1H NMR Spectroscopy: A Solution to China’s Gutter Oil Crisis

by Zhang Ruixian

© Jen Leung 
Available from: https://www.flickr.com/photos/jennikokodesu/4459701002/

Imagine yourself enjoying a pancake bought from a Chinese street hawker. Tasty, isn’t it? However, what you don’t know is that what you’re eating might contain poisonous gutter oil.

Recently, researchers in China have discovered an efficient way to identify gutter oil with the help of NMR (Nuclear Magnetic Resonance) spectroscopy. By looking for 1H NMR fingerprints that are unique in the spectra of gutter oil1, the research has shed light on solving this long-term food safety issue in China.

Worries concerning the presence of gutter oil in food have been around China for the past three years or so. Gutter oil, or illegal cooking oil, is secondhand oil refined from cooking waste, gutters, drains, and animal fat. Through a series of simple processes, such as collection, preliminary filtration and boiling, refining, and the removal of adulterants, illegal gutter oil is packed and sold to low-end restaurants and small canteens2.

Considered germy and carcinogenic, gutter oil is strictly prohibited in China. Even so,
the reality is that it accounts for a horrifying 10 percent of the cooking oil used in China, according to Chinese state media. In 2011, the police in China seized over 100 tons of illegal cooking oil and arrested 32 people across 14 provinces3. In January this year, a man was sentenced to death for selling gutter oil with a total worth of 52.4 million yuan (£5 million) to 17 dealers in Shandong and Shanxi provinces4.

Now, a team of researchers from the University of Chinese Academy of Sciences, Peking University and the Chinese Academy of Inspection and Quarantine has found an effective way to detect gutter oil by using NMR spectroscopy, in support of the Chinese government in this food-hazard elimination campaign.

In the research, the team tested 60 different oil samples, including those of gutter oil, and obtained their spectra. By applying mathematical methods, they found 12 key parts of the spectrum for the identification of gutter oil. Therefore, with the data of the 60 samples tested at hand, they were able to come up with a determination function that could be used to do the testing. The results for the two blind tests including altogether 69 samples were quite satisfactory, with accuracies of 91.9% and 93.8% respectively1.

1H NMR spectrum and chemical shift —— the fingerprints of gutter oil

1H NMR spectroscopy, which the team has applied, takes advantage of the spinning of nuclei (protons) in hydrogen atoms. A nucleus in a hydrogen atom has two possible spin states (up and down, or 1/2 and -1/2), which each produces its own magnetic moment (a magnet-like property). Under normal conditions, the two states have the same energy. However, when the nucleus is placed in a magnet field, the interaction between magnetic moments and the magnet field leads to a difference in the energy of the two spinning states, and only by the absorption of electromagnetic radiation (light) of a specific wavelength can the two spinning states interconvert. This property is called magnet resonance, and can be detected by the NMR spectrometer, which records the absorption as peaks in the spectrum.

NMR Spectrometer © EMSL
Available from: https://www.flickr.com/photos/emsl/4281863613/

What the researchers specifically focused on to identify gutter oil was the chemical shifts of the H atoms in the spectrum. When a magnetic field is applied, the electrons around a proton will generate a magnetic field themselves to lower the strength of the applied field, which we call shielding effect. With different chemical structures, the electron densities around protons are different, leading to different shielding effects when the same magnetic field is applied. As a result, the energy difference between the two spin states of protons in different hydrogen atoms may vary, which results in different chemical shifts and is reflected on the absorption spectrum5. The team took advantage of these chemical shifts, and recognize those that can serve to identify gutter oil.

1H NMR Spectrum of Lineoleic Acid (A kind of Fatty Acid)
© Zhang Ruixian

The reason that gutter oil molecules have difference chemical shifts is because they have been used for cooking already, and in processes such as heating, they can easily become oxidized. As a result, the number of carbon atoms and the degrees of unsaturation (carbon/carbon double bond) in their molecules will decrease, and this change in their structures leads to different chemical shifts in their spectra.

“Gutter oil is secondhand cooking oil, which has been heated and processed. Therefore, its chemical structures are different from those of regular cooking oil,” says Professor He Y. J. from the University of Chinese Academy of Sciences. “We check the molecular structures of the oil samples tested by looking at their 1H NMR spectrum. If certain parts of the spectrum are anomalous, then the sample is suspected to be gutter oil.”6

By studying the spectra of 60 known oil samples, including gutter oil and normal cooking oil, the team identified 12 key chemical shifts where the absorption peaks of the gutter oil and legal cooking oil are different. Then with these 12 chemical shifts, they were able to sort all 60 samples into eight categories based on the characteristics of their spectra and came up with a determination function to do blind tests on unknown samples1.

“With the data of 60 known samples, we found 12 chemical shifts important for the identification of gutter oil,” He said.

Two groups of blind tests, each including over 30 samples, were conducted using the determination function. By inputting the spectrum of a tested sample, the function could give the category to which the sample most likely belongs. Overall, the results showed great prospects, with accuracies of 91.9% and 93.8%1.

1H NMR spectroscopy — an advantageous identification method

Other methods of identifying gutter oil existed before the 1H NMR method was introduced. One of these methods is testing the cholesterol content in the oil. Gutter oil is secondhand oil, which has attained animal fat during the cooking process. As a result, its cholesterol content is much greater than ordinary cooking oil, which are produced from vegetables or nuts. Some scientists took advantage of this composition difference, and developed a method to identify gutter oil with the application of gas chromatography7.

“Some other methods also give quite accurate testing results, but what they focus on is the difference in some contents other than the oil component itself,” said He. “The composition of gutter oil from different underground factories may vary, in which case the testing results might not be reliable. 1H NMR focuses on the differences in the structure of the oil molecules, which does not have this problem.”6

Compared with the cholesterol-testing method, 1H NMR spectroscopy method is also less time-consuming, with a less complicated sample preparation process where such procedures as heating and washing are not required.

Future prospects of the technology

The accuracy of the 1H NMR spectroscopy method can be further improved by expanding the database of oil spectrum and by introducing more sophisticated determination function, which are believed by the researchers to be the two factors contributing to the errors in the two blind tests. Professor He believes that the cost of the method can be greatly reduced if all samples can be tested at one time. “The cost can be just a few dollars per sample, and it also takes much less time.”6

Up till now, the team has already made progress on the promotion of the technology, and they believe it will soon be put into mass usage.

References:

1. Cai BT, Yuan .F, Zhou Y, XU XL, Yang Z, Liu QJ, Huang J.C, Zhong WK, Wang EQ and He YJ. Identification of illegal cooking oils by using 1H NMR fingerprints combined with multivariate analysis. Science China Chemistry 2013; 43(5): 558-567.

2. Ye Q. and Pei XF. Methods for differentiating recycled cooking oil needed in China. Your Global Fats and Oils Connection. (undated). [Accessed 2014 March 6]. http://www.aocs.org/Membership/FreeCover.cfm?itemnumber=18028

3. Mass arrests in China illegal ‘gutter oil’ police sting 13 September 2011. BBC News, Asia-Pacific. [Accessed 2014 March 6] http://www.bbc.co.uk/news/world-asia-pacific-14894070

4. Man in China sentenced to death for selling illegal cooking oil 08 January 2014. The Independent News, World, Asia. [Accessed 2014 March 6] http://www.independent.co.uk/news/world/asia/man-in-china-sentenced-to-death-for-selling-illegal-cooking-oil-9046781.html

5. Pivia D, Lampman G, Kriz G, Vyvyan J. Introduction to Spectroscopy. 4th ed. Cengage Learning; 2008. 105-142 p.

6. Sun AM. “He ci gong zhen” rang di gou you xian yuan xing: mang ce shi yan zheng que lv gao da 93.8% [“NMR” reveals gutter oil: blind test accuracy as high as 93.8%]. 19 February 2013. China Science Daily. [Accessed on 2014 April 6] http://news.sciencenet.cn/dz/upload/201321932619329.pdf

7. Zhang R, Zu LY, Fan T, Zhao KP. Distinguishing hogwash oil in edible vegetable oil by determination of cholesterol content. China Oils and Fats 2006; 31(5): 65-67.

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