The Australian Wine Research Institute

Background and terminology

Traditionally malolactic fermentation (MLF) has been carried out post-primary fermentation, through either the addition of certain lactic acid bacteria or by natural or spontaneous fermentation. This is typically termed sequential MLF.

Another option for winemakers is co-inoculation, where lactic acid bacteria are added during alcoholic fermentation. Recent work has shown that the time for MLF to be completed or MLF efficiency can be improved via co-inoculation (Costello et al. 2019). The AWRI helpdesk takes many calls each year on the unpredictable nature of MLF, with these often being related to stuck or sluggish sequential MLFs which stretch out weeks or even months longer than expected. These wines can be significantly affected by spoilage yeasts or bacteria due to the lack of SO₂ present while waiting for MLF to be completed.

Application and helpful hints

Irrespective of the timing of MLF inoculation, the AWRI helpdesk team has found that preparation of a MLF starter culture using a protocol that acclimatises the bacteria to the harsh wine conditions provides the highest chance of a successful MLF. If using freeze-dried bacteria, suppliers should be consulted on choosing a malolactic bacteria strain that is most compatible with the fermentation yeast used.

The addition of freeze-dried bacteria to a primary fermentation during alcoholic fermentation, can be made at any stage. The 2018 Cabernet Sauvignon winemaking treatment trial used a protocol of co-inoculating with rehydrated freeze-dried bacteria 48 hours after primary fermentation had commenced in all but one of the treatments. The exception was the treatment where sequential MLF was conducted.