Taming High Acidity in Cold-Climate Grapes: MLF and Deacidification

Cold-climate hybrids like Frontenac, La Crescent, and Marquette ripen beautifully in short seasons – but they carry a heavy acid load, often 10-16 g/L total acidity (TA) at harvest, compared to 6-8 g/L in warm-climate Cabernet Sauvignon. That sharp, mouth-puckering bite is the defining winemaking challenge for northern home vintners. The good news: you have four practical tools – malolactic fermentation, chemical deacidification, cold stabilization, and careful blending – and most hybrid wines benefit from using at least two of them together.

I’ve been growing Frontenac and Marquette in Wisconsin (USDA Zone 4b) for over a decade, and acid management has been the steepest part of the learning curve. This guide pulls together what UMN Extension, Cornell Enology, and Iowa State viticulture programs actually recommend – plus what I’ve learned the hard way on my own crush pad.

Why Cold-Climate Grapes Are So Acidic

Grapes contain two primary organic acids: tartaric acid and malic acid. In warm climates, malic acid is metabolized by the vine during ripening – warm nights are especially important for this process. In cold climates like Minnesota or Wisconsin (Zones 3-6), short growing seasons and cool night temperatures slow that malic acid breakdown dramatically. You end up with fruit that has all the sugar you need for fermentation, but still carries a large malic acid burden.

According to UMN Extension viticulture research, cold-hardy hybrids routinely show:

• Total acidity (TA): 10-16 g/L (expressed as tartaric acid equivalents) – versus a target finished wine range of roughly 5.5-7.5 g/L for reds, about 6.5-8.5 g/L for whites
• pH: 3.0-3.2 at harvest in many hybrids – lower than most warm-climate grapes (pH 3.4-3.8)
• Malic acid: often 4-8 g/L of that total, making it a prime target for reduction

Tartaric acid is more stable and harder to reduce without chemistry; malic acid is the winegrower’s lever. Once you understand that distinction, the strategies below make a lot more sense.

Related: How to Know When Your Grapes Are Ready to Harvest – getting Brix and pH right at pick time is the first line of acid defense.

Strategy 1: Malolactic Fermentation (MLF)

Malolactic fermentation is the most powerful acid-reduction tool available to the home winemaker working with high-acid hybrids. It converts sharp malic acid into softer lactic acid through the action of lactic acid bacteria – most commonly Oenococcus oeni. The chemical equation is straightforward: one molecule of malic acid (dicarboxylic, two acid groups) becomes one molecule of lactic acid (monocarboxylic, one acid group) plus carbon dioxide. You lose half the acid groups – and the sensory result is a noticeably softer, rounder mouthfeel.

What MLF actually does to your numbers:

• Reduces TA by roughly 1-2 g/L typically, sometimes up to 3 g/L in high-malic cold-climate fruit, depending on how much malic acid was present at the start of MLF
• Raises pH – often by 0.1-0.3 units – because lactic acid is weaker than malic acid. For very low-pH hybrids starting at 3.0-3.1, this pH rise is often welcome and actually helps microbial stability
• Adds buttery, creamy, or nutty aromatic compounds (primarily diacetyl) – desirable in fuller red styles, sometimes unwanted in crisp aromatic whites like La Crescent

When to Use MLF (and When Not To)

Reds (Frontenac, Marquette, Petite Pearl): MLF is standard practice, not optional. The high malic acid load in these varieties almost always benefits from the conversion. Inoculate with a commercial MLF starter (Chr. Hansen, Lallemand/Scott Labs, White Labs all make suitable strains) after primary fermentation completes, or co-inoculate 24-48 hours after yeast pitch if you’re confident in your sulfite levels. Target temperature 65-72°F (18-22°C) – MLF stalls below 59°F (15°C).

Whites (La Crescent, Itasca, Brianna): More nuanced. Full MLF is often too much – it can strip the fresh fruit aromas and floral character that make these varieties interesting. A partial MLF (arrest with SO₂ once you’ve reduced TA by 1-1.5 g/L) or no MLF with other deacidification methods is a legitimate strategy. Iowa State Extension’s home winemaking notes specifically caution against routine MLF on aromatic white hybrids when you’re trying to preserve varietal character.

Track it: Use paper chromatography (Accuvin test strips work too) or an Accuvin malic acid test to confirm MLF completion. Don’t assume it finished because the wine stopped bubbling – incomplete MLF in bottle = re-fermentation, burst corks, and ruined wine.

Strategy 2: Chemical Deacidification

When MLF alone isn’t enough, or when you’re working with a white that you don’t want to put through MLF, chemical deacidification is your next tool. The two main agents are:

Potassium Bicarbonate (KHCO₃)

This is the most commonly recommended option for home winemakers. It reacts with tartaric acid (preferentially) and malic acid to form carbon dioxide and potassium salts, which drop out of solution over time. Practitioner guidance from winemaking extension programs and commercial suppliers consistently puts the dose at approximately 0.67-0.9 g of potassium bicarbonate per liter to lower TA by about 1 g/L – with the lower end (~0.67 g/L) common in WineMaker Magazine’s bench-trial guidance and the upper end (~0.9 g/L) reported for higher-buffered hybrid must. Always bench-trial first; actual results vary by wine pH, buffering capacity, and initial acid composition.

Best practice: Bench trial first. Take 100 mL samples, add calculated doses in small increments (0.5 g/L steps), measure TA and pH after each addition, then scale up. Always deacidify in stages – it’s much easier to remove more acid than to add it back. Add cold-stabilize after treatment to precipitate the potassium tartrate salts formed in the reaction.

Calcium Carbonate (CaCO₃) and “Acidex” (Double-Salt)

Calcium carbonate is cheaper and widely available, but it works differently: it has a stronger affinity for tartaric acid and can strip too much tartrate if overdosed, leaving the wine with a “chalky” or flat character. A specialized product called Acidex (marketed under various names) combines calcium carbonate with a seed crystal that preferentially triggers the “double-salt precipitation” of both tartaric and malic acid simultaneously – a useful approach when you need to reduce a very large malic acid load in one step before fermentation.

Important caution: Chemical deacidification raises pH – sometimes significantly. A higher pH reduces the effectiveness of SO₂ as a preservative (you need more free SO₂ to protect the wine at higher pH) and can create microbial instability. Always recalculate your SO₂ additions after any deacidification treatment.

Strategy 3: Cold Stabilization

Cold stabilization is a gentler form of deacidification that home winemakers in northern climates can often do for free. Chilling the wine to near-freezing – 28-32°F (-2 to 0°C) – causes potassium bitartrate (cream of tartar) crystals to precipitate out of solution. This removes tartaric acid and has a modest effect on TA: typically a reduction of 0.5-1.5 g/L depending on initial potassium and tartrate levels.

If you’re in Minnesota or Wisconsin, this is almost a free step in late November – move your carboy to the unheated garage or an outbuilding for two to four weeks, then rack off the crystals. The effect on TA is smaller than MLF or chemical deacidification, but cold stabilization also improves clarity and long-term tartrate stability in the bottle (no gritty crystals surprising your guests).

Strategy 4: Amelioration and Blending

Amelioration (Water Addition)

In very high-acid years – or with particularly tart lots of Frontenac – adding water and sugar (amelioration) is a legitimate and traditional home-winemaking practice. It dilutes acid alongside everything else: flavors, color, body. UMN Extension fruit winemaking guidelines and the classic First Steps in Winemaking (C.J.J. Berry) both treat amelioration as a standard hybrid-grape technique, not a compromise.

As a rough guide, a 10% water addition reduces TA by roughly 10% – so if you’re starting at 14 g/L, that might get you to 12.6 g/L, still needing other interventions. Amelioration is best used in combination with MLF or chemical deacidification, not as a standalone fix. Commercial wineries in many U.S. states have legal limits on water addition; home winemakers producing for personal use have more flexibility.

Blending

Blending a high-acid lot with a lower-acid grape – or adding a small amount of a neutral fruit wine made from lower-acid fruit (apple, pear, or a warm-climate grape) – can smooth the final balance without adding water or chemicals. This is how many successful hybrid winemakers in Minnesota’s commercial sector tame La Crescent: blending 15-20% with a variety that has naturally lower TA achieves balance while preserving the variety’s aromatic signature. Also consider: picking earlier or later can shift your starting acid profile – harvest timing decisions are the first deacidification decision you make each year.

See also: Pete’s home wine-making walkthrough – covers the full process from crush to bottle.

My High-Acid Frontenac Plan: A Practical Walkthrough

Here’s how I approach a typical high-acid Frontenac harvest in Wisconsin. This is what the process looks like in practice, not just on paper.

Step 1 – Measure before you do anything. At crush, I take TA and pH readings on the must. I use the standard NaOH titration method with phenolphthalein indicator (endpoint = pink). If TA is above 12 g/L and pH is below 3.1, I know I need a multi-step approach.

Step 2 – Pre-ferment potassium bicarbonate addition (optional). If TA is extremely high (14+ g/L), I’ll add potassium bicarbonate to the must before fermentation – targeting a TA reduction of 1.5-2 g/L, keeping a 1 g/L safety margin from my target. I do a bench trial first on 200 mL of must. Stir, wait 30 minutes, re-measure.

Step 3 – Primary fermentation. Pitch yeast (I use Lalvin 71B for Frontenac – it’s known to metabolize some malic acid on its own, typically 20-40%, which gives a small bonus reduction). Monitor temperature; keep it in the 65-75°F (18-24°C) range.

Step 4 – Inoculate for MLF. After pressing and racking off gross lees, I pitch Lallemand Enoferm Alpha or a similar Oenococcus oeni culture. Keep the wine warm (65-70°F / 18-21°C), minimize SO₂ until MLF completes. Paper chromatography every two weeks to track progress.

Step 5 – Cold stabilize. Once MLF is confirmed complete and I’ve added 30-40 ppm free SO₂, the wine goes outside (or into my cold garage) for four to six weeks in November-December. Rack off the tartrate crystals in January.

Step 6 – Final TA/pH check and adjustment. Measure again. If TA is still above target (my target for a Frontenac red is 6.5-7.5 g/L), I’ll do a second small potassium bicarbonate addition. If it’s already in range, I leave it. Then I recalculate and adjust free SO₂ for the final pH.

To do this reliably, you need a decent pH meter and a titration kit. Here’s what I reach for:

Related tools on this site: Brix to Alcohol Calculator – convert your Brix reading to estimated alcohol so you can plan sugar additions before and after any water amelioration.

Frequently Asked Questions

How much does malolactic fermentation lower acidity in hybrid wines?

MLF typically reduces total acidity (TA) by 1-2 g/L in cold-climate hybrids, sometimes up to 3 g/L in high-malic cold-climate fruit, depending on how much malic acid was present before fermentation. Varieties like Frontenac and La Crescent, which are naturally high in malic acid, often see reductions toward the higher end of that range. MLF also raises pH by roughly 0.1-0.3 units, which can help stability in very low-pH wines.

Can I use potassium bicarbonate on grape must before fermentation?

Yes – pre-fermentation potassium bicarbonate addition is a standard practice. Practitioner guidance puts the dose at approximately 0.67-0.9 g per liter to lower TA by about 1 g/L (bench-trial first, since actual results vary by buffering capacity and initial acid profile). The reaction releases CO₂ and forms potassium salts that will drop out with cold stabilization. Avoid over-treating: raising the pH too high before fermentation can cause microbial stability problems.

Should I do MLF on La Crescent or other aromatic white hybrids?

It depends on your style goal. Full MLF on La Crescent will reduce TA and increase pH, but it also adds buttery/diacetyl notes and strips some of the variety’s signature tropical and citrus aromas. Iowa State Extension and UMN Extension both suggest that aromatic white hybrids are better served by partial MLF (arrested early with SO₂) or chemical deacidification, rather than full MLF. Many commercial Minnesota producers making La Crescent in an off-dry style prefer to skip MLF entirely and control acid chemically.

What’s the difference between tartaric and malic acid in grapes?

Both are naturally occurring organic acids in grapes. Tartaric acid is more stable – it doesn’t metabolize during ripening and is harder to remove without chemical intervention. Malic acid is “apple acid” (it’s the dominant acid in green apples) and decreases as grapes ripen, particularly in warm climates. Cold-climate hybrids retain high malic acid because cool nights and short seasons limit the vine’s natural malic acid metabolism. MLF targets malic acid directly; cold stabilization primarily removes tartaric acid as potassium bitartrate crystals.

Is it safe to do both MLF and potassium bicarbonate addition?

Yes, and for very high-acid hybrids it’s often necessary to use both. The typical approach is to use potassium bicarbonate pre-fermentation to bring TA down from, say, 14 g/L to 11-12 g/L, then rely on MLF post-fermentation for a further 1-2 g/L reduction. The important caution is that each step raises pH, so monitor pH carefully throughout and recalculate your free SO₂ requirements after each intervention. The molecular SO₂ needed to protect wine at pH 3.5 is very different from what’s needed at pH 3.1.

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