Impact of malolactic fermentation on red wine color
The color of a red wine is an important sensory attribute that originates primarily from anthocyanins. However, development of stable red wine color is impacted by compounds such as p-coumaric acid, caffeic acid, catechin, and quercetin that are involved in copigmentation reactions as well as acetaldehyde and pyruvic acid. While it is known that yeast can alter the concentrations of some of these compounds, little is known regarding the impact malolactic bacteria may have on red wine color development. This project investigates the effect of the malolactic fermentation (MLF) on red wine color and the ability of malolactic bacteria to degrade compounds important to the development of stable red wine color. Pinot noir and Merlot wines produced in 2008 were stored at 13°C, sampled every three months for twelve months and analyzed for a number of different color parameters. Pinot noir wines that had undergone MLF, including wine produced with a simultaneous alcoholic and MLF, had the lowest color throughout storage. The pH adjusted control (pH 3.67) gave the highest color at each time point analyzed, and consistently had higher color than the control wine (pH 3.53). Pigmented polymer for the Pinot noir wines showed similar trends to the color analysis in that the control wines gave the highest values while the wines that had undergone MLF had lower values. Overall, pigmented polymer values increased overtime. For the Merlot, similar trends were noted as for the Pinot noir. Wines that underwent MLF had lower color values at each time point with the pH adjusted control giving the highest color. The control wines also had higher pigmented polymer values than wines that had undergone MLF with pigmented polymer values increasing over time. In addition to the analysis performed on 2008 wines, Pinot noir and Merlot wines were again produced in 2009. A portion of the Pinot noir and Merlot wines underwent a simultaneous alcoholic and malolactic fermentation (S. cerevisiae VQ15 + O. oeni VFO) while the remainder were only inoculated with S. cerevisiae VQ15. At dryness, all wines were pressed and sterile filtered (0.45 ?m). For both the Pinot noir and Merlot wines, three carboys were inoculated with either O. oeni strain VFO, Alpha (Lallemand), or VP- 41 (Lallemand) at approximately1 x 106 cfu/mL. The remaining carboys of wine were not inoculated with O. oeni. All wines were kept at 20°C. Some of the wine that had not undergone MLF was pH adjusted to the same final pH of wines that had completed MLF. Samples were taken before and after MLF for analysis and wines were sterile filtered, bottled, and stored at 13°C. Prior to MLF, all wines had very similar concentrations of acetaldehyde and pyruvic acid although wines that had undergone a simultaneous alcoholic and malolactic fermentation had lower acetaldehyde and pyruvic acid concentrations. In wines inoculated for MLF post alcoholic fermentation, acetaldehyde and pyruvic acid concentrations decreased during the course of the MLF. Strains VFO and Alpha more completely degraded acetaldehyde than VP41 while VFO degraded more pyruvic acid than did the other two strains tested. The same trends were also observed for the Merlot wines although lower initial concentrations of acetaldehyde were observed compared to the Pinot noir wines. These results show that all the O. oeni strains tested degraded both pyruvic acid and acetaldehyde during the MLF in both Pinot noir and Merlot wines including during a simultaneous fermentation. Future experiments aim to determine the mechanism by which red wine color is impacted by MLF. This will include investigating the role that bacterial degradation of acetaldehyde and pyruvic acid plays as well as whether adsorption of anthocyanins to ML bacteria cell walls is occurring. In addition, experiments varying the time when wines are inoculated for MLF may suggest the best time to inoculate for MLF in order to maximize color development and stability.