Database can distinguish gutter oil from edible oil

Chinese researchers have established a “spectral database” that can be used to determine counterfeit cooking oil and gutter oil, a new paper reveals.

Writing in the journal Food Chemistry, the researchers said the emergence of gutter oils – used or waste cooking oil collected from sources such as restaurant fryers, grease traps, slaughterhouse waste and sewage from sewer drains and relabelled as normal edible oil – has the potential to cause health problems and there is growing concern over the practice.

“The quality and safety of edible oil products in the market are not secured with widespread edible oil frauds in recent years,” the researchers said, noting the practice of counterfeit oil where cheaper oils replace more expensive ones, oil adulteration where cheaper oil is mixed in with edible oil and recycling used oil for resale.

“Identification of gutter oils is a challenging analytical task, since gutter oils have very diverse preparation processes and sources and it may not be effective to rely on detection of targeted toxic substances or specific food marker compounds for determination of gutter oils,” the researchers said.

In a bid to address this challenge, the researchers used matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to speedily establish a comprehensive spectral database for analysis of edible oils.

“Although MALDI-MS has been extensively used to analyse edible oils, so far there has been no systematic investigation and establishment of a comprehensive MALDI spectral database of various edible oils, which will definitely benefit the technique and the analysis of edible oils,” the researchers said.

In the study, more than nine hundred edible oil samples were collected and analysed using a previously developed MALDI-MS protocol. The MALDI-MS spectra obtained were stored in an in-house developed system, and a comprehensive MALDI-MS spectral database including more than thirty edible oil species was then established, which included oils such as butter, canola, flaxseed, olive and rapeseed.

The gutter oil samples were provided by China-based Syscan Technology Holdings Limited.

By comparing the MALDI-MS spectra of the samples with the reference spectra in the database a rapid classification of edible oils could be generated.

According to the results of the study, different oils showed spectra variations with characteristic peaks and intensity ratios, which allowed differentiation between the oils.

The mass spectra of castor oil and fish oil, for example, were easily distinguished from those of other edible oils where castor oil showed a very simple mass spectrum while fish oil possessed a more complex mass spectrum.

Edible oils were then divided into eight groups based on their characteristic spectral patterns and principal component analysis results.

When it came to classification of 435 edible oil products, 97.2 per cent of the tested samples were classified correctly and only 12 out of the 435 tested samples were misclassified.

Meanwhile, for commonly used edible oils such as olive oil, peanut oil and canola oil, correct classification rates of 100 per cent were achieved.

Differentiation of counterfeit edible oils, repeatedly cooked edible oils and gutter oils from normal edible oils could also be achieved based on the MALDI-MS spectra. While gutter oils have diverse compositions and so no universal spectral pattern for identification, the differences in the triacylglyceride contents and the presence of extra peaks of the oxidised products could provide clues for distinguishing gutter oils from normal edible oils, the researchers said.

“As gutter oils have to be labelled as normal edible oils in order to be sold in the market, identification of mislabelling is a reasonable strategy to screen out gutter oils,” the researchers said, concluding that “MALDI-MS analysis combined with the established spectral database and statistical analysis could be an effective approach for rapid classification of edible oils”.