Читать книгу X-Ray Fluorescence in Biological Sciences - Группа авторов - Страница 43

Coffee processing begins with green coffee grains undergoing a thermal procedure (roasting). As a result, fried coffee grains are produced. The composition of green coffee grains depends on several factors such as variety, origin, climate, soil quality, treatment on fields, and collection and storage procedures. During the roasting process, the beans undergo physical changes and go through several chemical reactions that alter, generate, and decompose the coffee aroma. It is noted that the chemical composition of fried coffee beans plays an important role in determining the quality of the coffee drink [81]. The quality of beans depends on the species, geographical origin, content of defective beans, and other substances added (accidentally or intentionally), i.e. coffee husks and stems, corn, barley, chicory, wheat middlings, brown sugar, soybean, rye, etc. [25].

Orlic et al. [82] published one of the first papers on the use of XRF to research of samples of coffee. Let's review their results. The authors used the X‐ray spectrometer PW 1010/30 from Philips with X‐ray tube with Mo‐anode, and two modes of a fluorescence excitation: direct and secondary excitation mode (Zr, Mo, and Ag pure foils). Coffee grains were air dried at 85 °C for 24 hours, then ground in an agate mortar manually or mechanically. Coffee powder was placed in Teflon holders or pressed into tablets. The results of analysis of some coffee products (grains, pulp and cascarilla) are shown in Table 3.2. Thirteen elements were found in the concentration range from 0.2 ppm to 1.56%. The K concentration in grains is higher than in pulp (~1.5 and 1%, respectively) and significantly lower in cascarilla (0.07%). A typical Ca concentration is 0.15% in grains, 0.5% in pulp and 0.06% in cascarilla. Elements other than Zn and Fe show no significant changes in concentrations. The Zn content is generally higher in pulp (60–70 ppm) than in grains and cascarilla. Fe concentrations vary from 50 to 92 ppm (in the text for some reason up to 1300 ppm) in various grain samples, while Cu concentrations range are from 15 to 20 ppm. It is obtained that the As content is in the range of concentrations between 0.2 to 10 ppm and Pb is about 1 ppm. In some pulp samples, extremely high (~180 ppm) concentrations of Pb were found, but these coffee trees were treated with insecticides for experimental purposes. Concentrations of As and Pb, elements which are of interest were close to their minimum detection limits in some samples, and therefore the results obtained for these elements should be taken with caution. For these cases, the authors pre‐tested two methods of concentration: chemical decomposition followed by precipitation and low temperature ashing. The first method gives particularly good results for all non‐volatile elements such as Fe, Ni, Cu, Zn, Cd, Hg, and Pb. On the other hand, low temperature ashing can be effectively used for both high Z metals and volatile elements, among which As is the most important in coffee analysis.

Table 3.2 Concentrations of elements in some coffee products (ppm).

Element	Sample
Grain 1	Grain 2	Grain 3	Pulp	Cascarilla
K	15 645	15 304	14 700	10 370	725
Ca	1482	1372	1514	5390	621
Ti	—	—	1.2	11	—
Mn	38	36	22	30	11
Fe	54	54	92	45	70
Ni	0.3	0.5	—	0.3	0.6
Cu	18	17	18	27	22
Zn	5.8	5.9	10	57	6
As	—	0.2	1.2	—	0.6
Se	—	0.2	0.4	—	0.1
Br	1.1	1.0	1.2	1.4	1.9
Rb	39	40	38	—	3.3
Pb	—	0.9	0.3	17.2	—

Haswell et al. [11] discusses the use of the EXTRA II TXRF spectrometer (Germany) for the classification of coffee and wine. In the first step of analysis preparation, the purified quartz reflectors were scanned to check for contamination. The quartz glass reflectors were processed with silicone solution (to prevent wetting on the disc and to aid droplet formation). For the analysis, samples of Cabernet Sauvignon wine from different countries and instant coffee of several brands were used. In preparing the specimens, a solution containing element V (internal standard) and deionized water was added to the wine and coffee samples. An aliquot of the sample with the added internal standard was then pipetted onto the prepared reflector. Measurements for each sample were performed for five hundred seconds, six times. Relative standard deviation for Ni turned out to be more (10 to 50%) than for other elements – K, Ca, Cr, Mn, Fe, Cu, Zn, Br, Rb, and Sr (up to 10%). Based on the results of the determination petal diagrams and dendrograms were built that showed a unique shape depending on chemical composition. The proposed technique allowed researchers to accurately determine the country of wine production and the brand of coffee.

Ninomiya [83] used TXRF to test the quality of instant coffee. A drop of 1 μl of coffee sample was pipetted onto the Si substrate, then dried and measured. Excitation conditions: 40 kV and 30 mA, Mo Kα radiation of X‐ray tube anode, sample measurement time – 1000 seconds. Spectra obtained during measurement of two samples of instant coffee showed the presence in the coffee spectrum of a doubtful quality of emission line P and Cl, while in the spectrum of ordinary coffee line P is absent, and a weak peak Cl is visible. The Si peak due to the use of the Si substrate was noted. According to the gas chromatography and inductively coupled plasma mass spectrometry method, the difference between the spectra for the two coffee samples is due to the presence in the coffee sample of a doubtful quality of dichlofos, containing P and Cl.

Akamine et al. [84] carried out XRF analyses for 102 samples of fried and green coffee beans from Brazil, Colombia, Guatemala, Indonesia, Tanzania, and Vietnam. The Epsilon 5 polarized ED spectrometer was used. In combination with a 100 kV high voltage generator, this spectrometer allows the determination of high Z element contents with better sensitivity. To build a calibration graph, CRM plants were used: tomato leaves (SRM 1573a), spinach leaves (SRM 1570a), apple leaves (SRM 1515), wheat flour (SRM 1567a), white rice flour (CRM 7502a), and rice flour‐unpolished (CRM No.10‐a). Preparation of the samples for analysis consisted of lyophilization, grinding, and pressing of the material. Authors investigated the effect of tablet thickness (0.5–4.5 g) on the fluorescence intensity of elements with high Z. It was found that the radiation intensities of the Kα‐lines of Rb and Sr increase with increasing tablet weight and become constant at 2.5 g weight (tablet thickness – 6 mm). When the fluorescence intensities were normalized to the intensity of non‐coherently scattered primary radiation, it is obtained that the intensities of the Kα‐lines Rb and Sr do not change practically at the weight of the sample of 1.5 g (thickness – 4 mm). The effect of the secondary target material on fluorescence intensity has been investigated. It was obtained that the optimum secondary target for Mn, Fe, Ni – Ge (sixty minutes), for Rb and Sr – Mo (three hundred seconds), for Ba – Al₂O₃ (sixty minutes). The measurement time of one sample was two and a half hours. A fundamental parameter method was used to determine the concentration of elements. Detection limits were obtained in ppm: 0.4 for Mn, 0.4 for Fe, 0.4 for Ni, 0.2 for Rb, 0.2 for Sr, and 0.8 for Ba. Table 3.3 shows the concentrations of elements in coffee zeroes by country of cultivation. The maximum levels of Mn (47 ppm), Sr (12 ppm), Ba (9 ppm) were recorded in coffee samples from Colombia, Fe (53 ppm), Ni (3 ppm) in Vietnamese coffee, Rb in coffee from Indonesia (82 ppm) and from Tanzania (67 ppm). It has been found that roasting does not affect the content of analytes in coffee. The use of analysis results for Fe, Mn, Ni, Rb, Sr, and Ba allowed for the successful identification coffee's country of origin.

Table 3.3 Concentrations (mean ± standard deviation) of some elements in coffee beans, depending on the country of origin (ppm) Adapted from [84].

Element	Concentration (mean ± standard deviation)
Mn	39.7 ± 4.9	47.1 ± 8.5	29.3 ± 2.3	34.6 ± 20.1	37.5 ± 4.8	39.7 ± 10.1
Fe	35.5 ± 3.8	30.7 ± 1.4	52.9 ± 13.2	32.8 ± 3.5	28.3 ± 2.7	29.7 ± 1.5
Ni	nd	0.5 ± 0.3 (n = 10)	3.5 ± 0.9	nd	nd	nd
Rb	22.2 ± 5.7	23.5 ± 7.4	42.7 ± 13.8	82.5 ± 13.4	67.2 ± 9.5	37.7 ± 7.7
Sr	3.4 ± 0.7	11.8 ± 8.3	3.3 ± 0.7	5.7 ± 1.1	6.4 ± 2.9	6.6 ± 1.8
Ba	3.1 ± 1.0	8.6 ± 3.8	2.3 ± 1.4 (n = 15)	4.3 ± 2.8	7.7 ± 4.1	6.3 ± 2.3
Number of samples (n)	17	17	17	10	7	7
Production country	Brazil	Colombia	Vietnam	Indonesia	Tanzania	Guatemala

Note – nd: not detected; n: number of samples

Debastiani et al. [85] used the PIXE version to investigate the elemental composition of roasted coffee grains and roasted ground coffee. The chemical composition of the packaged roasted ground coffee and the composition of the similarly roasted coffee grains ground immediately prior to the analysis were also compared. Measurements were performed at the Institute of Physics of the Federal University of Rio Grandi do Sul (Brazil, Ion Implantation Laboratory). A beam of protons with an energy of 2 MeV and Si (Li) a detector with energy resolution of 160 eV for radiation with an energy of 5.9 keV were used. C, O, and N are measured by the Rutherford Backscatter (RBS) method. The results showed that the coffee grain matrix consisted mainly of C, followed by O and N. PIXE results for roasted ground coffee and roasted coffee grains showed that K and Mg are the elements with the highest concentration, Zn and Sr are trace elements. Comparison of these two types of coffee showed that the contents of certain elements, particularly Mg, Cl, Ca, Fe, Zn, and Rb, are statistically significantly different. No clear picture of the origin of such differences has been revealed. Because of this, the correlation between these differences and the quality of the final drink has not been established. The authors note the possibility that factors such as processing and negligent harvesting leading to the inclusion of twigs and defective beans in frying processes and crushing may have influenced the results. The results obtained by the authors for Cl showed that packaged ground roasted coffee contained more of this element, compared with fresh ground coffee. Cl‐based compounds such as chloroanisoles are known to impart unpleasant odors to beverages such as wine and coffee, and these results are consistent with the fact that freshly ground coffee is generally thought to produce a better‐quality beverage. Green and roasted grains have the same chemical composition, indicating that none of the elements studied in this work are lost during the roasting process. All elements except Fe have the same concentrations in both kinds of grains. The distribution of elements over the surface of the grains is uniform, while the distribution within the grains shows certain patterns for different elements.

In the work of Hernández et al. [86], an ED X‐ray spectrometer (Rh anode, silicon drift detector) was used to determine the contents of several elements from 11 to 38: Na, P, S, Cl, K, Ca, Mn, Fe, Cu, Zn, Br, Rb, and Sr. The sample material was dried at room temperature and the tablets were pelletized. The authors analyzed 11 samples of commercial ground coffee and compared the obtained data with a sample of instant coffee and two other samples of ground coffee. Calibration and validation of content determination accuracy was assessed by analyzing CRM‐certified NIST 1547 (peach leaves), 1570a (spinach leaves), 1573a (tomato leaves), and 1571 (orchid leaves). Generally, the measured content of the elements was little different for all coffee samples and did not exceed toxic levels. Nevertheless, differences between the contents of the elements are shown and discussed. The authors recommend collecting a more complete collection of coffee samples for future work. In such a collection it is necessary to significantly expand the range of samples for instant coffee.

In a recently published article Debastiani et al. [87] continued research on Brazilian coffee by PIXE version. The authors investigated the change in the chemical composition of Brazilian coffee at various stages of the drip brewing process. For this purpose, more than 140 samples of 8 different Brazilian brands of ground coffee (original samples of ground coffee, material of spent coffee, and coffee beverage after paper filtration) were analyzed. Major conclusions from Debastiani et al. [87] are:

 Confirmed the remark of Debastiani et al. [85] about significant differences in concentrations of certain elements between coffee samples of different brands and between different batches of the same brand.

 K, Mg, P, Ca, and S proved to be elements with a higher concentration in Brazilian coffee.

 Analysis of spent coffee showed that the extraction coefficients are specific for each element (the highest extraction coefficients from ground coffee are obtained for Cl and K, then Rb and P).

 Paper filters do not transfer elements to coffee beverage.