Continuous High-Heat Processing Degrades Cold-Pressed and Refined Edible Oils

A new laboratory study from India has put one of the most familiar claims in food marketing to a controlled stress test: that cold-pressed oils are nutritionally superior to refined oils. Researchers at the ICMR-National Institute of Nutrition in Hyderabad heated twelve widely consumed edible oils continuously at 230 ± 5 °C and tracked a battery of physicochemical and nutritional markers over four hours. Their findings, published in Frontiers in Nutrition, show that prolonged high-heat exposure degrades every oil tested, but that the degree of degradation depends far more on how unsaturated an oil is to begin with than on whether it was cold-pressed or refined. Cold-pressed groundnut, sunflower, rice bran and palm oils actually retained more polyunsaturated fatty acids during the first hour of heating than their refined counterparts, yet suffered greater losses after four hours of continuous thermal stress.

The experimental design was deliberately grounded in real-world conditions. The team purchased seven cold-pressed oils (groundnut, sunflower, rice bran, palm, mustard, sesame and coconut) and five refined oils (groundnut, sunflower, rice bran, palm and soybean) from three local markets in Hyderabad in January 2025. For each oil type, three commercial brands were bought from each market, yielding nine individual units that were pooled into a single composite sample. The heating temperature and durations of one, two and four hours were chosen after surveying street vendor stalls, where oil is commonly kept hot all day; the four-hour exposure was designed to simulate the thermal stress a single batch of oil experiences in one day of continuous frying. Each composite oil was heated in 100 mL aliquots in open stainless-steel containers on a calibrated hotplate, with three independently heated batches per time point serving as replicates.

Fatty acid profiling by gas chromatography revealed the central pattern of the study. Across all oils, continuous heating depleted total polyunsaturated fatty acids while proportionally enriching saturated and, in most cases, monounsaturated fractions. This redistribution reflects oxidative cleavage of the carbon-carbon double bonds that make polyunsaturated fatty acids the preferred targets of thermal free-radical attack, since their bis-allylic hydrogens are far more weakly bonded than those in monounsaturated molecules. After one hour, cold-pressed rice bran oil held its polyunsaturated content at 31.35 percent from a starting 33.10 percent, and refined sunflower oil declined from 59.44 to 56.19 percent, while refined palm and cold-pressed coconut oil showed the steepest early losses. By four hours, refined soybean oil had fallen from 61.20 to 40.65 percent polyunsaturated fatty acids, and cold-pressed sesame oil from 40.84 to 25.55 percent.

The most striking nutritional finding concerns trans fatty acids, which were negligible in unheated cold-pressed oils but accumulated progressively in every oil during heating. Refined soybean oil showed the sharpest rise, climbing from 0.76 to 5.88 percent after four hours, followed by cold-pressed sesame oil, which rose from 0.16 to 4.05 percent. Cold-pressed coconut oil recorded the lowest accumulation at just 0.38 percent. Trans isomers form when oxidative radical attack breaks the π bond of a double bond, generating unstable allyl radicals that rotate freely around the adjacent single bond before re-forming a trans-configured double bond, which is thermodynamically more stable than the cis form. The authors note that high dietary trans fatty acid intake is associated with increased risk of coronary heart disease, dyslipidemia, cardiovascular disease and metabolic dysfunction-associated steatotic liver disease.

Peroxide value, a standard marker of early-stage lipid oxidation, displayed oil-specific trajectories that reveal the two-stage chemistry of thermal degradation. All unheated oils fell within Codex limits except cold-pressed palm oil at 15.39 mEq O2/kg. Refined sunflower oil exceeded the limit after four hours, reaching 15.81 mEq O2/kg, while both rice bran oil types declined continuously to 0.01 mEq O2/kg after two hours. Cold-pressed coconut oil spiked to 28.62 mEq O2/kg after one hour before falling back. The authors interpret these declines as evidence that unstable lipid hydroperoxides fragment under continued heating, releasing free radicals that rearrange into conjugated diene and triene structures. Indeed, conjugated dienes rose sharply in most oils, with refined soybean oil reaching 14.07 after two hours, and conjugated trienes climbed steadily throughout, though they remained below diene levels in all samples. The transition from primary to secondary oxidation marks the advanced stage of oil deterioration.

One counterintuitive result was the consistent decline in free fatty acids and acid values across all oils. Cold-pressed palm oil, which began with the highest values at 8.26 percent oleic acid and 16.44 mg KOH/g, fell to 0.18 percent and 0.37 mg KOH/g after four hours. The explanation lies in volatilization: free fatty acids possess higher vapor pressure than intact triglycerides and escape from the oil matrix at elevated temperatures, a mechanism that parallels industrial deodorization, where palm oil refined at 230 to 260 °C achieves final free fatty acid content below one percent. Saponification value, which reflects average fatty acid chain length, peaked in cold-pressed coconut oil at 321 ± 0.68 mg KOH/g, consistent with chain scission into shorter fatty acids, and was lowest in cold-pressed sunflower oil at 213 ± 0.83 mg KOH/g. Refined rice bran oil showed marked fluctuation, which the authors attribute to a temporary induction period conferred by its native antioxidants, oryzanol and tocopherols.

Iodine value, a measure of total unsaturation, declined progressively in all oils, with the largest relative reduction in cold-pressed coconut oil and substantial decreases of roughly 30 percent in both palm oil types, while refined soybean oil showed the minimal decline of 6.72 percent. Refractive index generally rose with heating duration, reflecting the formation of conjugated double-bond systems and high molecular-weight oxidation polymers that increase the electronic polarizability of the oil. Colour measurements told a simpler story: the lightness value, L*, decreased continuously in every oil, with cold-pressed sesame oil showing the most pronounced darkening. The authors attribute this progressive browning to oxidative polymerization of unsaturated fatty acids into polar dimers and oligomers, degradation of natural pigments such as carotenoids, and condensation of aldehydes and ketones into brown polymers through aldol condensation and radical-mediated polymerization.

Principal component analysis consolidated the dataset into a coherent chemical narrative. The first two components explained a cumulative 64.4 percent of variance, with the first axis separating highly saturated oils from polyunsaturated ones through strong loadings on iodine value, refractive index and polyunsaturated content against saturated content. The second axis captured hydrolytic and isomerization changes, driven strongly by trans fatty acid loading. Unheated oils clustered near lightness, polyunsaturated content, iodine value and acid value, while oils heated for two and four hours shifted toward trans fatty acids, conjugated dienes and trienes. Coconut and palm oils separated distinctly from all others irrespective of heating duration, anchored by their saturated profiles, underscoring that fatty acid composition, not processing method, was the primary determinant of thermo-oxidative behavior in this experiment.

The study carries important caveats. Because nine commercial units of each oil were pooled into a single composite sample, the findings reflect the thermal response of that composite rather than the variability of individual brands. The design did not include matched cold-pressed and refined pairs for every botanical oil, so mustard, sesame, coconut and soybean serve as characterizations of individual oils rather than processing comparisons, and the refractive index interaction effect was interpreted cautiously after failing the Bonferroni-adjusted significance threshold. Definitive frying safety recommendations would also require endpoints not measured here, such as total polar compounds, TOTOX values and polymerized triacylglycerols. Nonetheless, the conclusion is clear: continuous high thermal processing degrades oils through early hydrolysis and primary oxidation followed by secondary oxidation and isomerization, and saturated oils such as coconut and palm showed comparatively slower thermo-oxidative change while polyunsaturated-rich soybean and sesame oils accumulated trans fatty acids fastest. For consumers and street vendors alike, the message is that how long an oil stays hot matters as much as which oil is chosen, and that cautious thermal application is needed to preserve oil quality.

Subject of Research: Effect of continuous high thermal processing on physico-chemical and nutritional alterations in cold-pressed and refined edible oils

Article Title: Effect of continuous high thermal processing on physico-chemical and nutritional alterations in cold-pressed and refined edible oils

Article References: Nandipeta, V., Chava, S. L., Mergu, N., & Rajendran, A. (2026). Effect of continuous high thermal processing on physico-chemical and nutritional alterations in cold-pressed and refined edible oils. Frontiers in Nutrition, 13, Article 1949768. https://doi.org/10.3389/fnut.2026.1949768

Image Credits: AI Generated

DOI: 10.3389/fnut.2026.1949768

Keywords: Effect, continuous, high, thermal, processing, physico-chemical, nutritional, alterations, cold-pressed, refined, edible, oils