What oils should you not heat and why it matters for your health?

Overheating certain cooking oils can release harmful free radicals and toxic compounds that can damage cells in the body. To protect your health and the integrity of your food, it is crucial to know what oils should you not heat, especially for high-temperature cooking methods like frying.

Quick Summary

Heating oils beyond their smoke point can produce harmful toxins and deplete nutritional value. This guide covers which specific oils to avoid heating and explores the reasons behind their instability for healthier cooking choices.

Key Points

• Low Smoke Point: Avoid heating flaxseed and walnut oils, as their low smoke points cause them to break down and become rancid, losing their health benefits.

• Unstable Fats: Standard seed oils high in polyunsaturated fats, like corn and sunflower, are unstable and can form harmful compounds when heated, especially repeatedly.

• Refined vs. Unrefined: Unrefined oils have lower smoke points than their refined counterparts and are generally better for low-to-medium heat cooking or cold applications.

• EVOO Use: While more stable than perceived, extra virgin olive oil is best for moderate-heat cooking rather than high-heat frying to preserve its antioxidants.

• Toxicity: Reheating any cooking oil, particularly for deep frying, significantly increases the concentration of harmful, toxic compounds like aldehydes.

• Proper Storage: Store delicate oils like flaxseed and walnut in the refrigerator and in dark bottles to prevent oxidation and rancidity.

The Science Behind Oil Instability

When cooking oil is heated, it eventually reaches a specific temperature known as its 'smoke point.' At this point, the oil begins to break down, releasing free fatty acids and producing visible smoke. This process not only imparts an acrid, burnt flavor to your food but can also generate toxic and potentially carcinogenic compounds such as aldehydes and lipid peroxides. Repeatedly heating oil, a common practice in many commercial kitchens, significantly increases the concentration of these harmful byproducts.

The Fatty Acid Connection

The stability of an oil is largely determined by its fatty acid composition. Polyunsaturated fats (PUFAs) are more fragile than monounsaturated or saturated fats because their chemical structure contains multiple double bonds, which are susceptible to oxidation when exposed to heat, light, and air. This is why oils high in PUFAs are a poor choice for high-heat applications.

Oils to Avoid Heating

Flaxseed Oil

Flaxseed oil is one of the most unstable and heat-sensitive oils due to its high concentration of omega-3 fatty acids, a type of PUFA. With a very low smoke point (around 225°F or 107°C), it should never be used for cooking or frying. Heating flaxseed oil destroys its delicate nutritional properties and can lead to the formation of harmful compounds. This oil is best used raw, drizzled over finished dishes or added to salad dressings and smoothies.

Walnut Oil

Similar to flaxseed oil, walnut oil is rich in PUFAs and has a low smoke point. It offers a delicate, nutty flavor that is best preserved by using it in cold applications. Heating walnut oil will cause it to go rancid quickly, spoiling both its taste and health benefits. Like flaxseed oil, it should be refrigerated and used for cold dishes or as a finishing oil.

Certain Seed and Vegetable Oils

Some widely used seed and vegetable oils, including standard sunflower, corn, and soybean oils, are high in omega-6 polyunsaturated fats. While some brands offer 'high-oleic' versions that are more stable, the standard versions are prone to oxidation when heated, especially for prolonged or repeated frying. Many restaurants use and reuse these oils, concentrating the level of toxic aldehydes and other byproducts over time. For most home cooking, there are better, more stable options.

Extra Virgin Olive Oil for High Heat

Extra virgin olive oil (EVOO) has a lower smoke point (350–410°F or 177–210°C) than its refined counterpart. While it is more stable than many believe due to its high antioxidant content, using it for high-heat searing or deep frying risks passing its smoke point and diminishing its potent anti-inflammatory properties. EVOO is a superior choice for sautéing, baking, or roasting at moderate temperatures.

Refined vs. Unrefined Oils

The refining process plays a significant role in an oil's heat tolerance. Unrefined oils, like extra virgin olive oil or cold-pressed sesame oil, contain more impurities (like proteins and other compounds) that cause them to smoke and break down at lower temperatures. Refined oils have these impurities removed, resulting in a cleaner oil with a higher smoke point. However, this process can also strip away some of the oils' beneficial nutrients. Always check the label and choose the right oil for the specific heat level you intend to use.

Best Practices for Cooking with Heat-Sensitive Oils

• Use low-heat oils correctly: Reserve flaxseed, walnut, and unrefined sesame oil for finishing dishes or making salad dressings. They add flavor and nutrients best when uncooked.
• Store oils properly: Heat, light, and air accelerate rancidity in all oils, but especially delicate ones. Store your oils in cool, dark cabinets, preferably in dark glass bottles. Consider buying smaller bottles of delicate oils and refrigerating them to prolong freshness.
• Choose the right oil for the job: For high-heat cooking like frying and searing, opt for refined, high smoke point oils such as avocado oil (520°F/271°C) or refined coconut oil (450°F/232°C).

The Problem with Reused Oil

Many restaurants reuse frying oil multiple times, creating an environment where toxic compounds accumulate significantly. While at-home cooking is less frequent, reusing oil from a previous high-heat cooking session should still be avoided. This practice intensifies oxidation and further degrades the oil, making it an unhealthy choice. After deep frying or a particularly hot sauté, it is best to dispose of the used oil properly and start with a fresh batch for your next meal.

Comparison Table: Oil Stability and Usage

Oil Type	Refined or Unrefined	Typical Smoke Point	Best Cooking Use
Flaxseed Oil	Unrefined	225°F / 107°C	Salad dressings, dips, finishing dishes (never heat)
Walnut Oil	Unrefined	320°F / 160°C	Salad dressings, sauces, cold dishes (never heat)
Standard Sunflower Oil	Refined/Unrefined	225-450°F / 107-232°C	Avoid for high heat due to high PUFA content
Extra Virgin Olive Oil	Unrefined	350-410°F / 177-210°C	Sautéing, roasting (low to medium heat)
Refined Coconut Oil	Refined	450°F / 232°C	Frying, searing, high-heat cooking
Avocado Oil	Refined	520°F / 271°C	Frying, grilling, high-heat cooking

Conclusion

Understanding an oil's smoke point and fatty acid profile is key to making healthier cooking decisions. Heating oils like flaxseed and walnut beyond their tolerance point destroys their nutritional benefits and introduces harmful byproducts. For robust, high-heat cooking, rely on stable fats like avocado or refined coconut oil. For moderate-heat sautéing and dressings, extra virgin olive oil is an excellent choice that offers beneficial antioxidants. By choosing the right oil for the right application, you can ensure both the safety of your food and the preservation of its flavor. As a final rule of thumb, always avoid reheating cooking oil to prevent the accumulation of toxic compounds.

For more guidance on healthy eating, refer to the World Health Organization's advice on fats.

Proper Storage of Unstable Oils

To maximize the shelf life and preserve the quality of your most delicate oils, follow these storage tips:

• Refrigerate: Oils high in PUFAs, like flaxseed and walnut, should be stored in the refrigerator to slow down the oxidation process that leads to rancidity.
• Use Dark Bottles: Light is another catalyst for oxidation. Ensure your heat-sensitive oils are in dark, opaque bottles to protect them from light exposure.
• Buy Small Quantities: Because these oils have a shorter shelf life, purchasing smaller bottles ensures you will use them before they go rancid.
• Seal Tightly: Air is a primary threat to oil freshness. Always replace the cap tightly after each use to minimize air exposure.

Frequently Asked Questions

What happens if you heat flaxseed oil?

Heating flaxseed oil causes its delicate omega-3 fatty acids to degrade rapidly and oxidize, leading to a loss of nutritional value and the formation of potentially harmful compounds. It should only be used in cold applications.

Is it safe to cook with extra virgin olive oil at high temperatures?

While extra virgin olive oil is more stable than many believe due to its antioxidants, its moderate smoke point means it is best suited for low to medium-heat cooking. High-heat applications risk degrading its beneficial properties and flavor.

Why are some seed oils bad for high-heat cooking?

Standard seed oils like corn, soy, and sunflower are high in polyunsaturated fats, which are chemically unstable when exposed to high heat. This instability leads to oxidation and the production of toxic compounds.

Can I reuse cooking oil?

Reusing cooking oil is not recommended, as each heating cycle causes further degradation and concentrates toxic compounds. This is especially true for deep-frying oil, which should be discarded after use.

Which oil is best for deep frying?

For deep frying, you should use a refined oil with a very high smoke point and good oxidative stability. Recommended options include refined avocado oil or refined coconut oil.

What is the difference between refined and unrefined oils regarding heating?

Refined oils have had impurities removed, giving them a higher smoke point and making them more suitable for high-heat cooking. Unrefined oils contain more natural compounds, resulting in a lower smoke point and making them better for cold or low-heat uses.

How should I use walnut oil?

Walnut oil has a low smoke point and is best reserved for cold applications. Use it in salad dressings, pestos, or drizzled over finished meals to add its signature nutty flavor.