Last Month's Most Accessed Feature: Compromised coffee: Soybean and corn adulterants by CE-MS

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  • Published: Dec 1, 2017
  • Categories: Base Peak
thumbnail image: Last Month's Most Accessed Feature: Compromised coffee: Soybean and corn adulterants by CE-MS

Cheating with coffee

The adulteration of roasted coffee beans with coffee husks, corn or soybean has been detected using capillary electrophoresis-mass spectrometry to measure the monosaccharides originating from the constituent polysaccharides present in the beans.

The sheer amount of coffee produced and sold around the world makes it an ideal target for food fraudsters. December production alone in 2016 was 10.17 million bags, according to the International Coffee Organisation, with an estimated 151.6 million bags expected to be produced in the whole of that year. The most common variety is Arabica, accounting for about 47% of annual production, followed by Robusta at about 27%.

Slip a little cheaper coffee into a more expensive one, or mix the beans with coffee husks, corn or soybean, and the profit to be made is very tempting to criminal organisations. Once the coffee and its adulterants have been roasted and ground, it is very difficult to see whether or not the coffee is pure. There are a number of established analytical techniques to detect coffee adulteration based on chromatography, mass spectrometry and spectroscopy but a team of Brazilian scientists has developed a novel procedure based on capillary electrophoresis-mass spectrometry (CE-MS).

Claudimir Lucio do Lago and Fernando Silva Lopes from the University of São Paulo with Vagner Bezerra dos Santos and Daniela Daniel from the Federal University of Pará, Belém and Agilent Technologies based their method on the sugar content of coffee beans. When the polysaccharides present are hydrolysed with acid, the resulting monosaccharides are indicative of the natural material that might be present.

The official method recommended by The Association of Analytical Chemists employs high-performance anion-exchange chromatography coupled to pulsed amperometric detection but is hampered by a run time of up to 80 minutes. The new approach reduces the time to about 15 minutes by optimising the acid reagent and introducing a simple neutralisation step to reduce the conductivity of the sample to make it compatible with CE.

Sugar separation

The researchers set up the method using standard solutions of monosaccharides that derive from coffee and typical adulterants. So, a mixture of mannose, fructose, galactose and arabinose is typical of unadulterated coffee beans. Conversely, xylose is produced in raised amounts from the hydrolysis of coffee husks and elevated levels of glucose and xylose together are indicative of pure corn.

Soybeans are characterised by raised amounts of galactose, glucose and arabinose but also by the presence of fucose. Even though this is found in small amounts following acid hydrolysis, it did not occur in any other samples tested, so can be considered a unique marker of the presence of soybeans in roasted coffee.

The best CE separation of a total of nine monosaccharides, plus inositol as an internal standard, was achieved at high pH with aqueous triethylamine as a background electrolyte under an applied voltage of 25 kV. They separated as anionic species and were detected in the mass spectrometer in negative-ion mode with multiple reaction monitoring of one selective transition for each analyte. Under these conditions the lower limit of quantification was 0.01 mmol/L or less for each analyte.

Corn in coffee

The optimised method was applied to samples of 100% arabica medium roasted coffee beans that were purchased from local stores, as well as to pure corn and soybeans. After manual inspection to remove any impurities, the polysaccharides present were hydrolysed with sulphuric acid over 150 minutes and the resulting solution was neutralised with barium hydroxide that produced a barium sulphate precipitate. The filtered solution was diluted and analysed.

The electropherograms displayed the monosaccharides derived from each sample and the percentages of each were calculated for use in a principle components analysis to distinguish between the coffee and two adulterants. The first two components of the analysis explained more than 99.7% of the variance, confirming the strong discriminatory capacity.

The PCA successfully detected the adulterants in coffee beans mixed in the lab with ground roasted corn or soybeans at levels from 2.5-20% which are realistic limits that might be used in practice. Any lower and it is not particularly viable for the fraudsters, any higher and the adulterants might be visible to consumers.

A further 14 commercial roasted ground coffee samples were tested, the detection of glucose revealing adulteration by corn in three of the cases. This proportion matches that from an earlier study on adulterated Brazilian coffee.

The team concluded that their method "has the potential to be used as a routine approach for the adulteration detection and authenticity checking for ground, roasted coffee. According to obtained results fucose, xylose, and glucose can be used as a potential marker to identify adulteration in coffee sample."

Related Links

Food Chemistry 2018, 243, 305-310: "Detection of coffee adulteration with soybean and corn by capillary electrophoresis-tandem mass spectrometry"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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