Sensory, what does malolactic fermentation highlight?

Abstract

Lactic acid bacteria (LAB) drive malolactic fermentation (MLF) which modulates the sensorial profile of wines through the decarboxylation of malic acid into lactic acid. Beyond deacidification and the associated softer flavors, LAB also synthesize secondary metabolites, including diacetyl, esters and sulphur compounds, that contribute to wine bouquet. However, the organoleptic role of MLF remains controversial, as it depends on the wine matrix, winemaking processes and LAB strains. This study evaluated the impact of various Oenococcus oeni strains through two pilot scale assays in winery. First, seven commercial strains and indigenous LAB conducted sequential MLF in merlot. Results demonstrated that MLF influenced color, aromatic composition and sensory profiles of wines. After MLF, the wines were perceived brighter with ruby shades and higher concentrations of diacetyl and ethyl lactate, while preserving the ester content. Comparisons between strains revealed that indigenous bacteria produced significantly higher levels of diacetyl, ethyl lactate and high-alcohol acetates in accordance with the sensory profiling. These wines were characterized by amylic and buttery notes, which masked fruity notes. While differences between commercial strains were more subtle, variabilities in ester and diacetyl production led to distinct wines though their fruit intensity. The second assay compared two O. oeni strains along with two inoculation strategies in merlot: co-inoculation (24 h after yeast addition) and sequential inoculation (post-alcoholic fermentation). Inoculation timing proved to be a dominant factor after two months of aging. Sequential inoculation favoured the synthesis of substituted or hydroxylated ethyl esters, acting as fruit enhancers and resulting in wines with jammy and pastry notes. Conversely, co-inoculated wines were associated with brightness, fresh fruit, floral and vegetal notes. In this inoculation modality, a strain-specific effect was more pronounced, differentiating wines based on their concentrations of C13-norisoprenoids and floral aromas versus fatty acid ethyl esters, diacetyl, ethyl lactate and toasted notes. Ours studies show that LAB are not just tools for malic acid degradation but are essential for sensory modulation and complement the action of yeasts. The diversity of selected strains and the strategic timing of inoculation offer winemakers precise levers to fine-tune the analytical and organoleptic profiles of their red wines.