Coffee adulterant quantification by derivative thermogravimetry and chemometrics analysis

Coffee quality is determined by analyzing its chemical, physical, sensory and hygienic-sanitary characteristics. It is a product consumed worldwide with high economic value. Thus, it can undergo several types of adulteration (either fraudulent or accidental). It is commonly adulterated through blending with cheaper products that are not easily detected by consumers, which in turn modify its appearance or taste. The main adulterants found in ground coffee are processing byproducts, such as husks, straws and sticks and cereals, such as corn, rye, rice, barley and wheat due to their low prices, in addition to being rather similar to coffee and widely available. Therefore, the present work aims to quantify adulterants (corn, coffee husk and coffee straw) found in ground coffee using thermogravimetry and the multivariate regression analysis. Coffee and adulterants have been toasted at 200 °C, ground, sieved and mixed in 22 different ratios (1–50%, m/m). Blends have been investigated by derivative thermogravimetry analysis (DTG) by employing two different types of dynamic gas atmospheres (nitrogen and synthetic air). Then, its results were used to develop multivariate models using the partial least squares to quantify the adulterants. TGA curves show samples with higher overlapping thermal events; however, DTG results present a selective coffee event between 350 and 430 °C that enables quantification. There were also fewer prediction errors using nitrogen as dynamic gas (2.6%, 1.4% and 7.7% m/m for corn, coffee husk and coffee straw, respectively) with 3–4 latent variables. As for synthetic air, there were greater prediction errors (4.7% and 4.6% m/m for corn and coffee husk, respectively), but fewer if compared to another gas atmosphere for coffee straw (4.8% m/m) using 4–5 latent variables. A combination of thermogravimetry and chemometrics proved to be quite promising and suitable for quantifying common materials used to adulterate ground coffee.