Interaction of pH, ethanol concentration and wine matrix on induction of malolactic fermentation with commercial ‘direct inoculation’ starter cultures

The effect of the interaction between pH (2.9-3.5), alcohol concentration (12.5-14.5% v/v) and wine matrix (Chardonnay, Grenache blend and Cabernet Sauvignon) on malolactic fermentation (MLF) induced by two commercially available directly inoculated Oenococcus oeni starter cultures was determined. Wine matrix had greatest impact on the rate of MLF, followed by pH and alcohol; the matrix effect was significantly modified by pH and bacterial culture. The rate of L-malic acid degradation by starter culture A was 3.3-61.3% in the Grenache blend and 3.3-4.5% in the Chardonnay wine compared to the Cabernet Sauvignon wine, for the pH/alcohol ranges 2.9/14.5 to 3.5/12.5. L-malic acid degradation rate by starter culture B was lower but less affected by wine matrix. The rate of L-malic acid degradation was closely related to culture viability, with a high rate of L-malic acid degradation being associated with bacterial growth and vice versa. Treatment of two wines in which MLF had previously failed to complete with various combinations of nutrients, carbon fining and heating, restored MLF, suggesting that a growth-limiting nutrient was absent and/or a toxic substance not related to pH, alcohol or SO2 concentration was present. This work demonstrated that the wine matrix may affect culture viability more significantly than either pH or alcohol concentration. A high rate of MLF was shown to depend on factors permitting bacterial growth, however, the two strains studied responded differently to the wine matrix, suggesting that malolactic bacteria vary in tolerance to various stresses in the wine environment.