Surface-engineered Saccharomyces cerevisiae displaying α-acetolactate decarboxylase from Acetobacter aceti ssp xylinum

To convert alpha-acetolactate into acetoin by an alpha-acetolactate decarboxylase (ALDC) to prevent its conversion into diacetyl that gives beer an unfavourable buttery flavour. We constructed a whole Saccharomyces cerevisiae cell catalyst with a truncated active ALDC from Acetobacter aceti ssp xylinum attached to the cell wall using the C-terminal anchoring domain of alpha-agglutinin. ALDC variants in which 43 and 69 N-terminal residues were absent performed equally well and had significantly decreased amounts of diacetyl during fermentation. With these cells, the highest concentrations of diacetyl observed during fermentation were 30 % less than those in wort fermented with control yeasts displaying only the anchoring domain and, unlike the control, virtually no diacetyl was present in wort after 7 days of fermentation. Since modification of yeasts with ALDC variants did not affect their fermentation performance, the display of alpha-acetolactate decarboxylase activity is an effective approach to decrease the formation of diacetyl during beer fermentation.