How much to press white grapes: the secret your glass never tells
The difference between an elegant white wine and one that ruins your morning may come down to the final minutes at the press. Here we explain what happens at the chemical level — and why every bar of pressure matters in our winery.
There is a question we hear often during winery visits: “What do they put in wine that gives me a headache?” The honest answer is that the cause is almost never something that was added — it is something that was poorly extracted during production. And one of the most critical moments — and the least visible to the consumer — happens at the pneumatic press, when the decision is made about how hard to squeeze white grape pomace.
Understanding this process requires no winemaking degree. You only need to know that the grape, like any fruit, holds different juices in different parts. And that not all those juices are created equal.
The white grape is not a homogeneous sponge
When harvest arrives at the winery and white grapes enter the press, the first juice to fall is what we call free-run must: the juice released simply by the weight of the grapes themselves, before any pressure is applied. This must is the cleanest, the freshest, the most aromatic. It is the pure essence of the variety.
As pressure increases, what we call press fractions begin to emerge. The problem is that these fractions do not only bring more juice — they also bring compounds from parts of the grape we do not want in the wine.
0 – 0.3 bar. Gravity-released juice, zero pressure. Maximum aromatic delicacy, minimal extraction of unwanted compounds. The gold standard of pressing.
0.3 – 0.8 bar. Excellent quality. Well-balanced in acidity and aromas. The vast majority of fine white wines are made from this fraction alone.
0.8 – 1.5 bar. Intermediate quality. Phenolic compounds begin to appear. Used in lower-tier products or discarded entirely.
Above 1.5 bar. This is where the problems begin. High extraction from seeds, stems, and compounds that add bitterness and can cause discomfort.
“Free-run must is like squeezing an orange with an open hand. The tail press is like crushing the whole fruit under a rock.”— A classic analogy in white wine enology
What compounds are released under excessive pressure?
This is the part that matters most to consumers who want to understand why certain wines disagree with them. The short answer: high pressure breaks open the seeds and the toughest tissues of the berry, releasing compounds that have no business being in wine.
Phenolic compounds from the seed
Grape seeds contain catechin tannins and proanthocyanidins. In the skin these tannins are soft and ripe; in the seed they are astringent, bitter, and in some cases pro-inflammatory when consumed in excess. A white wine produced with a high press fraction often shows that characteristic dry bitterness at the back of the palate.
Biogenic amines
Histamine, tyramine, and putrescine are amines that form during fermentation, especially when the must carries a high solids load. These suspended solids act as a substrate for the bacteria that generate these amines. A sensitive person may experience headaches, facial flushing, or digestive discomfort even in wines with low alcohol content.
Biogenic amines are not unique to wine — they are also present in aged cheeses, cured meats, and fermented foods. Anyone with histamine sensitivity will notice it across multiple foods, not just wine. However, poorly pressed must can significantly amplify these levels in the finished wine.
Oxidized lipids
High pressure ruptures the cell membranes of the grape skin and seed, releasing fatty acids that oxidize rapidly. These compounds contribute rancid notes to the wine, diminish aromatic freshness, and can interfere with fermentation by inhibiting yeast activity.
The pressure table: a practical reference guide
As a practical summary, here is how we work in the winery and how we interpret each pressure range:
| Pressure range | Fraction | Quality | Risk |
|---|---|---|---|
| 0 – 0.3 bar | Free-run must | Exceptional | No risk |
| 0.3 – 0.8 bar | First press | Very good | No risk |
| 0.8 – 1.2 bar | Second press (low) | Acceptable | Monitor |
| 1.2 – 1.5 bar | Second press (high) | Low | Increasing |
| Above 1.5 bar | Tail press | Not recommended | High risk |
Why don’t all wineries work the same way?
The answer is fundamentally economic. Using only free-run must and first press means discarding between 15% and 25% of the potential wine volume per pressing cycle. For an industrial-scale winery, that percentage translates to millions of litres per year. The financial pressure to use every last drop is enormous.
On top of that, the use of fining agents, aggressive filtration, and sulfur dioxide adjustments can partially “clean up” a low-quality must — masking problems that nonetheless remain in the wine as compounds undetectable by nose or palate, but felt by the bodies of sensitive individuals.
Small-production wines, single-estate labels, and single-varietal whites focused on freshness tend to come from wineries that strictly control their press fractions. References to “free-run must,” “low-temperature fermentation,” or “slow-cycle pneumatic press” in technical sheets are all good signs to look for.
What we do
At our winery we work with membrane pneumatic presses, which distribute pressure evenly and gradually, minimizing mechanical rupture of the seeds. Our standard protocol for varietal whites is never to exceed 0.8 bar, and for our premium lines we work exclusively with free-run must and first press.
After pressing, we subject the must to cold static settling at low temperature (between 6 and 8 °C) for 12 to 18 hours, allowing suspended solids to sediment before fermentation begins. This step is key to reducing the potential biogenic amine load and ensuring a clean, precise fermentation.
- 1Membrane pneumatic press with a programmed cycle and pressure recorded per lot.
- 2Fraction separation: free-run must and first press go into separate tanks from the very first drop.
- 3Cold static settling before alcoholic fermentation begins.
- 4Turbidity monitoring (NTU) to ensure the must entering the fermentation tank is within optimal range.
- 5Tail press fractions are directed to pisco production or discarded entirely — they never appear in our wine labels.
In summary
A white wine that agrees with you is no accident. Behind that glass lie precise technical decisions: at what pressure the press was stopped, how the must was clarified, what temperature was used during fermentation. These are decisions that never appear on the label, but that the body registers with perfect clarity.
The next time you choose a white wine, look for producers who speak openly about how they make it. A winemaker who explains their pressing process is a winemaker who has something worth saying.
Questions about how we make our white wines?
Visit our winery or get in touch. We love sharing our process — glass in hand.
