Oil purification (stripping) by a solvent-free aluminum oxide-based procedure
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Edible oils are primarily composed of triacylglycerols and can be stripped of other minor components, making the oil a blank matrix to study the oxidation and rancidity of lipids.
This study aimed to evaluate the effectiveness of aluminum oxide-based stripping in removing minor components, lipid oxidation products, and tocopherols of Baltic herring (Clupea harengus membras) oil and microalgae (Schizochytrium sp) oil.
The peroxide value and p-Anisidine value were measured before and after stripping. Tocopherol content in both non-stripped and stripped samples was analyzed with ultra-high performance liquid chromatography with fluorescence detection. Headspace solid-phase microextraction/gas chromatography–mass spectrometry was employed for the analysis of volatiles and secondary oxidation products. Both non-stripped and stripped samples were incubated at 45 °C for 27 hours to accelerate the oxidation. Thin-layer chromatography techniques were applied to identify the lipid classes. The peroxide value (meq/kg) and p-anisidine value of non-stripped Baltic herring oil decreased from 2.57±0.08 to 0.69±0.09, and from 11.042±1.29 to 1.38±0.14, respectively, and similar depletion was observed in microalgae oil. The total tocopherol content in non-stripped microalgae oil, 7738.0±765.0 mg/kg, declined to 1070.8±86.2 mg/kg after stripping. Similarly, in Baltic herring oil, the total tocopherol concentration reduced from 1188.15±109.87mg/kg to 208.4±45.1mg/kg. In the oxidized state, volatiles formed due to the decomposition of hydroperoxides, such as 1-octen-3-one, 2-butenal, and 2-pentenal, in the stripped oil showed a higher peak area. However, fewer volatiles, such as nonanal, octanal, and 1-octen-3-ol, were present in non-oxidized oils with less abundance, indicating mild oxidation. Interfacial tension was found to be the same for all samples. Stripping significantly decreased levels of phospholipids and free fatty acids. The statistical analysis shows a significant difference between the stripped and non-stripped samples (t-test and Mann-Whitney U test, p < 0.01). Overall, from the reduced peroxide value, p-Anisidine value, and total tocopherol content in oil after stripping, it can be concluded that the aluminum-oxide-based approach for stripping is applicable for the removal of minor components. Moreover, the removal of minor components and the increase in incubation temperature accelerate oxidation and volatile formation, as expected.