Refining research helps optimise flavours of edible oils

Rapeseed and canola oil is valued worldwide for its nutritional properties and distinctive flavour, but crude oil contains free fatty acids, phospholipids, pigments and trace metals that can reduce its stability and limit its applications. Industrial refining of these oils commonly includes degumming, alkali refining, decolourisation and deodorisation. During alkali refining, sodium hydroxide removes free fatty acids by converting them into soapstock; but, exposure to high pH and elevated temperatures may also trigger secondary reactions that generate undesirable off-flavours. Although the flavour composition of cold-pressed rapeseed oil has been extensively characterised, the molecular origins of the soapy and metallic off-flavours after alkali refining have remained unclear.

Now, a research team has identified the odour-active compounds responsible for the soapy and metallic off-flavours when cold-pressed rapeseed oil undergoes alkali refining. The findings could help edible-oil manufacturers optimise refining conditions, monitor more informative chemical markers and preserve the desirable sensory qualities of cold-pressed rapeseed oil.

The study published in Food Innovation and Advances 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-flavours 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 odour, the results indicated that alkalinity, rather than heating alone, was primarily associated with off-flavour formation.

The researchers then isolated volatile flavour compounds using solvent-assisted flavour evaporation and analysed them through gas chromatography-mass spectrometry and gas chromatography-olfactometry. Aroma extract dilution analysis detected 46 odour-active compounds and ranked their sensory potency. Quantification and odour 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 odour 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 odour activity values, their high flavour-dilution factors suggested a combined sensory effect.

To verify causality, the team reconstructed the aroma of alkali-refined oil by adding the identified odourants to a neutral oil matrix. Removing aldehydes significantly reduced both soapy and metallic perceptions, while omitting ketones specifically weakened the metallic odour. These experiments confirmed that multiple aldehydes jointly generate the soapy off-flavour and that epoxy-aldehydes and vinyl ketones create the metallic off-flavour.

The study identifies alkali-refining as a critical control point for preserving the flavour quality of refined rapeseed oil. Shorter alkaline-contact times or lower processing temperatures may help limit the decomposition of lipid hydroperoxides into potent secondary odourants. 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 odour-active breakdown products that directly affect consumer perception.

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