The Hidden Truth About Heat-Sensitive Oils: Why Some Plant Oils Don’t Like the “Heat & Hold” Method
- Oct 7
- 6 min read
When we think of natural oils, it’s easy to assume they’re all created equal. After all, they’re just oils, right? Not quite. Some are robust little workhorses that can handle heat, while others are incredibly delicate — bursting with beneficial compounds that are easily destroyed by excessive temperature. In formulation, understanding which oils fall into which category can make the difference between a product that stays stable for months and one that oxidizes within weeks.
Let’s break it down.
The Science Behind Oil Sensitivity
Every oil is a blend of fatty acids and unsaponifiable matter (the “extra goodies” like vitamins, phytosterols, and antioxidants). Two key factors determine how heat-sensitive an oil is:
Degree of Saturation
Saturated fats (like coconut or palm) have no double bonds, making them very stable under heat.
Unsaturated fats (like sunflower, rosehip, or borage) contain double bonds — the more double bonds, the more reactive and prone to oxidation.
Iodine Value (IV) The iodine value tells you how many double bonds are present.
A low IV (<70) means the oil is mostly saturated and very stable.
A moderate IV (70–100) indicates some unsaturation — these oils can tolerate gentle heating.
A high IV (>100) signals heavy unsaturation — these oils are extremely sensitive and should never be exposed to prolonged heat.
Why the “Heat & Hold” Method Isn’t Always Ideal
The classic “heat and hold” method (heating both phases to 158 °F and holding for 20 minutes) is great for emulsifier consistency and microbial safety, but it can quietly destroy delicate oils behind the scenes. High-iodine-value oils can oxidize and degrade when exposed to sustained heat — but heat is only one piece of the puzzle.
Oxidation — the slow chemical reaction that turns beautiful golden oils into rancid, off-smelling ones — is accelerated by four main culprits:
1. Heat
Heat provides the energy needed to break double bonds in unsaturated fatty acids. The higher the iodine value, the more vulnerable those double bonds are. Once they’re broken, free radicals form, and oxidation takes off like a chain reaction.
Avoid: prolonged heating (especially above 140 °F)
Do: add sensitive oils during cool-down and minimize time at high temperature. I heat up my low IV oils first, then add my high IV oils after I've removed the beaker from the heat.
2. Oxygen
Oxygen exposure is a slow killer. Even at room temperature, every time you open a bottle or whip a formula, air mixes in and begins reacting with unsaturated lipids. That’s why you’ll notice high-linolenic oils like rosehip or borage turn rancid long before your coconut oil ever will.
Avoid: whipping or aerating during cool-down. Use an overhead stirrer if you have one. This is a slow, sweeping method of cool down.
Do: use airless packaging, nitrogen flushing, or tightly sealed containers
Include: chelators like disodium EDTA (0.05–0.2%) to neutralize metal ions that catalyze oxidation
3. Light (especially UV)
UV light acts as a catalyst, breaking molecular bonds and generating free radicals within the oil. It’s why you’ll see premium cosmetic oils packaged in amber or opaque bottles. Even short exposure to daylight can cause photo-oxidation — changing the oil’s color and destroying natural actives like carotenoids, chlorophylls, and tocopherols.
Avoid: clear jars and direct lighting during storage or photography
Do: use amber glass, opaque PET, or aluminum containers
4. Trace Metals
Tiny amounts of metal contamination (from mixing tools, pigments, or even tap water) can act like accelerants in oxidation reactions. Iron, copper, and nickel are the main offenders. Even a few parts per million can drastically shorten shelf life.
Avoid: uncoated metal tools and reactive pigments
Do: always add EDTA or sodium phytate as a chelator, and choose stainless steel or silicone tools when working with oils
5. The Solution: Antioxidant & Packaging Strategy
You can’t stop oxidation completely — but you can dramatically slow it down. Combine several strategies:
Add Antioxidants:
Mixed tocopherols (0.2–0.5%)
Rosemary extract (0.05–0.1%)
Ascorbyl palmitate (0.05–0.1%)
Control Oxygen: minimize headspace and use airless pumps
Block Light: store in dark, cool places
Chelate Metals: EDTA or sodium phytate in the water phase
Together, these steps create a hurdle system that helps protect your delicate oils long-term — even those with high iodine values.
Specialty Oils That Dislike Heat
These beauties offer potent skin benefits but hate extended heat exposure. Their iodine values typically exceed 100, making them highly unsaturated and oxidation-prone:
Black currant seed oil 107-123 IV
Borage seed oil 140-155 IV
Camelina seed oil 124-153 IV
Evening primrose oil 147-155 IV
Kukui nut oil 136-175 IV
Rosehip oil 152-169 IV
Sea buckthorn oil 130-200 IV
Walnut oil 135-151 IV
Handle them gently — add them during cool-down, ideally under 45 °C, and always pair with antioxidants like mixed tocopherols or rosemary extract.
Comparing High vs. Low–Medium Iodine Value Oils
Property | High Iodine Value Oils (>100) | Low–Medium Iodine Value Oils (<100) |
Examples | Rosehip • Borage • Evening Primrose • Black Currant • Kukui Nut • Sea Buckthorn • Camelina • Walnut | Coconut • Babassu • Jojoba • Meadowfoam • Olive • Avocado • Macadamia • Shea Butter |
Fatty Acid Profile | Rich in polyunsaturated fatty acids (PUFAs) — especially linoleic, linolenic, and gamma-linolenic acids | Rich in monounsaturated or saturated fatty acids, like oleic, palmitic, lauric |
Iodine Value Range | Typically 100–190 | Typically <70–100 |
Degree of Unsaturation | Very high (many double bonds) | Low to moderate |
Oxidative Stability | Poor — oxidizes rapidly, even at room temperature | Strong — resists oxidation and rancidity |
Heat Tolerance | Low — should not be heated or held above 113-122 °F | High — can withstand 158-176 °F heating for emulsification |
Recommended Usage | Add during cool-down phase (1–5 %) as actives | Safe for heated oil phase and bulk formulation (10–90 %) |
Unsaponifiable Content | Often high, rich in carotenoids, tocopherols, phytosterols, and squalene | Moderate to low; more stable but fewer “nutrients” |
Shelf Life | Short (3–9 months, even shorter if poorly stored) | Long (1–3 years with proper storage) |
Storage Needs | Refrigerate or store in dark, airtight amber bottles; always use antioxidants | Cool, dry storage; antioxidants optional |
Best Formulation Role | Functional or treatment oils, high-value actives, facial serums, after-emulsification enrichers | Structural oils and emollients in lotions, butters, balms, cleansers |
Typical Sensory Feel | Light, fast-absorbing, “dry,” may feel slightly unstable over time | Smooth, cushiony, more consistent texture |
Key Takeaway
High iodine value = high unsaturation = high sensitivity. Use those oils as your luxury touchpoints in a formula, not your backbone. Let the low-to-medium iodine oils carry the structure and heat load — then layer in your specialty oils cold for maximum skin benefit and shelf stability.
Don’t Forget the Unsaponifiables
The unsaponifiable fraction — all those wonderful components like phytosterols, squalene, tocopherols, and carotenoids — is what makes specialty oils so valuable.
Unfortunately, these exact compounds are the ones most easily changed or destroyed by heat and prone to degradation. Preserving them is the whole point of cold-processing your formula.
So while high-IV oils are “nutrient dense,” they’re also the ones you have to baby. Think of them as delicate actives, not structural oils. and prone to degradation.
Balancing Stability and Benefit
A good rule of thumb for emulsion or balm formulation:
Use stable oils (low IV, like coconut, jojoba, meadowfoam, or hydrogenated esters) in your heated phase.
Use delicate, high-IV oils in your cool-down phase, at small percentages (1–5%), supported with antioxidants.
Store finished products away from light, air, and heat.
This approach maintains emulsion stability and protects the therapeutic compounds that make these oils special.
Final Thoughts
Not all plant oils can take the heat — and that’s okay. When you understand iodine values, fatty acid saturation, and the importance of the unsaponifiable fraction, you can build smarter, more stable formulations that truly perform. That's not to say that you CAN'T heat your specialty oils, just be cautious when doing so.
BUT, don't take MY word for anything. Do some independent research from reputable sources, NOT chatgpt, NOT facebook groups, NOT mommy bloggers.
One fantastic source is https://chemistscorner.com/ , filled with REAL cosmetic chemists.
Another source is https://labmuffin.com/ , Michelle Wong is a cosmetic chemist & PhD.
The 2 ladies behind this source https://chemistconfessions.com/ Gloria & Victoria, are cosmetic chemists and VERY funny.
You can look up individual ingredients here: https://www.cir-safety.org/
You can find a wealth of information here: https://www.cosmeticsinfo.org/product-category/skin-care/
~Lissa~
