What is stone-pressed olive oil? Stone mills, presses and what the label means

In Turkish markets, village bazaars and online listings, the phrase taş baskı zeytinyağı, "stone-pressed olive oil", turns up again and again, usually as shorthand for a more natural, better oil. There is a real production method behind the phrase. But that method is not defined in the regulations, and on its own it guarantees nothing about quality.

In short

"Stone-pressed" is the everyday name for two separate operations: crushing the olives in a granite stone mill and squeezing the resulting paste in a hydraulic press after spreading it on mats or bags. The "stone" part describes the crushing step; the "pressed" part describes the step in which the oil is separated from the solids.

In most mills in Türkiye today, the olives are broken up in a metal crusher, the paste is kneaded in a malaxer and the oil is separated in a horizontal centrifuge (a decanter). We describe that layout in how cold-pressed olive oil is made. Stone pressing is the name for the traditional setup that came before this continuous system.

What happens in a stone mill

In a traditional mill, two or three large granite stones turn inside a stone basin, grinding the olives, pits included, into a paste. A metal crusher does the same job far faster with knives, hammers or discs.

A stone mill also partly mixes the paste as it grinds. The geographical indication document for Milas olive oil says as much, indirectly: according to the document, kneading the paste after crushing is needed "especially when metal mills are used". We explain why kneading matters in what is a malaxer.

The pressing step: bags and the hydraulic press

The paste from the stone mill is spread onto round bags or mats, which are stacked and squeezed under hydraulic pressure. Oil and vegetation water seep out through the mats, while the solids (pomace) stay inside. The oil and water are then separated, either by letting them settle naturally or with a centrifuge.

The method still appears in official documents. The production specification of the North Aegean Olive Oils designation of origin, registered under number 392, lists three ways of extracting the oil: hydraulic pressing, percolation and continuous centrifugation. Pressing is defined as spreading the paste inside filter bags and applying hydraulic pressure to it; if water is used during the process, its temperature must not exceed 30 °C. The same document says the crushing may be done either in metal crushers or in stone mills (North Aegean olive oils).

That leads to an important point: the stone and the press are independent of each other. One mill may grind with stones and separate with a decanter; another may crush with metal and separate in a press. The words "stone-pressed" on a label do not tell you which of these was done.

Is stone-milled oil better? What the measurements show

Two studies that compared crushing methods directly point in the opposite direction from popular belief.

In a study carried out with the Aglandau variety in the Vaucluse region of France and published in 2009 in Comptes Rendus Chimie (volume 12, issue 8), Veillet and colleagues processed olives with a stone mill and with a hammer crusher:

The oil from the hammer crusher had about 35 per cent more phenolic compounds and a markedly stronger antioxidant activity. The researchers' micrographs showed that the hammer crusher cut the fruit cells more cleanly, while in the stone mill the cell layers were crushed and damaged. On acidity, both methods stayed far below the extra virgin limit.

A study by Preziuso and colleagues, published in 2010 in the European Journal of Lipid Science and Technology (volume 112, issue 12), compared three setups in Italy: a metal crusher with moving knives, a granite stone mill, and the two used together. It found no statistically significant difference in oil yield or in quality parameters; every oil qualified as extra virgin. Once again the difference showed up in the phenolic compounds: the metal crusher gave oils with more phenols that were more bitter and pungent, while the stone mill gave a milder oil.

So the real difference of a stone mill lies in flavour: a less bitter, less pungent, softer oil. That is a matter of preference. But the claim that stone-milled oil is "more nutritious" is not supported by the measurements; on phenolic content the picture can actually be reversed. We explain where bitterness comes from in why is olive oil bitter and pungent, and what phenols do in oleocanthal and polyphenols.

The weak point of the press: clean mats

In a press system, the factor that most determines quality is not the machine but the mats. The defect guide of the California Olive Oil Council (COOC) notes that the defect tasters call "winey", or vinegary, used to be more common before modern mills replaced hydraulic presses. The cause was mats that had not been properly cleaned: fresh olive paste came into contact with the residues of earlier batches that had fermented in the mats, in other words with acetic acid, ethyl acetate and ethanol. The guide's advice is plain: if a hydraulic press is used, the mats must be properly cleaned or replaced with new ones.

That is why "it came off a press" is not a measure of quality on its own. A well-kept press can turn out excellent oil; a neglected one can turn even the best olives into a defective oil. The full list of tasting defects is in tasting olive oil.

What "stone-pressed" guarantees on a label

Nothing. In the labelling article of Türkiye's Food Codex Communiqué on Olive Oil and Olive Pomace Oil (No. 2017/26), the only defined wording related to pressing is "first cold pressing": it may be used only for extra virgin and virgin olive oils obtained below 27 °C from the first mechanical pressing of the olive paste with a hydraulic press. Article 5 of the European Union's Implementing Regulation No 29/2012 sets the same condition. A producer who prints that phrase must be able to show both the press and the temperature limit. The details are in what does first pressing olive oil mean.

"Stone-pressed", by contrast, does not appear in the communiqué, so there is no defined condition it has to meet. Even so, Türkiye's Food Codex Regulation on Food Labelling and Consumer Information states that the information on a label must not mislead about the method by which a food was manufactured or obtained. Printing "stone-pressed" on oil that came out of a centrifuge mill conflicts with that principle.

The Ministry of Agriculture and Forestry's list of counterfeit and adulterated foods shows that the name does not protect quality either. The list currently published on the ministry's website contains 55 olive oil entries dated between 22 April and 12 October 2026; in 51 of them seed oils had been mixed in. Two entries carry the words "stone-pressed" in the product name: both belong to two different companies in the Efeler district of Aydın, are dated 22 July and 20 August 2026, and in both the finding was the addition of seed oils. The most recent batch is broken down in the 12 October adulteration list.

What to check when buying stone-pressed oil

1. Grade: does the label say "extra virgin olive oil"? That is the binding term.

2. Harvest year and best-before date: freshness comes before method.

3. "First cold pressing": if present, it is a defined claim that requires a press and the 27 °C limit.

4. Analysis: if acidity is printed, peroxide value and the other parameters must be printed too.

5. Smell: a sour, vinegary or winey note may point to a defect from the mats.

6. Ask the producer: was the crushing done with stones or a metal crusher, and was the oil separated in a press or a decanter? The two answers are separate.

To read the whole label, see how to read an olive oil label; to follow the traces of fake oil, see olive oil fraud and adulteration.

A note of caution

Studies comparing crushing methods are few, and each covers a single variety and a single season. The results can change with the ripeness of the olives and with the length and temperature of kneading. The figures above do not show that one method is superior in every situation; they show that the word "stone", on its own, is not a measure of quality.