Updated: Dec 9, 2020
Malolactic fermentation (MLF) is a microbial process that can play a tremendous role in the taste, smell and overall quality of your wine. When complete, the malic acid in the wine will have been converted to lactic acid and CO2 with a few bonus aromatic compounds.
Most red wines that you will find in the store have undergone malolactic fermentation. White wines like chardonnay owe their buttery smell and silky mouth feel to the process. Not all wines will benefit from malolactic fermentation though. A white wine that is intended to be crisp and refreshing can become dull if the malic acid is converted to lactic acid. Getting a handle on this secondary form of fermentation is a big step towards making better wines at home.
The Acids in Grapes
The main acids in wine grapes are tartaric acid, malic acid, and citric acid. Tartaric acid is generally found in the highest concentration and is usually the best choice for making acid adjustments. Malic acid is second to tartaric (except in the case of a few hybrid grapes). Malic acid is the primary acid of an apple and can contribute a crisp freshness to a wine, or a sour harshness. Citric acid is found only in small quantities in a grape. While citric is the cheapest of the acids, it should generally be avoided for any acid adjustments, especially if the wine is expected to go through malolactic fermentation.... we'll get into that later.
The Good Microbes on a Grape
Straight off the vineyard a grape cluster is a microbial oasis. Without adding anything to a wine must, you will still find many strains of Saccharomyces Cerivasaie (Wine Yeast) and Oenococcus Oeni, the preferred Lactic Acid Bacteria (LAB). Wine yeast has evolved to convert sugars into alcohol and CO2. This creates a harsh environment that wipes out most other microbes that came in with the grapes, with the exception of a select few. Among these select few winners are usually a strain or two of malolactic bacteria.
Even though malolactic bacteria is probably in your must or on your barrel already, I prefer to add a known strain from a freeze dried packet. This helps to ensure a complete fermentation and reduce the odds of a spoilage yeast or bacteria taking a hold over the wine during the aging period.
What Exactly Happens During Malolactic Fermentation?
The difference between malic and lactic acid is only one carboxyl group. As malolactic bacteria metabolizes malic acid, a "decarboxylation" occurs. When malic acid goes in, lactic acid and CO2 come out, with the occasional Diacetyl. Diacetyl is responsible for the buttery smell of a red or white wine. When malolactic fermentation is complete, the primary acids of the resulting wine will be tartaric and lactic acid. Lactic acid is a little more approachable than malic acid and helps to create a more savory red wine.
Citric acid will occasionally be targeted by malolactic bacteria after the malic acid has been depleted. Citric is initially metabolized into acetic acid and pyruvic acid. The pyruvic acid is then secondarily metabolized into diacetyl, contributing yet more butteriness. If citric acid is used for acid adjustments though, excess diacetyl and acetic acid can result. Too much diacetyl can lean your wine towards a spoiled milk smell instead of butter or butterscotch. This is especially true in a white wine. Many of the more inexpensive kit wines and juice buckets have been adjusted with citric acid to save money.
Controlling the Butter
As a winemaker, you have a good bit of control over the diacetyl levels. More diacetyl, means more butter, while less diacetyl means less butter.
To encourage higher diacetyl levels, you can do the following:
1. Inoculate with malolactic bacteria AFTER primary fermentation is complete, and after racking off of the majority of the lees. Yeast irreversibly breaks down diacetyl, so the less yeast that is involved in the process, the more diacetyl retention.
2. Take steps to encourage a slow malolactic fermentation. The easiest method is to store the wine in a cooler place when malolactic fermentation is underway. A small sulfite addition will slow the fermentation and temporarily bind up the diacetyl, to be released later on. Lower pH also favors a slow malolactic fermentation.
3. Choose a malolactic bacteria strain that naturally produces more diacetyl
4. Discourage the wine from going too reductive during malolactic fermentation. This can be accomplished by an occasional stirring of the fine lees. Under reductive conditions malolactic bacteria will produce aromatically neutral acetoin and 2,3-butanediol instead of diacetyl (2,3-butanedione).
5. Add malic acid in conjunction with tartaric acid when making acid adjustments prior to malolactic fermentation.
If you are not looking for a distinct buttery aroma, then consider adding malolactic bacteria to the wine must BEFORE the completion of alcoholic fermentation. This, in addition to warmer storage conditions and malolactic strain selection can create a wine with little to no perceptible butter aroma.
71B yeast can be used to decrease malic acid which will in turn decrease diacetyl production. Beware that 71B will create a small amount of isoamyl acetate which can give the wine an artificially fruity smell though.
Preventing Malolactic Fermentation
How do you prevent malolactic fermentation? Malolactic bacteria is very sensitive to pH and SO2. In a lower pH wine, malolactic fermentation can be prevented all together by maintaining sufficient SO2 levels. This is common practice for many white wines and fruit wines. Products like lysozyme have also proven effective at preventing malolactic fermentation. The most reliable way to prevent malolactic fermentation is to remove the bacteria through sterile filtration. This is generally not performed until near bottling time though, so malolactic fermentation must be delayed by SO2, refrigeration, lysozyme, or some combination until the wine is settled enough for filtration. True sterile filtration is very difficult to achieve at home, but is possible with products like the Buon Vino Mini Jet.
pH Shift from Completed Malolactic Fermentation
How much does the pH change during malolactic fermentation? It is worth noting that the pH of your wine will generally increase by 0.15 to 0.30 after once the malic acid has been converted to lactic acid. When making acid adjustments, this should be figured into the calculations.
Additional benefits of Malolactic Fermentation
Another major benefit of malolactic fermentation is increased microbial stability. A wine that has fermented dry and completed malolactic fermentation will have very little sugar, malic acid, citric acid, or micronutrients for spoilage bacteria to feed on. With a little SO2, this wine can go into bottle without sterile filtration and see little to no risk of in-bottle spoilage.
Testing for Complete Malolactic Fermentation
In a lab environment, a spectrophotometer can be used to test for the absence or presence of malic acid in a wine. For home winemakers and small wineries, paper chromotography can be used without breaking the bank quite as bad. Another option is the Accuvin malic acid test strips. If using a more aggressive malolactic bacteria strain and keeping sulfite levels low for the first month or two of aging, it is unlikely that the malolactic fermentation will not run to completion but it doesn't hurt to test anyways if you are making larger batches of wine.
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