Polyunsaturated plant-based cooking oils, such as rapeseed oil, sunflower oil, corn oil and olive oil, are increasingly being used as healthier alternatives to animal-derived fats for preparing fried foods. However, these health benefits are undermined if the oil used is not pure.
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The increasing prevalence of food fraud has meant that the risk of inadvertently buying adulterated vegetable oils is on the rise. There is consequently a growing need for effective analyses to readily detect adulteration of edible plant-based oils.
Research indicates that electron spin resonance spectroscopy (ESR) could provide the answer. A study of several plant-based cooking oils successfully discriminated adulterated vegetable oils from their pure counterparts using ESR spectra.
Adulteration of Cooking Oils
Some unscrupulous retailers are increasing their profit margins by bulking up products with cheaper alternatives. In the case of vegetable cooking oils, the oil is mixed with oil that has already been used. Such deception, in addition to defrauding the consumer and being detrimental to the reputations and livelihoods of honest producers, can result in serious health consequences.
Oils containing high amounts of unsaturated fatty acids, such as the plant-based cooking oils, are highly susceptible to thermal oxidation1. This means that used vegetable oils contain a range of oxidation products. These include various polymers, trans-fatty acids and compounds with oxygen-containing groups that are detrimental to human health. It follows that if the oil is already contaminated with used oils containing these oxidation products, the production of more during the frying process will increase their concentrations further, posing an even greater risk to health2.
To protect consumers from such potential health risks, edible vegetable oil manufacture is strictly regulated. The quality checks required to ensure that an oil meets the necessary standard, however, is complicated by the lack of convenient, efficient and reliable detection methods for adulteration with previously used cooking oil. Although several methods are available to detect the products of oxidation, they tend to be labor-intensive, require time-consuming sample preparation and/or be of insufficient sensitivity.
Nuclear magnetic resonance, which provides rapid analysis with minimal sample preparation, has been used to identify adulteration of edible vegetable oils, but the limits of detection for many different oils were high3.
Electron Spin Resonance Spectroscopy
ESR is a powerful tool for the identification of free radicals. It detects the unique response of radicals with unpaired electrons to a magnetic field. Indeed, it is well established as a valuable tool for the detection and quantification of lipid-free radicals. Such free radicals are produced by the thermal oxidation of cooking oil and so will be present in used cooking oil.
Unfortunately, the free radicals are so unstable that they quickly combine with other molecules in their immediate environment to make more stable compounds. The concentration of free radicals at a given time is thus very low, making them difficult to detect.
This can be addressed by using a diamagnetic compound. The free radicals react with this to form a more stable species that accumulates and can be readily detected by ESR4. The most commonly used spin trap for lipid analysis of foodstuffs is α-phenyl-N-tert-butylnitrone (PBN) due to its high lipophilic and reactivity characteristics. The oxidative stability of lipids in food has been widely evaluated using such ESR spin-trapping methodologies5,6.
The technique has now been applied to the analysis of plant-based cooking oils to identify contamination with used plant-based oils7.
Detecting Edible Plant Oil Adulteration using ESR
Pure samples of several different plant oils, rapeseed oil, soybean oil, peanut oil, corn oil and olive oil were mixed with 1% to 80% used frying oil. Each sample was analyzed by ESR at 140℃ using a Bruker EMXplus-10/12 spectrometer operating at 9.85 GHz and fitted with the Bruker variable temperature control unit. Samples of used frying oil, as well as each of the pure vegetable oils, were analyzed as controls. PBN was used as the spin trap to stabilize any free radicals produced to enable detection of contamination with used frying oil. In addition, acid values and peroxide values were determined using the AOCS official method4
Peroxide values and the proportion of polar compounds, which includes the products of oxidation, polymerization, and hydrolysis, were considerably higher in the used frying oil than in the pure oils.
The initial signal intensities of the ESR analysis and the rate at which they increased were both higher in the used frying oil than in the pure commercial oils. The increase became less marked as heating continued due to an increase in decay rates. It was shown that comparison of the ESR signal intensities at 2 minutes provided an accurate indication of adulteration7. Furthermore, the amount of adulteration could be determined from the ESR signal intensity at 2 minutes.
This data demonstrates that electron spin resonance provides a simple and convenient tool for ensuring the authenticity of plant-based cooking oils through the rapid detection of free radical production.
References
- Zhang Q, et al. Chemical alterations taken place during deep-fat frying based on certain reaction products: A review. Chemistry & Physics of Lipids 2012;165(6):662-681.
- Yin H, et al. Free Radical Lipid Peroxidation: Mechanisms and Analysis. International Journal of Advances in Engineering & Technology 2011;111(10):5944-5972.
- Zhang Q, et al. Discrimination of Edible Vegetable Oil Adulteration with Used Frying Oil by Low Field Nuclear Magnetic Resonance. Food & Bioprocess Technology 2013;394 6(9):2562-2570.
- Andersen ML, et al. Analysis of lipid oxidation by ESR spectroscopy. In Analysis of lipid oxidation 2005. AOCS Press.
- Jerzykiewicz M, et al. Pro- and Antioxidative Effect ofα-Tocopherol on Edible Oils, Triglycerides and Fatty Acids. Journal of Oil 359 & Fat Industries 2013;90(6):803-811.
- Ottaviani MF, et al. Electron paramagnetic resonance investigations of free radicals in extra virgin olive oils. Journal of Agricultural & Food Chemistry 2001;49(8):3691-3696.
- Chen H, et al. High sensitive and efficient detection of edible oils adulterated with used frying oil by electron spin resonance, Food Control (2016), doi: 10.1016/j.foodcont.2016.08.050.