Heated Cow Ghee Releases Acrolein and a Mystery Ion, Lab Study Finds

Cow ghee, the clarified butter that anchors South Asian kitchens and Ayurvedic preparations, has now been put through one of the most complete thermal interrogations any food fat has received. In an open-access study published in Discover Chemistry, researchers at Swarnandhra College of Engineering and Technology in Andhra Pradesh, India, heated 50-gram batches of ghee across three carefully staged temperature regimes, from gentle warming to a furnace set point of 500 degrees Celsius, and tracked every volatile that escaped. What they found is a chemical biography of ghee that shifts dramatically with heat: quiet hydrolysis at kitchen-warming temperatures, a surge of the toxic aldehyde acrolein near the smoke point, and, at pyrolytic extremes, a fast-decaying ion at mass-to-charge ratio 27 that the team could not identify. The work is the first to map a single food fat continuously from hydrolysis through pyrolysis, a range that previous ghee studies, focused on shelf life and clarification below 130 degrees Celsius, never approached.

The experimental design is deceptively simple but methodologically deliberate. Stage 1 held ghee at 60 to 70 degrees Celsius for 30 minutes in a water bath, mimicking warm storage and culinary preheat. Stage 2 ramped the sample from 220 to 340 degrees Celsius in 20-degree steps, each held for five minutes, under flowing nitrogen to suppress oxidative side reactions. Stage 3 dropped the sample directly into a muffle furnace set to a nominal 500 degrees Celsius for just 90 seconds, a condition meant to model the pyrolytic shock of a cooking flare-up rather than ordinary frying. The authors are candid about a key caveat here: no thermocouple sat at the sample, and a 50-gram liquid charge cannot reach furnace temperature in 90 seconds, so the reported set point is an upper bound on what the ghee actually experienced. Five independent replicates were run for each stage and each temperature step, giving the statistical backbone for the comparisons that followed.

At the gentle end of the scale, the chemistry was quiet but measurable. Infrared spectroscopy of the residue showed a broadening O-H stretch and a growing carbonyl feature, the fingerprints of ester hydrolysis, the slow cleavage of the fat’s triglyceride bonds by water. Titration confirmed it: free fatty acid content rose from 0.42 percent to 0.71 percent over the 30-minute hold, a statistically significant increase, while free glycerol, released from the same broken esters, accumulated to 0.18 percent of sample mass. Crucially, no volatile carbonyls escaped at this temperature. Glycerol was liberated but not yet dehydrated, meaning the fat was ageing without yet becoming a source of airborne toxins. The team notes this mirrors the mild hydrolytic ageing documented in butter during warm storage, and that the two-step cascade they propose, hydrolysis first and dehydration later, remains plausible but unproven because the stages used separate aliquots of ghee.

The intermediate stage is where the story turns toxic. As the temperature climbed through the 280 to 320 degrees Celsius window, acrolein, a pungent, irritating aldehyde flagged by food scientists and air-quality regulators alike, surged to a maximum of 112.6 micrograms per gram of ghee at 300 degrees Celsius, accounting for 64.2 percent of all carbonyls detected by derivatisation and high-performance liquid chromatography. Above 300 degrees the acrolein yield fell, consistent with the molecule itself breaking apart into smaller fragments. The peak sits squarely inside the 280 to 340 degrees Celsius window known for catalytic glycerol dehydration, and the glycerol backbone of triglycerides, not the fatty-acid chains, is the established precursor. In other words, the glycerol freed during low-temperature hydrolysis becomes the feedstock for acrolein once the ghee approaches its smoke point, a two-act mechanism the data support circumstantially if not yet causally.

Stage 3 delivered the study’s most tantalising puzzle. Within a minute of insertion into the hot furnace, the headspace flooded with volatile fragments, and proton-transfer-reaction time-of-flight mass spectrometry caught a base peak at nominal mass-to-charge ratio 27 that rose between 45 and 60 seconds and decayed to background by about 180 seconds, with a 1/e decay time of 90 to 140 seconds. The signal matched none of the carbonyl reference standards. The team systematically eliminated the obvious candidates: vinyl alcohol, ketene and propene would all appear at different masses in proton-transfer ionisation, and the absence of any protonated parent at mass 43 rules out ketene and propene outright. The falling intensity across masses 27, 28 and 29 suggests hard, electron-impact-like fragmentation rather than soft proton transfer. Definitive identification would have required exact-mass measurement to distinguish the C2H3+ ion at 27.0229 from other formulas, but the raw high-resolution data files were not retained and cannot be recovered. The researchers therefore report an unassigned transient ion, an unusually honest label in a field where over-claiming is common.

Conventional bag sampling told a complementary story. Tedlar-bag gas chromatography-mass spectrometry of the Stage 3 headspace detected acrolein at 245 micrograms per gram, along with benzene, toluene, 1,3-butadiene and 2-methylfuran, a roster of light aromatics and dienes consistent with radical cracking of fatty-acid chains above 400 degrees Celsius. Notably, the transient mass-27 ion was absent from the bag samples, consistent with a lifetime shorter than the storage interval, exactly the class of short-lived reactive species that conventional cooking-emission studies, which typically operate at 150 to 250 degrees Celsius and rely on bag capture, have never catalogued. Mass-balance accounting showed progressive loss of material through the stages, with the lowest recovery at Stage 3, the missing fraction inferred to be non-condensable gases such as hydrogen, methane and carbon monoxide that the protocol did not capture.

The most unexpected experiment came last. The team injected 100 millilitres of pyrolysis gas into a 2.5-litre humid chamber held at 25 degrees Celsius and 65 percent relative humidity, alongside blank and nitrogen-injection controls, and watched what happened to the water. Gravimetric uptake on desiccant cartridges reached 1.3 milligrams after 30 minutes in the pyrolysis condition, against 0.3 milligrams for the blank and 0.4 for the nitrogen control, a difference that held up statistically against the blank and was marginal against the nitrogen control. Raman spectroscopy of the chamber’s O-H stretching envelope told the same story from a different angle: under pyrolysis gas the envelope maximum shifted 4 to 7 wavenumbers toward lower frequency, and roughly 8 percentage points of intensity migrated from the higher-frequency sub-band to the lower-frequency one, a redistribution consistent with oxygenated organic species hydrogen-bonding to water.

The authors are careful, almost to a fault, about what this chamber result does and does not mean. The Raman envelope peaks near 3400 wavenumbers, the signature of condensed water, not the two narrow gas-phase stretches near 3657 and 3756, so the signal reflects an integrated chamber water response dominated by condensed films and droplets rather than resolved gas-phase clusters. The 100-millilitre injection is a 4 percent by-volume perturbation, several orders of magnitude above ambient cooking-VOC mixing ratios, and with only three replicates per condition the design detects only large effects. The team explicitly frames the experiment as a laboratory feasibility observation carrying no atmospheric inference. Still, the coincidence of gravimetric uptake, spectral shift and sub-band redistribution is a genuine first: no prior study has tested whether high-temperature lipid pyrolysis products perturb a water-vapour hydrogen-bonding network at all, and the answer, provisionally, is that they do something measurable.

The practical implications reach from the stovetop to the atmosphere. Cooking is a newly recognised heavyweight in urban air chemistry, accounting for up to 20 percent of anthropogenic volatile organic compound emissions in some PTR-MS field studies, with cooking organic aerosol emission factors spanning 0.1 to 6 grams per kilogram of food. In domestic cooking, where temperatures rarely exceed 250 degrees Celsius, the study suggests the acrolein window opens as ghee nears its smoke point, reinforcing long-standing concerns about overheating clarified butter. The Stage 3 regime applies only to flare-ups, oil fires and industrial high-heat frying, but it is precisely there that short-lived reactive species, the kind no bag sample can catch, are released. The authors lay out a clear roadmap: a continuous temperature ramp tracking free fatty acid, glycerol and acrolein simultaneously to confirm the hydrolysis-dehydration cascade; high-resolution mass spectrometry with isotope-pattern resolution to finally name the mass-27 ion; and chamber experiments at parts-per-billion mixing ratios with condensed water controlled, ideally in vessels above 100 litres with cryogenic trapping. Until then, the humble jar of ghee carries a chemical secret that only extreme heat, and now careful instrumentation, can reveal.

Subject of Research: Thermal degradation chemistry of cow ghee from hydrolysis to pyrolysis and its pyrolysis products' interaction with water vapour

Article Title: Thermal decomposition of cow ghee by multi-stage heating and preliminary study of interactions of pyrolysis products with water vapour

Article References: Dorepalli, N. R., Prasad, M. S. V. K. V., & Pappoppula, S. (2026). Thermal decomposition of cow ghee by multi-stage heating and preliminary study of interactions of pyrolysis products with water vapour. Discover Chemistry, 3(1), Article 564. https://doi.org/10.1007/s44371-026-01008-6

Image Credits: AI Generated

DOI: 10.1007/s44371-026-01008-6

Keywords: cow ghee, acrolein, pyrolysis, glycerol dehydration, PTR-ToF-MS, FTIR, HPLC-DNPH, cooking emissions, indoor air quality, water vapour, Raman spectroscopy, free fatty acids