Olive oil consumption is associated with a myriad of benefits to human health, attributed to the monounsaturated fatty acids, α- and γ-tocopherols, phytosterols, phenolic compounds, and flavonoids it contains. In ancient Greece, Hippocrates called it “the great healer,” and Homer referred to it as “liquid gold.” It is an essential component of the widely popular Mediterranean diet, and is marketed to consumers as a healthy option, often with images of olive groves in idyllic Mediterranean locales. Unfortunately, olive oil is an ingredient most at risk for food fraud, and the daydream associated with this marketing quickly dissolves when the bottle is shown to contain a mix of olive oil and any of several less expensive oils such as soybean, corn, or rapeseed. Since a story about the adulteration of olive oils was published in The New Yorker in 2007, it has continued to make headlines. However, the American olive oil market remains one of the largest outside Europe, and a variety of quality tests, methods, and limits have been included in United States Department of Agriculture (USDA) regulations to protect consumers. Other techniques have been developed to support rapid, simplified testing.
Not All Olive Oil Is Created Equal
Olive oil is very similar to a freshly squeezed juice. Processing begins when olives are harvested by hand or by machines that shake olives from the tree into drop nets below. The olives are then washed and processed within 24 hours to minimize oxidation. Stone mills— and more recently, motorized mills—are used to crush the olives into a paste, which is then mixed so that various enzymes may begin to produce aromas. The paste is then pressed with a hydraulic press and the remaining liquid is centrifuged to separate the oil from water.
According to the USDA, for a product to be classified as extra virgin olive oil (EVOO), it must be obtained solely by mechanical or physical means, under thermal conditions that do not lead to any alterations in the oil, and no treatments other than washing, decantation, centrifugation, and filtration may be employed. Alternatively, products classified as olive oil must be obtained solely from the fruit of the olive tree (Olea europaea L.).
Olive pomace is a solid residue that results from pressing or centrifugation. Oil extracted from pomace, called crude olive pomace oil, is refined with solvents and other physical treatments prior to human consumption. Olive pomace oil consists of a blend of refined pomace oil and virgin olive oils, and while safe for consumption, lacks the polyphenols found in EVOO.
Why and How Oil Adulteration Occurs
Xylella fastidiosa is a bacterial plant pathogen, commonly referred to as olive leaf scorch, or olive tree leprosy. It colonizes the xylem network within the plant, preventing the flow of water from the roots to the leaves, killing the plant. It was first detected in olive trees in the Lecce province in Apulia, Italy in 2013, and is estimated to have destroyed up to one million trees. The resulting poor harvest may have contributed to increased sales of low quality or adulterated oil.
Criminal organizations have also been suspected of adulterating olive oil for profit. In May 2019, Europol’s Intellectual Property Crime Coordinated Coalition and the Italian NAS Carabinieri, and the Tribunal of Darmstadt in Germany arrested 20 individuals and seized 150,000 liters of fake olive oil—sunflower oil with added chlorophyll, beta-carotene, and soybean oil.
According to Stefan Tordenmalm, market manager, Processed Food, PerkinElmer, Inc., “Adulteration of olive oil (a product that is not only high-value but also highly popular) occurs anywhere in the world when a dishonest player in the supply chain looks to make more money by diluting expensive EVOO with other lower cost oils like sunflower oil, soybean oil, rapeseed oil, corn oil, or others. This could happen anywhere in the value chain from milling of the olives to the bottling and labeling of the oil. Everyone in the industry, therefore, needs to stay vigilant and test the oil they receive at their part of the chain.”
What This Means for Consumers
Olive oil adulteration has implications for the consumer and the company selling the product. “The consumer will end up with a lower quality product than they paid for, without the distinctive flavor and healthy characteristics of EVOO like antioxidants,” explains Tordenmalm. “It can also damage the company’s brand reputation and pose financial and legal issues.”
Contamination with other compounds can also pose health risks to the consumer. Routine testing is performed for other contaminants such as pesticides and heavy metals, as well as contaminants that are introduced during processing, such as 3-MCPD (3-monochloropropane-1,2-diol esters) and glycidyl esters. These are heat-induced contaminants that form during the deodorization step of refining, and can be used to determine if refined and pomace olive oil was added to virgin olive oil.
There are regulations in place in the US and the EU to ensure olive oil quality. “USDA regulation §52.1539 describes quality criteria for different grades of olive oil,” explains Tordenmalm, and Commission Regulation (EU) No 61/2011 “defines exactly how EVOO can be produced and what the quality criteria are.” A major component of quality assurance is testing.
Olive Oil Testing Methods
Tordenmalm describes two different methods of analysis. “Different oils have different compositions, such as the fatty acid profile for example, and by analyzing the composition in detail it is possible to determine whether the oil is pure EVOO. If adulteration has occurred, some components will be present in higher or lower concentrations than what would occur in pure EVOO.” This detailed composition can be determined with a combination of liquid (LC) and gas chromatography (GC), which are also suitable for identifying organic contaminants. Heavy metals may be tested with inductively coupled plasma mass spectrometry. Alternatively, infrared (IR) spectroscopy may be used to perform “fingerprinting of the oil to look at specific patterns rather than whether individual components are too high or too low,” he explains.
GC and LC methods offer some advantages, and are outlined in the USDA regulations. Chromatographic analysis is ideal for separating the individual components of complex mixtures, offering a high degree of resolution and precision, and requires a small sample volume. “These methods need to be used by olive oil producers for analysis of finished oil which is sold,” explains Tordenmalm. However, he notes that “while these are standardized methods, they have the disadvantage of being laborious, and some of them take several hours including sample preparation steps. In many situations that is not a problem, but for an olive refinery, they are not suitable to ensure the authenticity of every truckload.”
More rapid techniques, such as IR spectroscopy, enable analysis within minutes and do not require highly trained technical staff. “By comparing the infrared spectrum of an oil sample with a spectral library of different oils, pure and adulterated, an infrared instrument can detect the addition of oils other than the olive oil itself,” says Tordenmalm.
UV-visible spectroscopy (UV-Vis) can also be used to differentiate between oils in a sample. “Lower quality oils that may have been mixed in contain unsaturated hydrocarbons that absorb UV light in the 200-300 nm spectral range,” explains Tordenmalm. “Therefore, a high absorption within this wavelength range points to a lower quality oil.”
Industry leaders and regulators rely on a range of instrumentation to ensure olive oil—and countless other food products—meet quality standards to protect consumers. Of these, rapid testing techniques like IR and UV-Vis spectroscopy are ideal for identifying fraudulent products at numerous points in the supply chain.