What Is Actually in Your Cooking Oil? Refining, Plastic, and Heat?

If you are looking for a non-toxic cooking oil, you will find plenty of fear and very little evidence. This post looks at what actually ends up in a bottle of cooking oil: what the factory adds or removes, what the heat in your pan adds, and what the container and the farm leave behind. It shows what the research has measured and where the measuring stops, so you can choose with more information.

The evidence comes from two 2025 papers. A review in OCL, a journal on oils and fats, looked at how oil is pulled from seeds and how much solvent is left behind. A study in the Journal of Food Science and Technology tested 18 commercial olive oils from the Spanish market, sold in six kinds of packaging, for plastic chemicals called phthalates and for metals. Both are small bodies of evidence, and this post says where each one runs out.

At a Glance: What the Research Has Measured •  A 2025 review found that pulling oil from seeds today relies on a solvent, and hexane is still the most efficient one. •  Across six earlier studies of oils for human consumption, 105 samples were tested for hexane. Ten had more than 1 mg/kg, and six of those ten came from one study of 10 oils. •  The most recent EFSA opinion on hexane dates to 1994. •  A 2025 study of 18 commercial olive oils found the phthalate DEHP in 6, at 0.1 to 0.23 mg/kg. The EU limit is 0.6 mg/kg. •  One extra virgin olive oil sold in PET plastic went over EU migration limits. Phthalates also turned up in some oils sold in glass, porcelain, and cardboard. •  Copper was found in all 18 oils, and above the 0.1 mg/kg limit in several. Arsenic and cadmium were not detected in any.

What Refining Does to a Cooking Oil

Olive oil is made by pressing the fruit. Most seed oils are not. The OCL review found that taking oil from seeds today relies on a solvent, and that hexane, a solvent made from petroleum, is still the most efficient one for the companies that crush oilseeds.

The natural worry is that the hexane stays in the bottle. The measured evidence is mostly reassuring. The review pulled together earlier work on refined oils. Of the nine studies on the subject, six covered oils meant for people to eat. Together they tested 105 samples and found ten with more than 1 mg/kg of hexane. Six of those ten came from one study of 10 oils in Iran, and a second Iranian study of 40 oils found nothing above 42.6 micrograms per kilogram. A British study of 23 refined oils found none above its detection limit of 50 micrograms per kilogram. An older study found that hexane was no longer detectable once the oil went through the final heating step of refining, called deodorizing.

That does not prove hexane is harmless in a bottle. The review points out that the latest EFSA opinion on hexane dates to 1994 and rests on a 90-day toxicology study from 1989, which cannot show what very small doses do over many years. The data is thin in both directions. The honest summary is that residues in properly refined oil are usually very low, and that nobody has measured them well.

For most bottles, the bigger difference is what refining takes out. Refined oils lose most of their color, smell, and flavor, and many of their natural plant compounds go with them. For how that plays out in olive oil, see Natural Olive Oil vs Refined Olive Oil: Which One Is Better?.

What Heat Adds That You Did Not Put In

Heat is the one thing you add to every cooking oil yourself. Every oil changes when it gets hot, and some change more than others. The details, including what forms when seed oils are heated to 180 degrees Celsius, are in How Seed Oils Compare to Olive Oil on Inflammation and Fat Quality. This post does not repeat those studies.

For whether extra virgin olive oil can handle a pan, see Can You Cook With Extra Virgin Olive Oil?. For what heat does to olive oil polyphenols in particular, see Does Heating Olive Oil Destroy Its Polyphenols?.

What matters here is simple. The oil you choose and how hot you take it both count, and the second one is in your hands. If the oil is smoking, the pan is too hot. Lower the heat, and if the oil smells burnt, start again with fresh oil.

What the Bottle Adds: Plastic, Glass, and the Cap

Phthalates are chemicals added to plastic to make it flexible. They are not locked into the plastic, and they dissolve easily in fat, which makes oil one of the foods where they would be expected to move. The authors of the Journal of Food Science and Technology study note that heat, sunlight, and long storage make the move easier.

The researchers bought 18 olive oils from Spanish shops over two harvest seasons. The oils came in glass, PET plastic, tin, porcelain, aluminum, and cardboard. The phthalate DEHP showed up in 6 of the 18, at 0.1 to 0.23 mg/kg, which is below the EU limit of 0.6 mg/kg for that chemical. Two other phthalates showed up in a few oils. One extra virgin olive oil in a PET bottle went over the EU migration limits, with one phthalate at 2.85 mg/kg against a limit of 1.8.

The findings do not make plastic a clean villain or glass a clean hero. Phthalates were found most often in PET, but also in glass, porcelain, and cardboard. The authors say that points beyond the bottle itself, to caps and to the production line, where oil passes through tubing and machinery before it is packed. Antimony, a metal used to make PET, was not detected in any of the oils.

What this supports is modest. Dark glass is still a sensible container because it protects oil from light, and plastic is the packaging with the clearest record of moving chemicals into oil. It does not support the idea that any bottle guarantees zero. Once the bottle is open, where you keep it matters too. See How to Store Olive Oil to Maximize Freshness and Polyphenol Retention.

What the Farm Leaves Behind

The same study measured metals. Copper turned up in every oil, and above the 0.1 mg/kg limit set by the International Olive Council in several of them. The authors tie this to copper-based sprays that growers use against a fungal disease of olive leaves, and say the sprays were sometimes applied too close to harvest. Zinc and aluminum also showed up. Neither has a legal limit in olive oil. Arsenic and cadmium were not detected in any of the 18 oils.

None of this is visible on a label. USDA organic certification covers how the olives were grown. For what it does and does not guarantee, see USDA Organic Extra Virgin Olive Oil: What the Certification Actually Guarantees.

How to Spot a Cooking Oil You Can Trust

No home test can reliably find hexane, phthalates, or copper in a bottle. What you can do is buy on process and traceability. A single-origin oil from a producer that says where the olives were grown and how fast they were pressed gives you more to check than a blend with no story. The studies above did not test whether this lowers contaminants, but a producer that answers those questions is one you can hold to account.

For a shopping method that works in any store, see How to Walk Out of Any Store With the Right Bottle of Olive Oil. For why some bottles are not what the label says, see Olive Oil Fraud: The Industry Problem and How to Protect Yourself. For why pressing speed matters to the oil inside, see Why Olive Oil Pressed Within Hours Is Superior.

What the Research Cannot Tell You Yet

No study here shows that swapping your cooking oil improves your health. The amounts of hexane and phthalates that researchers measured were mostly small. The hexane safety opinion is more than 30 years old. The packaging study tested 18 oils from one country and does not name brands, so one sample cannot speak for a product.

The strongest health evidence for olive oil comes from eating it as part of a diet, not from avoiding contaminants. For that evidence, see Olive Oil and Heart Health: What 40 Years of Research Has Found. Choosing a better cooking oil means knowing more about what goes into your food, enjoying better flavor, and getting the unique beneficial compounds naturally found in olives. It is about making a more informed choice about what you use every day.

 

Why Four Hour Olive Oil Takes Guesswork Out of the Bottle Polyphenols, the plant compounds behind much of olive oil’s flavor and benefit, start fading the moment olives are picked. Much grocery store oil has lost a large share of them before it reaches your shelf, and refined oils have lost most of them. Four Hour Olive Oil is pressed, not refined. It presses Ayvalik olives within 4 hours of harvest on the Aegean coast of Turkey. It is USDA Organic, single origin, and bottled in Italian dark glass, which makes it a true high-polyphenol olive oil with real flavor. Premium is made for everyday cooking. Ultra Premium is made for finishing and raw use. If you are going to swap your cooking oil, the oil should actually contain what you are choosing it for. Ultra Premium EVOO  |  Premium EVOO  |  Duo

Frequently Asked Questions

What is the healthiest cooking oil?

Extra virgin olive oil has the strongest body of research for heart health, and it holds up well in a pan. No single oil is best for every job, and the best choice also depends on how it was made and how fresh it is.

What is a non-toxic cooking oil?

There is no official definition. What can be said is that pressed oils skip the solvent step used for most seed oils, and that an oil from a single origin in dark glass has fewer unknowns. No oil is guaranteed free of every trace contaminant.

Is hexane in cooking oil dangerous?

In the studies reviewed, hexane left in refined oil was usually undetectable or very low. The most recent EFSA opinion dates to 1994, so the data is thin. Extra virgin olive oil is made by pressing, so no solvent is used.

Do plastic bottles leach chemicals into cooking oil?

A 2025 study of 18 olive oils found phthalates in several, most often in oils sold in PET plastic, and one PET oil went over EU migration limits. Most amounts were below legal limits. Heat, sunlight, and long storage make the move easier.

Is olive oil in glass bottles safer than plastic?

Dark glass protects oil from light, which matters for freshness. The same study still found phthalates in some oils sold in glass, which the authors link to caps and the production line. Glass is a sensible choice, not a guarantee.

What is the healthiest oil to cook eggs with?

Extra virgin olive oil works well for eggs at normal pan heat. Four Hour Premium Extra Virgin Olive Oil is made for this kind of everyday cooking. Keep the heat at medium and lower it if the oil starts to smoke.

What is the healthiest cooking oil for frying?

For pan-frying, extra virgin olive oil is a good choice. Deep frying is the hardest test for any oil because the heat is high and it lasts a long time, so we do not name a best oil for it.

What is the healthiest oil to cook with at high heat?

Extra virgin olive oil has a smoke point of about 375 to 405 degrees Fahrenheit, which covers sautéing, roasting, and pan cooking. For very high heat or long frying, no oil is a healthy option, and the best step is to keep the cooking short.

Are seed oils toxic?

That has not been proven. The research on omega-6 fats and inflammation is mixed, and the heat question is real. Our seed oils guide lays out both sides.

Is USDA organic olive oil free of pesticides?

USDA Organic rules bar synthetic pesticides in growing, but no certificate promises zero of everything. The 2025 study found copper in every oil it tested and linked it to fungicide sprays used on olive trees.

Which Four Hour oil should I use for cooking?

Use Four Hour Premium Extra Virgin Olive Oil for everyday cooking and Four Hour Ultra Premium Extra Virgin Olive Oil for finishing and raw use. Both are USDA organic, single origin, and pressed within 4 hours of harvest.

How should I store cooking oil?

Keep it in a cool, dark cupboard away from the stove, with the cap closed. Dark glass helps protect the oil from light.

References [1] Carré, T., et al. "Solvent Solutions: Comparing Extraction Methods for Edible Oils and Proteins in a Changing Regulatory Landscape. Part 7: Overall comparison between solvent solutions." OCL 32: 9. 2025. View study [2] Charfi, A., Moya López, A.J., Sánchez Villasclaras, S. "Food safety in the production of olive oils. Presence of heavy metals and phthalic acid esters using different types of packaging." Journal of Food Science and Technology. 2025. View study

The studies cited in this post are peer-reviewed and publicly available. This content is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional before making changes to your diet or health regimen.