Scott Labs Malolactic Bacteria Choosing Guide

Last Updated: 5/2024

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

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

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.

Alcohol Level

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.

SO₂

In all forms, SO₂ can be problematic to 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 favorable conditions are: free SO₂ <10 ppm, total SO₂<45 ppm, and molecular SO₂ <0.3 ppm.