By the end of October the mills in the south are running, and two questions come back to Swiss kitchens that are the same every year. The first: my olive oil is cloudy and there is sediment at the bottom of the bottle — is something wrong with it? The second: I put it in the fridge and it turned thick and buttery — does that mean it is the real thing?

The short answer to both: cloudiness is usually not a defect, and the fridge tells you precisely nothing about whether an oil is genuine. Both can be backed up, and both are worth knowing, because one case asks you to use the bottle up quickly while the other asks only for a little patience.

Two kinds of cloudiness with nothing in common

Cloudy is not one thing. In practice there are exactly two causes, and they differ on a single decisive point: one goes away again, the other stays.

 Cold hazeUnfiltered oil
Triggercold: fridge, cold cellar, balcony in winterbottling without filtration after pressing
Looka veil, white flakes, a buttery massevenly hazy, often greenish, sediment at the bottom
Reversibleyes, completely, at room temperatureno, the sediment stays and keeps settling
Effect on qualitynonea shorter window in which to use it

Cold haze is pure physics. As the oil cools, part of the triglycerides crystallises out. In the laboratory that shows up in differential scanning calorimetry as heat released, and on warming the same crystals melt again as heat absorbed — Chiavaro and colleagues described both directions in monovarietal extra virgin oils, the cooling in 2007 and the heating in 2008 in the «Journal of Agricultural and Food Chemistry». That is exactly why a cold oil turns clear again at room temperature without anything having changed about its grade. For the bottle you cook with every day, a dark cupboard at 14 to 18 °C is the right place, not the fridge.

Why unfiltered oil is cloudy — and ages faster

The second kind of cloudiness is deliberate. An oil bottled as «unfiltered» or «non filtrato» keeps its suspended matter: tiny droplets of vegetation water plus particles of flesh and pit. Carlotta Breschi and colleagues describe exactly that composition in the «Italian Journal of Food Science» (2021), and make the point that turbidity is not a yes-or-no property but a scale made up of different proportions of water, insoluble solids and microbial load.

There is a price for it. The same group stored four variants of one oil for 240 days: naturally cloudy, filtered, with the solids only and with the vegetation water only. Hydrolysis — the splitting of the fats that drives acidity up — was marked in the cloudy and in the watery variant, while the filtered oil changed less. The cloudy samples also showed appreciable microbial activity, which produced volatile compounds and sensory defects. In work by Ciafardini and Zullo («Foods», 2022), more than 90 percent of the suspended solids in a freshly pressed Coratina oil settled to the bottom of the bottle within the first three months.

The picture is not entirely uniform: a study at the University of Perugia found no worse oxidative stability in cloudy oil than in filtered oil, and saw the rancid defect appear only after six months of storage. The International Olive Council recommends filtration all the same, because it removes solids and water and so extends shelf life; the great majority of commercial extra virgin olive oils are therefore filtered. A clear bottle on a Swiss shelf is the normal case, not grounds for suspicion.

For the kitchen that boils down to one simple rule: a naturally cloudy bottle is not a faulty product, but it is not a stockpile item either. Buy small, keep it on the table, finish it in weeks rather than in a year. When the new, mostly unfiltered oil reaches Switzerland and how to recognise a genuine harvest date is covered in our guide to new harvest oil.

The fridge test proves nothing

Which brings us to the second question. Since an American television appearance in February 2013, a rule of thumb has been doing the rounds: put the bottle in the fridge; if the oil goes solid it is real olive oil, if it stays liquid it has been cut with something else. The Olive Center at the University of California, Davis, tested that and published the result under the title «Refrigeration Is Not Reliable».

The experiment: seven samples of different composition in a laboratory fridge at 4.7 °C (40.5 °F) — roughly where a domestic fridge sits, since the German Federal Institute for Risk Assessment recommends no more than 7 °C and preferably below 5 °C. After 60 hours no solidification was visible in any sample. After 120 hours those samples containing at least 50 percent virgin olive oil showed a slight sediment, which became more obvious after 180 hours. Not one sample turned fully solid. The predominantly refined oils stayed liquid. The centre's conclusion: the test says neither whether an oil is really made from olives nor anything about its quality.

The reason lies in the chemistry. What sets the solidifying point is the fatty acid and triglyceride composition, not the trade grade. Paul Vossen, the olive oil specialist at the University of California, put it this way in 2013: solidification comes down to the saturated fatty acids, and how much of them an oil contains depends first of all on the olive varieties and only secondarily on the growing area. It says nothing about freshness, purity, flavour or the extra virgin classification. Chiavaro and colleagues showed the matching point: a higher share of unsaturated fatty acids shifts the onset of crystallisation to lower temperatures. Two faultless extra virgin oils therefore go solid at different temperatures.

That leaves the counter-test: does an adulterated oil pass? It does. The same research group blended high-oleic sunflower oil into extra virgin olive oil and found barely any difference on cooling in the calorimeter: the onset of crystallisation — which is exactly what you would see as «it is going solid» — shifted significantly only at 40 percent foreign oil («Journal of Food Lipids», 2009). Smaller additions, from around 20 percent, could be identified at all only because the researchers analysed the shape of the exotherms in the thermogram. What a precision instrument finds only in the shape of a curve, nobody sees with the naked eye on a bottle in the fridge. The North American trade association NAOOA accordingly calls the test completely false and misleading, and the sensory specialist Richard Gawel names the logical flaw: the test would only work if olive oil contained exclusively monounsaturated fatty acids and every other oil exclusively polyunsaturated ones. An olive oil with some rapeseed oil blended in goes solid in the fridge and passes the test without trouble.

What to do with a cloudy or solid bottle

  • Leave it standing. At room temperature a cold oil becomes clear and liquid again within a few hours. Nothing more is needed.
  • Not the microwave and not a hot water bath. If you are in a hurry, stand the bottle in hand-warm water. Heat speeds up every chemical reaction — as a rule of thumb the rate doubles for every 10 °C — and that costs aroma without gaining anything.
  • Do not make the fridge a permanent solution. For the oil you use daily, 14 to 18 °C in a dark cupboard is better; the fridge is worth it at most for a spare bottle you will not open for months.
  • With unfiltered oil, leave the sediment in the glass. Pour off the clear oil and do not shake the bottle before use; the sediment is the most fragile part.
  • In winter, think about the cold corners. A garage, an unheated cellar, a balcony or a larder against an outside wall drop easily below fridge temperature during a cold Swiss week. The bottle goes flaky there — a cosmetic flaw, not damage.

What does uncover real defects and real fraud

If the eye proves nothing, what does? At home, the nose. Rancid, fusty and musty are the three defects you can identify yourself in two minutes, and none of them shows up visually: an oil can be crystal clear and rancid, and a cloudy oil can smell impeccable. The step-by-step method is in our two-minute smell test.

Everything beyond that — whether an oil has been cut, wrongly declared or merely badly stored — is decided by a laboratory and a tasting panel: acidity, peroxide value, fatty acid and sterol profiles, plus the sensory assessment by trained tasters. Who runs the controls and how adulteration actually works is covered in our piece on olive oil fraud.

In practice that means: trust the verdict that comes out of a laboratory and a tasting, not the one from your fridge. The two oils that earned a «very good» in the 2025 Kassensturz and K-Tipp test are ordinary bottles from the Swiss shelf — their grade came from analysis and tasting.

What a litre currently costs on the Swiss shelf is in our overview of olive oil prices in Switzerland.

Frequently asked questions

Is cloudy olive oil bad?

Usually not. Either it was bottled unfiltered, in which case the cloudiness is intended, or it has been cold, in which case it disappears again at room temperature. What matters is the smell, not the look: a clear oil can be rancid and a cloudy one faultless.

My olive oil went solid in the fridge — does that make it genuine?

No. The UC Davis Olive Center kept seven samples at 4.7 °C for 180 hours; not one turned fully solid, and the test said nothing about whether an oil was made from olives. The solidifying point is set by the fatty acid composition, so above all by the olive variety — not by authenticity.

Can an adulterated olive oil go solid in the fridge?

Yes. In a 2009 study, blending in high-oleic sunflower oil shifted the onset of crystallisation significantly only at 40 percent foreign oil; smaller shares, from around 20 percent, could be identified only from the shape of the calorimeter curve, not from when the oil went solid. An oil with some rapeseed or sunflower oil blended in behaves in the fridge like a pure olive oil.

What is the sediment at the bottom of the bottle?

In unfiltered oil it is vegetation water plus particles of flesh and pit — «morchia» in Italian. Normal, but fragile: water and solids encourage hydrolysis and microbial activity, which is why a naturally cloudy oil should be used up promptly and not shaken up.

How do I get solid olive oil liquid again quickly?

Take the bottle out of the cold and leave it at room temperature; if you are in a hurry, stand it in hand-warm water. Not the microwave and not boiling water — the heat costs aroma and speeds up oxidation.

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