Timing of Malolactic Fermentation
The malolactic conversion may occur spontaneously before the yeast fermentation, may occur during the yeast fermentation or may happen well after the yeast fermentation is completed. If the juice has a sufficiently high pH and is held at warm temperatures (above 18°C/64°F) and no sulfite is used, the wild lactic acid bacteria present on the grapes may initiate growth and the malolactic conversion before the yeast are able to start fermentation. The conversion of malate to lactate may be completed before the onset of the alcoholic fermentation. However early lactic acid bacterial activity may lead to arrest of the yeast fermentation. This is generally not due to depletion of nutrients but to the production of acetic acid and fatty acids that are inhibitory to yeast. These acids generally are not inhibitory until ethanol concentration reaches a high level so the impact of the early malolactic conversion may not be seen until the last phase of the yeast fermentation.
The malolactic conversion may also initiate spontaneously during the yeast fermentation. This usually happens when yeast metabolic activity has raised the tank temperature to an inhibitory level (above 40°C/104°F) and the alcoholic fermentation is arrested. The bacteria are more heat tolerant and can commence growth at this time. Their growth is encouraged by attempts to encourage the arrested yeast: nutrient addition and aeration. Often under these circumstances the yeast will struggle reinitiating and completing fermentation and may remain arrested due to the production of inhibitory acids by the bacteria.
It is more common for a spontaneous malolactic fermentation to occur after the yeast fermentation is completed. This is due in large part to the need to dissipate the yeast-produced sulfite and, in the absence of nutrient addition, the need for yeast cell lysis to replenish the nutritional content of the juice.
Inoculation can be used to dictate the timing of the malolactic conversion. Nutrients can be added to stimulate the malolactic conversion, but if this has been done then the benefit of the malolactic conversion in consumption of nutrients that could be otherwise consumed by spoilage yeast and bacteria may be compromised.