Introduction

In the intricate world of winemaking, stuck malolactic fermentations (MLFs) can pose significant hurdles for winemakers. However, fear not! In this comprehensive guide, we address the common challenges encountered during stuck MLFs and provide practical solutions to overcome them. From low cell counts to physiological conditions, low amino nitrogen to phenolic inhibition, and even pesticide residues, we explore each issue to equip winemakers with the knowledge and techniques necessary to achieve successful MLFs.

Six Common Problems for Winemakers

1.            Stuck MLF due to Low Cell Count: One of the primary causes of stuck MLF is a low cell count of Oenococcus oeni, the MLF bacteria. To tackle this issue, it is crucial to measure the cell count of the wine through microbiology laboratory testing. If the cell count is low, it is essential to investigate inhibitory compounds. The solution lies in checking the wine conditions, removing inhibitory factors, and (re)inoculating with a suitable MLF strain. Additionally, dosing with Bactiv-Aid can aid the health of the wine bacteria population.

2.            Physiological Conditions of Wine - pH, Temperature, T-SO2, and % Alcohol: MLF bacteria require specific physiological conditions to thrive and metabolize. Parameters such as wine pH, total SO2, temperature, and alcohol percentage play a vital role in MLF success. Diagnosing these conditions can be easily done through simple wine chemical tests. By cross-referencing the tested parameters with the employed MLF strain, winemakers can determine the suitability. Selecting the appropriate MLF strain and considering adjustments, such as removing excess SO2 or blending, can help resolve issues arising from unfavorable physiological conditions.

3.            Stuck MLF due to Low Amino Nitrogen: A population of MLF bacteria requires an adequate supply of Amino Nitrogen to implant and complete MLF successfully. Testing the concentration of Amino N in the problematic wine using enzymatic tests is crucial for diagnosis. Low Amino N levels in wine during MLF are often a result of low levels in the starting fruit. Using yeast strains with high nitrogen demands can exacerbate the issue. The solution lies in treating the wine with a specific nutrient for wine bacteria, such as Bactiv-Aid, which provides the necessary Amino Nitrogen to facilitate MLF completion.

4.            Stuck MLF due to Medium-Chain Free Fatty Acids: Medium-Chain Fatty Acids (MCFA) found in wines, specifically C8, C10, and C12 MCFA, can be toxic to wine bacteria, leading to stuck MLF. Diagnosis of MCFA presence requires advanced techniques such as HPLC or knowledge of the yeast strain used during primary fermentation. Winemaking conditions and stress response of Saccharomyces cerevisiae strains play a role in MCFA production. Treating a wine with stuck MLF due to MCFA can be achieved by using Bactiv-Aid, which binds these compounds. Avoiding yeast strains known for producing high levels of MCFA can also help prevent the issue.

5.            Stuck MLF due to Phenolic Compounds: Certain tannins and phenolic compounds naturally present in grapes and wine can be toxic to MLF bacteria. Advanced tannin assays and sensory examination aid in diagnosing the concentrations of specific phenols in a given wine. High levels of extraction and certain grape varieties can contribute to toxic concentrations. Fining the problematic wine with  PVPP can reduce the concentration of phenols. Co-inoculation is an effective preventive measure to minimize the risk of stuck MLF due to phenolic inhibition.

6.            Stuck MLF due to Pesticide Residues: Pesticides used in viticulture can sometimes find their way into wine, potentially inhibiting MLF due to their toxicity. Diagnosing pesticide residues requires specialized laboratory testing. Preventing the issue is best achieved by ensuring viticulturists respect pesticide dosage and withholding periods. In cases where pesticide residues are present, fining the wine with active carbon has shown to be effective in reducing residue levels.

Conclusion

Troubleshooting stuck MLFs requires a systematic approach and an understanding of the various challenges that may arise. By addressing low cell counts, physiological conditions, low amino nitrogen, medium-chain free fatty acids, phenolic inhibition, and pesticide residues, winemakers can navigate through these obstacles and achieve successful MLFs. Armed with the knowledge and solutions provided in this guide, winemakers can troubleshoot like professionals and elevate their winemaking skills to new heights.