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Background and Aims Diacetyl accumulation in wine, which has undergone malolactic fermentation by Oenococcus oeni, is often associated with aromatic off-flavours. Characterisation of the diacetyl-related metabolic pathway helps to explain bacterial diacetyl formation during winemaking. The present study describes the time-dependent formation of diacetyl during the vinification process after simultaneous, induced malolactic fermentation by freeze-dried O. oeni on the basis of gene-expression analysis of the citrate- and pyruvate-derived pathways. Methods and Results After simultaneously induced malolactic fermentation in Pinot Blanc by O. oeni, the dynamics of diacetyl formation were compared with citrate consumption and gene-expression of the diacetyl-related metabolic pathways. Diacetyl concentration showed two maxima: the first increase was primarily influenced by the activity of Saccharomyces cerevisiae; however, the second increase was induced only by O. oeni and correlates with bacterial citrate degradation. Expression of the alsS gene showed two significant responses; however, only the second response was affected by the citrate-associated genes maeP and citE. Additionally, alsD and butA2 were found to be continuously underexpressed during the winemaking process. Conclusions Taken together, we suggest that diacetyl accumulation during the vinification process by O. oeni is affected by citrate fermentation. The diacetyl-related alsS gene, however, is also overexpressed independently by other substrates, which may also increase the intracellular pyruvate level resulting in diacetyl formation. Significance of the Study Characterisation of the time-dependent diacetyl accumulation and its degradation during the vinification process is essential for the development of strategies that focus on the suppression of the diacetyl concentration below the sensory threshold. |
