Malolactic fermentation (MLF) is the bacterial conversion of L-malic acid to L-lactic acid and carbon dioxide. Use it only when it fits the wine style and stability plan, measure the conditions before inoculation, and confirm completion by testing malic acid—not by bubbles, taste, or elapsed time.
Separate MLF from alcoholic fermentation
Yeast alcoholic fermentation converts sugars into ethanol and carbon dioxide. Malolactic fermentation is carried out by lactic acid bacteria, commonly a selected Oenococcus oeni culture, and consumes L-malic acid. It may be inoculated during alcoholic fermentation or afterward, depending on the wine, organisms, and supplier protocol.
MLF usually reduces titratable acidity, raises pH, and changes aroma and mouthfeel. It can improve microbial stability against later metabolism of malic acid, but the higher pH and the period before post-MLF protection can also increase spoilage risk. “Softer” does not automatically mean safer or better.
Keep separate records for sugar and malic acid. A wine can finish alcoholic fermentation while MLF has not begun, or finish MLF before all sugar is gone in a co-inoculated process. Stable Brix cannot certify MLF, and falling malic acid cannot certify alcoholic completion.
Decide whether MLF belongs in the style
MLF is widely used in red wine for acid balance and microbial stability. It is also used selectively in white and sparkling base wines. A crisp aromatic white may be designed to retain malic acid, while a Chardonnay may use full or partial MLF for a different texture and aroma profile.
| Decision | Questions to answer | Evidence required |
|---|---|---|
| Encourage full MLF | Does the style benefit, and can conditions support the selected culture? | Malic acid, pH, alcohol, temperature, SO₂, strain compatibility |
| Avoid MLF | Is retained acidity central to the style, and can later growth be prevented? | Stability plan, filtration capability where applicable, pH and SO₂ management |
| Partial or blended MLF | Can separate lots be measured and stabilized reliably? | Lot-specific malic analysis and controlled blending records |
Do not decide from grape color alone. Review starting malic acid, titratable acidity, pH, alcohol, desired sensory profile, packaging plan, and the ability to verify completion or exclusion. When MLF is not desired, professional stabilization may require filtration and carefully managed sulfur dioxide; this is not achieved by simply storing the wine cool.
Measure the conditions before inoculation
The Australian Wine Research Institute identifies temperature, pH, alcohol, and sulfur dioxide as the main interacting controls. Its general fact sheet describes 18–22°C (64–72°F), pH 3.3–3.5, total SO₂ below 30 mg/L, and alcohol below 13% v/v as favorable conditions, while values outside those ranges can make MLF increasingly difficult. These are AWRI planning categories, not a universal culture specification.
Multiple moderate stresses add together. A culture might tolerate one of low pH, high alcohol, cool temperature, or elevated SO₂, yet fail when several occur together. Use the technical sheet for the exact strain and measure the wine. Do not estimate pH from taste or infer SO₂ from the amount added; bound and free forms behave differently.
UC Davis similarly states that lactic acid bacteria generally require warmer conditions than wine yeast and that higher pH permits a broader range of bacteria, including organisms that may be undesirable. Adjusting pH merely to favor MLF can affect color, oxidation, flavor, and microbial stability, so it belongs in a complete enology plan.
Choose inoculation timing deliberately
Post-alcoholic inoculation separates the two fermentations and is familiar to many winemakers, but the finished wine has higher ethanol and fewer nutrients for the bacteria. Co-inoculation can shorten the total unprotected period and may succeed under conditions that would challenge a later inoculation, yet compatibility and process risks must be evaluated.
AWRI’s red-wine fact sheet lists potential co-inoculation concerns including high sulfur dioxide, competition, yeast–bacteria incompatibility, and acetic acid risk when alcoholic fermentation becomes stuck. Follow the culture manufacturer’s timing, rehydration, nutrient, and compatibility directions. Do not combine a yeast procedure from one supplier with a bacteria procedure from another without confirming compatibility.
If primary fermentation is already sluggish, diagnose it before adding MLF bacteria. UC Davis advises against adding malolactic bacteria at the same time as an attempted alcoholic restart. Use the stuck-wine workflow to establish what has actually stopped.
Monitor malic acid, not just activity
MLF may release fine bubbles, create turbidity, or change aroma, but these are not specific measurements. Carbon dioxide can remain from alcoholic fermentation, and other microbes can produce gas or haze. Taste can suggest acid change but cannot quantify the remaining L-malic acid.
Use paper chromatography as a qualitative home or cellar screen only if you understand its detection limits and chemical-safety instructions. Enzymatic analysis or a qualified wine laboratory provides a quantitative result. Penn State Extension recommends laboratory confirmation where process and stability decisions depend on the result.
Record malic acid, method, detection limit, temperature, pH, alcohol estimate, SO₂ result, date, culture, inoculation lot, and sensory observations. Plot malic acid over time. A declining trend demonstrates conversion; a single faint chromatography spot is not a precise concentration.
Confirm completion with a defined analytical endpoint
AWRI’s November 2024 red-wine fact sheet recommends “not detected,” typically below 0.05 g/L by enzymatic analysis, as the preferred endpoint and states that 0.1 g/L or less is low enough to consider MLF complete. Those values are tied to the stated method and wine-production context. Use the laboratory’s reporting limit and the applicable production standard.
After a confirming result, complete the process’s post-MLF protection without unnecessary delay. AWRI warns that delay can expose wine to oxidation, volatile-acidity increase, acetic acid bacteria, and Brettanomyces. The exact sulfur dioxide or filtration decision depends on pH, wine style, turbidity, jurisdiction, and measurement; this guide does not prescribe a universal dose.
When co-inoculation finishes MLF before alcoholic fermentation, AWRI advises waiting for alcoholic completion before post-MLF stabilization. Confirm both endpoints separately and do not bottle from one favorable lab result alone.
Troubleshoot slow or absent MLF
- Verify the test. Confirm that the method can detect the expected malic-acid range and that a new sample agrees.
- Measure the four controls. Check wine temperature, pH, alcohol, and free and total SO₂ as appropriate.
- Review the culture. Check strain tolerance, lot, storage, expiry, rehydration, inoculation rate, and yeast compatibility.
- Review timing and nutrition. Note lees contact, racking, nutrient plan, and when bacteria were added relative to alcoholic fermentation.
- Check for spoilage risk. Use sensory screening plus laboratory analysis where volatile acidity or other microbial activity is suspected.
Penn State Extension and AWRI both emphasize that low temperature, low pH, high alcohol, and sulfur dioxide can inhibit MLF. Do not correct all four at once. A rescue starter must be adapted to the actual wine conditions using a validated laboratory, extension, or manufacturer protocol.
A worked decision example
A red wine has completed alcoholic fermentation. The winemaker wants full MLF, but the cellar is cool and the wine’s pH, alcohol, and SO₂ have not been measured. The correct next step is not to add bacteria and wait for bubbles. First obtain the four measurements, compare them with the selected culture’s specification, and decide whether conditions can be adjusted safely.
After inoculation, malic acid is measured on a schedule. It declines but does not disappear. The wine therefore remains in MLF management even if it tastes softer. When quantitative analysis reaches the defined completion criterion, alcoholic fermentation is also confirmed complete, and no spoilage issue is detected, the winemaker proceeds promptly with the validated stabilization and packaging plan.
Safety and scope
Do not bottle a wine with unconfirmed alcoholic or malolactic fermentation in ordinary still-wine bottles; renewed fermentation can produce carbon dioxide, haze, sediment, and dangerous pressure. Chemical reagents, sulfur dioxide, acids, bases, and microbial cultures require current labels, accurate measurement, suitable protective equipment, and proper storage.
MLF is a technical production choice, not a home remedy or health treatment. Commercial wine requires jurisdiction-specific process controls, laboratory methods, records, labeling, and worker-safety practices. Consult an enology laboratory or extension specialist when measurements conflict or spoilage is suspected.