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A new study has identified the specific volatile compounds responsible for the soapy and metallic off-flavors that develop in rapeseed oil during industrial alkali refining, giving processors concrete targets for quality control.
Published on 27 March 2026 in Food Innovation and Advances (DOI: 10.48130/fia-0026-0010), the research was carried out by a joint team from China Agricultural University and the Wilmar (Shanghai) Biotechnology Research and Development Center, with Professors Jihong Wu and Chunming Liu serving as co-corresponding authors.
Rapeseed oil is prized worldwide for its nutritional profile and distinctive flavor. However, crude oil contains free fatty acids, phospholipids, pigments, and trace metals that compromise stability and limit downstream applications. Industrial refining typically proceeds through degumming, alkali refining, decolorization, and deodorization. During alkali refining, sodium hydroxide neutralizes free fatty acids by converting them into soapstock, yet exposure to high pH and elevated temperatures can also trigger secondary reactions that generate off-flavors. While the flavor chemistry of cold-pressed rapeseed oil has been extensively documented, the molecular origins of the soapy and metallic notes that emerge after alkali treatment had remained unclear until now.
To address that gap, the researchers compared four samples: crude cold-pressed oil, a thermally treated control, water-degummed oil, and alkali-refined oil. A panel of 12 trained assessors conducted quantitative descriptive analysis across seven sensory attributes. The crude oil showed pronounced cabbage-like, green, and pungent notes, while the alkali-refined sample registered significantly higher soapy and metallic intensities, scoring 3.8 and 3.5 respectively. Because the heat-treated control developed only a slight metallic odor, the team concluded that alkalinity, rather than heating alone, was the principal driver of off-flavor formation.
Volatile flavor compounds were isolated using solvent-assisted flavor evaporation and analyzed by gas chromatography-mass spectrometry and gas chromatography-olfactometry. Aroma extract dilution analysis identified 46 odor-active compounds, which were then ranked by sensory potency. Quantification combined with odor activity value (OAV) calculations revealed striking shifts during refining. Trans-4,5-epoxy-(E)-2-decenal rose from 0.50 µg/kg in crude oil to 8.50 µg/kg after alkali refining, lifting its OAV from below 1 to 7. (Z)-1,5-octadien-3-one increased from 0.20 to 2.50 µg/kg, and 1-octen-3-one climbed from 4.0 to 35.2 µg/kg. Nonanal rose from 160.5 to 256.8 µg/kg, while decanal moved from 20.1 to 55.4 µg/kg. Although several saturated aldehydes had low individual OAVs, their high flavor-dilution factors pointed to a cumulative sensory effect.
To confirm causality, the team reconstructed the aroma profile of alkali-refined oil by spiking the identified odorants into a neutral oil matrix. Removing aldehydes significantly reduced both soapy and metallic perceptions, while omitting ketones specifically weakened the metallic odor. These recombination and omission experiments confirmed that multiple aldehydes jointly produce the soapy off-flavor, while epoxy-aldehydes and vinyl ketones drive the metallic off-flavor.
The study positions alkali refining as a critical control point for preserving flavor quality in refined rapeseed oil. The authors recommend shorter alkaline-contact times and lower processing temperatures to limit the breakdown of lipid hydroperoxides into potent secondary odorants. They also advise processors to monitor trans-4,5-epoxy-(E)-2-decenal and nonanal as more flavor-relevant indicators than peroxide values alone, which may not adequately reflect the odor-active degradation products that most directly shape consumer perception.
The work was supported financially by the Arawana Charity Foundation.
Source: Newswise