF1 expression leading to enhanced ester concentrations in the final fermentation product.