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Researchers Identify Organic Coffee Through NMR-Based Metabolic Profiling

In recent years, there has been an interest from consumers who are willing to pay a premium for organic food products, who may be interested in the potential environmental or nutritional benefits. Organic coffee is grown without the use of man-made chemical treatments such as chemical fertilizers, pesticides, and herbicides and instead, the soil is enriched through compost, manure, and crop rotation.

Given that the soil environment can greatly influence the metabolic composition of the food, can organic and non-organic foods be distinguished based on these profiles? Research has already demonstrated that this can be applied to foodstuffs including tomatoes, potatoes, and oranges. In a recent study, researchers applied NMR to explore whether the same could be achieved for coffee.

The team sourced 68 samples of roasted and ground C.arabica coffee. Of these, 26 were organic and 42 were conventionally grown. One hundred milligrams of each sample were placed in buffered deuterated water solution and centrifuged. They were then analyzed with 1H NMR using a Bruker Avance spectrometer.

Overall, the researchers found that the NMR spectra for the different classes of coffee appeared very similar. In particular, they were characterized by the presence of compounds such as chlorogenic acids, trigonelline, and caffeine in the aromatic region, and by organic acids, fatty acids, and sucrose in the aliphatic region.

In order to achieve separation between the two classes of coffee, the researchers applied chemometrics. They found that applying PLS-DA (Partial Least Square – Discrimination Analysis) generated a plot that separated the samples by farming practice fairly well and allowed them to create a model with good predictive ability.

However, they achieve further distinction by applying OPLS-DA (Orthogonal Projection to Latent Structures – Discriminant Analysis), which allowed them to remove noise from their data. The researchers then were able to use this approach to further inspect the metabolites that were responsible for the distinction between the two coffee types.

This identified several metabolites in each type of coffee that had the highest discriminatory power. Many of these are involved in the flavor profile of roasted coffee. They included trigonelline, which is correlated with a sour taste, and was a characterizing metabolite in conventionally grown coffee. They also identified that quinic acid, one of the most bitter-tasting compounds in coffee, is distinctive in organic coffee.

Other important metabolites included fatty acids, β-(1-3)-D-galactopyranose, and quinides in organic coffee, and chlorogenic acid in conventionally grown coffee.

The researchers were able to produce these results despite the varied and unknown geographic origins of the products, and without any information on how they had been roasted. They took advantage of the nontargeted nature of NMR, which facilitates the monitoring of multiple compounds within the same experiment. They say that their findings demonstrate how the metabolic content of the samples contains all the information needed to distinguish between conventional and organically grown coffee, as well as the efficacy of NMR as a method for its characterization.

Consonni R, Polla D & Cagliani LR. Organic and conventional coffee differentiation by NMR spectroscopy. Food Control 2018; 94: 284-288. doi: 10.1016/j.foodcont.2018.07.013.