A real-world lesson in testing your must and changing your plan when the grapes demand it

It was a perfect day for crushing grapes.

We were preparing 660 pounds of Sangiovese and Merlot grown nearby in Castelnuovo di Farfa, in Italy’s Sabina region. That is me on the left looking perfectly at ease. The sun was shining, the setting was beautiful, and the grapes tasted wonderful. We were surrounded by ripe fruit, old stone buildings, and everything you might picture when imagining a day of making wine in the Italian countryside.

Look at this scene. Look at this setting. Look at these grapes. Even the cat lounging in the background was relaxed. What could possibly go wrong?

As the day unfolded, we crushed the grapes and meticulously removed the remaining stems by hand. Hours later, our palms were stained purple. We were tired, the tanks were finally filled, and I was ready to take my hydrometer reading.

That was when the screeching strings from Psycho started playing in my head.

The hydrometer rose past 1.110 and kept going.

That was where the scale ended.

Curiously, I had recently written another article called “How to Make Better Wine From Fresh Grapes.” Its central argument was simple: you should not decide what kind of wine you are going to make until you test the grapes in front of you.

Apparently, the grapes wanted to make sure I practiced what I preached.

When the Hydrometer Runs Out of Numbers

I have been making wine for many years, but I had never encountered a reading this high.

The hot, dry summer was probably part of the explanation. Some of the grapes were visibly dehydrated. As berries lose water, the sugar remaining in their juice becomes more concentrated. If enough dehydrated fruit was present throughout the harvest, it could have raised the sugar concentration of the entire must.

I cannot prove that the weather was solely responsible, but it was a likely contributor.

Either way, I could not determine the exact starting gravity because my hydrometer apparently had not anticipated grapes with quite this much ambition. Based on what I could observe, the must may have contained enough sugar to produce a wine approaching 15.5% or even 16% alcohol.

Even the cultured yeast I had selected was not rated to reliably ferment a wine that strong.

I felt more than a few pangs of panic. I needed to solve the problem, and I needed to pivot quickly. Six hundred and sixty pounds of grapes were already crushed and waiting for me. This was not a small experimental batch I could shrug off if things went badly.

I went into research mode.

Imagining the Alternative

As I worked through the problem, I imagined what these grapes might have become in the hands of someone who never tested them.

Crush the grapes. Let nature take its course. Assume they will become wine because that is what grapes are supposed to do.

In all likelihood, it would have been a mess.

For a spontaneous fermentation to finish this must dry, an unusually alcohol-tolerant native yeast would have needed to be present, outcompete everything else, take control, remain healthy, and continue working beyond the rated tolerance of the cultured yeast I had deliberately selected.

Could all of that have happened? Theoretically, yes. Realistically, it was a terrible bet.

I was not going to risk 660 pounds of grapes on it.

The more likely result would have been a fermentation that started normally, slowed as the alcohol increased, and stopped with considerable sugar remaining. The struggling yeast could have produced sulfur compounds and other unpleasant aromas and flavors. The residual sugar would then have left the wine vulnerable to unwanted microbial activity and volatile acidity.

If it eventually became drinkable at all, it probably would have been a flawed, semisweet table wine.

There is nothing wrong with semisweet wine when you intend to make one. Accidentally producing one because the yeast could not finish is another matter. That is not a stylistic decision. It is an unfinished fermentation.

How I Made the Correction

I had approximately 300 kilograms, or 660 pounds, of crushed grapes and needed to act quickly. Based on the estimated volume of the must, I decided to add 18 liters of water, an addition of roughly 7%. Because the original gravity was beyond my hydrometer’s scale, I could not calculate the correction with perfect precision. I chose a conservative amount and planned to let the next reading tell me whether it was enough.

Adding plain water would have lowered the sugar while also diluting the acidity. That is why winemakers use acidulated water, which replaces some or all of the acidity that would otherwise be lost.

A typical red must contains approximately 6 to 9 grams per liter of titratable acidity. Matching even the low end of that range in 18 liters of water would have required about 108 grams of tartaric acid. The calculation I received recommended more acid, but I deliberately reduced the addition to 80 grams. That produced acidulated water containing approximately 4.4 grams of tartaric acid per liter.

This was a conservative decision, not an attempt to establish an exact pH. Because the water represented only about 7% of the total must volume, using 80 grams instead of 108 reduced the overall acidity by only about one-tenth of a gram per liter compared with fully matching a TA of 6 g/L. That was a very small tradeoff, and one I was comfortable making.

My immediate priority was to lower the sugar without overcorrecting the acidity before fermentation. Skin contact, yeast activity, potassium, and tartrate precipitation can all change pH and titratable acidity. If the finished wine needs more acid, I can measure it and add tartaric acid later. Removing an excessive addition would be a much larger problem.

I also had two sensory clues that the must already contained substantial acidity. By the following day, it had spent hours on the skins, allowing enough pigment to enter the juice for me to evaluate its color. Lower-pH red must generally appears brighter and redder, while higher-pH must may look duller, grayer, or more purple. The deep ruby red color I saw was consistent with what I have learned to recognize as a suitable range for fermentation.

The taste reinforced that judgment. Even with an extraordinarily high concentration of sugar, I could still perceive a pleasant, rounded acidity. Sweetness suppresses the perception of acid, so if the acidity remained clearly evident through that much sugar, it was another strong indication that the must was not acid-deficient.

These are not judgments I would recommend that every winemaker make without testing. Grape variety, skin contact, oxidation, extraction, temperature, and sugar can all influence what you see and taste. I am also a painter with a particularly sensitive eye for color, and I have accumulated years of sensory memories from making wine.

A book can explain what different colors and flavors may indicate, but it cannot give you that memory. You develop it by tasting must, observing its color, following it through fermentation, and remembering what the finished wine became. Over time, those experiences begin to form recognizable patterns.

Had the color appeared gray, or had the acidity disappeared beneath the sweetness, I would have stopped, tested the pH and titratable acidity, and made another adjustment before pitching the yeast. If you do not have the experience to make that sensory judgment confidently, testing is the right course. A starting pH around 3.6 is a reasonable target for many red musts, provided the titratable acidity is also appropriate.

After incorporating the acidulated water, I tested the sugar again and confirmed that the specific gravity had fallen below 1.110. The wine will still be substantial, but the yeast now has a much more realistic chance of finishing it cleanly.

Testing Is Only Useful If It Can Change Your Mind

Had I skipped the original hydrometer reading, fermentation might have begun vigorously enough to convince me that everything was fine. The trouble would have appeared later, after the rising alcohol began overwhelming the yeast while substantial sugar remained.

By then, I would not have been making a controlled correction. I would have been trying to rescue 660 pounds of grapes from a fermentation already in distress. The smell might have announced the problem first. Stressed yeast can produce hydrogen sulfide, which smells like rotten eggs. Trust me, you do not want to encounter it.

That is why you test.

Measurements do not make wine for you. They tell you when the wine you planned to make may not be possible. They give you time to replace panic with research and prevent a difficult must from becoming a ruined one.

I told other winemakers to let the grapes lead. This year, mine made certain I was listening by sending my hydrometer off the scale, and my anxiety with it.