Malolactic conversion (MLC) is a key process in winemaking that generally follows alcoholic fermentation and converts the malic acid in wine into lactic acid. This conversion is led by lactic acid bacteria (LAB), the primary one being Oenococcus oeni.
MLC is a pivotal way to reduce the wine’s overall acidity, as it degrades acids and the newly converted lactic acid is perceived as softer and less sharp than malic acid. Additionally, MLC enhances the wine's mouthfeel and flavour profile.
UNDERSTANDING MALOLACTIC CONVERSION
Natural vs. Induced MLC
Malolactic conversion can happen spontaneously under suitable conditions or be encouraged through specific winemaking practices. In natural cases, indigenous lactic acid bacteria present in the winery environment initiate the conversion. Alternatively, winemakers can add cultures of lactic acid bacteria to ensure the process occurs consistently and with controlled outcomes.
Effects of Malolactic Conversion on Wine
The impact of MLC on wine is multifaceted:
Reduction of Total Acidity: MLC results in a decrease in the total acidity of the wine. This reduction occurs as some acid molecules are transformed during the conversion process.
Altered Acid Perception: Malic acid, known for its sharp taste, is converted into lactic acid, which has a softer taste and creates a sensation of a smoother, rounder palate.
Enhanced Body: Wines undergoing MLC typically exhibit a fuller body. Lactic acid imparts a creamier texture compared to the lean and refreshing sensation associated with malic acid.
Change in Fruit Aromas: The fruit profile of the wine shifts from fresh, crisp notes like green apple and pear to riper, more tropical flavours such as mango and melon. LABs also reduce or even completely remove the green-grassy aromas in wine.
Buttery Flavour: One of the most notorious impacts of MLC on wine flavour is the production of diacetyl, which imparts a distinctive buttery, creamy or popcorn character, enhancing its complexity.
Increased Microbial Stability: Lactic acid is more resistant to microbial spoilage than malic acid, therefore MLC makes the wine more stable. However, in some cases, the rise in pH to certain levels (>3.5) caused by MLC can increase the risk of bacterial spoilage, requiring careful management.
Grape Varieties and Styles suitable for MLC
Malolactic conversion (MLC) might be encouraged or avoided, depending on the wine style or grape variety nature. Most red wines are encouraged to undergo MLC. While in white wines or sparkling wine bases, it is applied more selectively.
For example, in red wines made from Cabernet Sauvignon or Merlot, MLC can reduce the vegetal character and make the tannins taste less astringent. Grapes that have a neutral aromatic character tend to combine better with malolactic fermentation. For example, Chardonnay and Chenin Blanc wines aspiring for a buttery, full-bodied profile, can profit from extra complexity and a softer acidity.
On the other hand, wines from aromatic grape varieties generally do not undergo MLC, to avoid the loss of fresh fruit or floral notes as well as their refreshing acidity. For example, in Riesling, MLC is mostly avoided to preserve the crisp, vibrant acidity that defines this variety. Also, for Sauvignon Blanc wines MLC is often skipped to maintain the sharp, zesty character which is the signature of this grape.
Controlling Malolactic Conversion
Winemakers can either encourage or inhibit MLC depending on the style of wine they aim to produce. Techniques to prevent MLC include:
Temperature Control: Storing wine at temperatures below 12°C will inhibit lactic acid bacteria action.
Acidity Management: Wines with a pH lower than 3 will rarely undergo malolactic conversion.
Sulphur Dioxide Levels: Keeping sulphur dioxide concentrations above 50 mg/L during and after fermentation is likely to avoid malolactic conversion.
Lees Removal: Transferring the wine off its lees immediately after alcoholic fermentation removes lactic acid bacteria and its nutrients.
Sterile Filtration: Filtering the wine in a sterile manner will remove all microorganisms, including acid-lactic bacteria.
Promoting Malolactic Conversion
Conversely, to promote MLC, winemakers might:
Adjust pH Levels: Ensuring that the wine's pH is above 3.3 encourages lactic acid bacteria activity.
Reduce Addition of Sulphur: Minimising or completely avoiding the addition of sulphur dioxide during and after alcoholic fermentation.
Nutrient Management: Keeping the wine on its lees to provide nutrients for lactic acid bacteria.
Addition of LAB: Inoculating the wine with lactic acid bacteria (LAB).
Conclusion
Malolactic conversion is a definitive key process in winemaking that not only affects the acidity and flavour profile of wine but also its overall sensory characteristics and stability. However, the outcomes are not always desired, especially when wines are aimed to showcase their varietal expression. Therefore, through careful management of processes, winemakers can craft wines that meet specific stylistic and quality goals, whether aiming for a crisp and vibrant Riesling or a smooth and full-bodied Chardonnay.
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