I have previously discussed the malolactic fermentation (MLF) of wine both as a 'natural" occurrence and being induced through inoculation. In that treatment I discussed MLF as occurring after alcoholic fermentation (AF) in a sequential process. The gap between the conclusion of AF and the initiation of MLF is a risky period as no sulfur can be applied to the wine due to its toxic effect on the bacteria that facilitate the latter. During this time, the wine is exposed to the potential of oxidation and attack by spoilage organisms. While inoculation reduces the duration of the gap, it does not completely eliminate the risk.
Under the "right" circumstances, the lactic acid bacteria (LAB) can themselves pose a risk to wine quality. In a normal AF, all of the fermented sugars are consumed by the yeasts, with residual sugars coming in at < 2g/l. Also, the fermented sugars will have been fully consumed prior to the full depletion of the malic acid. In cases of stuck or sluggish fermentations, however, the malic acid may be fully depleted while fermentable sugars are still available. LABs are heterofermentative and while they preferentially consume malic and citric acids, they "possess the metabolic machinery to utilize glucose and fructose for the production of energy." The by-product of such energy production is acetic acid, an undesirable characteristic in wine.
Co-inocualtion, an emerging approach, has been employed as a strategy to minimize the identified risks. The figure below shows MLF considerations and the options in the inoculation space.
Co-inoculation involves introducing selected LAB into the wine within 12 to 48 hours of its inoculation with AF yeasts. If the LAB is introduced earlier than 12 hours post yeast inoculation, 99% of the viable cells would be killed off by free sulfites. In the early stages of AF, however, the yeasts produce compounds which detoxify the free sulfites, rendering the environment more hospitable for the LAB.
Advantages of Co-inoculation
Following are some of the benefits associated with co-inoculation:
- Better control over the winemaking process in terms of time management
- Shorter AF-MLF cycle
- Bacteria are introduced into a low-ethanol, high-nutrient environment with the lowest pH the wine will probably experience
- Bacterial activity is suppressed during active fermentation but cells acclimatize their membranes to the rising alcohol during this time
- Microbiological activity of both the yeasts and the bacteria work to limit development of contaminating organisms
- As yeasts begin to die at the end of AF, they release nutrients into the wine just as malic bacterial production begins to transition from lag to log phase
- When MLF occurs under the reductive conditions of AF, lower levels of diacetyl and higher levels of fruity esters manifest in the wine.
