Scott Labs Malolactic Bacteria Choosing Guide
Last Updated: 7/2021
Applies to: Winemakers looking for assistance in selecting and comparing Scott Labs malolactic bacteria and nutrients. This guide contains a downloadable pdf choosing chart with information on each bacteria strain and nutrient offered.Download
Selecting Bacteria for Success
Understanding the Impact of Wine Conditions on Malolactic Fermentation
The success of malolactic fermentation (MLF) can be largely attributed to four parameters: pH, temperature, ethanol, and SO2. These parameters cannot be viewed independently since they have a compounding effect on the growth and metabolism of malolactic bacteria. For example, a wine with low pH and high SO2 will be more antagonistic to bacteria than low pH alone.
Sensory contributions of bacteria are important (see bacteria descriptions on the following pages for more information). However, the selected bacteria strain must be compatible with specific wine conditions:
pH affects the rate at which bacteria will consume malic acid. The optimal pH for MLF is >3.5 and when pH is lower, MLF can be inhibited. Additionally, pH impacts what forms of SO2 are present in wine. The lower the pH, the more molecular (antimicrobial) SO2 is present which adversely affects ML bacteria.
Temperature impacts both the growth rate of bacteria and the malic acid degradation rate. The temperature range ML bacteria can withstand is highly influenced by alcohol concentration. The higher the alcohol in the wine, the lower the MLF temperature should be. The ideal temperature (during sequential inoculation) is 20°C (68°F). Temperatures more than 25°C (77°F) can be lethal, while temperatures <10°C (50°F) can inhibit MLF.
Just like with yeast, ethanol can destabilize the bacteria cell membrane and bacteria strains vary in their ability to tolerate ethanol. Ensure the chosen bacteria strain can tolerate the alcohol content of the wine.
In all forms, SO2 can be problematic to malolactic bacteria. It is antimicrobial, especially at lower pHs, and can cause varying levels of damage to bacteria, up to and including cell death. Free SO2 is inhibitory to bacteria, but bound SO2 is also a problem. SO2 can be loosely bound to acetaldehyde which bacteria can consume, thereby releasing and increasing free SO2. For that reason, it is always important to measure both free and total SO2 prior to adding bacteria. Different bacteria strains have different tolerances to SO2, though optimal conditions are: free SO2 <10 ppm, total SO2<45 ppm, and molecular SO2 <0.3 ppm.