Rapid fraud detection of cocoa powder with carob flour using near infrared spectroscopy

Highlights

• Near infrared spectroscopy can detect cacao adulterated with carob flour.

• The method is rapid, useful and simple for quality cacao analyses.

• PLS-DA distinguishes pure cocoa powders from those adulterated with carob flour.

• A PLS analysis has the potential to quantify the percentage of adulteration.

Abstract

Cocoa powder is a highly valuable global product that can be adulterated with low-cost raw materials like carob flour as small amounts of this flour would not change the color, aroma and taste characteristics of the final product. Rapid methods, like NIR technology combined with multivariate analysis, are interesting for such detection. In this work, unaltered cocoa powders with different alkalization levels, carob flours with three different roasting degrees, and adulterated samples, prepared by blending cocoa powders with carob flour at several proportions, were analyzed. The diffuse reflectance spectra of the samples of 1100–2500 nm were acquired in a Foss NIR spectrophotometer. A qualitative and a quantitative analysis were done. For the qualitative analysis, a principal component analysis (PCA) and a partial least squares discriminant analysis (PLS-DA) were performed. Good results (100% classification accuracy) were obtained, which indicates the possibility of distinguishing pure cocoa powders from adulterated samples. For the quantitative analysis, a partial least squares (PLS) regression analysis was performed. The most robust PLS prediction model was obtained with one factor (LV), a coefficient of determination for prediction (R_P²) of 0.974 and a root mean square error of prediction (RMSEP) of 3.2% for the external set. These data allowed us to conclude that NIR technology combined with multivariate analysis enables the identification and determination of the amount of natural cocoa powder present in a mixture adulterated with carob flour.

Introduction

Cocoa powder, thanks to its characteristic and pleasant flavor and aroma, is one of the most valued commodities around the world (Bonvehí, 2005). Among its applications in the food industry, the formulation of beverages, confectionery, bakery and pastry products stands out (Shankar, Levitan, Prescott, & Spence, 2009). Apart from flavor and aroma, cocoa is highly appreciated as a natural coloring agent, partly because of the current tendency to restrict the use of artificial colors.

During cocoa processing, it is possible to modify cocoa color and aroma through roasting and/or alkalization processes. Roasting consists of exposing cocoa beans to temperatures of 130–150 °C for 15–45 min. It is used to inactivate microorganisms and to develop the characteristic brown color, mild aroma and texture of commercial natural beans (Afoakwa, Budu, Mensah-Brown, Felix & Ofosu-Snsah, 2014; Bonvehí, 2005; Krysiak, 2006). Alkalization is an optional operation to reduce acidity, bitterness and astringency, and to darken cocoa's color. This procedure involves using an alkali (generally potassium carbonate) in combination with oxygen, water and high temperatures. These extreme conditions provoke, among others, Maillard reactions and polyphenol oxidations and polymerizations, which end up with flavor and color modifications from light brown (natural) to red, dark brown or extremely black (Li et al., 2012; Miller et al., 2008).

In recent years, the demand for cocoa powder has increased and its supplies have tightened, thus its price has steadily grown (Fadel, Mageed, Samad, & Lotfy, 2006). Consequently, there is a demand to develop cocoa substitutes. Some studies suggest that cocoa-like aromas can be found in roasted carobs (Arrighi, Hartman, & Ho, 1997). Carob pods are characterized by a high sugar content (around 50%), composed essentially of sucrose. This high sugar content favors the same chemical reactions that occur during the roasting and alkalization of cocoa: caramelization of high sugar content and Maillard reactions between amino acids and sugars (Fadel et al., 2006). In this way, toasted carob can provide similar aromas to cocoa.

Bearing in mind this striking aromatic and visual similarity between carob flour (natural or toasted) and cocoa (natural or alkalized), some traders have seen that selling carob (average price of 940 US$/tonne) as cocoa (1945 US$/tonne), by omitting this substitution, is a profitable option to increase their benefits (ICCO, 2017). However, this deliberate, intentional and undeclared substitution of one product for another with a lower price is food fraud that not only affects producers and consumers, but also the physico-chemical properties of the manufactured product. Some studied examples comprise milk chocolates and chocolate cakes, in which some percentages of cocoa powder have been substituted for carob flour (Rosa, Tessele, Prestes, Silveira, & Franco, 2015; Salem & Ohaad Fahad, 2012).

To detect food adulteration, the three most widespread technologies are liquid chromatography, infrared spectroscopy and gas chromatography (Moore, Spink, & Lipp, 2012). Liquid and gas chromatography analyses need long sample preparation times, method optimization, and high-cost materials and reagents, while infrared spectroscopy is fast, reliable, less expensive and a chemical-free alternative (Ellis et al., 2012). Near infrared spectroscopy (NIR) is an infrared spectroscopy type characterized by recording reflectance or transmittance spectra within the region from 750 nm to 2500 nm. These spectra act as a ‘fingerprint’ that is characteristic of a particular sample molecule and allows its identification. Some examples of using NIR and multivariate analyses in the cocoa sector include the prediction of basic food components, such as moisture, carbohydrate, fat, protein, theobromine and catechin and total polyphenol content (Huang et al., 2014; Veselá et al., 2007; Álvarez et al., 2012). In other sectors, NIR in combination with a multivariate analysis has been employed to detect starch in onion powders, acid whey, starch, maltodextrin in skim powder milk, sudan dyes in chili powders, and talcum powder in teas (Lohumi, Lee, Lee, & Cho, 2015; Capuano, Boerrigter-Eenling, Koot, & van Ruth, 2015; Haughey, Galvin-King, Ho, Bell, & Elliott, 2015; Li, Zhang, & He, 2016).

In this context, the aim of this work was the rapid detection of the adulteration of cocoa powders, regardless of their alkalization level, with carob flours by applying NIR and a multivariate analysis.