ABSTRACT

This study aimed to evaluate the performance of the infrared spectrum in the range of 4000−650 cm⁻¹ for characterizing and differentiating dried and ground coffee cherry pulp of different varieties. The spectral data were subjected to first and second derivative treatments to perform the statistical analyses. Three varieties of coffee pulp were previously characterized for color, water activity, moisture, chlorogenic acids, and caffeine. The results of principal component analysis (PCA) showed that Fourier transform infrared (FTIR) spectroscopy is a viable technique for characterizing and differentiating dried and ground coffee cherry pulp among different varieties, showing the best differentiation with treatment of data from the first derivative, which was mainly associated with the caffeine content and chlorogenic acids. This study is the first investigation of FTIR spectroscopy with attenuated total reflectance for characterizing dried and ground coffee cherry pulp from coffee varieties grown in Colombia.

KEYWORDS
Coffee cherry by-products; coffee chemical compounds; infrared spectrum analysis

INTRODUCTION

Coffee is one of the most widely consumed food products globally and one of the most known and traded commodities (Velasquez et al., 2018Velasquez S, Peña N, Bohórquez JC, Gutiérrez N (2018) Determination of the complex permittivity of cherry, pulped, green, and roasted coffee using a planar dielectric platform and a coaxial probe between 0.3 and 6 GHz. International Journal of Food Properties 21(1):1332-1343. DOI: http://dx.doi.org/10.1080/10942912.2018.1490320
http://dx.doi.org/10.1080/10942912.2018.... ; Sezer et al., 2018Sezer B, Apaydin H, Bilge G, Boyaci IH (2018) Coffee arabica adulteration: Detection of wheat, corn and chickpea. Food Chemistry 264:142-148. DOI: https://doi.org/10.1016/j.foodchem.2018.05.037
https://doi.org/10.1016/j.foodchem.2018.... ). It is an important crop that provides livelihoods for millions of producers worldwide (Estevez et al., 2017Estevez CL, Bhat MG, Bray DB (2017) Commodity chains, institutions and domestic policies of organic and fair-trade coffee in Bolivia. Agroecology and Sustainable Food Systems 42:299-327. DOI: http://dx.doi.org/10.1080/21683565.2017.1359737
http://dx.doi.org/10.1080/21683565.2017.... ). Nearly 25 million farmers in 50 countries around the world depend on coffee for their subsistence (Ghosh & Venkatachalapathy, 2015Ghosh P, Venkatachalapathy N (2015) Changes in physico-chemical properties of coffee due to hot air assisted microwave drying. International Journal of Processing and Post Harvest Technology 6(1)69-79. DOI: http://dx.doi.org/10.15740/HAS/IJPPHT/6.1/69-79
http://dx.doi.org/10.15740/HAS/IJPPHT/6.... ). The coffee cherry is composed of an external red skin attached to the soft yellowish, fibrous, and sweet pulp. This part is commonly referred as “pulp”. This is followed by a translucent, colorless, thin, viscous, and highly hydrated layer of mucilage (Esquivel & Jimenez, 2012Esquivel P, Jiménez VM (2012) Functional properties of coffee and coffee by-products. Food Research International 46(2):488-495. DOI: http://dx.doi.org/10.1016/j.foodres.2011.05.028
http://dx.doi.org/10.1016/j.foodres.2011... ). During the typical processing, the pulp is removed mechanically, whereas the mucilage remains attached to the coffee beans during the fermentation process; the mucilage is then removed by washing. In the first processing stages of coffee production, wastes are generated (Selvam et al., 2014Selvam K, Govarthanan M, Kamala-Kannan S, Govindharaju M, Senthilkumar B, Selvankumar T, Sengottaiyan A (2014) Process optimization of cellulase production from alkali-treated coffee pulp and pineapple waste using Acinetobacter sp. TSK-MASC. RSC Advances 25:12625-13200. DOI: http://dx.doi.org/10.1039/c4ra00066h
http://dx.doi.org/10.1039/c4ra00066h... ) in the form of skin, pulp, and mucilage (Poltronieri & Rossi, 2016Poltronieri P, Rossi F (2016) Challenges in specialty coffee processing and quality assurance. Challenges 7(2):19. DOI: http://dx.doi.org/10.3390/challe7020019
http://dx.doi.org/10.3390/challe7020019... ); depending on the process used, different amounts of these components are produced. Coffee pulp is the main residue obtained during wet and semi-dry processing; it is essentially composed of sugars, proteins, and minerals. It also contains appreciable amounts of tannins, polyphenols, and caffeine, which are considered toxic in nature (Bonilla-Hermosa et al., 2014Bonilla-Hermosa VA, Duarte WF, Schwan R F (2014) Utilization of coffee by-products obtained from semi-washed process for production of value-added compounds. Bioresource Technology 166:142-150. DOI: http://dx.doi.org/10.1016/j.biortech.2014.05.031
http://dx.doi.org/10.1016/j.biortech.201... ). The large volume of coffee produced and processed by the industry results in the generation of a range of waste and by-products, resulting in the contamination of water bodies and lands around the production units; this represents a serious environmental problem for coffee-producing countries (Hikichi et al., 2017Hikichi SE, Andrade RP, Dias ES, Duarte WF (2017) Biotechnological applications of coffee processing by-products. Handbook of Coffee Processing By-Products 221-244. DOI: http://dx.doi.org/10.1016/B978-0-12-811290-8.00008-6
http://dx.doi.org/10.1016/B978-0-12-8112... ).

Colombia is the world's leading exporter of soft coffee. In the year 2017/18, Colombia produced an average of 14 million 60 kg bags (OIC, 2019OIC (2019) International Coffee Organization: Historical data on the global Coffee trade information. Available: http://www.ico.org/new_historical.asp?section=Statistics. Accessed Jan 13, 2019.
http://www.ico.org/new_historical.asp?se... ), for which 361,200 tons of fresh pulp were generated. Fresh coffee cherries contain over 430 g of coffee cherry pulp per kg and represent 30% of the dry matter of the coffee berry (Heeger et al., 2017Heeger A, Kosinska-Cagnazzo A, Cantergiani E, Andlauer W (2017) Bioactives of coffee cherry pulp and its utilisation for production of Cascara beverage. Food Chemistry 221:969-975. DOI: http://dx.doi.org/10.1016/j.foodchem.2016.11.067
http://dx.doi.org/10.1016/j.foodchem.201... ). Therefore, alternatives to transform the by-products remaining after harvesting coffee are necessary. These can provide added value to the waste product while offering new renewable materials (Collazo-Bigliardi et al., 201875Collazo-Bigliardi S, Ortega-Toro R, Chiralt BA (2018) Isolation and characterisation of microcrystalline cellulose and cellulose nanocrystals from coffee husk and comparative study with rice husk. Carbohydrate Polymers 191:205-215. DOI: https://doi.org/10.1016/j.carbpol.2018.03.022
https://doi.org/10.1016/j.carbpol.2018.0... ). Coffee pulp has been used as a food product, for example, in cascara beverages (Heeger et al., 2017Heeger A, Kosinska-Cagnazzo A, Cantergiani E, Andlauer W (2017) Bioactives of coffee cherry pulp and its utilisation for production of Cascara beverage. Food Chemistry 221:969-975. DOI: http://dx.doi.org/10.1016/j.foodchem.2016.11.067
http://dx.doi.org/10.1016/j.foodchem.201... ), a flour type obtained from dried and ground coffee cherry pulp (Ramirez & Jaramillo, 2013Ramirez VA, Jaramillo LJC (2013) Proceso para la obtención de miel y/o harina de café a partir de la pulpa o cascara y el mucilago del grano de café. Patent: WO2013088203A1, 20 Jun 2013.; Gonzalez-Rios et al., 2017Gonzalez-Rios O, Suarez QML, Monroy RJA, Mortera CM, Sánchez MM, Hernandez HMC, Basterra GA, Santillan DEA, Garcia OA, Angel SS, Bautista MM (2017) Proceso para elaboración de harina a partir de pulpa de café. Patent: MX2015018016, 20 Jun, 2017.), as a mix for cookies (Linxia, 2014Linxia S (2014) Premixed coffee cookie flour. Patent: CN103548932, 05 Feb 2014.; Shuyuan, 2016Shuyuan L (2016) Coffee cookie. Patent: CN105265544(A), 27 Jun 2016.) and in other applications such as production of bioethanol (Shenoy et al., 2011Shenoy D, Pai A, Vikas RK, Neeraja HS, Deeksha JS, Nayak C, Rao CV (2011) A study on bioethanol production from cashew apple pulp and coffee pulp waste. Biomass & Bioenergy 35(10):4107-4111. DOI: http://dx.doi.org/10.1016/j.biombioe.2011.05.016
http://dx.doi.org/10.1016/j.biombioe.201... ; Menezes et al., 2013Menezes EGT, Carmo JR, Menezes AGT, Alves JGLF, Pimenta CJ, Queiroz F (2013) Use of different extracts of Coffee pulp for the production of bioethanol. Applied Biochemistry Biotechnology 169:673-687. DOI: http://dx.doi.org/10.1007/s12010-012-0030-0
http://dx.doi.org/10.1007/s12010-012-003... ), and cellulase (Selvam et al., 2014Selvam K, Govarthanan M, Kamala-Kannan S, Govindharaju M, Senthilkumar B, Selvankumar T, Sengottaiyan A (2014) Process optimization of cellulase production from alkali-treated coffee pulp and pineapple waste using Acinetobacter sp. TSK-MASC. RSC Advances 25:12625-13200. DOI: http://dx.doi.org/10.1039/c4ra00066h
http://dx.doi.org/10.1039/c4ra00066h... ).

It is important to characterize the dried and ground coffee cherry pulp as a raw material for the preparation of different food products. Techniques using Fourier transform infrared (FTIR) spectroscopy have been successfully applied for characterizing a range of agricultural products (Amir et al., 2011Amir RM, Anjum FM, Khan MI, Khan MR, Pasha I, Nadeem M (2011) Application of Fourier transform infrared (FTIR) spectroscopy for the identification of wheat varieties. Journal Food Science Technology 50:1018-1023. DOI: http://dx.doi.org/10.1007/s13197-011-0424-y
http://dx.doi.org/10.1007/s13197-011-042... ). FTIR spectroscopy can also be used non-destructively and quickly to obtain biochemical fingerprints that provide information about the molecular structure and composition of the entire sample (Cebi et al., 2017Cebi N, Yilmaz MT, Sagdic O (2017) A rapid ATR-FTIR spectroscopic method for detection of sibutramine adulteration in tea and coffee based on hierarchical cluster and principal component analyses. Food Chemistry 299:517-526. DOI: https://doi.org/10.1016/j.foodchem.2017.02.072
https://doi.org/10.1016/j.foodchem.2017.... ). However, it has not been implemented as an analysis strategy for dried and ground coffee cherry pulp. The main aims of this study were to characterize the caffeine and chlorogenic acid profile of dried coffee cherry pulp using high-performance liquid chromatography (HPLC) and to evaluate the application of the infrared spectrum with attenuated total reflectance (ATR—FTIR) as an effective tool for obtaining chemical information from the coffee cherry pulp as a postharvest byproduct and its differentiation among different coffee varieties by principal component analysis (PCA).

MATERIAL AND METHODS

Coffee pulp samples

Nine coffee cherry pulp samples (Coffea Arabica L.) of the Castillo, Colombia, and Caturra varieties, from the Huila region of Colombia, were processed at the South Colombian Coffee Research Center (CESURCAFÉ) of the Universidad Surcolombiana. The ripe cherries were harvested selectively by submerging in water to remove the light or vain fruits (Koskei et al., 2015Koskei RK, Patrick M, Simon M (2015) Effects of coffee processing technologieson physico-chemical properties and sensory qualities of coffee. African Journal of Food Science 9(4):230-236. DOI: http://dx.doi.org/10.5897/AJFS2014.1221
http://dx.doi.org/10.5897/AJFS2014.1221... ). The fruits were subsequently depulped using the Gaviota Gv 300 (INGESEC) equipment; subsequently, the coffee pulp samples were dehydrated at 65°C for 24 h in an oven (UF55−Memmert). The dried samples were then ground in a Bunn electric mill (G3 HD BLK, Springfield, Illinois, USA) to obtain a fine particle size between 150 and 250 μm. A generic USB digital microscope (1000x optical zoom) was used to capture images of the dried and ground coffee cherry pulp (Gabriel-Guzmán et al., 2017Gabriel-Guzmán M, Rivera MV, Cocotle-Ronzón Y, García-Díaz S, Hernandez-Martinez E (2017) Fractality in coffee bean surface for roasting process. Chaos, Solitons and Fractals 99:79-84. DOI: http://dx.doi.org/10.1016/j.chaos.2017.03.056
http://dx.doi.org/10.1016/j.chaos.2017.0... ) as shown in Figure 1.

Moisture content and water activity

The moisture content was measured using an infrared moisture analyzer (OHAUS MB 45; Zanin et al., 2016Zanin RC, Corso MP, Kitzberger CSG, Scholz MBS, Benassi MT (2016) Good cup quality roasted coffees show wide variation in chlorogenic acids content. LWT - Food Science and Technology 74:480-483. DOI: http://dx.doi.org/10.1016/j.lwt.2016.08.012
http://dx.doi.org/10.1016/j.lwt.2016.08.... ), with the standard method at 103°C for 10 min. All tests were performed in triplicates. For measuring water activity (a_w), 2 to 3 g of dried and ground coffee cherry pulp were placed inside a vapor sorption analyzer (VSA Aqualab Decagon Devices, Inc. Pullman, WA). Before measurement, the a_w dewpoint sensor was verified using four saturated aqueous salt solutions 13.41 M LiCl (0.25 ± 0.003 a_w), 8.57 M LiCl (0.50 ± 0.003 a_w), 6.0 M NaCl (0.76 ± 0.003 a_w), and 2.33 M NaCl (0.92 ± 0.003 a_w), purchased from the instrument manufacturer (Schmidt & Lee, 2012Schmidt SJ, Lee WJ (2012) Comparison between water vapor sorption isotherms obtained using the new dynamic dewpoint isotherm method and those obtained using the standard saturated salt slurry method. International Journal of Food Properties 15:236-248. DOI: http://dx.doi.org/10.1080/10942911003778014
http://dx.doi.org/10.1080/10942911003778... ).

Color

The color parameters of the dried and ground coffee cherry pulp samples were determined using a Konica Minolta colorimeter (CR-410, N.J. USA). A standard white plate was used to calibrate the equipment (Y = 87.0, x = 0.3160, y = 0.3231). The results were expressed according to the CieLab color system (L*: lightness, a*: redness, and b*: greenness) (Homez-Jara et al., 2018Homez-Jara A, Daza LD, Aguirre DM, Muñoz JA, Solanilla F, Váquiro HA (2018) Characterization of chitosan edible films obtained with various polymer concentrations and drying temperatures. International Journal of Biological Macromolecules 113:1233-1240. DOI: https://doi.org/10.1016/j.ijbiomac.2018.03.057
https://doi.org/10.1016/j.ijbiomac.2018.... ).

Aqueous extraction

Sample extraction was conducted in hot water to simulate the conditions of food product preparation. Different particle sizes were separated using sieves of 1.4 mm and 0.71 mm; only the particles retained in the 0.71 mm sieve were used for extraction (Heeger et al., 2017Heeger A, Kosinska-Cagnazzo A, Cantergiani E, Andlauer W (2017) Bioactives of coffee cherry pulp and its utilisation for production of Cascara beverage. Food Chemistry 221:969-975. DOI: http://dx.doi.org/10.1016/j.foodchem.2016.11.067
http://dx.doi.org/10.1016/j.foodchem.201... ). For aqueous extraction, samples (1.0 g) were added to 20 mL of Milli-Q water for 15 min at 85°C in a water bath (TC-250, Brookfield) and stirred on a magnetic plate at 500 rpm for 10 min. Then, 1.5 mL of the water extracts were centrifuged at 9,000 rpm for 10 min in an Eppendorf Microcentrifuge Heraeus Pico 17 (Thermo Scientific). Extracts were prepared in triplicates. For HPLC analysis, the extracts were filtered using Minisart 0.2 μm nylon filters (Germany).

HPLC-diode array detector analysis of chlorogenic acids (CGAs) and caffeine

HPLC analysis was performed using 1.5 mL of the obtained aqueous extract. Determinations were performed using an Agilent 1260 Infinity II series liquid chromatograph (Agilent Technologies). Santa Clara. CA. USA) with a Poroshell 120-C180 (2.7 μm. 4 μm− 4.6 × 150 mm) column. The sample injection volume was 20 μL. Elution was carried out with a gradient of 100% methanol (eluent A) and water with 0.2% acetic acid (eluent B). Separation started with 80% of A for two min, followed by 2−10 min (A-56%, B-44%), and 10−14 min (A-80%). Detection was performed with a diode-array detector at 280 nm and 324 nm. Chlorogenic acids and caffeine were identified by comparing their retention times and the UV-spectra of the standards, which were prepared in Milli-Q water at concentrations of 100, 200, 300, 400, and 500 mg L^—1 for CGAs and 10, 20, 30, 40, and 50 mg L^—1 for caffeine.

ATR-FTIR measurements and spectral collection

The spectral measurements were made with an ART-FTIR CARY 630 spectrometer (Agilent, Santa Clara, CA, USA), between the wavelengths 4000−650 (cm⁻¹), with a resolution of 8 cm⁻¹ and with 20 scanners. The ATR-FTIR measurements were performed in a dry atmosphere at room temperature (20 ± 0.5 °C) (Bahamón et al., 2018Bahamón MAF, Parrado MLX, Gutierrez-Guzman N (2018) ATR-FTIR for discrimination of espresso and americano Coffee pods. Coffee Science 13(4):1499. DOI: http://dx.doi.org/10.25186/cs.v13i4.1499
http://dx.doi.org/10.25186/cs.v13i4.1499... ; Barrios et al., 2020Barrios RYF, Salas CKT, Giron HJ (2020) Comparison of sensory attributes and chemical markers of the infrared spectrum between defective and non-defective Colombian coffee samples. Coffee Science 15: e151659. DOI: https://doi.org/10.25186/cs.v15i.1659
https://doi.org/10.25186/cs.v15i.1659... ); approximately 1 g of the dried and ground coffee cherry pulp was placed in the sampling accessory and pressed; the background data was obtained from readings of the accessory without any sample. Once the spectra were obtained, they were exported to the Excel format for analysis. All samples were analyzed in triplicates.

Statistical analysis

The results of moisture content, water activity measurements, and HPLC determinations were assessed using analysis of variance (one-way ANOVA) with a confidence level of 95%. Mean comparison analyses were performed to identify statistically significant differences in the parameters evaluated between different categories. Statistical procedures were carried out using StatGraphics Centurion XVI. (Manugistics Inc., Rockville, MD, USA). Processing techniques were applied to the data obtained from the infrared spectra to compensate for any change in the experimental conditions and to improve the results. The pretreatments were first derivative and second derivative, obtained through the Resolutions Pro software (Agilent−USA, 2015). Principal components analysis (PCA) was carried out using these results as well as the raw data, to observe a better differentiation between the different types of dried and ground coffee cherry pulp. Matrices of size 45 × 900 were constructed such that each row corresponded to a sample and each column represented the spectral data at a given wave number. The analysis was carried out using R-statistical software (version 3.6.3, R statistics, St. Louis, MO, USA). Treatments of the first and second derivatives of the spectrum were used because they allowed an increase in e spectral resolution as well as greater differentiation in favor of the fine structures of the spectrum. The increase in resolution allowed resolution of bands that were too close and overlapped in the normal absorption spectrum, and minimized the baseline deviations caused by dispersion effects.

RESULTS AND DISCUSSION

Moisture content and water activity are factors that can affect the quality of flours; their increase can accelerate the caking of powdered foods. Thus, when storage moisture increases, the caking rate increases significantly (Carter et al., 2015aCarter PB, Galloway TM, Campbell SG, Carter HA (2015a) The critical water activity from dynamic dewpoint isotherms as an indicator of pre-mix powder stability. Food Measure 9:479-486. DOI: http://dx.doi.org/10.1007/s11694-015-9256-1
http://dx.doi.org/10.1007/s11694-015-925... ). Table 1 shows the statistically significant differences in water activity and moisture content between the dried and ground coffee cherry pulp of different varieties; further, these values were consistent for each variety. The Caturra variety exhibited higher values of water activity and moisture content, continued for Colombia and Castillo, respectively. Duangjai et al. (2016Duangjai A, Suphrom N, Wungrath J, Ontawong A, Nuengchamnong N, Yosboonruang A (2016) Comparison of antioxidant, antimicrobial activities and chemical profiles of three coffee (Coffea arabica L.) pulp aqueous extracts. Integrative Medicine Research 5(4):324-331. DOI: https://doi.org/10.1016/j.imr.2016.09.001
https://doi.org/10.1016/j.imr.2016.09.00... ) reported similar moisture content values for dried coffee pulp powder.

The results in Table 1 can be related to the adsorption isotherms reported in pre-mix powder by Carter et al. (2015aCarter PB, Galloway TM, Campbell SG, Carter HA (2015a) The critical water activity from dynamic dewpoint isotherms as an indicator of pre-mix powder stability. Food Measure 9:479-486. DOI: http://dx.doi.org/10.1007/s11694-015-9256-1
http://dx.doi.org/10.1007/s11694-015-925... ), in wheat flour by Carter et al. (2015bCarter PB, Morrls FC, Weaver LG, Carter HA (2015b) The case for water activity as a specification for wheat tempering and flour production. Cereal Foods World 60(4):166-170. DOI: http://dx.doi.org/10.1094/CFW-60-4-0166
http://dx.doi.org/10.1094/CFW-60-4-0166... ), and in other food products by Schmidt & Lee (2012Schmidt SJ, Lee WJ (2012) Comparison between water vapor sorption isotherms obtained using the new dynamic dewpoint isotherm method and those obtained using the standard saturated salt slurry method. International Journal of Food Properties 15:236-248. DOI: http://dx.doi.org/10.1080/10942911003778014
http://dx.doi.org/10.1080/10942911003778... ). In general, the samples were below the critical value of water activity for microorganism proliferation (a_w < 0.61) reported by Tapia et al. (2008Tapia SM, Alzamora MS, Chirife J (2008) Effects of water activity (aw) on microbial stability: As a hurdle in food preservation. In: Barbosa-Cánovas GV, Fontana Junior AJ, Schmidt SJ, Labuza TP (eds). Water activity in foods, fundaments and applications. Blackwell Publishing and the Institute of Food Technologists, p239-271. DOI: https://doi.org/10.1002/9780470376454.ch10
https://doi.org/10.1002/9780470376454.ch... ) as well as the maximum value of a_w for the main multipurpose flours category reported by Schmidt & Fontana (2008)Schmidt SJ, Fontana AJJ (2008) Water activity values of select food ingredients and products. Appendix E. In: Barbosa-Cánovas GV, Fontana Junior AJ, Schmidt SJ, Labuza TP (eds). Water activity in foods, fundaments and applications. Blackwell Publishing and the Institute of Food Technologists, p407-420. DOI: https://doi.org/10.1002/9780470376454.app5
https://doi.org/10.1002/9780470376454.ap... .

Color is a crucial factor with regard to consumer acceptance; the values in Table 2 show that the obtained L* values ranged from 29.7 to 30.3. The highest L* value was achieved for the Caturra variety and the lowest values were obtained for the Colombia variety. There were statistically significant differences that can be attributed to the different varieties studied.

The a* (green-red) values were between 7.45 and 7.727, and b* (blue-yellow) ranged from 10.562 to 10.704. There were no statistically significant differences in the a* and b* values between the varieties of dried and ground coffee cherry pulp, indicating that the samples did not show differences in redness and greenness.

Table 3 shows the contents of chlorogenic acids (CGAs) and caffeine compounds in the samples of dried and ground coffee cherry pulp.

The highest content of total chlorogenic acids amounting to 2.141 mg g⁻¹ was observed in the Castillo variety; the other varieties showed values between 0.506 to 0.722 mg g⁻¹. The CGAs content in the Castillo variety presented statistically significant differences compared to the Colombia and Caturra varieties. Heeger et al. (2017Heeger A, Kosinska-Cagnazzo A, Cantergiani E, Andlauer W (2017) Bioactives of coffee cherry pulp and its utilisation for production of Cascara beverage. Food Chemistry 221:969-975. DOI: http://dx.doi.org/10.1016/j.foodchem.2016.11.067
http://dx.doi.org/10.1016/j.foodchem.201... ) reported similar values of CGA content in dried coffee cherry pulp of the Caturra variety and mentioned high values of CGAs in the Bourbon-Congo variety (∼2.15 mg g⁻¹), which can be associated with our results for the Castillo variety. The total caffeine content between the varieties showed no statistically significant differences and showed consistent with those reported by Heeger et al. (2017Heeger A, Kosinska-Cagnazzo A, Cantergiani E, Andlauer W (2017) Bioactives of coffee cherry pulp and its utilisation for production of Cascara beverage. Food Chemistry 221:969-975. DOI: http://dx.doi.org/10.1016/j.foodchem.2016.11.067
http://dx.doi.org/10.1016/j.foodchem.201... ); however, their different mean values correspond with the absorbance peaks described in Figure 2, which presents the spectra of the dried and ground coffee cherry pulp of the Castillo, Caturra, and Colombia varieties, and their absorbance peaks for each wavelength.

The infrared spectrum obtained for each of the samples of dried and ground coffee cherry pulp shown in Figure 2 is similar to the spectrum of roasted coffee reported in literature (Wang et al., 2011Wang N, Fu Y, Lim L (2011) Feasibility study on chemometric discrimination of roasted. Journal of Agricultural and Food Chemistry 59(7):3220-3226. DOI: http://dx.doi.org/10.1021/jf104980d
http://dx.doi.org/10.1021/jf104980d... ; Rivera et al., 2013Rivera W, Velasco X, Rincón CA (2013) TGA and FTIR evaluation of composition changes produced by roasting of Coffee beans. Revista Colombiana de Física 45(3):205-208. Available: https://www.researchgate.net/publication/287273545_TGA_and_FTIR_evaluation_of_composition_changes_produced_by_roasting_of_coffee_beans. Accessed Jun 13, 2019.
https://www.researchgate.net/publication... ; Niya & Lim, 2012Niya W, Lim L-T (2012) Fourier transform infrared and physicochemical analyses of roasted Coffee. Journal of Agricultural and Food Chemistry 60(21):5446-5453. DOI: http://dx.doi.org/10.1021/jf300348e
http://dx.doi.org/10.1021/jf300348e... ). Peaks of interest associated with different chemical compounds such as caffeine, carbohydrates, water, and proteins have been identified, which have been reported in different studies on coffee (Ribeiro et al., 2010Ribeiro JS, Salva T J, Ferreira MMC (2010) Chemometric studies for quality control of processed brazilian coffees using drifts. Journal of Food Quality 33(2):212-227. DOI: http://dx.doi.org/10.1111/j.1745-4557.2010.00309.x
http://dx.doi.org/10.1111/j.1745-4557.20... ; Ribeiro et al., 2011Ribeiro JS, Ferreira MMC, Salva TJG (2011) Chemometric models for the quantitative descriptive sensory analysis of Arabica coffee beverages using near infrared spectroscopy. Talanta 83(5)1352-1358. DOI: http://dx.doi.org/10.1016/j.talanta.2010.11.001
http://dx.doi.org/10.1016/j.talanta.2010... ; Amir et al., 2011Amir RM, Anjum FM, Khan MI, Khan MR, Pasha I, Nadeem M (2011) Application of Fourier transform infrared (FTIR) spectroscopy for the identification of wheat varieties. Journal Food Science Technology 50:1018-1023. DOI: http://dx.doi.org/10.1007/s13197-011-0424-y
http://dx.doi.org/10.1007/s13197-011-042... ; Reis et al., 2013aReis N, Franca SA, Oliveira LS (2013a) Performance of diffuse reflectance infrared Fourier transform spectroscopy and chemometrics for detection of multiple adulterants in roasted and ground Coffee. LWT- Food Science and Technology 53(2)395-401. DOI: http://dx.doi.org/10.1016/j.lwt.2013.04.008
http://dx.doi.org/10.1016/j.lwt.2013.04.... ; Craig et al., 2014Craig AP, Franca AS, Oliveira LS, Irudayaraj J, Ileleji K (2014) Application of elastic net and infrared spectroscopy in the discrimination between defective and non-defective roasted coffees. Talanta 128:393-400. DOI: http://dx.doi.org/10.1016/j.talanta.2014.05.001
http://dx.doi.org/10.1016/j.talanta.2014... , Barrios et al., 2020Barrios RYF, Salas CKT, Giron HJ (2020) Comparison of sensory attributes and chemical markers of the infrared spectrum between defective and non-defective Colombian coffee samples. Coffee Science 15: e151659. DOI: https://doi.org/10.25186/cs.v15i.1659
https://doi.org/10.25186/cs.v15i.1659... ). The spectra of the three varieties showed similarity in the number of peaks present but showed differences in absorbance values, wherein the majority of the peaks showed lower absorbance values for the spectrum corresponding to the Castillo variety compared to the spectrum of the other two varieties. Amir et al. (2011Amir RM, Anjum FM, Khan MI, Khan MR, Pasha I, Nadeem M (2011) Application of Fourier transform infrared (FTIR) spectroscopy for the identification of wheat varieties. Journal Food Science Technology 50:1018-1023. DOI: http://dx.doi.org/10.1007/s13197-011-0424-y
http://dx.doi.org/10.1007/s13197-011-042... ) reported peaks for water in the range of 3300−1640 cm⁻¹. In this study, water peaks were observed at a wavelength of 3280 cm⁻¹, and corresponding to Table 1 indicating the values of water activity and moisture content for the three varieties and their statistically significant differences, high values of absorbance were observed for the Caturra, followed by the Colombia and Castillo varieties, respectively.

Reis et al. (2013a)Reis N, Franca SA, Oliveira LS (2013a) Performance of diffuse reflectance infrared Fourier transform spectroscopy and chemometrics for detection of multiple adulterants in roasted and ground Coffee. LWT- Food Science and Technology 53(2)395-401. DOI: http://dx.doi.org/10.1016/j.lwt.2013.04.008
http://dx.doi.org/10.1016/j.lwt.2013.04.... reported that the content of caffeine in coffee husk is similar to that in coffee beans, and that the peaks expressed at the wavelength 2922−2855 cm⁻¹ are likely to be primarily associated with caffeine. Further, caffeine is reported to be detected in the range of 1650−1600 cm⁻¹ in the infrared spectrum (Craig et al., 2014Craig AP, Franca AS, Oliveira LS, Irudayaraj J, Ileleji K (2014) Application of elastic net and infrared spectroscopy in the discrimination between defective and non-defective roasted coffees. Talanta 128:393-400. DOI: http://dx.doi.org/10.1016/j.talanta.2014.05.001
http://dx.doi.org/10.1016/j.talanta.2014... ), as shown in Figure 2 (1602 cm⁻¹). The region around 2100 cm⁻¹ is associated with carbohydrates, chlorogenic acids (CGAs), and proteins (Craig et al., 2014Craig AP, Franca AS, Oliveira LS, Irudayaraj J, Ileleji K (2014) Application of elastic net and infrared spectroscopy in the discrimination between defective and non-defective roasted coffees. Talanta 128:393-400. DOI: http://dx.doi.org/10.1016/j.talanta.2014.05.001
http://dx.doi.org/10.1016/j.talanta.2014... ). Further, CGAs are a family of esters formed between certain transcinamic acids and chemical acids, and are associated with the presence of absorption peaks in the region of 1450−1000 cm⁻¹ (Lyman et al., 2003Lyman D, Benck R, Dell S, Merle S, Murray-Wijelath J (2003) FTIR-ATR Analysis of brewed Coffee: Effect of roasting conditions. Journal of Agricultural and Food Chemistry 51(11):3268-3272. DOI: http://dx.doi.org/10.1021/jf0209793.
http://dx.doi.org/10.1021/jf0209793... ). In the present study, this zone presented some absorption peaks (Figure 2), and those with the highest absorbance were associated with the dried and ground coffee cherry pulp of the Colombia and Caturra varieties. According to Ribeiro et al. (2010Ribeiro JS, Salva T J, Ferreira MMC (2010) Chemometric studies for quality control of processed brazilian coffees using drifts. Journal of Food Quality 33(2):212-227. DOI: http://dx.doi.org/10.1111/j.1745-4557.2010.00309.x
http://dx.doi.org/10.1111/j.1745-4557.20... ), wavenumbers ranging from 1700−1600 cm⁻¹ are highly related to chlorogenic acid and caffeine concentration in coffees. In this case, the wavelength of 1603 cm⁻¹ is typical for chlorogenic acid (Capek et al. 2014Capek P, Paulovičová E, Matulová M, Mislovičová D, Navarini L, Suggi-Liverani F (2014) Coffea arabica instant coffee-chemical view and immunomodulating properties. Carbohydrate Polymers 103:418-426. DOI: http://dx.doi.org/10.1016/j.carbpol.2013.12.068
http://dx.doi.org/10.1016/j.carbpol.2013... ). As described above, the results of the present study show a higher absorbance value in the peaks for caffeine and CGAs for the samples corresponding to the coffee pulp of the Caturra and Colombia varieties, which agrees with the results shown in Table 3, showing a lower content of these compounds in the Castillo variety; this indicates that it is convenient to implement this technique to obtain rapid results for these types of compounds in dried and ground coffee cherry pulp.

Peaks of absorbance were evident at 1237, 1014, and 776 cm⁻¹ and may be associated with saccharose in the range of 1242–1218 cm⁻¹ (Ribeiro et al., 2011Ribeiro JS, Ferreira MMC, Salva TJG (2011) Chemometric models for the quantitative descriptive sensory analysis of Arabica coffee beverages using near infrared spectroscopy. Talanta 83(5)1352-1358. DOI: http://dx.doi.org/10.1016/j.talanta.2010.11.001
http://dx.doi.org/10.1016/j.talanta.2010... ), arabinogalactans at 1065−1020 cm⁻¹ (Craig et al., 2018Craig AP, Botelho BG, Oliveira LS, Franca AS (2018) Mid infrared spectroscopy and chemometrics as tools for the classification of roasted coffees by cup quality. Food Chemistry 245:1052-1061. DOI: https://doi.org/10.1016/j.foodchem.2017.11.066
https://doi.org/10.1016/j.foodchem.2017.... ), and carbohydrates at 1500−700 cm⁻¹ wavelength (Reis et al., 2013bReis, N, Franca AS, Oliveira LS (2013b) Discrimination between roasted Coffee, roasted corn and Coffee husks by diffuse reflectance infrared fourier transform spectroscopy. LWT - Food Science and Technology 50(2)715-722. DOI: http://dx.doi.org/10.1016/j.lwt.2012.07.016
http://dx.doi.org/10.1016/j.lwt.2012.07.... ). Thus, the dried and ground coffee cherry pulp could be considered as a product rich in chemical compounds. Further, the FTIR technique allows rapid detection of the differences between these compounds across different varieties of coffee.

Determination of spectral variations across the different varieties of dried and ground coffee cherry pulp was determined by means of PCA, which reduced the dimensionality of the IR spectra and facilitated visualization of the data set. Figure 3 shows the PCA biplot from each of the pre-treatments of data used (first derivative and second derivative) as well as the results of the raw data. The best differentiation was found with the processing of data from the first derivative (Figure 3B), which shows a grouping of the samples of dried and ground coffee cherry pulp of the Castillo variety on the left side of CP1, whereas the dried and ground coffee cherry pulp samples of the Caturra variety were grouped on the right side of CP1. Further, samples of dried and ground coffee cherry pulp of the Colombia variety were grouped in the lower part of CP2. The raw data (Fig. 3A) and those treated with the second derivative (Fig. 3C) did not show a clear differentiation between the different varieties of coffee pulp. In both cases, the dried and ground coffee cherry pulp samples of the Caturra and Colombia varieties were overlapped and were separated from the samples of the Castillo variety.

The results show that the infrared spectrum can be used to differentiate between the varieties of dried and ground coffee cherry pulp, by treatment of the first derivative data. They also confirm, in a concrete manner, small differences in the absorbances observed in the spectra described in Figure 2 among the three varieties of dried and ground coffee cherry pulp, which may be related to the data described in Table 2, regarding caffeine and chlorogenic acids. These results show that this technique is a simple but versatile tool for characterizing and differentiating the dried and ground coffee cherry pulp from different varieties, as demonstrated in other studies that have used PCA of infrared spectra as a technique to determine differences between food matrices (Reis et al., 2013bReis, N, Franca AS, Oliveira LS (2013b) Discrimination between roasted Coffee, roasted corn and Coffee husks by diffuse reflectance infrared fourier transform spectroscopy. LWT - Food Science and Technology 50(2)715-722. DOI: http://dx.doi.org/10.1016/j.lwt.2012.07.016
http://dx.doi.org/10.1016/j.lwt.2012.07.... , Craig et al., 2012Craig AP, Franca AS, Oliveira LS (2012) Evaluation of the potential of FTIR and chemometrics for separation between defective and non-defective coffees. Food Chemistry 132(3):1368-1374. DOI: https://doi.org/10.1016/j.foodchem.2011.11.121
https://doi.org/10.1016/j.foodchem.2011.... ). Notably, this technique provides coherent information on the differences between the chemical characteristics of different varieties of coffee pulp, which have been confirmed by chemical analyses. Thus, in future, determination of the infra-red spectral information can be considered a factor of relevance for application in the food industry, as it indicates that each coffee variety should be given specific treatment based on the differences found between them.

CONCLUSIONS

Coffee pulp is a waste generated during the processing of coffee, and has high potential as a raw material with multiple uses in the agri-food industry; therefore, it is necessary to establish strategies for its characterization and application. The FTIR spectra showed fingerprints related to the chemical compounds present in the dried and ground coffee cherry pulp as reported in other food products, indicating that the dried and ground coffee cherry pulp is a matrix rich in biomolecules, with great potential in the food industry. These results allowed us to conclude that the FTIR technique facilitates quick identification of the chemical composition of dried and ground coffee cherry pulp from different varieties, and shows small differences in their absorbance peaks, mainly associated with caffeine and chlorogenic acids. We found that the variety Castillo presented lower contents of the chemical compounds according to the analyzed spectrum. The PCA results of the data obtained from the infrared spectrum showed that it is possible to discriminate the dried and ground coffee cherry pulp according to its variety, by means of the first derivative treatment. Finally, these results indicate that the ATR-FTIR technique provides satisfactory results for characterization and differentiation of coffee varieties, and can be an important tool in the food industry.

ACKNOWLEDGEMENTS

This work was supported by the South Colombian Coffee Research Center CESURCAFÉ – Universidad Surcolombiana.

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