Critical Reviews in Analytical Chemistry

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Coffee Adulteration: More than Two Decades of Research Aline Theodoro Toci, Adriana Farah, Helena Redigolo Pezza & Leonardo Pezza To cite this article: Aline Theodoro Toci, Adriana Farah, Helena Redigolo Pezza & Leonardo Pezza (2016) Coffee Adulteration: More than Two Decades of Research, Critical Reviews in Analytical Chemistry, 46:2, 83-92, DOI: 10.1080/10408347.2014.966185 To link to this article: http://dx.doi.org/10.1080/10408347.2014.966185

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CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY 2016, VOL. 46, NO. 2, 83–92 http://dx.doi.org/10.1080/10408347.2014.966185

Coffee Adulteration: More than Two Decades of Research Aline Theodoro Tocia, Adriana Farahb, Helena Redigolo Pezzaa, and Leonardo Pezzaa

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a Instituto de Qu
ımica, UNESP-Universidade Estadual Paulista “J
ulio de Mesquita Filho,” Araraquara, Brazil; bN
ucleo de Pesquisa em Caf
e Prof. Luiz Carlos Trugo, Instituto de Nutri¸c~ao, UFRJ-Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil

ABSTRACT

KEYWORDS

Coffee is a ubiquitous food product of considerable economic importance to the countries that produce and export it. The adulteration of roasted coffee is a strategy used to reduce costs. Conventional methods employed to identify adulteration in roasted and ground coffee involve optical and electron microscopy, which require pretreatment of samples and are time-consuming and subjective. Other analytical techniques have been studied that might be more reliable, reproducible, and widely applicable. The present review provides an overview of three analytical approaches (physical, chemical, and biological) to the identification of coffee adulteration. A total of 30 published articles are considered. It is concluded that despite the existence of a number of excellent studies in this area, there still remains a lack of a suitably sensitive and widely applicable methodology able to take into account the various different aspects of adulteration, considering coffee varieties, defective beans, and external agents.

Analytical methods; coffee adulteration; coffee quality

Introduction Coffee is globally one of the most widely used food products and is vital to the economies of countries involved in its production and export. In the past two decades there has been continual growth in the global consumption of coffee, driven by new products and drink formulations, new domestic coffee machines, and high street coffee shops, together with changes in the profile of the typical consumer (Associa¸c~ao Brasileira da Ind
ustria de Caf
e [ABIC], 2014). Accompanying this trend, a growing number of articles have been published concerning the adulteration of coffee, since growth in the market has made it necessary to implement regulation and control strategies. The adulteration of roasted coffee is both frequent and diversified. It can involve the quality of the beans (considering species, geographical origin, and defective beans), as well as the addition of other substances (coffee husks and stems, maize, barley, chicory, wheat middlings, brown sugar, soybean, rye, triticale, and a¸c a
ı) to coffee blends in order to make them less expensive. There are nearly 100 coffee species in the world; the most commercialized are Coffea arabica (arabica coffee) and Coffea canephora (robusta coffee), with arabica having higher commercial value due to its aromatic superiority. Among these species, there are a variety of cultivars that can be grown and spread throughout the world, such as the Typica and Bourbon varieties (arabica), but there are also some more linked to their producing countries, such as Novo Mundo and Catuai (Brazil), Jimma and Harar (Ethiopia), and Villa Sarchi (Costa Rica). The quality of the fresh coffee beans, the proportion of defective beans, and the type of roasting and grinding process all influence the final product (Toci and Farah, 2008, 2014; Toci et al., 2008). Due to these factors, and considering that after roasting and grinding, the addition of others materials cannot CONTACT Aline Theodoro Toci © 2016 Taylor and Francis Group, LLC

[email protected]

be detected visually (ABIC, 2014), investigation of the adulteration of roasted and ground coffee is highly complex. For the regulation of coffee adulteration, various different norms have been adopted by national and international committees and organizations. For example, the International Coffee Council (ICC) published Resolution 399 on 24 May 2001, encouraging ICO member countries (45 members) to take measures to remove defective coffees from the marketplace. A document was issued outlining a framework for action needed to implement a Coffee Quality-Improvement Programme (CQP). The Programme sets target standards for exportable coffee, stating that exporting countries shall strive not to export coffee that has the following characteristics: for arabica, in excess of 86 defects per 300 g of sample (New York green coffee classification/Brazilian method, or equivalent); for robusta, in excess of 150 defects per 300 g (Vietnamese, Indonesian, or equivalent). The main concern about defective beans is the possible presence of ochratoxin A (OTA). To this end, the Food and Agriculture Organization (FAO) was appointed to oversee a project sponsored by the ICO, with the aim of trying to eliminate or reduce the formation of molds. In 2004, the Brazilian Association of Coffee Industries (ABIC), together with the Brazilian Deliberative Council for Coffee Policy (CDPC), created the Coffee Quality Program (PQC). This program certifies the purity of roasted coffees, which is regulated by the Ministry of Agriculture, Livestock, and Supply (MAPA). Normative Instruction number 16 (24 May 2010) was issued with the objective of ensuring the quality of roasted ground coffee, and sets a maximum 1% limit for the combined total content of impurities, including husks and stems, sediment (stones, agglomerates, and sand), and other substances (such as maize, rye, barley, a¸c a
ı seeds, and sugar). The PQC also sets the quality

Rua Prof. Francisco Degni 55, Jardim Quitandinha, 14800-060, Araraquara, SP, Brazil.

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of coffee, using the classifications traditional, superior, and gourmet. In addition to considering the sensorial characteristics of the drink, this program sets a limit of 40% for the quantity of defective coffee beans. However, this program does not yet cover the entire national territory, because it is a private initiative that is not supported by a segment of the coffee roasting industry, notably the market-leading brands that do not require certificates to improve their visibility. Global data about coffee adulteration are practically nonexistent, mainly because it involves the domestic economic situation of each country. In Brazil, which is the world’s largest coffee producer, an inspection conducted by ABIC analyzed 2400 brands, and among these 583 were adulterated with husks, maize, rye, a¸c a
ı seeds, or brown sugar, representing 25% of the national brands (Peixoto, 2009). Most of these brands were not certified by ABIC. However, this problem is even greater in some Brazilian states, as in the state of Minas Gerais, which owns 50% of the Brazilian coffee production. In this state, the adulterations reach 47% of regional brands (Peixoto, 2009). The conventional methods that are most widely used in laboratories to identify the adulteration of roasted and ground coffee involve the use of optical and electron microscopy. Complementary physicochemical analyses are also performed, including moisture content, mineral residues, ether-extractable substances, and caffeine. The analyses based on microscopy are frequently slow and subjective, and can produce conflicting results. Other techniques have therefore been studied in order to provide analyses that are more reliable and reproducible and able to identify the many different types of possible adulteration. These alternative techniques include chromatographic analysis and infrared spectroscopy. More recently, polymerase chain reaction (PCR) techniques have been used to characterize the DNA of samples. The present review considers the physical, chemical, and biological techniques that have been used to identify coffee adulteration. A total of 30 articles are described (Table 1), almost all of which used statistical tools to complement the analytical technique (these statistical tools are not the focus of the present work).

Spectrometric methods Microscopy Starting in the 1950s, methods based on optical microscopy were introduced for the detection of adulterated roasted and ground coffee. These techniques require sample pretreatment, such as degreasing with an organic solvent, drying, and sifting (Menezes and Bicudo, 1958). Optical microscopy is based on the use of objective and ocular lenses to focus on the substance being studied. The analyses depend on the degree of agreement obtained between the characteristics of the unknown substance and those of roasted coffee particles. Resolution can be improved by illuminating the object with radiation at a wavelength shorter than that of visible light. The depth of field is inversely proportional to the magnification, so that perfect smoothness of the surface under observation is ideally required, which is not feasible for coffee analysis. In this case, the microscope slides are prepared using chemical reagents, and the

quantification of impurities is based on comparison of the aqueous extract percentage of the sample with that of pure coffee (Menezes and Bicudo, 1958). The method requires considerable technical ability, and is therefore subjective. Scanning electron microscopy (SEM) uses a beam of electrons in place of the photons used in conventional optical microscopy. SEM can provide rapid information concerning the morphology of a solid sample, as well as identify the chemical elements present. The principle of SEM is based on the use of a small-diameter beam of electrons to explore the surface of the sample, point by point, along successive lines, and transmit the detector signal (which can consist of electrons or photons) to a cathodic screen whose scan rate is perfectly synchronized with that of the incident beam. The method does not require prior sample preparation, but as in optical microscopy, it entails a series of comparisons between samples and potential adulterants (Lopez, 1983), which makes it as subjective as optical microscopy. Infrared spectroscopy Spectrometric techniques, previously used only for the identification of analytes, have been greatly improved with the introduction of the direct injection (direct infusion) technique, which avoids the need for prior separation of analytes and provides an accurate digital representation of the sample. Infrared (IR) spectroscopy is one of the most widely used physical techniques for the identification of adulterants. Its advantages include reduced analysis time, less sample manipulation, and less chemical waste generation than conventional methods. Although the IR spectra of coffee products are complex, due to strong overlapping of peaks originating from many chemical species, studies have demonstrated the viability of this analytical technique for the detection of adulteration in roasted coffee. Briandet et al. (1996) presented an efficient method using Fourier transform-infrared (FT-IR) spectroscopy for the detection of adulteration of freeze-dried instant coffees. Adulteration with glucose, starch, and chicory was investigated using two different FT-IR procedures: diffuse reflectance and attenuated total reflectance. Three different statistical treatments of the spectra were carried out. First, the spectra were compressed by principal component analysis (PCA), and a linear discriminant analysis (LDA) was performed. With this approach, a 98% classification success rate was achieved. Second, a simultaneous partial least squares (PLS) regression was carried out for the content of three added carbohydrates (xylose, glucose, and fructose) in order to assess the potential of FT-IR spectroscopy for determining the carbohydrate profile of instant coffee. Last, the discrimination of pure from adulterated coffee was performed using an artificial neural network (ANN), which achieved a perfect assignment rate. The performance of the ANN was validated using an independent data set and 100% correct classification was once again achieved. Pizarro et al.(2007) studied the adulteration of C. arabica blends with the C. robusta variety using near-infrared spectroscopy (NIRS) combined with multivariate calibration methods. A total of 108 arabica blends were used, with C. robusta percentages ranging from 0 to 60% (w/w). A method employing PLS regression and wavelet-based preprocessing was developed,

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Table 1. List of papers with different analytical approaches (physical, chemical, and biological) to the identification of coffee adulteration. Method

No.

Physical

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

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Chemical

Biological

Technique used Infrared

Multispectral imaging Mass spectroscopy Photoacoustic Thermal lens Photothermal NMR HPLC-UV HPLC-UV-vis HPAEC HPAEC-PAD HPLC-HPAEC-PAD HPLC with fluorescence DH-GC-MS GC SPME-GC-MS PCR

Authors

Year

Adulterants investigated

Briandet et al. Pizarro et al. Tavares et al. Reis et al. Assad et al. Sano et al. Gon¸c alves et al. Amorim et al. Garrett et al. Cesar et al. Fontes et al. Fontes et al. Ciampa et al. Blanc et al. Davis et al. Pauli et al. Stober et al. Garcia et al. Domingues et al. Jham et al. Ruiz et al. Valdenebro et al. Jham et al. Toci and Farah Toci and Farah Oliveira et al. Martellossi et al. Ferreira et al. Spaniolas et al. Spaniolas et al.

1996 2007 2012 2013 2002 2003 2007 2009 2012 1984 2001 2006 2010 1989 1990 2011 2001 2009 2014 2007 1995 1999 2008 2008 2014 2009 2005 2012 2006 2008

Glucose, starch, chicory Robusta Husks Husks, maize Barley, maize, stems Coffee husks and straw, maize, brown sugar, soybean Husks, straw Defective seeds Robusta Coffee husks, maize, barley Maize Maize Robusta Coffee husks, maltodextrin, caramelized sugar Coffee husks Coffee husks, starch Legumes Coffee husks, maize Triticale, a¸c a
ı Maize Maize, barley Robusta Maize Defective coffee seeds Defective coffee seeds Barley Robusta Maize, rice, barley Robusta Robusta

called OWAVEC, which was used to simultaneously perform two crucial preprocessing steps in multivariate calibration: signal correction and data compression. Several other preprocessing methods (mean centering, first derivative, and two orthogonal signal correction methods, OSC and DOSC) were additionally applied in order to obtain calibration models with the best possible predictive capacity and to evaluate the performance of the OWAVEC method, comparing the qualities of different regression models. The calibration model developed after preprocessing derivative spectra using OWAVEC provided high-quality results (0.79% root mean squared error of prediction [RMSEP]). The percentage of the C. robusta variety present was predicted with greater reliability than with models constructed from the raw spectra or using other orthogonal signal correction methods. The OWAVEC model also offered greater simplicity. The work reported was only a feasibility study, and the authors suggested that further research would be needed before it could be used for authentication purposes. More recently, Tavares et al. (2012) used mid-infrared spectroscopy to distinguish 13 coffee blends containing different percentages (0.5 to 30%) of coffee husks. Samples adulterated with husks were identified by PCA, and quantitative estimation of adulteration was achieved by PLS regression. The minimum quantity detectable by this method (0.5%) was lower than the maximum level (1.0%) of impurities permitted by law. Reis et al. (2013) evaluated the feasibility of employing diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for discrimination among roasted coffee, roasted maize, and coffee husks. Arabica coffee beans, coffee husks, and ground maize kernels were submitted to light, medium, and dark roasts. Principal component analysis of the DRIFTS

spectra enabled separation of the samples into three groups: coffee, coffee husks, and maize. The calibration set consisted of a total of 116 samples: 33 samples of roasted coffee, 27 samples of roasted coffee husks, 30 samples of roasted maize, and 26 samples of adulterated coffee, with adulteration levels ranging from 10 to 50% of one or both adulterants. Classification models based on linear discriminant analysis provided complete discrimination (100% recognition and prediction) among roasted coffee, pure adulterants (maize and coffee husks), and adulterated coffee samples. Multispectral imaging Assad et al. (2002) developed a method based on reflectance to identify adulteration with barley, maize, and stems at percentages ranging from 1 to 50%. The method was based on image analysis, considering that different organic materials found in ground coffees present distinct spectral signatures. Multispectral images of coffee samples were generated using a glass magnifier connected to a charge coupled device (CCD) camera. The camera captured images of the sample in visible spectral bands (RGB: red, green, and blue), which were then analyzed using a digital image processing program to calculate the relative proportions of the components of the sample. Capture of the images was fast, but sample preparation steps were required, including cleaning, drying, and homogenization. Nevertheless, this method achieved a minimum accuracy of 95% for quantification of adulteration in powdered coffee. In an extension of the work, the same method was proposed for the identification of adulteration in arabica coffee blends mixed with coffee husks and straw, maize, brown sugar, and soybean (Sano et al., 2003).

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The following concentrations of contaminants were prepared: 10, 20, 30, 40, 50, 100, 200, 300, 400, and 500 g/kg. The percentage areas of the contaminants in each image were calculated by the maximum likelihood supervised classification technique. Best-fit equations relating weight percentages (g/kg) to the percentage areas were obtained for each coffee contaminant. To test the method, 247 coffee samples with different amounts and types of adulterants were analyzed in the laboratory. It was concluded that the method could provide fast, nondestructive, and precise analysis of a wide range of adulterants present at different concentrations in ground coffee powders. The high correlations obtained for samples containing two different contaminants also indicated that the method could be successfully employed when more than one adulterant was present in the sample. This method was subsequently validated for the selective identification of husks and straws (Gon¸c alves et al., 2007), with detection and quantification limits of 0.03 and 0.05%, respectively. The method was considered to be valid, although for regulatory purposes it was suggested that precision in the concentration range 0.2–2.2% would need to be improved. Mass spectroscopy Direct infusion electrospray ionization mass spectrometry, in both negative (ESI(–)-MS) and positive (ESI(C)-MS) ion modes, has been used to distinguish pure green and roasted arabica coffees according to bean characteristics (green, ripe, and overripe [defective seeds]) and post-harvesting processes (dry, wet, and semi-wet), as well as coffees with different cup qualities (Amorim et al., 2009). Statistical analysis using PCA showed that in the ESI(–)-MS mode, ions from chlorogenic acids and short-chain organic acids derived from sugars were most important for the discrimination of defective beans. In the ESI(C)-MS mode, discrimination mainly employed low m/ z ions, such as protonated pyridine and alkylpyridines resulting from trigonelline degradation. Preliminary results showed that both ESI(C)-MS and ESI(–)-MS modes were able to differentiate the cup qualities of roasted arabica coffees, and the ions used to perform discrimination were the same as those employed in ripeness and post-harvesting evaluations. In subsequent work by the same group, Garrett et al. (2012) evaluated the use of direct-infusion ESIMS data, combined with the PLS multivariate calibration technique, to detect and quantify the adulteration of arabica coffee by robusta coffee. Five robusta/arabica blends, with robusta percentages of 0, 25, 50, 75, and 100%, were used for the construction of a calibration curve, and samples with robusta concentrations of 20, 40, 60, and 80% were evaluated. A total of 16 PLS models were built using ESI(§) quadrupole time-of-flight (QTOF) and ESI (§) Fourier transform ion cyclotron resonance (FT-ICR) MS data for hot aqueous extracts of certified coffee samples. ICR is a phenomenon related to the movement of ions in a magnetic field. It is used for accelerating ions in a cyclotron and for measuring the masses of ionized analytes in mass spectrometry. The model using the 30 most abundant ions detected by ESI (C) FT-ICR MS produced the most accurate coffee blend percentage prediction and was later successfully employed to predict the blend composition of commercial robusta and arabica

coffees. In addition, ESI(§) FT-ICR MS analysis enabled the identification of 22 compounds in the arabica coffee and 20 compounds in the robusta coffee, most of which were phenolics. Other spectroscopic methods Other promising physical methods that have been proposed over the years include photoacoustic, photothermal, thermal lens, and nuclear magnetic resonance (NMR) techniques. Some of these methodologies have not yet been fully studied or validated. Nevertheless, they deserve attention due to the good results that have been achieved, as described below. The photoacoustic (PA) effect has been recognized in the past few years as an important tool for studying the optical absorption properties of crystalline, powdered, and amorphous solids. Cesar et al. (1984) used this method to evaluate the adulteration of coffee by coffee husks, maize, and barley. The success of this spectroscopic technique is essentially due to the fact that only the absorbed light is converted into pressure fluctuations in the gas cell. The primary source of the acoustic signal in the cell arises from the periodic heat flow from the solid to the surrounding gas as the solid is cyclically heated by the absorption of pulsed light. Only a relatively thin layer of gas adjacent to the surface of the solid is assumed to respond thermally to the periodic heat flow from the solid to the surrounding gas. This boundary layer of gas then acts as a piston, generating the acoustic signal detected by the microphone. Since the magnitude of the periodic pressure fluctuations in the cell is proportional to the amount of heat emanating from the solid absorber, there is a close correspondence between the strength of the acoustic signal and the amount of light absorbed by the solid. The feasibility of this method for detection of coffee adulteration was demonstrated. The technique avoided any need for sample preparation and required only that the sample exhibit a fairly homogeneous particle size distribution. A linear relationship was obtained between the PA signal and the mass concentrations of all the adulterants, and excellent mathematical models were obtained for maize and husks. However, for barley, the mathematical model was not as satisfactory, perhaps due to the choice of a response wavelength very close to that of the pure coffee. Although it was possible to establish an adequate methodology for the detection of different adulterants in powdered coffee, there remain difficulties related to sample uniformity and compaction. In order to obtain compact and uniform coffee samples, it is necessary to control not only the grain size and compaction pressure, but also the moisture content of the sample. Considering the problems involved in sample uniformity, the same group published two other studies. The first investigated the viability of thermal lens spectrometry (TLS) to detect the presence of an adulterant (maize) in brewed coffee (Fontes et al., 2001). Briefly, this technique involves the illumination of a sample by a modulated light beam and subsequent measurement of the temperature fluctuation induced in the sample as a result of non-radiative de-excitation processes within the material. Since the photothermal signal is derived only from the absorbed light, the effects of scattered light play no significant role in these spectroscopic techniques, which should therefore

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be suitable for studies involving powdered samples. Thermal lens experiments were performed using brewed coffee made with a pure commercial coffee and a commercial coffee mixed with 4 wt.% of roasted maize meal. The concentrations of the samples investigated (pure and adulterated) ranged from 0.04 to 20 wt.%. The results indicated that the dn/dT (the analytical expression for absolute determination of the thermo-optical properties of the sample) and pH values exhibited similar behavior as a function of the coffee brew concentration and were equally sensitive for detecting the presence of adulterants. Even though these preliminary experiments were limited to well-controlled samples with a single adulterant concentration, the results indicated that the combination of the two detection techniques (thermal lens spectrometry and pH measurements) could be useful for the routine detection of coffee adulterants, once a wider range of adulterants and concentrations has been tested. The second work reported an alternative photothermal approach for analysis of the adulteration of coffee by maize (Fontes et al., 2006), which was actually an extension of the work initiated in 2001. The approach differed from the previous photothermal study in two fundamental ways. First, the proposed method was non-spectroscopic, in that no wavelength scanning was performed. Instead, the parameter monitored was the temperature coefficient of the refractive index of the coffee brews at the fixed pumping beam wavelength. Second, the new method was based on the use of coffee brews, instead of powdered samples. This not only eliminated the need for careful control of sample compaction, but at the same time also ensured that the samples used were always homogeneous. The thermal lens analyses were accompanied by measurements of the pH of the different coffee brews. From a quantitative perspective, the results suggested that the monitoring of dn/dT as a function of the coffee brew concentration could discriminate between pure and adulterated brews for brew concentrations above 0.6%. Even though the results presented in this article were limited to well-controlled samples, the thermal lens and pH data indicated that the combination of these two techniques could be successfully applied to detect adulterants in brewed coffee. Finally, Ciampa et al. (2010) demonstrated the potential of high-resolution magic angle spinning nuclear magnetic resonance (HR-MAS NMR) spectroscopy for discrimination between arabica and robusta coffees. The innovative approach to high-resolution magic angle spinning involved the use of a novel NMR probe head that enabled highly resolved spectra to be obtained for gel-like or suspended samples. Variations of the concentrations of relevant species (caffeine, trigonelline, sugar, amino acids, acrylamide, pyrazines, melanoidins, and chlorogenic acids) were monitored as a function of roasting temperature (from green to completely roasted beans). The results showed that monitoring of analytes such as caffeine, chlorogenic acids, and sugars might be able to be used to predict the amount of robusta coffee in the blends. Nonetheless, further studies are still needed to confirm this possibility. The HRMAS NMR technique was demonstrated to be a powerful tool for fast chemical composition measurements, opening up perspectives for novel applications of this approach in coffee quality control.

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Instrumental separation methods (chromatographic methods) The most widely used chemical techniques reported for the analysis of coffee adulteration are liquid chromatography (seven relevant studies) and gas chromatography (five studies). Carbohydrates are the analytes most frequently used to distinguish between pure and adulterated coffee.

High-performance liquid chromatography (HPLC) One of the first studies to employ liquid chromatography for analysis of carbohydrates in order to identify adulteration was undertaken by Blanc et al. (1989). HPLC-UV was used to characterize the carbohydrate profiles of 122 soluble coffee samples, pure and adulterated with husks, maltodextrin, and caramelized sugar. The results showed that pure soluble coffee contained maximum levels of ~0.3% total xylose and sucrose, no maltose, and about 2% total glucose. Higher levels of total xylose could be explained by the co-extraction of coffee husks or parchment, while the levels of free fructose and glucose distinguished whether unroasted or roasted husks/parchments had been added. Elevated levels of maltose and total glucose indicated the addition of maltodextrins, and high levels of sucrose and total glucose reflected the addition of caramelized sugar. It was concluded that analyses of both free and total sugar contents were required in order to obtain information on the nature of the adulterants. In further work by the same group (Davis et al., 1990), mannitol, a polyhydric sugar alcohol, was identified in coffee products for the first time using HPLC. Mannitol is frequently found in exudates of plants such as flowering ash, olive, and plane trees, and is present in marine algae at concentrations in excess of 20%. It was identified in the carbohydrate fraction of dried coffee husks at concentrations of 1.61–2.03%. Its presence in some commercial soluble coffees at levels above 0.30% was indicative of adulteration by coffee husks. A total of 145 samples from different countries were evaluated, and it was concluded that the majority of soluble coffee powders sold globally were manufactured from good quality coffees, although more than half the samples contained mannitol at concentrations exceeding 0.3%. In 2001, Stober et al. published a preliminary study concerning the detection of legume tissues in soluble coffees by oligosaccharide profiling combined with protein analyses, using high-performance anion exchange chromatography (HPAEC; Stober et al., 2001). This type of chromatography separates carbohydrates according to specific interactions between the hydroxyl groups of the glycan and the stationary phase of the column, at high pH. The glycans are separated chromatographically as anionic species, and their interaction with the column is based on their size, composition, and linkage. Twenty soluble authentic coffees and blends from different manufacturers were analyzed. A peak with the retention time of stachyose (a tetrasaccharide typically found in leguminous plants) was identified in the chromatograms for the commercial coffee blends, corresponding to concentrations ranging from about 0.1 to 0.2 g/100 g. No peaks with the same retention time were observed in the HPAEC chromatograms for the authentic soluble coffees. In order to identify

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the stachyose, purification of the oligosaccharide fraction was performed by classical reversed-phase (RP)-HPLC, followed by methylation and analysis by GC-MS. It was found that about 50% of this fraction consisted of stachyose. The authors concluded that the unusual HPAEC oligosaccharide profile, together with high protein content and the confirmed presence of stachyose, was indicative of the use of an adulterant derived from a legume. This could lead to a new tool for the screening of soluble coffee authenticity. Other work published in 2007 used HPLC with fluorescence detection for the quantification of g-tocopherol, used as a marker for the adulteration of Brazilian coffee (Coffea arabica L.) with maize (Jham et al., 2007). This study characterized the profiles of a-, b-, g-, and d-tocopherol in six coffee varieties and six maize samples. The results showed a higher concentration of g-tocopherol in maize samples (91.3 mg/kg) than in pure coffee samples (61.7 mg/kg). Evaluation was also made of four coffee/maize blends prepared in the laboratory, together with six commercial blends. The results indicated that g-tocopherol could be used as a marker for the adulteration of Brazilian coffee with maize. Nevertheless, further detailed examination would be needed of factors that could affect tocopherols, such as coffee variety, sample origin, storage, processing, and the presence of other adulterants. Garcia et al. (2009) reported the carbohydrate profiles of roasted and ground arabica coffees and their adulteration by coffee husks and maize, using high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD). In this technique, non-derivatized analytes are detected by applying various potentials to the working electrode over a specific time period. The hydroxyl groups in carbohydrates are oxidized on the electrode surface and the resulting current is measured. Sensitivity at picomole to femtomole levels can be achieved with PAD, making it one of the most sensitive detection techniques. The results showed higher levels of galactose and mannose in pure coffee, with concentrations of 8.25 and 9.65% (w/w), respectively. However, in pure coffee husks, the main carbohydrates were mannitol (0.64%), arabinose (4.24%), and xylose (3.40%). The highest concentration of glucose was detected in the maize sample (52.53%, w/w). Chemometric methods were applied in order to identify patterns for adulteration by coffee husk and maize, with different amounts of these contaminants added to the coffee in accordance with a simplex-centroid statistical design. The results were used to produce linear models describing the effects on carbohydrate levels caused by the addition of adulterants to the coffee. Factor analysis, supported by hierarchical cluster analysis, enabled the discrimination of four distinct groups that differed according to the compositions of the mixtures of raw materials used. In subsequent work, Pauli et al. (2011) validated a chromatographic method for the determination of total carbohydrates in soluble coffee, using a HPLC-UV-vis system with post-column derivatization, in order to detect adulterant additions. The validated method was shown to be accurate and robust. Adulteration with coffee husks was associated with increases in the levels of xylose, glucose, and starchy products in the samples, together with decreases of galactose and mannose, which are characteristic carbohydrates present at high

concentrations in soluble coffees produced from arabica and robusta coffee beans. Recently, the same group evaluated the performance of two different chromatographic systems, HPLC-HPAEC-PAD and HPLC-UV-vis with post-column derivatization, used for carbohydrate determination (ISO method 11292) in order to detect and quantify adulterants in coffee (Domingues et al., 2014). Total carbohydrate analyses performed using both methodologies were effective in determining the concentrations of monosaccharides evaluated in roasted and ground coffee adulterated with triticale and a¸c a
ı, considering the original constituents of the matrices. Although the determination of carbohydrates using the HPLC-UV-vis system with post-column derivatization resulted in numerically different concentrations, with poorer chromatographic resolution, sensitivity, and predictive model fitting than the HPLC-HPAEC-PAD system, the former technique is faster, easier to use, and could be performed in most laboratories possessing a UV-vis detector. This system therefore appears to offer the potential for use in routine quality control screening of adulterants in coffee. For the purposes of quantification and forecasting by mathematical modeling, the HPLC-HPAEC-PAD technique was shown to be superior, but is more expensive and requires specialist knowledge of electrochemical techniques. The simplex-centroid experimental design for the three components of the mixtures (arabica coffee, triticale, and a¸c a
ı) was used to obtain correlations between the two chromatographic systems. Principal component analysis of the carbohydrates revealed similar trends for each of the matrices. Galactose was a characteristic component of the arabica coffee matrix. Glucose and xylose were the predominant carbohydrates in triticale, while at higher concentrations mannose characterized the a¸c a
ı matrix. Gas chromatography The gas chromatography (GC) technique has also been used to detect adulteration in roasted coffee, albeit to a less extent than liquid chromatography. One of the first studies using this technique was undertaken by Ruiz et al. (1995). The profiles of the volatile components of Colombian coffee at two grades of roasting, together with those of roasted maize and roasted barley, were determined by dynamic headspace sampling followed by GC separation and mass spectrometric detection. The volatile components present at highest concentrations in the maize samples were furfural and 5-methylfurfural, while in barley the most prevalent compounds were 2-methylbutanal, 3-methylbutanal, and furan. Sensorial analysis was also performed, which enabled detection of adulteration with 20% cereal when the coffee was roasted for 8 min. It was difficult to perceive the cereal aroma when the roasting was performed for 10 min, especially in the case of adulteration with barley. The authors suggested that these compounds could be used as indicators of the adulteration of coffee with cereals in quality control procedures designed to ensure the quality and purity of Colombian coffee. Valdenebro et al. (1999) proposed a method for determination of the percentage of arabica coffee in blends of roasted arabica/robusta coffee, based on their sterol contents. Thirteen blends were prepared, using concentrations of arabica coffee ranging from 40 to 100% (m/m). Commercial roasted coffee

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samples were also analyzed. The sterol content was determined by extracting the coffee oil, saponifying the lipids, and separating the sterols present in the unsaponifiable fraction using thin-layer chromatography. The sterols were then converted to trimethylsilyl derivatives, which were analyzed by gas chromatography. Twelve sterols were determined in roasted coffee samples consisting of mixtures of the arabica and robusta classes. Using the sterol profiles in the blends as chemical descriptors, application of principal component regression (PCR) then provided a method for determination of the composition of mixtures of roasted commercial coffees. The results showed that when the descriptor D5avenasterol was used alone, it was possible to predict the percentage of arabica in coffee mixtures with greater reliability than with the use of the first PC (which included inputs from all the variables and was therefore more complex). Using the same approach, Jham et al. (2008) studied the use of fatty acid profiles as potential markers for the adulteration of Brazilian roasted coffee with maize. Six C. arabica varieties were used (Catuai, Catuca
ı, Bourbom, Mundo Novo, Rub
ı, and Top
azio), together with three blends containing 5, 10, and 20% of maize grains. The fatty acid methyl ester (FAME) compositions of the different varieties were determined for the first time. The method proved to be very fast, with complete characterization (>99%) of the sample being possible in less than 6 min. Although the linoleic/stearic acid ratios were significantly different for coffee and maize, it was suggested that in order for a compound to serve as a marker of adulteration in coffee, a very large difference (at least a factor of ~30) should exist between its concentrations in pure and adulterated coffee samples. Hence, this probe could not be used as a marker to detect maize adulteration in commercial coffees. Toci and Farah (2008, 2014) conducted two studies using solid-phase microextraction (SPME)-GC-MS to investigate the volatile compound profiles of defective roasted coffee beans in order to be able to detect substantial defects in coffee blends. Application of the SPME-GC-MS technique is straightforward and no previous sample preparation is required, which makes it potentially promising for routine chromatographic analysis. In general, defective beans showed higher numbers and concentrations of volatile compounds than control beans. These compounds included pyrazines, pyrroles, and phenols. Several potential volatile compound markers for defective beans were proposed. Butyrolactone and hexanoic acid were generally observed only in raw and roasted defective beans, respectively; 3ethyl-2-methyl-1,3-hexadiene was a marker for raw black beans; b-linalool and 2-butyl-3,5-dimethylpyrazine were indicators for roasted defective beans in general; and 2-pentylfuran was a marker for roasted black beans. An additional 16 compounds were suggested as indicators of poor quality. In addition to demonstrating the ability of the technique to identify defective coffee beans, it was suggested that the compounds used as indicators of quality could also be used in quantitative investigations to determine the percentages of defective beans in commercial coffee blends. Oliveira et al. (2009) evaluated the potential of the SPMEGC-MS technique to detect the adulteration of ground roasted coffee with roasted barley, using pure coffee and coffee mixed

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with 1, 5, and 50% of barley. Discrimination between unadulterated and adulterated samples was achieved using PCA. Pyridine was the substance that showed the greatest influence in the case of roasted coffee, and the relative peak intensity increased with roasting time. An unexpected finding was that adulterated samples could be more easily discriminated at the highest level of roasting, which enabled detection of adulteration with as little as 1% (w/w) roasted barley in dark roasted coffee samples. The authors also suggested that the use of aromatic compounds to identify adulterants was poorly explored and inconclusive, and that further investigations were required. Capillary electrophoresis One of the first articles reporting the use of capillary electrophoresis to identify adulterants in coffee was published by Nogueira and Lago (2009). The proposed method was based on the controlled acid hydrolysis of the polysaccharides xylan and starch present in some plant-based adulterants, followed by analysis of the corresponding monosaccharides (xylose and glucose, respectively). Acid hydrolysis by HCl increases the ionic strength of the sample, which impairs the electrophoretic separation. A neutralization step based on anion exchange resin was therefore necessary. The best separations were obtained using NaOH (80 mmol/L), cetrimonium bromide (CTAB) (0.5 mmol/L), and methanol (30%, v/v). The high pH of this electrolyte resulted in the separation of the monosaccharides in the form of anionic species. The two adulterants evaluated were maize and coffee husks. The LOQ (limit of quantification) for both monosaccharides was 0.2 g in 100 g of dry matter, which conformed to acceptable limits. The capillary electrophoresis technique has been shown to be as efficient as liquid chromatography for determination of carbohydrate profiles of adulterants such as coffee husks and maize grains. Nevertheless, the need for a hydrolysis step makes the technique slower than HPLC. There have been few publications describing the use of this technique, which therefore merits further investigation.

Biological methods Contemporary techniques such as polymerase chain reaction (PCR) offer the potential for the unequivocal identification of DNA, and the use of DNA molecular markers has been employed to identify different species or varieties. Molecular markers such as microsatellites have been used to good effect in the characterization of Coffea species, enabling discrimination between robusta and arabica and detection of the presence of adulterants. Martellossi et al. (2005) demonstrated that PCR-grade DNA can be obtained from roasted beans and even from instant coffee. This would permit the analysis of commercial samples, provided that suitable markers for species/ variety identification could be found. A total of 25 samples of green coffee beans and 12 roasted coffees (3 arabica, 3 robusta, and 6 instant coffees) were used. It was demonstrated that sufficient DNA survives during roasting and freeze-drying to enable successful extraction and subsequent amplification, provided that a suitable protocol is

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applied. Various methodological considerations were described for the successful amplification of the DNA, and the effect of the presence of contaminants that could inhibit PCR was noted. Notwithstanding, once suitable markers are found, use of these extraction methods should enable assessment of the authenticity of coffee varieties, determination of the percentage of robusta in blends, and detection of adulterants. The same analytical approach was adopted by Spaniolas et al. (2006), who used the PCR technique to determine the genomic profiles of arabica and robusta coffee. Samples were also analyzed with a lab-on-a-chip capillary electrophoresis system. The choice of target was based on a phylogenetic study in which a number of single-nucleotide polymorphisms (SNPs) were found in several Coffea species, indicating the existence of different chlorotypes in the arabica and robusta coffees. One of these SNPs resides in a PsuI restriction site, resulting in the site being present in robusta but absent in arabica. The purpose of these studies was to evaluate whether this SNP could be exploited for the qualitative or quantitative detection of robusta contamination of arabica using a PCR-restriction fragment length polymorphism (RFLP) approach. In this study, a total of 11 roasted coffee samples (8 arabica and 3 robusta) with different origins were used. A 5% limit of detection was achieved for coffee powder mixtures analyzed using this technique. The chloroplast target used was the trnL(UAA)-trnF(GAA) intergenic spacer region, which was found to be discriminatory for all of the arabica and robusta varieties used in this study. However, although the beans originated from a variety of geographical regions, further studies will be required to confirm that the plastid copy number remains relatively constant across a wider range of varieties, and that there are no significant influences of factors related to the environment or type of cultivation. More recently, Ferreira et al. (2012) developed a DNAbased method to detect and quantify the adulteration of roasted coffee with barley, maize, and rice. The amplification capacity was determined by random amplified polymorphic DNA (RAPD)-PCR and the products were analyzed by horizontal electrophoresis in agarose gel. Marker genes for coffee, barley, maize, and rice were obtained from the National Centre of Biotechnology Information (NCBI, USA). In order to confirm the specificity of the chosen genes, they were statistically analyzed with respect to their similarity with those of other species using the Basic Local Alignment Search Tool (BLAST). The regions of markers that showed similarity to other species were discarded and specific regions were selected as DNA templates to design the primers. The marker genes for barley (cytochrome C), maize (zein), and rice (hypothetical protein Chromosome 8) showed no similarity to organisms of the C. arabica and C. canephora species. Each primer pair provided specific amplification only with the corresponding target (adulterant) DNA sequences. This promising method showed the ability to detect the presence of the aforementioned adulterants, indicating that it could be used to ensure coffee quality in accordance with international market specifications.

Final considerations This review presents an overview of recent advances in the detection of adulterants in coffee. The greatest numbers of published articles concern spectrometric and instrumental separations techniques equally, with 13 articles. Spectrometric methods, such as infrared and multispectral imaging techniques, have practical advantages in that they are fast and do not require prior sample preparation. In particular, the infrared method can be used to study a variety of external adulterants, including glucose, starch, chicory, and maize, as well adulteration involving different coffee varieties. Mass spectroscopy has also been shown to be a promising technique for the detection of defective beans and adulteration with different coffee species. Instrumental separation methods used in this area are based on chromatographic separation employing liquid chromatography, gas chromatography, and capillary electrophoresis. Despite being time-consuming and more expensive than spectrometric methods, these techniques have delivered highly promising results for the detection of a wide range of adulterations involving different coffee species, coffee husks, maltodextrin, caramelized sugar, legumes, maize, starch, triticale, a¸c a
ı, and barley. The advantages of chromatographic methods based on chemical screening performed on the samples permits, in many cases, the identification of potential adulterants’ marker compounds. The carbohydrates found most abundantly in adulterants have been the class more cited as potential markers. Among them can be mentioned xylose and mannitol (husks), stachyose (leguminous), mannose (a¸c a
ı seeds), sucrose, glucose, and frutose (brown sugar). Other classes of compounds also showed relevance as markers, like tocopherols (D5avenasterol and g-tocopherol, for arabica and robusta distinction and maize, respectively), fatty acids (lenoleic/stearic acid profile for maize), and volatile compounds (furfural and 5-methylfurfural, for maize; 2-methylbutanal, 3-methylbutanal, and furan, for barley; butyrolactone, hexanoic acid, 3-ethyl-2-methyl-1,3-hexadiene, b-linalool, and 2-butyl-3,5-dimethylpyrazine and 2pentylfuran, for defective seeds). Biological methods based on the PCR technique have shown considerable advances, despite their relatively low level of uptake compared to the physical and chemical methods. It has been shown that complete DNA still remains after the coffee roasting process (in both roasted and instant coffee), which indicates that the technique is viable, despite difficulties associated with amplification of the DNA signal. However, this method still requires full validation of the DNA data bank and remains expensive for use in routine analyses. In general terms, it can be concluded that despite the existence of valuable studies in this area, it still remains necessary to develop a widely applicable and sensitive methodology that can address the various aspects of coffee adulteration. This includes discrimination between species, detection of defective beans, and identification of the presence of external agents. This task will not be straightforward because of the complexity of the issues involved. Nonetheless, the prospects remain promising, due to both scientific advances and the interest of regulatory agencies. The latter include the Institute for Scientific Information on Coffee (ISIC), Coop
eration Internationale en Recherche Agronomique pour le D
eveloppement (CIRAD), the

CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY

International Coffee Organization (ICO), the Food and Agriculture Organization (FAO), and the Food Safety and Inspection Service of the United States Department of Agriculture (FSI/USDA).

Funding The authors would like to thank the Brazilian National Research Council (CNPq) for financial support.

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