Must Preparation: Getting Your Juice Ready to Ferment

What Is Must Preparation and Why It Matters

Must is the term for the freshly crushed grape juice along with the skins, seeds, and pulp that will become wine. Must preparation refers to everything you do between crushing your grapes and pitching your yeast, including testing sugar and acid levels, making chemical adjustments, adding sulfites, and ensuring the must is in optimal condition for a healthy fermentation.

This step is where winemaking truly begins to diverge from simply squeezing fruit. The decisions you make during must preparation determine the alcohol level, acidity balance, color intensity, and overall flavor profile of your finished wine. Skipping or rushing this stage is one of the most common reasons home wines turn out thin, flabby, overly acidic, or otherwise unbalanced.

The Window of Opportunity

You have a limited window to make adjustments to your must before fermentation takes over. Once yeast is actively fermenting, adding acid or sugar becomes more difficult and less predictable. Most adjustments should be completed within the first 12 to 24 hours after crushing, before yeast inoculation. This timeline also allows any sulfite additions to dissipate to safe levels for your yeast.

Testing Your Must: Essential Measurements

Accurate testing is the foundation of good must preparation. You cannot make proper adjustments without knowing your starting numbers.

Measuring Sugar Content (Brix)

Brix measures the sugar concentration in your must as a percentage of weight. Use a hydrometer or refractometer to take this reading. A hydrometer is more accurate for must that has already been crushed and mixed, while a refractometer is handy for field testing individual berries.

To take a hydrometer reading, fill a test jar with must, insert the hydrometer, and read the scale at the meniscus (the bottom of the curved liquid surface). Target Brix levels depend on your desired wine style:

• Light white wines: 20 to 22 Brix (producing approximately 11 to 12 percent alcohol)
• Full-bodied whites: 22 to 24 Brix (approximately 12 to 13.5 percent alcohol)
• Light to medium reds: 23 to 25 Brix (approximately 12.5 to 14 percent alcohol)
• Full-bodied reds: 25 to 27 Brix (approximately 14 to 15 percent alcohol)

Record your Brix reading along with the temperature of the sample, as hydrometers are calibrated to a specific temperature (usually 60 degrees Fahrenheit). Adjust your reading by adding 0.3 Brix for every 10 degrees above the calibration temperature.

Measuring pH

The pH of your must tells you how acidic it is on a logarithmic scale. A pH meter is far more accurate than test strips and is strongly recommended. Calibrate your pH meter with pH 4.0 and 7.0 buffer solutions before each use.

• Target pH for white wines: 3.1 to 3.4
• Target pH for red wines: 3.3 to 3.6

pH affects microbial stability, color stability, sulfite effectiveness, and the perception of freshness in the finished wine. A must with a pH above 3.7 is at elevated risk for bacterial spoilage and will need more sulfite to remain stable.

Measuring Titratable Acidity (TA)

While pH tells you the strength of acidity, titratable acidity (TA) measures the total amount of acid present, expressed in grams per liter (g/L) of tartaric acid equivalent. You need an acid testing kit with sodium hydroxide (NaOH) solution and phenolphthalein indicator.

• Target TA for white wines: 6.0 to 8.0 g/L
• Target TA for red wines: 5.5 to 7.0 g/L

Both pH and TA should be considered together. A must can have a low pH (seemingly acidic) but also a low TA, or vice versa. The balance between the two is what creates the sensation of proper acidity in the finished wine.

Making Chemical Adjustments to Your Must

Once you have your measurements, you can make targeted adjustments to bring the must into the ideal range for fermentation.

Adjusting Sugar Levels

If your Brix is too low, you can add sugar through a process called chaptalization. Dissolve 1.5 ounces of table sugar per gallon to raise the Brix by approximately 1 degree. Dissolve the sugar in a small amount of warm must before stirring it into the main batch. This is legal for home winemaking in most regions and is common in cooler climate grape growing areas.

If your Brix is too high (above 27 Brix), the resulting high alcohol can stress yeast and result in a stuck fermentation. You can dilute the must with clean, distilled water at a rate of approximately 1 gallon of water per 10 gallons of must to lower Brix by about 2 to 3 degrees. However, dilution also reduces flavor concentration, so it should be used sparingly.

Adjusting Acidity

If your must is too low in acid (high pH), add tartaric acid at a rate of 1 gram per liter to lower the pH by approximately 0.1 units. For a 5-gallon batch (about 19 liters), this means adding 19 grams (roughly 4 teaspoons) to achieve a 0.1 pH reduction. Always make additions incrementally, adding half the calculated amount, stirring thoroughly, waiting 15 minutes, and retesting before adding more.

If your must is too high in acid (low pH), you can add potassium bicarbonate at a rate of 1 gram per liter to raise pH by approximately 0.1 units. Alternatively, calcium carbonate (precipitated chalk) can be used at 2.5 grams per liter to reduce TA. Make acid reductions cautiously, as over-correction produces flat, lifeless wine.

Adding Sulfites

Add potassium metabisulfite immediately after crushing to suppress wild yeast, bacteria, and oxidation. The standard dose is 50 ppm for healthy grapes, which equals approximately 1/4 teaspoon (1.4 grams) per 5 gallons of must. For grapes with visible mold or damage, increase the dose to 75 ppm.

Measure sulfite additions precisely using a digital scale that reads in grams. Sulfite effectiveness depends on pH: at a pH of 3.0, only 6 percent of total SO2 is in the active molecular form, while at a pH of 3.8, less than 2 percent is active. This means high-pH musts need proportionally more sulfite for the same level of protection.

Nutrient Additions and Enzyme Treatments

Beyond the basic chemical adjustments, several additives can improve fermentation performance and wine quality.

Yeast Nutrients

Grape must naturally contains some nutrients for yeast, but supplementation ensures a healthy, complete fermentation. Add a yeast nutrient complex containing diammonium phosphate (DAP) and organic nitrogen sources at crush. A typical addition is 1 gram of Fermaid-O or equivalent per gallon of must, added in stages: half at the start of fermentation and half at one-third sugar depletion (when Brix drops by one-third from the starting value).

The total yeast assimilable nitrogen (YAN) target for a healthy fermentation is 200 to 300 ppm. If you can test your must's native YAN level, supplement only the difference. Under-nourished yeast produces hydrogen sulfide (rotten egg smell) and other off-flavors, while over-nourishing can stimulate spoilage organisms.

Pectic Enzymes

Adding pectic enzyme (also called pectinase) at crush breaks down the pectin in grape skins, improving juice yield, color extraction, and clarity. Add pectic enzyme at a rate of 1/2 teaspoon per 5 gallons of must, stirring well. The enzyme works best at temperatures below 80 degrees Fahrenheit and needs at least 8 to 12 hours to work before fermentation begins. For white wines, pectic enzyme is especially valuable because it helps settle out solids during the cold settling (debourbage) step before fermentation.

Oak Additions at Must Stage

Some winemakers add oak chips or powder to the must during fermentation to build complexity and soften tannins early. If you plan to do this, add 1 to 2 ounces of medium-toast oak chips per 5 gallons at the must stage. This is not a substitute for barrel or oak alternative aging later but can contribute a foundation of vanilla, spice, and toast notes that integrate well during fermentation.

Must Preparation for White Wines vs. Red Wines

The must preparation process differs significantly between white and red wines.

White Wine Must Preparation

For white wines, the goal is to separate clean juice from the skins and solids as quickly as possible. After crushing (or whole-cluster pressing), the juice undergoes cold settling (debourbage) for 12 to 24 hours at 40 to 50 degrees Fahrenheit. During this time, solid particles settle to the bottom of the vessel.

After cold settling, rack the clear juice off the sediment into a clean fermenter. The cleaner the juice going into fermentation, the more aromatic and fruit-forward the resulting wine will be. White must typically needs more acid adjustment than red must, particularly in warm climates where grapes lose acidity quickly during ripening.

Red Wine Must Preparation

For red wines, the skins and seeds remain in the must throughout fermentation to extract color, tannin, and flavor. After crushing and destemming, the entire must goes into the primary fermenter. Red must preparation focuses on ensuring adequate tannin extraction, managing pH and acidity, and preparing for either a cold soak or direct yeast inoculation.

If you plan a cold soak, chill the must to 45 to 55 degrees Fahrenheit and hold it there for 2 to 5 days before adding yeast. This extracts color and flavor from the skins without the tannin extraction that occurs during alcoholic fermentation. Whether cold soaking or not, ensure your sulfite addition is made promptly and your nutrient additions are planned.

Common Must Preparation Mistakes

Making Too Many Adjustments at Once

The most frequent error is trying to correct multiple parameters simultaneously. Make one adjustment at a time, retest, and then evaluate whether further changes are needed. Acid, sugar, and sulfite additions can interact with each other, and making them all at once can lead to overshooting your targets.

Relying on Taste Alone

While tasting your must is valuable, it cannot replace precise measurements with calibrated instruments. Must tastes very different from finished wine, and your palate cannot accurately gauge pH, TA, or Brix to the precision needed for good winemaking decisions. Always test with instruments and use taste as a supplementary check.

Neglecting Temperature Control

Failing to control the temperature of your must during preparation invites problems. Warm must above 70 degrees Fahrenheit can begin spontaneous fermentation before you are ready, oxidizes faster, and provides favorable conditions for spoilage organisms. Keep your must cool during the preparation phase.

Frequently Asked Questions

How long can I wait between crushing and pitching yeast?

With a proper sulfite addition of 50 ppm, you can safely wait 12 to 24 hours between crushing and yeast inoculation. If you plan a cold soak, the must can be held for 2 to 5 days at temperatures below 50 degrees Fahrenheit with sulfite protection. Without sulfite, wild yeast and bacteria will begin working within hours.

Do I need to add water to my grape must?

Generally, no. High-quality wine grapes contain sufficient juice and do not need dilution. Adding water dilutes flavor, color, and body. The exception is when Brix is excessively high (above 27 degrees), in which case careful dilution prevents stuck fermentation. For fruit wines made from non-grape fruits, water addition is standard and expected.

What happens if I skip must preparation entirely?

Skipping must preparation means going into fermentation blind, without knowing your sugar, acid, or pH levels. You may end up with wine that is too alcoholic, too tart, too flat, or microbiologically unstable. While grapes that arrive in perfect condition occasionally produce good wine with minimal intervention, this is the exception rather than the rule.

Can I prepare must the night before and add yeast in the morning?

Yes, this is a common and practical approach. Crush and destem in the evening, add sulfites and pectic enzyme, take your measurements, and make any necessary adjustments. Cover the must and keep it cool overnight. In the morning, retest if needed, prepare your yeast starter, and inoculate. The 12-hour gap between sulfite addition and yeast pitching is ideal.

Should I add all my yeast nutrients at once during must preparation?

No. Add yeast nutrients in two or three staged additions rather than all at once. Add the first portion at yeast inoculation and the remaining portions at one-third and two-thirds sugar depletion. Staged nutrient additions keep yeast healthy throughout fermentation and reduce the risk of producing off-flavors from nutrient shock.

How do I handle must from grapes that arrived damaged or moldy?

Sort aggressively, removing all visibly damaged or moldy fruit. Increase your sulfite addition to 75 to 100 ppm to combat the higher microbial load. Add lysozyme at 250 ppm if you are concerned about lactic acid bacteria. Consider adding a higher dose of pectic enzyme to compensate for the reduced juice quality. Despite these measures, heavily compromised fruit will produce inferior wine, and it may be better to seek a replacement source if more than 15 to 20 percent of the fruit is damaged.

What is the difference between free-run juice and press juice?

Free-run juice is the juice that drains from the must under gravity alone after crushing, without any pressing. It is generally higher quality, with softer tannins and more delicate flavors. Press juice is extracted by applying pressure to the skins after free-run has been collected. Press juice is darker, more tannic, and more concentrated. Many winemakers keep these fractions separate during fermentation and blend them back together (or not) after tasting.