5 Steps to a Succesful Malolactic Fermentation (MLF)02/22/2012
Malolactic bacteria have a reputation as being decidedly more difficult to work with than yeast, however many of the problems often encountered stem from a lack of understanding the appropriate conditions necessary for the bacteria to successfully complete its job. One reason this might be the case is that there really isn’t a single variable that can be controlled to ensure success, à la: “make sure you don’t sulfite until after the MLF has completed and all will be well”. In fact, the real answer to better being able to successfully complete an MLF is a bit more complex than that and actually lies in understanding the synergistic relationship between the following five elements: A) alcohol (ethanol), B) temperature, C) pH, D) SO2 (sulfite), and E) nutrients and lees management:
A) Alcohol (ethanol)
Alcohol, at the levels desired in most finished wines (usually around 12%–14%) is in itself toxic to most organisms, including ML bacteria. However, unlike most other organisms, with the proper nutrition and environmental conditions, ML bacteria can adapt to successfully survive in this medium.
Yet, an important factor to note is that higher temperatures aggravate this alcohol toxicity, and even ML bacteria adapted to the wine will start to feel the effects of alcohol toxicity if the wine’s temperatures become too elevated. On the other hand, if the wine’s temperature becomes too cool, then the ML bacteria stop reproducing and the secondary fermentation will slow and potentially shut-down altogether (until the wine warms-up again). Therefore, the answer to “what is the ideal temperature to conduct an MLF” lies in a compromise:
Red wines: have an optimum temperature for a favourable MLF of around 70º F (20º C), which is cool enough to limit alcohol toxicity and yet warm enough to maintain full activity.
White wines: are often fermented at the same temperatures as the reds, but some strains will allow the winemaker to work at the even cooler temperatures of around 58º F (15º C). This might make it easier to maintain the cooler handling conditions often desired for white winemaking, but it will cause the process to work at a slower pace and therefore the fermentation will take longer to complete.
Note that if the temperature of the wine will be falling colder than the recommended range before the MLF has finished (for example: it is not temperature controlled and the cellar temperature drops during the winter), it is important that the ML bacteria has a chance to at least establish itself as the dominant strain in the wine at the recommended temperatures before the wine gets cold. In other words, having one or two weeks at 70º F and then having the temperature slowly drop is better than trying to get the MLF under way at 57º–60ºF right from the start.
The pH of the wine and how it affects ML bacteria is actually one of the most straightforward of the five elements. Basically, if the wine has a pH that is too low, it will exacerbate the already harsh conditions of the wine and it will inhibit the bacteria’s survival. However, if the pH of the wine is too high, then while the bacteria have an easier time thriving, the wine also becomes more susceptible to a greater number of spoilage bacteria. So, the ideal pH range recommended for a wine undergoing an MLF is therefore based on a compromise between ideal sanitary conditions on the low end and levels that are high enough to facilitate growth and survival on the upper end, and this usually equates to a range of between 3.1 pH* and 3.6 pH.
Note that these thresholds are strain dependent and therefore may differ slightly between different cultures. Some strains may indeed be able to work at a pH of 3.0/3.1, albeit not as comfortably as it would at a pH of 3.2.
D) SO2 :
Most winemakers know that a high “free” SO2 level can inhibit ML bacteria, and that if you want to carry out an MLF then you usually don’t sulfite the wine until after the fermentation has completed. However, it is crucial to realize that “bound” SO2 also has a negative affect on the bacteria. This is because while “bound” SO2 is 5 to 10 times less active than “free” SO2 , at high enough levels it too can hinder bacterial growth. So, if you want to do an MLF on a particular wine not only do you need to be aware of how much “free” SO2 is in the wine, you also need to keep tabs on the portion that is “bound” as well. These two portions combined are referred to as “total” SO2 and the following SO2 levels are recommended by Lallemand as being favourable MLF conditions: 0–10ppm “free”, and 0–30ppm “total”.
Note that in general, if you crush and add a single 50ppm “total” SO2 addition up front, by the time the fermentation is over you will usually have around 20–25ppm as “total” and 0–10ppm as “free”. In other words, you should be well within the recommended ranges. (Keep in mind, however, that this amount will vary with different must compositions and fermentation temperatures).
E) Nutrients and Lees Management:
While it is true that ML bacteria convert malic acid into lactic acid, they do need a nice, rounded set of nutrients in order to remain healthy throughout the entire fermentation process. Winemakers provide this nutrition by first adding nutrients to the hydration water, and then by maintaining a small amount of leftover yeast (“light lees”) in the wine for the bacteria to feed off during the fermentation. Let’s take a closer look at each of these steps.
Adding nutrients during hydration
Recent research from Lallemand has shown that, just as we now know to use Go-Ferm during the yeast’s hydration, ML bacteria also benefit from the use of nutrients during the hydration process. Therefore we recommend adding Acti-ML to the hydration water in order to ensure that our bacteria get the best start possible. (Complete instructions for doing this will be covered a little later.)
Nutrients in the lees
In addition to the Acti-ML addition during hydration, a major source of nutrients is found in the spent yeast at the bottom of the vessel, i.e.: the “lees”. Beginning towards the end of the alcoholic fermentation and continuing on until the wine is racked from the lees, the spent yeast cells begin to autolyse (or burst open on their own) and release their innards into the wine. This not only releases a series of polysaccharides which gives the wine a greater mouthfeel and complexity, it also contains a rich variety of nutrients and minerals that the bacteria can use as a complete nutrient set, and for the majority of most fermentations this is all that they need*. The only trick is that, over time, both the lees and the bacteria can settle-out and compact on the bottom of the vessel, effectively burying the bacteria alive and lowering the ratio of working bacteria to wine volume. This does neither the winemaker, nor the bacteria any good. So, in order to counteract this phenomenon, the lees (along with the bacteria) need to be stirred back up into the wine 1–2 times a week until the MLF comes to a completion. This process is called “bâtonnage”, in French (for the stick, or “baton”, used to stir) and will ensure that the highest percentage of food and working bacteria will remain suspended and in contact with the wine at all times. Obviously, strict sanitation and oxygen management (i.e.: flushing with inert gas) are a must during the stirring period.
*Note that there are indeed times when the lees may not have enough of the nutrients that the ML bacteria need during fermentation and the winemaker will need to add a nutrient-set that has been expressly designed for them (“Acti-ML”, for example). Yet, this should only be done if you know that the situation warrants it up-front, for example:
• You know from past experience or through lab analyses that the vineyard/must has some nutritional deficiencies.
• You have started the ML fermentation and all of the other parameters are within the correct levels, yet the fermentation is very slow or stops. In this case we can add another dose of Acti-ML to the wine at a rate of: .75 – 1.0 grams per gallon (possibly along with some yeast hulls as well).
• You had to rack off of the lees for some reason (maybe a H2S problem) and you need a new, clean source of nutrients for the ML bacteria.
These are all good times to add the ML food. However, the reason why one doesn’t just add it arbitrarily at it the beginning of every ML fermentation is because while the goal is to have enough nutrients for the ML bacteria to comfortably finish their fermentation, the winemaker ideally wants the wine to be as nutrient depleted as possible once they’re done. Any nutrient source left over after the ML bacteria have finished is available to any spoilage organism that may possibly find its way into the wine. A nutrient-desert is a great insurance policy, and while it may not be completely practical for all winemaking styles, it still is worth keeping in mind when deciding about nutrient additions for an MLF
A final nutritional note: ML bacteria do not take up DAP; so do not use it as a part of an ML nutritional regimen. The DAP will only be available to potential spoilage organisms, as well as give the wine a salty taste at high enough concentrations!
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