Rapeseed oil is valued worldwide for its nutritional properties and distinctive flavor, but crude oil contains free fatty acids, phospholipids, pigments, and trace metals that can reduce its stability and limit its applications. Industrial refining therefore commonly includes degumming, alkali refining, decolorization, and deodorization. During alkali refining, sodium hydroxide removes free fatty acids by converting them into soapstock. However, exposure to high pH and elevated temperatures may also trigger secondary reactions that generate undesirable off-flavors. Although the flavor composition of cold-pressed rapeseed oil has been extensively characterized, the molecular origins of the soapy and metallic off-flavors after alkali refining have remained unclear.
A study (DOI: 10.48130/fia-0026-0010) published in Food Innovation and Advances on 27 March 2026 by a joint team from China Agricultural University and Wilmar (Shanghai) Biotechnology Research and Development Center, with Jihong Wu and Chunming Liu as co-corresponding authors, identifies aldehydes and ketones accumulating during alkali refining as the principal contributors to soapy and metallic off-flavors in cold-pressed rapeseed oil, a conclusion validated by odour activity values and aroma recombination and omission experiments.
The team compared four samples representing controlled processing conditions: crude cold-pressed oil, a thermally treated control, water-degummed oil, and alkali-refined oil. A panel of 12 trained assessors first performed quantitative descriptive analysis using seven sensory attributes. The crude oil displayed strong cabbage-like, green, and pungent notes, whereas alkali-refined oil had significantly stronger soapy and metallic intensities, reaching scores of 3.8 and 3.5, respectively. Because the heat-treated control developed only a slight metallic odor, the results indicated that alkalinity, rather than heating alone, was primarily associated with off-flavor formation. The researchers then isolated volatile flavor compounds using solvent-assisted flavor evaporation and analyzed them through gas chromatography-mass spectrometry and gas chromatography-olfactometry. Aroma extract dilution analysis detected 46 odor-active compounds and ranked their sensory potency. Quantification and odor activity value calculations showed that trans-4,5-epoxy-(E)-2-decenal increased from 0.50 μg/kg in crude oil to 8.50 μg/kg after alkali refining, raising its odor activity value from below 1 to 7. (Z)-1,5-octadien-3-one increased from 0.20 to 2.50 μg/kg, while 1-octen-3-one rose from 4.0 to 35.2 μg/kg. Meanwhile, nonanal increased from 160.5 to 256.8 μg/kg and decanal from 20.1 to 55.4 μg/kg. Although some of these saturated aldehydes had low individual odor activity values, their high flavor-dilution factors suggested a combined sensory effect. To verify causality, the team reconstructed the aroma of alkali-refined oil by adding the identified odorants to a neutral oil matrix. Removing aldehydes significantly reduced both soapy and metallic perceptions, while omitting ketones specifically weakened the metallic odor. These experiments confirmed that multiple aldehydes jointly generate the soapy off-flavor and that epoxy-aldehydes and vinyl ketones create the metallic off-flavor.
The study identifies alkali-refining as a critical control point for preserving the flavor quality of refined rapeseed oil. Shorter alkaline-contact times or lower processing temperatures may help limit the decomposition of lipid hydroperoxides into potent secondary odorants. The researchers also suggest monitoring trans-4,5-epoxy-(E)-2-decenal and nonanal rather than relying solely on peroxide values, which may not accurately reflect the odor-active breakdown products that directly affect consumer perception.
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References
DOI
10.48130/fia-0026-0010
Original Source URL
https://doi.org/10.48130/fia-0026-0010
Funding information
Thanks for the financial support from Arawana Charity Foundation.
About Food Innovation and Advances
Food is essential to life and relevant to human health. The rapidly increasing global population presents a major challenge to supply abundant, safe, and healthy food into the future. The open access journal Food Innovation and Advances (e-ISSN 2836-774X), published by Maximum Academic Press in association with China Agricultural University, Zhejiang University and Shenyang Agricultural University, publishes high-quality research results related to innovations and advances in food science and technology. The journal will strive to contribute to food sustainability in the present and future.