Blocking MLF — Lysozyme, SO₂, and Sterile Filtration
Three complementary tools give winemakers precise control over malolactic fermentation, protecting natural acidity and freshness in aromatic whites, rosés, and cool-climate sparklers.
Winemakers block malolactic fermentation (MLF) using three primary tools: lysozyme (an enzyme derived from hen egg white that lyses lactic acid bacteria cell walls), sulfur dioxide (which inhibits O. oeni via molecular SO₂ toxicity), and sterile filtration (typically 0.45 µm membranes that physically remove bacterial populations). Each method operates on a different biological mechanism and offers distinct trade-offs in cost, timing, and sensory impact. Blocking MLF is essential for preserving crisp acidity, fresh aromatics, and residual sugar stability in aromatic white wines and dry rosés.
- Lysozyme (EC 3.2.1.17) is a muramidase enzyme extracted from hen egg white that cleaves β-1,4-glycosidic bonds in bacterial cell wall peptidoglycan, lysing gram-positive lactic acid bacteria including Oenococcus oeni, Lactobacillus, and Pediococcus spp.
- Effective lysozyme doses range from 100–500 mg/L; 250–300 mg/L is generally sufficient to delay or prevent MLF, while 500 mg/L is the OIV and EU-approved maximum and is used for more aggressive inhibition
- SO₂ inhibits O. oeni at molecular SO₂ levels above approximately 0.8 mg/L; because molecular SO₂ is pH-dependent, the required free SO₂ addition varies widely with wine pH — roughly 35–50 mg/L free SO₂ at typical white wine pH ranges
- Sterile filtration at 0.45 µm is the wine industry standard for removing bacteria before bottling; absolute-rated membrane cartridges offer the most reliable bacterial removal, and wines should be bottled immediately after sterile filtration to prevent recontamination
- EU maximum total SO₂ limits are 150 mg/L for dry red wines and 200 mg/L for dry white and rosé wines, with higher allowances for sweet wines; the U.S. TTB permits up to 350 mg/L total SO₂
- Lysozyme is approved by the OIV and the European Commission and is listed as an allergen under EU food legislation (Regulation 1266/2010/CE), requiring declaration on wine labels; the TTB permits its use in the U.S. on an experimental basis with advance approval
- Blocking MLF is standard practice for aromatic whites (Sauvignon Blanc, Riesling, Albariño, Grüner Veltliner) and dry rosés where fresh fruit aromatics and bright acidity are defining stylistic goals; MLF also needs blocking in wines with residual sugar to prevent in-bottle refermentation
What It Is and Why It Matters
Blocking MLF is the deliberate prevention of malolactic fermentation, the bacterial conversion of tart malic acid to softer lactic acid and CO₂. While MLF is encouraged in most red wines and some whites (including Chardonnay and white Burgundy), blocking it preserves the natural acidity, volatile thiols, and green-fruit esters that define cool-climate aromatic whites. The three core techniques operate through distinct mechanisms: lysozyme attacks bacterial cell walls enzymatically, SO₂ poisons bacterial metabolism through its molecular form, and sterile filtration physically removes viable bacteria. Winemakers may combine these methods for synergistic protection, or select a single tool based on wine style, pH, budget, and local regulations.
- Preserves malic acid, retaining a sharper, more refreshing acidity profile compared to MLF-converted wines
- Maintains volatile thiols and green-fruit esters characteristic of Sauvignon Blanc, Albariño, and Grüner Veltliner
- Prevents lactic acid bacteria from generating biogenic amines such as histamine and tyramine, and avoids in-bottle CO₂ production from post-bottling MLF
How Each Method Works
Lysozyme cleaves the β-1,4-glycosidic linkages in bacterial peptidoglycan cell walls, causing osmotic lysis and cell death. It is active specifically against gram-positive bacteria such as Lactobacillus, Pediococcus, and Oenococcus oeni, and does not inhibit yeast or gram-negative bacteria like Acetobacter. Importantly, some strains of lactic acid bacteria can be resistant to lysozyme. SO₂ works through its molecular (unionized) form, which penetrates bacterial cell membranes and disrupts cellular metabolism; since the proportion of molecular SO₂ rises sharply as pH falls, its antimicrobial efficacy is highly pH-dependent. Sterile filtration physically removes microorganisms by passing wine through a membrane with pores typically rated at 0.45 µm, the industry standard for bacteria removal in winemaking; wines must be bottled immediately after filtration to prevent recontamination.
- Lysozyme: enzymatic lysis of gram-positive LAB only; effective within 1–3 days; requires bentonite fining to remove residual protein from white and rosé wines; does not protect against oxidation
- SO₂: molecular SO₂ is the active antimicrobial species; free SO₂ targets of approximately 0.8 mg/L molecular SO₂ are used to inhibit O. oeni; SO₂ also provides antioxidant protection that lysozyme cannot
- Sterile filtration: physically removes bacteria immediately; absolute-rated 0.45 µm membrane cartridges provide the most reliable results; filter integrity should be validated by a bubble-point or diffusion test before use
Effect on Wine Style and Sensory Profile
Wines with blocked MLF retain their natural malic acid, producing a sharper, more refreshing midpalate compared to MLF-converted examples. This preserves bright citrus and green-fruit aromatics, the volatile thiols associated with Sauvignon Blanc, and the mineral tension valued in Riesling and Albariño. Blocking MLF also prevents the production of diacetyl (a buttery compound) and reduces the risk of lactic, creamy secondary aromas developing. The trade-off is that blocked wines may feel less texturally round; some producers choose to allow a small fraction of the blend to undergo MLF and then blend back, achieving a balance between freshness and body. In wines with residual sugar, blocking MLF is essential to prevent in-bottle refermentation.
- Sauvignon Blanc: preserved volatile thiols enhance grass, gooseberry, and citrus aromas; malic acidity provides a crisp, nervy finish
- Albariño and Riesling: saline minerality, stone fruit, and floral aromatics remain vivid; acidity supports long aging potential
- Risk of reductive sulfur notes if free SO₂ is poorly managed; lysozyme offers no antioxidant protection, so SO₂ additions remain necessary alongside it
When Winemakers Use It
Blocking MLF is standard for aromatic whites where acidity and fresh fruit are primary stylistic goals: Loire Sauvignon Blancs, Muscadet, German Riesling, New Zealand Sauvignon Blanc, and dry rosés from Provence and Bandol. It is also essential for any wine with residual sugar, where malic acid remaining after primary fermentation could feed a spontaneous MLF in bottle, producing CO₂ and off-aromas. Some Champagne houses, notably Gosset, Krug, and Alfred Gratien, deliberately block or limit MLF in base wines to preserve high acidity and aging potential before prise de mousse; Bollinger and the majority of Champagne houses, by contrast, do allow MLF in their base wines. Blocking decisions are typically made immediately post-primary fermentation and require monitoring of free SO₂, pH, and bacterial populations.
- Timing: SO₂ additions are made promptly after primary fermentation completes, targeting molecular SO₂ thresholds adjusted for wine pH
- Monitoring: paper chromatography or enzymatic malic acid assay confirms whether MLF has begun; regular free SO₂ measurements guide re-dosing
- Temperature: keeping wine below approximately 15°C slows lactic acid bacterial growth and supports chemical inhibition methods
Regional Practices and Producer Approaches
In the Loire Valley, producers of Sancerre and Pouilly-Fumé typically rely on SO₂ additions post-fermentation to block MLF and preserve the crisp Sauvignon Blanc character. Muscadet producers similarly block MLF to retain the salinity and mineral freshness of Melon de Bourgogne. German Riesling estates use SO₂ management combined with cold cellar temperatures to block MLF and maintain residual sugar stability across Kabinett and Spätlese styles; their low pH values (often 2.9–3.2) make SO₂ particularly effective at low free doses. In Champagne, houses such as Gosset, Krug, and Alfred Gratien are known for blocking or limiting MLF in their base wines to preserve acidity and ensure long aging potential, while most Champagne houses, including Bollinger, conduct full MLF as part of their house style. New Zealand Sauvignon Blanc producers broadly block MLF to ensure vintage-to-vintage freshness and shelf stability in export markets.
- Champagne houses blocking MLF (Gosset, Krug, Alfred Gratien): acidity preservation and extended aging potential are the key drivers
- Champagne houses conducting MLF (Bollinger and the majority): roundness, complexity, and microbiological stability before prise de mousse
- German Riesling: low pH facilitates SO₂ efficacy at modest free SO₂ doses; cold storage is a complementary tool for blocking MLF in off-dry and sweet styles
Best Practices and Risk Management
A layered approach combining SO₂ management, cold storage, and lysozyme (where permitted) provides the most robust MLF inhibition with minimal sensory impact. Because lysozyme has no antioxidant activity and its effectiveness diminishes over time, it should always be used alongside adequate free SO₂ rather than as a replacement. Winemakers should allow at least 24 hours between lysozyme and SO₂ additions, as simultaneous use can reduce the efficacy of both. Residual lysozyme protein must be removed from white and rosé wines by bentonite fining before bottling to prevent haze. Sterile filtration at bottling provides a final physical barrier and is especially important for wines with any residual malic acid or sugar. All SO₂ additions should be documented for regulatory compliance, targeting total SO₂ within legal limits: 200 mg/L for EU dry whites/rosés, 150 mg/L for dry reds, and 350 mg/L in the United States.
- Lysozyme protocol: rehydrate in water (5–10 times its weight) before addition; remove residual protein with bentonite fining (5–10 g/hL) in white and rosé wines before bottling
- SO₂ dosing: calculate free SO₂ targets from wine pH to achieve approximately 0.8 mg/L molecular SO₂; use Ripper method or aeration-oxidation for accurate measurement
- Sterile filtration: validate membrane integrity with a bubble-point or diffusion test before use; bottle immediately after filtration to prevent post-filtration bacterial recontamination
- Contingency: if unexpected MLF begins, promptly raise free SO₂, chill the wine, and assess whether lysozyme addition and sterile filtration are needed to arrest the fermentation
Wines with blocked MLF show a bright, precise aromatic character: citrus (lemon zest, grapefruit), green fruit (crisp apple, gooseberry), and fresh herbal notes (cut grass, white pepper) that would be softened or displaced by malolactic conversion. On the palate, the retained malic acid produces a sharper, more angular acidity than the rounder feel of lactic acid, giving the wine energy and freshness through the finish. In cool-climate examples such as Loire Sauvignon Blanc or German Riesling, blocked MLF amplifies terroir-driven mineral tension and aromatic precision, making it an essential tool for producers whose house style depends on vibrancy and longevity.