How to conduct a Malolactic fermentation (MLF)


1)  Garbage in garbage out! Get the must dialed-in at crush, so that the subsequent wine will be in good shape post alcoholic fermentation for receiving the ML inoculation. A clean, healthy alcoholic fermentation means your ML bacteria will have an easier time getting started and finishing their job when it’s their turn to work in the wine:

  • Clean-out any moldy or raisined clusters (the mold makes toxins that can inhibit both yeast and ML bacteria, raisins will reconstitute in the must, boost the ºBrix, and lead to higher finished alcohol levels).

  • Get your sugars and ph/TA% in line so that the finished wine will not have a final alcohol above 15% (around 14% is better), and so that the pH will not be lower than 3.1/3.2 (3.2 is better). 
  • Make sure that the initial SO2  addition is around 50ppm “total”, or so (ideally you want to finish the fermentation with a maximum of 25–30ppm “total”, and 0–10ppm “free”. Less is better).
  • Take care of the yeast during the alcoholic fermentation (feed them and keep fermentation temperatures in line (below 85º F, 28º C), this limits their production of compounds that can later be possibly responsible for antagonizing the ML bacteria: H2S and VA, for example. Recent research shows that MLFs actually finish quicker and with less problems in wines made with yeast that are fed a complete set of nutrients during the alcoholic fermentation compared to those that are not. So remember: healthy yeast ultimately means healthy ML bacteria down the line.)

2)  Post Alcoholic Fermentation:

  • Wait until the must has reached 0º Brix before inoculating with the ML bacteria. ML bacteria, in the presence of residual sugars will also use this as a food source and one of the by-products of this pathway is VA. Ironically, high levels of VA in a must or wine can actually interfere with the bacteria’s ability to complete a Malolactic fermentation; regardless if they are the one’s who made it in the first place! And, of course, VA in detectible levels is considered a serious wine flaw. This possibility can therefore be greatly reduced by eliminating most of the sugars in the environment before they gain access to it.
  • Rack-off of the “gross” lees 24 hours post-press before inoculating the wine with the ML culture (As mentioned earlier, there is nothing helpful in the “gross” lees.  Remove them and remove potential problems, as well. There will be enough “light” lees remaining to feed the ML bacteria and you will keep the “being buried alive in the lees” factor to a minimum for the bacteria).

3)  ML inoculation preparation & handling: Prepare the ML culture: Some bacteria are labeled “direct-addition” and can be added to the wine directly from the pouch, while others require a 15–minute hydration period in clean, chlorine-free water before inoculating the wine. However, regardless of these differences all ML bacteria, including the “direct addition” and liquid ones, will benefit from a brief Acti-ML nutrient soak before going into the wine. Therefore we recommend treating any form of ML  bacteria you may be working with as if it required a 15–minute hydration before inoculation. This means that: For every 1 gram of bacteria being added to the wine, you will be adding 20g of Acti-ML to 100mL of distilled water at 77°F (25°C). After sitting for 15 minutes gently, yet, thoroughly stir this solution into your wine. The following example will use the 2.5g (66 gallons of wine) size ML bacteria packet to illustrate this. 

A) In a sanitized container: dissolve 50g of Acti-ML into 250mL of distilled water at 77°F (25°C). 

B)  Add the bacteria (2.5g) to the solution and gently stir/swirl to break up any clumps if needed. Wait 15 minutes.

C) Add the entire bacteria/nutrient solution into your wine and mix it throughout the entire wine volume. (Note: it is a good idea to stir the bacteria starter solution just before adding it into the wine to make sure that any of the nutrients and/or bacteria that may have settled-out during the 15 minute soaking period do not get left behind in the hydration vessel).

Inoculation and handling should take care to limit any oxygen exposure (the bacteria are anaerobic and depending on the strain may react negatively to various amounts of oxygen that may be introduced into the wine. In short, don’t splash when stirring the MLF and flush pumps and lines with inert gas before running a wine undergoing MLF through them. In general, it’s recommended not to rack a wine until the MLF is complete, however).

4) During the ML Fermentation:

  • Make sure the wine’s pH is at least around 3.1/3.2 (3.2 is better), if not adjust accordingly (Information on adjusting pH can be found in our Red (BK598) and White (BK597) Winemaking Manuals).
  • Keep the wine temperatures at around 70º F (20ºC) until the fermentation is complete (see section 5 below).
  • Stir the lees 1–2 times a week until completion (keep vessels topped-up and avoid oxygen. Flush any headspaces with inert gas).

5) Testing for Completion: Monitor with chromatography* (MT930), and once it seems to be finished, then run the first test. Often a MLF can slow or stop temporarily. If everything in the five elements checks out (alcohol, temperature, pH, SO2, and nutrients) and there is still no more progress within the week, then it’s time to consider adding an ML nutrient (such as Acti-ML) to the wine at a rate of .75–1.0 grams per gallon (possibly with a dose of yeast hulls, as well).

*Note that the sensitivity-threshold for the standard vertical test kit is around 70 mg/l, but it takes around 30 mg/l to be considered truly done. So, a good rule of thumb is to just wait an extra week or two after the test shows that you are done and that should be sufficient for a true completion.

6) Upon completion of the MLF: As soon as the MLF has completed, it is also a good idea to add SO2  immediately in order to stabilize and protect the wine. At this time, the wine should also be re-checked and the pH/TA% adjusted, if needed. If you are working with a red wine, then it is important to rack the wine at this point to counteract any of the reduction that may be remaining from the secondary fermentation. If you are doing a white, however, then you may choose to remain on the lees for more depth and complexity but continue to stir the lees once every 1–2 months.

For additional information on the malolactic fermenation, see our complete Guide to Malolactic Fermentation.


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