Fermentation restart protocol
Problems regarding malolactic fermentation (MLF) in wine can have different origins:
- Competition from residual yeasts.
- Wine toxicity: the presence of inhibiting compounds (ethanol, SO2, medium-chain fatty acids).
- Bacterial deficiency.
- Low level of nutrients necessary for the bacteria.
For each of these situations, there is a specific protocol:
As temperature increases, the toxic effects of ethanol on bacteria also raises, along with increased risk of bacterial spoilage and volatile acidity. It is especially important to keep an ideal temperature range during inoculation or preparation of the bacteria culture as bacteria is in its most sensitive stage during growth phase.
Both free SO2 and total SO2 have an inhibitory effect on bacterial growth, and it is essential to keep the concentration of SO2 low for MLF fermentation. As the molecular SO2 concentration increases when pH decreases, low pH is also unfavourable for bacterial growth, especially when pH is below 3.1.
Difficult MLF Conditions
Under harsh MLF condition, the availability of certain nutritional components can pose a significant impact on a smooth completion of fermentation. Oenococccus Oeni., the most common species of LAB used for MLF, has diverse requirement of nutrients, including organic source of nitrogen (amino acids and peptides), vitamins and trace elements (minerals). Inorganic form of nitrogen such as DAP can not be utilised by LAB bacteria.
Furthermore, although commercially available frozen bacteria packs are widely adopted in wineries due to their ease of application, especially for MLF in barrels, they are not the best strategy to adopt for wines with difficult MLF condition. Under such circumstances, it is advised to prepare a MLF starter culture and inoculate the wine in tank prior to barrelling down.