Food Chemistry 176 (2015) 175–183

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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem

Analytical Methods

A study on the selection of chemiluminescence system for the flow injection determination of the total polyphenol index of plant-derived foods Edyta Nalewajko-Sieliwoniuk, Julita Malejko ⇑, Marta S´wie˛czkowska, Agata Kowalewska Department of Analytical Chemistry, Institute of Chemistry, University of Białystok, Hurtowa 1, 15-399 Białystok, Poland

a r t i c l e

i n f o

Article history: Received 7 August 2014 Received in revised form 4 December 2014 Accepted 13 December 2014 Available online 23 December 2014 Keywords: Polyphenolic antioxidants Chemiluminescence Manganese(IV) Potassium permanganate Luminol Cerium(IV) Flow injection analysis Food

a b s t r a c t Different chemiluminescence systems based on luminol, permanganate, manganese(IV) and cerium(IV) reagents were compared regarding their sensitivity and selectivity to determine plant polyphenols. Among the seventeen systems tested, Mn(IV)-formaldehyde-hexametaphosphate was considered to be the most suitable for polyphenols detection. The developed flow injection method (FI-CL) based on enhancing effect of polyphenols on Mn(IV) chemiluminescence is characterised by low detection limit of gallic acid (0.02 lg L1) and high precision (RSD = 1.7%). The calibration graph was linear from 0.1 to 100 lg L1. The selectivity studies revealed that the FI-CL method ensures accurate determination of the total polyphenols content in food samples. The method was successfully applied to analysis of a variety of plant-derived foods (wine, tea, cereal coffee, fruit and vegetable juices, herbs and spices). The proposed method is superior to conventional spectrophotometric assays due to its higher sample throughput (195 samples h1), simplicity, sensitivity and, above all, higher selectivity. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction It is widely accepted that regular consumption of plant-derived foods, such as vegetables, fruits, tea and juices is beneficial to maintain human health. Some of these benefits may be due to the presence of polyphenolic compounds such as phenolic acids (hydroxybenzoic and hydroxycinnamic acids) and flavonoids (especially flavanols, flavonols and anthocyanins) which are commonly found in plants. A growing number of studies demonstrate correlations between the intake of polyphenol-rich foods and reduced incidence of cardiovascular disease, cancer and slower neuro-degeneration (Stevenson & Hurst, 2007). For quality control of plant-derived food products, it is important to assess the total polyphenol content and antioxidant activity. The methods commonly used for this purpose are spectrophotometric (e.g., total phenol assay by Folin–Ciocalteu method, ABTS radical cation decolorisation assay and DPPH radical scavenging assay) (Huang, Ou, & Prior, 2005; Karadag, Ozcelik, & Saner, 2009; Niki, 2010; Prior, Wu, & Schaich, 2005). However, none of them has been recognised as a standard method. Low sensitivity and selectivity, expensive reagents and long reaction time are the ⇑ Corresponding author. Tel.: +48 85 7457831; fax: +48 85 7470113. E-mail address: [email protected] (J. Malejko). http://dx.doi.org/10.1016/j.foodchem.2014.12.053 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.

main disadvantages of these methods. For example, Folin– Ciocalteu reagent can be reduced not only by phenols but also by many non-phenolic compounds, mainly sugars, aromatic amines, sulphur dioxide, ascorbic acid and Fe(II) (Prior et al., 2005). The total polyphenol content values obtained by Folin–Ciocalteu method are often higher than those calculated by the summation of the contents of all individual polyphenols determined by chromatography (Pérez-Jiménez, Neveu, Vos, & Scalbert, 2010). The antioxidant activity measured by DPPH and ABTS assays is underestimated due to interferences caused by the presence of coloured compounds in the sample (Arnao, 2000). In the majority of published papers, the spectrophotometric assays were carried out without taking possible interference into account. Therefore, there is a need to develop a simple and reliable method for the determination of the total polyphenolic/antioxidant level in a variety of foodstuffs of plant origin. Chemiluminescence (CL) is an efficient detection system for the determination of polyphenolic compounds offering a few orders of magnitude lower limits of detection in comparison with spectrophotometric methods. Possibility of high dilution of sample considerably reduces the interference from other constituents of food products. Simplicity and low cost of apparatus (no external light source is required) are another advantages of CL methods. Literature survey shows that the majority of chemiluminescence

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methods used for the antioxidant detection are manual. However, in order to achieve high precision and sensitivity of chemiluminescence determinations, measurements should be carried out in flow injection based systems. Automation of the analytical procedure significantly shortens the analysis time, ensures reproducible mixing of reagents and fast transport of the sample zone to the detector which is especially crucial for rapid chemiluminescent reactions. The review of the literature shows that antioxidants were determined in FI-CL systems mainly through the attenuation of luminol chemiluminescence generated in the following systems: luminolH2O2-horseradish peroxidase (Minioti & Georgiou, 2008), luminolONOO (Wada et al., 2011), luminol-HOCl and luminol-H2O2 (Erdemoglu, Turan, Cakici, Sener, & Aydin, 2006), luminol-H2O2Co(II)/EDTA (Fan et al., 2010; Giokas, Vlessidis, & Evmiridis, 2007), luminol-perborate-Co(II)/EDTA (Pulgarín, Bermejo, & Durán, 2012), luminol-H2O2-Fe(II) (Murillo Pulgarin, Garcia Bermejo, & Carrasquero Duran, 2010), luminol-I2 (Nalewajko-Sieliwoniuk, Nazaruk, Antypiuk, & Kojło, 2008). The mechanism of attenuation of the electromagnetic radiation emitted during luminol oxidation is very complex. The antioxidants can scavenge reactive oxygen species formed in the reaction as well as consume the oxidant (Giokas et al., 2007; Nalewajko-Sieliwoniuk et al., 2008). There are also examples of methods based on enhancing effect of polyphenols on KMnO4 (Costin, Barnett, Lewis, & McGillivery, 2003), nanocolloidal Mn(IV) (Malejko, Nalewajko-Sieliwoniuk, Nazaruk, Siniło, & Kojło, 2014; Nalewajko-Sieliwoniuk, Tarasewicz, & Kojło, 2010) and Ce(IV) (Nalewajko-Sieliwoniuk, Nazaruk, Kotowska, & Kojło, 2012) chemiluminescence. These oxidants react with analytes in acidic solutions to produce excited species (Adcock, Barnett, Barrow, & Francis, 2014; Adcock, Smith, et al., 2014; Nalewajko-Sieliwoniuk et al., 2012). In the case of potassium permanganate and manganese(IV) chemiluminescence, different enhancers have been employed. Among them formaldehyde is the most frequently used, but many other enhancers such as polyphosphates, formic acid, sodium sulphite, ethanol have been tested (Adcock et al., 2014; Adcock et al., 2014; Brown, Francis, Adcock, Lim, & Barnett, 2008). The red emission observed in manganesebased systems has been ascribed to excited Mn(II) species. In the case of Ce(IV)-based systems, the excited-state of Ce(III) is formed as a product of the reaction of Ce(IV) with polyphenols and a sensitizer such as rhodamine 6G (Cui, Zhang, Myint, Ge, & Liu, 2006). Then, Ce(III)⁄ transfers the excess of energy to rhodamine 6G, which emits radiation. The FI-CL methods described above demonstrate big diversity – they are based on different measurement conditions, standard phenolic compounds (gallic acid, apigenin, linarin, caffeic acid, 60 -caffeoylerigeroside, 6-hydroxyluteolin 7-O-glucoside) and expression of the results (e.g., IC50 – antioxidant concentration required to reduce in 50% the initial CL emission, equivalents of standard antioxidant). That makes the results impossible to compare. Moreover, in the majority of papers authors do not specify all analytical parameters and selectivity of the methods. Therefore, it is important to develop one FI-CL method for the determination of the total polyphenol index based on the most suitable chemiluminophore, carry out its full analytical characteristic and selectivity studies, and apply it for the analysis of diverse plant-derived foods (e.g., juices, wines, teas, coffees, herbs and spices). The application of one method for a variety of food samples will allow to compare the obtained results and to identify food products with the highest content of polyphenolic antioxidants. The objective of this study was to select chemiluminescence reagent the most suitable for the determination of polyphenols in various food samples. For this purpose, different CL systems, based on luminol, KMnO4, nanocolloidal Mn(IV) and Ce(IV) reagents, were compared regarding their sensitivity to determine polyphenolic compounds. To the best of our knowledge, this is

the first report on the comparison of various CL systems for their applicability in plant polyphenols detection. In the second part of this research, the most sensitive systems were selected and FI-CL methods for the determination of the total polyphenolic content were developed. Finally, considering analytical parameters and selectivity, the method based on enhancing effect of polyphenols on Mn(IV)-formaldehyde-hexametaphosphate chemiluminescence was chosen and applied to the analysis of food samples. The results were correlated with those obtained by spectrophotometric methods (Folin–Ciocalteu, ABTS and DPPH).

2. Materials and methods 2.1. Reagents and solutions Gallic acid, caffeic acid, (±)-catechin, rutin and quercetin were supplied by Sigma–Aldrich (Steinheim, Germany). Stock solution of gallic acid (1000 lg mL1) was prepared in water, whereas stock solutions of remaining polyphenols (1000 lg mL1) were prepared in methanol (from Sigma–Aldrich, Steinheim, Germany). These stock solutions were kept in the dark at 4 °C. Working solutions were prepared before analysis by dilution of the stock solutions with an appropriate carrier solution. Formaldehyde, potassium permanganate, sodium hydroxide, phosphoric acid, sulphuric acid, formic acid, citric acid, acetic acid, ethanol, iodine, potassium iodide, potassium hexacyanoferrate(III), potassium dihydrogen phosphate, potassium chloride, calcium chloride, sodium chloride, magnesium chloride, manganese(II) chloride, oxalic acid, sodium tetraborate, sodium metabisulphite, sodium sulphite and sodium carbonate were supplied by POCH (Gliwice, Poland). Caffeine, theobromine, theophylline, xanthin, proline, ascorbic acid, DL-malic acid, sucrose, glucose, fructose, tartaric acid, zinc chloride, iron(III) chloride, Trolox (6-hydroxy2,5,7,8-tetramethylchromane-2-carboxylic acid), sodium triphosphate pentabasic, sodium hexametaphosphate, sodium formate, DPPH (1,1-diphenyl-2-picrylhydrazyl) radical, ABTS (diammonium 2,20 -azino-bis(3-ethylbenzothiazoline-6-sulphonate)), potassium persulphate, cerium(IV) sulphate, tween 20, quinine, ammonium iron(II) sulphate, cobalt(II) chloride and horseradish peroxidase (HRP) were obtained from Sigma–Aldrich (Steinheim, Germany). Folin–Ciocalteu reagent, rhodamine 6G, rhodamine B, luminol, hydrogen peroxide (TraceSelect), copper and manganese standards for AAS (1000 mg/L in nitric acid) were supplied by Fluka (Steinheim, Germany). A transparent brown manganese(IV) solution was prepared according to the Jáky and Zrinyi (1993) method, which has been slightly modified. Manganese dioxide precipitate was formed by the reaction of potassium permanganate with sodium formate in the solution of pH 6.8. Then, it was collected on a glass Büchner funnel (with sintered disc porosity grade G-4) and rinsed three times with water. 0.4 g of wet manganese dioxide was added to 1 L of 6 mol L1 phosphoric acid. The mixture was ultrasonicated at 25 °C for 24 h and stored in the dark at room temperature for the next four days (Nalewajko-Sieliwoniuk et al., 2010). The concentration of nanocolloidal Mn(IV) solution determined by iodometric titration was 1.7  103 mol L1. Luminol stock solution (5  10–2 mol L1) was prepared by dissolving an appropriate amount of the compound in 0.2 mol L1 NaOH solution and stored in a refrigerator for 24 h prior to use. Working solutions of luminol were prepared before use by dilution of the stock solution with water. In the case of luminol-H2O2-HRP system, working solutions of luminol were prepared by dilution with buffer (1  102 mol L1 KH2PO4, pH 7.4). Stock solution of 80 IU mL1 HRP was prepared in the same buffer. Working solutions of HRP in buffer were prepared daily. Hydrogen peroxide

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working solutions were prepared daily by dilution of the 30% stock solution with water. A 5  103 mol L1 I2 stock solution was prepared in 50 mL of water containing 0.2 g of KI. Stock solutions of cerium(IV) sulphate (0.1 mol L1) and quinine (0.05 mol L1) were prepared in 0.5 mol L1 H2SO4 and 0.1 mol L1 H2SO4, respectively. ABTS was dissolved in water to a 7  103 mol L1 concentration. Trolox stock solution (2.5  103 mol L1) was prepared in methanol. Working standards of Trolox were prepared daily on dilution with methanol. A DPPH solution of concentration 6  105 mol L1 was prepared daily in methanol. The water used to prepare all solutions was purified in a Milli-Q Plus water purification system (Millipore S.A., Molsheim, France). 2.2. Sample pre-treatment of tea, cereal coffee, herbs and spices Eleven samples of different kinds of tea: Camellia sinensis (two black, two green, two white and pu-erh), two fruit teas (the first tea was a mixture of cranberry, chokeberry, hibiskus, rosehip and apple, the second tea was a mixture of black currant, cherry, chokeberry, gooseberry and apple), Aspalathus linearis (rooibos), Ilex paraguariensis (yerba mate), as well as four samples of dried herbs and spices (Curcuma longa, Salvia officinalis L., Thymus vulgaris L., Cistus incanus) and cereal coffee (roasted rye, chicory) were obtained from a commercial source. The tea and cereal coffee samples were prepared daily according to the procedure which simulated normal brewing conditions for a cup of tea or coffee. 1 g of tea/cereal coffee was extracted with 100 mL of hot water at a temperature of 80 °C for 10 min. The herb and spice extracts were prepared similarly: 1 g of dried herb/spice was extracted with 100 mL of boiling water for 10 min. After extraction, infusions were cooled to room temperature (25 °C) in water bath and filtered through a filter paper (if necessary). In order to determine the total content of polyphenolic compounds in extracts of tea, cereal coffee, herbs and spices by FICL method they were diluted with 1 mol L1 formaldehyde in order to fit the concentration of polyphenols to the linear calibration range of gallic acid (extracts of white tea were diluted 14,000 times, extracts of green tea, rooibos, yerba mate, sage, thyme and cistus were diluted 10,000 times, extracts of pu-erh tea were diluted 6500 times, extracts of black tea were diluted 4000 times, extracts of fruit tea, cereal coffee and curcuma were diluted 2500 times). The total content of phenols in infusions of tea, cereal coffee, herbs and spices was also determined spectrophotometrically with Folin–Ciocalteu reagent (F–C) (Djeridane et al., 2006). The samples were diluted with water to fit the concentration of polyphenolic compounds to the linear calibration range of gallic acid (extracts of green, white and black tea, sage, thyme and cistus were diluted 8 times; extracts of rooibos, yerba mate, pu-erh and fruit tea, cereal coffee and curcuma were diluted 4 times). In order to estimate the total antioxidant activity of investigated infusions DPPH free radical scavenging method (Miliauskas, Venskutonis, & van Beek, 2004) and ABTS decolorisation assay were used (Re et al., 1999). In DPPH method all samples before the analysis were

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diluted 10 times with water. In ABTS assay some of extracts required dilution with water to fit the concentration of analytes to the linear calibration range of Trolox (extracts of white tea were diluted 8 times; extracts of green, black and pu-erh tea, thyme and cistus were diluted 4 times; extracts of rooibos, yerba mate and sage were diluted 2 times). 2.3. Sample pre-treatment of wine, fruit and vegetable juices Five samples of fruit and vegetable juices (which included apple, red grapefruit, blackcurrant, tomato and carrot juice) and three grape wines of various types (red, white, rosé) were analysed. All investigated plant-derived beverages were randomly purchased from a local supermarket and kept in dark place in +4 °C. Tomato and carrot juice before analysis were filtered through a filter paper. In order to determine the total content of polyphenolic compounds by FI-CL method all samples were diluted with 1 mol L1 formaldehyde just before analysis (red wine was diluted 14,000 times, blackcurrant juice was diluted 8000 times, rosé wine was diluted 5000 times, apple juice was diluted 3900 times, tomato juice was diluted 3000 times, white wine was diluted 2000 times, carrot and red grapefruit juices were diluted 1300 times). The total content of phenols in investigated beverages was also determined spectrophotometrically by Folin–Ciocalteu method (Djeridane et al., 2006). The samples before analysis were diluted with water to fit the concentration of polyphenolic compounds to the linear calibration range of gallic acid (red wine was diluted 10 times; rosé wine, apple, blackcurrant and tomato juices were diluted 8 times; white wine, red grapefruit and carrot juices were diluted 4 times). In order to estimate the total antioxidant activity of samples DPPH free radical scavenging method (Miliauskas et al., 2004) and ABTS decolorisation assay were used (Re et al., 1999). In DPPH method all samples before analysis were diluted 10 times with water. In ABTS assay only some samples required dilution with water to fit the concentration of analytes to the linear calibration range of Trolox (red wine was diluted 5 times; rosé wine was diluted 4 times; apple and blackcurrant juices were diluted 2 times). 2.4. Chemiluminescence instrumentation and procedure The configuration of the FI-CL system used in this work is shown in Fig. 1. The solutions of carrier and reagents were continuously propelled by the peristaltic pump Minipuls 3 (Gilson, France). All flow lines were PTFE tubing (0.8 mm i.d.). The streams were merged in Perspex T-pieces. The standard and sample solutions were injected into a carrier stream using a rotary injection valve (Model 5041, Rheodyne, USA). The CL detection system consisted of a flow cell (a flat spiral of PTFE tubing: 0.8 mm i.d., 150 cm length) positioned directly in front of the window of photomultiplier tube. The PMT was located in a light-tight box and was operated at 1100 V. The data acquisition was performed by software provided by the manufacturer of the CL detector (KSP, Poland).

Fig. 1. Schematic diagram of the FI-CL system for the determination of the total polyphenolic content. PP: peristaltic pump; V: injection valve; M1, M2, M3: mixing points; RC1, RC2: reaction coils; FC: flow cell; PMT: photomultiplier tube; PC: computer; R1, R2, R3: reagents (details are in Section 2.4).

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The number of channels of FI-CL manifold was dependent on the chemiluminescence system used. In the case of luminol-based systems, the analyte solutions were injected into a carrier stream (Milli-Q water) and merged with the combined streams of luminol (R1) and oxidant (R2: I2 or K3Fe(CN)6 or H2O2). In the case of using catalyst, the fourth channel R3 (HRP or metal ions: Co(II) or Mn(II) or Fe(II) or Cu(II)) was installed in the manifold. After the injection of polyphenol standard or sample solution into the FI-CL manifold, a negative peak was measured with the CL detector. The FI-CL manifold with manganese-based detection had two channels: carrier (formaldehyde or formic acid or Na2SO3) and reagent R1 (potassium permanganate or manganese(IV) prepared in phosphoric acid). In some CL systems an additional enhancer was applied: sodium hexametaphosphate or sodium triphosphate pentabasic. These enhancers were dissolved in KMnO4 or Mn(IV) solution. The studies with using cerium(IV) chemiluminescence systems were conducted in a three-channel FI-CL manifold: carrier – H2O, R1 – Ce(SO4)2 in H2SO4, R2 – sensitizer (rhodamine 6G or rhodamine B or tween 20 or quinine). After the injection of polyphenol standard or sample solution into the FI-CL manifolds with manganese or cerium chemiluminescence detection, the increase in the CL intensity relative to the baseline was observed (a positive peak). The quantitative detection of polyphenols was based on the net CL intensity calculated according to the formula: DI = I0  IS (in the case of luminol-based systems) or DI = IS  I0 (in the case of manganese and cerium-based systems), where I0 is a background emission from the CL reaction and IS is an emission in the presence of polyphenolic compound. 2.5. Statistical analysis Data was analysed with Microsoft Office Excel software (version 2007, Microsoft Corp, USA). Each sample was analysed in triplicate and the results were reported as mean ± standard deviation. The precision of the results was assessed by calculating the confidence intervals, in which all measurements fall with a confidence level 1  a (a = 0.05, giving a probability of 95%) (Taverniers, De Loose, & Van Bockstaele, 2004). Outliers were checked by using Q-test. Correlations between the results obtained by FI-CL method and F–C, DPPH and ABTS assays were performed using a linear regression and a correlation coefficient (r). 3. Results and discussion 3.1. Preliminary experiments In the first part of this research, chemiluminescence response of five plant polyphenols in different CL systems was examined. These systems were based on luminol, KMnO4, nanocolloidal Mn(IV) and Ce(IV) reagents which had already been used for the chemiluminescence detection of polyphenols. The studies were conducted in FI-CL manifold presented in Fig. 1. Each polyphenol was injected into a carrier stream and then merged with appropriate reagents as detailed under Section 2.4. Five polyphenolic compounds with strong antioxidant activity were chosen for this comparative study: gallic acid, caffeic acid, catechin, quercetin and rutin. Five CL systems based on luminol reagent (luminol-I2, luminolK3Fe(CN)6, luminol-H2O2, luminol-H2O2-metal ion and luminolH2O2-HRP) were studied. Polyphenols have been known to attenuate chemiluminescence emission generated during reaction of luminol oxidation (Giokas et al., 2007; Murillo Pulgarin et al., 2010; Nalewajko-Sieliwoniuk et al., 2008). For all luminol-based

CL systems preliminary optimisation studies were conducted. The influence of reagents concentration on signal heights was examined. The tested ranges and the optimal values of these parameters are presented in Table 1. The optimal conditions were selected considering sensitivity of the measurements and stability of the baseline as it affected the precision. Fig. 2A shows the potential of luminol reagent for the detection of polyphenols. As it can be seen, the largest signals were obtained for caffeic acid in luminol-I2 system. This system is also the most sensitive for the determination of catechin. Gallic acid produced the greatest response in luminol-K3Fe(CN)6 system, however this system has been excluded from further studies since other polyphenols (rutin, quercetin, catechin), at the studied concentration range, did not give a measureable response. The lowest signals for all analytes were registered in luminol-H2O2 system. The luminol-based systems were also compared taking into account day-to-day reproducibility of CL signals. The best RSD values were obtained for luminol-I2 system (about 5%), the worst for systems where H2O2 was used as an oxidant (RSD P 7% in luminol-H2O2-HRP and RSD P 15% in luminolH2O2-Cu(II)). Thus, considering the intensity and reproducibility of CL signals, luminol-I2 system was chosen as the best for polyphenols determination. For this system full optimisation studies were conducted. In addition to concentration of reagents, the instrumental parameters of FI-CL system, including the length of reaction coils (RC1 and RC2, see Fig. 1), flow rate of solutions and volume of injected sample, were also optimised. Increasing the length of RC1 in the range 50–200 cm and RC2 in the range 50–150 cm resulted in a decrease (by 11–63%) in CL signals of polyphenols. As a compromise between the sensitivity of measurements and the stability of the baseline, the length of 100 cm for both coils was chosen. The influence of flow rate on analytical signals of polyphenols was examined in the range of 1.5– 4.5 mL min1. Significant increase (2.6–4.4 fold) in CL signal heights was observed with the increase of flow rate in the studied range. By reason of non-steady baseline observed for flow rates higher than 3.5 mL min1, this value was selected for further experiments. The sample volume was tested in the range from 0.3 to 0.9 mL by changing the length of the sample loop in the injection valve. The CL signals of polyphenols were rising with sample volume up to 0.5 mL and remained almost constant beyond this value. The volume of 0.5 mL was considered an optimal. Afterwards, the CL systems based on manganese compounds: KMnO4 and nanocolloidal Mn(IV) were tested for their sensitivity to determine plant polyphenols. It was proved that polyphenolic compounds are capable of enhancing chemiluminescence of manganese-based systems (Brown et al., 2008; Costin et al., 2003; Malejko et al., 2014; Nalewajko-Sieliwoniuk et al., 2010). The studied CL systems and the optimised parameters are summarised in Table 1. The signals of five polyphenols obtained in optimal concentrations of reagents are presented in Fig. 2B. Significant variations in detection sensitivity between the CL systems tested can be clearly observed. In the case of using permanganate reagent, the greatest response was obtained in KMnO4-formaldehyde-hexametaphosphate system. Much smaller signals were recorded in all other KMnO4-based systems. The application of two enhancers: formaldehyde and sodium hexametaphosphate in Mn(IV)-based system also provided the highest sensitivity. With these reagents, gallic acid produced the largest signals, followed by caffeic acid, quercetin, catechin and then rutin. Overall, among the CL systems tested, Mn(IV)-formaldehyde-hexametaphosphate showed the best signal intensities and was selected for further optimisation studies. A series of experiments were carried out on the selection of an optimal flow rate of solutions and volume of injected sample. These parameters were tested in the range 3.5–9 mL min1 and 0.2–0.9 mL, respectively. Increasing the flow rate from 3.5 to 7 mL min1 and the sample volume from 0.2 to 0.6 mL resulted

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E. Nalewajko-Sieliwoniuk et al. / Food Chemistry 176 (2015) 175–183 Table 1 The studied chemiluminescence systems for polyphenols detection. Optimized parameter Luminol-based systems 1. Luminol-I2 Concentration of luminol (mol L1) Concentration of NaOH (mol L1) Concentration of I2 (mol L1) 2. Luminol-K3Fe(CN)6 Concentration of luminol (mol L1) Concentration of NaOH (mol L1) Concentration of K3Fe(CN)6 (mol L1) 3. Luminol-H2O2 Concentration of luminol (mol L1) Concentration of NaOH (mol L1) Concentration of H2O2 (mol L1) 4. Luminol-H2O2-metal ions Type of metal Concentration of Cu(II) (mg L1) Concentration of luminol (mol L1) Concentration of NaOH (mol L1) Concentration of H2O2 (mol L1) 5. Luminol-H2O2-HRP Concentration of horseradish peroxidase (IU mL1) Concentration of luminol (mol L1) Concentration of NaOH (mol L1) Concentration of H2O2 (mol L1) Manganese-based systems 1. KMnO4-formaldehyde Concentration of KMnO4 (mol L1) pH (H3PO4) Concentration of formaldehyde (mol L1) 2. KMnO4-formaldehyde-triphosphate Concentration of KMnO4 (mol L1) pH (H3PO4) Concentration of sodium triphosphate pentabasic (%) Concentration of formaldehyde (mol L1) 3. KMnO4-formaldehydehexametaphosphate Concentration of KMnO4 (mol L1) pH (H3PO4) Concentration of sodium hexametaphosphate (%) Concentration of formaldehyde (mol L1) 4. KMnO4-Na2SO3-hexametaphosphate Concentration of KMnO4 (mol L1) pH (H3PO4) Concentration of sodium hexametaphosphate (%) Concentration of Na2SO3 (mol L1) 5. Mn(IV)-formaldehyde Concentration of Mn(IV) (mol L1) Concentration of H3PO4 (mol L1) Concentration of formaldehyde (mol L1) 6. Mn(IV)-formaldehyde-triphosphate Concentration of Mn(IV) (mol L1) Concentration of H3PO4 (mol L1) Concentration of sodium triphosphate pentabasic (%) Concentration of formaldehyde (mol L1) 7. Mn(IV)-formaldehydehexametaphosphate Concentration of Mn(IV) (mol L1) Concentration of H3PO4 (mol L1) Concentration of sodium hexametaphosphate (%) Concentration of formaldehyde (mol L1)

Studied range

Table 1 (continued) Optimal value

2  104–1  103 0.03–0.15 1  105–1  104

5  104 0.05 2  105

1  104–1  103 0.02–0.2 1  104–5  103

8  104 0.08 1  103

3

3

1  10 –0.01 0.01–0.2 2  104–5  103

5  10 0.02 2  103

Co(II), Mn(II), Fe(II), Cu(II) 0.05–0.2 1  104–1  103 5  103–0.05 1  103–8  103

Cu(II)

0.64–30

20

4

5  10 –1.9  10 8  103–0.2 5  104–0.01

2

0.1 5  104 8  103 5  103

0.01 0.05 5  103

1  104–1  103 1–4 0.1–1.2

5  104 2 0.4

1  105–1  103 1–4 0.5–6

1  104 2 4

0.1–1.2

0.4

1  105–1  103 1–4 0.5–2

5  104 2 1

0.1–1.2

0.4

1  105–1.5  103 1–4 0.5–2

1  103 2 1

1  104–5  103

1  103

2  104–1.7  103 3–6 0.1–1.2

1  103 6 0.4

2  104–1.7  103 3–6 4–8

1.7  103 6 6

0.1–1.2

0.4

2  104–1.7  103 3–6 0.8–4

1.7  103 6 3

0.1–1.2

1

Optimized parameter

Studied range

Optimal value

8. Mn(IV)-formic acidhexametaphosphate Concentration of Mn(IV) (mol L1) Concentration of H3PO4 (mol L1) Concentration of sodium hexametaphosphate (%) Concentration of formic acid (%)

2  104–1.7  103 3–6 0.8–4

1  103 6 3

1–4

2

5  103–0.05 0.1–2 1  105–1  103

0.03 0.2 5  105

5  103–0.05 0.1–2 1  105–2  104

0.03 0.2 5  105

5  105–0.01 0.05–0.5 5–13

1  104 0.1 9

1  104–0.01 0.05–0.5 1  103–0.05

No signals No signals No signals

Cerium(IV)-based systems 1. Ce(IV)-H2SO4-rhodamine 6G Concentration of Ce(IV) (mol L1) Concentration of H2SO4 (mol L1) Concentration of rhodamine 6G (mol L1) 2. Ce(IV)-H2SO4-rhodamine B Concentration of Ce(IV) (mol L1) Concentration of H2SO4 (mol L1) Concentration of rhodamine B (mol L1) 3. Ce(IV)-H2SO4-tween 20 Concentration of Ce(IV) (mol L1) Concentration of H2SO4 (mol L1) Concentration of tween 20 (%) 4. Ce(IV)-H2SO4-quinine Concentration of Ce(IV) (mol L1) Concentration of H2SO4 (mol L1) Concentration of quinine (mol L1)

in an increase in analytical signals of polyphenols. The pumping velocity of 7 mL min1 and the sample loop of 0.6 mL were chosen for further experiments, as above these values we did not observe a noticeable influence of studied parameters. The distance between the mixing point of all reagents and the flow cell (RC2, see Fig. 1) was 5 cm and was as short as possible. To explore the potential of Ce(IV) reagent for the chemiluminescence detection of polyphenols, we examined the response from five polyphenols with Ce(SO4)2 in sulphuric acid medium in the presence of various sensitizers: rhodamine 6G, rhodamine B, tween 20 and quinine. The tested parameters and the obtained signals are presented in Table 1 and Fig. 2C, respectively. As it can be seen from Fig. 2C, the sensitivity of systems based on cerium(IV) is considerably worse comparing to the previous systems. Therefore, these CL systems have been excluded from further studies. Taken together, the results revealed that two CL systems: luminol-I2 and Mn(IV)-formaldehyde-hexametaphosphate are the most promising systems for plant polyphenols detection. They have been selected for further studies to develop FI-CL methods of determination of the total polyphenolic content. 3.2. Analytical characteristic Under the optimal experimental conditions described above, analytical parameters of the FI-CL methods based on luminol-I2 and Mn(IV)-formaldehyde-hexametaphosphate chemiluminescence were evaluated. The linear ranges and equations of the calibration graphs obtained for standard solutions of gallic acid, caffeic acid, catechin, quercetin, and rutin are summarised in Table 2. The significant difference in detection sensitivity between methods can be observed. The slopes of the calibration graphs obtained in luminol-based method were much higher than those of Mn(IV)-based method. The luminol-based method exhibited the highest sensitivity for the detection of caffeic acid, while Mn(IV)-based method – for the detection of gallic acid. For that reason these compounds were chosen as standards for the determination of the total polyphenol index. The limit of detection for these polyphenols (expressed as signal-to-noise ratio of 3) was 0.02 lg L1.

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The Mn(IV)-formaldehyde-hexametaphosphate system has proved to be more sensitive for polyphenols detection than Mn(IV)-formaldehyde system which previously had been applied for the analysis of plant-derived beverages (NalewajkoSieliwoniuk et al., 2010). The application of sodium hexametaphosphate as an additional sensitizer resulted in nearly 2-fold increase in slope of the calibration graph of gallic acid. Moreover, the linearity range for gallic acid begins with 5-times lower concentration (0.1 lg L1) allowing higher dilution of the sample before analysis. The proposed Mn(IV)-formaldehyde-hexametaphosphate based FI-CL method provides lower limit of detection of gallic acid in comparison to the FI-CL methods developed by other authors (LOD was equal to 25.5 lg L1 in luminol-H2O2-HRP based method

45000

gallic acid

40000

caffeic acid catechin

35000

quercen

I [nA]

30000

run

25000 20000 15000 10000 5000

(Minioti & Georgiou, 2008), 8.5 lg L1 in KMnO4 based method (Costin et al., 2003), 0.5 lg L1 in luminol-AgNO3-Ag based method (Li et al., 2012) and 0.22 lg L1 in luminol-KMnO4 based method (Wang, Wang, & Yang, 2007)). The repeatability of the developed FI-CL luminol-based method, expressed as a relative standard deviation (RSD) of 15 subsequent measurements of 1 lg L1 caffeic acid, was 2.0%. In the case of Mn(IV)-based method the value of RSD was comparable and equal to 1.7% for 10 lg L1 of gallic acid. The reproducibility of the method, expressed as RSD of the slopes of the calibration graphs registered in different days, was better for Mn(IV)-based method (3.6% for gallic acid, n = 4) than for luminol-based method (5.3% for caffeic acid, n = 3). The sampling frequency was higher for Mn(IV)-based method (195 samples h1) than for luminol-based method (136 samples h1). The developed FI-CL methods are much faster than conventional batch spectrophotometric assays (in F–C method absorbance is measured 2 h after mixing of all reagents (Djeridane et al., 2006), in DPPH – 15 min after (Miliauskas et al., 2004), in ABTS – from 4 up to 6 min after, depending on the polyphenolic compound (Re et al., 1999)). Our methods are also faster than automated flow injection based spectrophotometric methods. Determination rate obtained for flow methods with ABTS, DPPH and F–C detection was in the range of 9–45 samples per hour (only one FI-ABTS method has sample throughput 120 h1) (Magalha˘es, Santos, Segundo, Reis, & Lima, 2009).

0 1

A

2

3

4

4000

gallic acid

3500

caffeic acid catechin

3000

quercen

2500

I [nA]

3.3. Selectivity studies

5

Luminol-based CL system

run 2000 1500 1000 500 0 1

B

2

3

4

5

6

Mn-based CL system

7

8

gallic acid 90

caffeic acid

80

catechin

I [nA]

70

quercen

60

run

50 40 30 20 10 0

C

1

2

3

Ce(IV)-based CL system

4

Fig. 2. CL signal intensities of polyphenols in: (A) luminol-I2 (1), luminol-K3Fe(CN)6 (2), luminol-H2O2 (3), luminol-H2O2-Cu(II) (4), luminol-H2O2-HRP (5) systems. Concentration of polyphenols: 10 lg L1; (B) KMnO4-formaldehyde (1), KMnO4formaldehyde-triphosphate (2), KMnO4-formaldehyde-hexametaphosphate (3), KMnO4-Na2SO3-hexametaphosphate (4), Mn(IV)-formaldehyde (5), Mn(IV)-formaldehyde-triphosphate (6), Mn(IV)-formaldehyde-hexametaphosphate (7), Mn(IV)formic acid-hexametaphosphate (8) systems. Concentration of polyphenols: 1 mg L1; (C) Ce(IV)-H2SO4-rhodamine 6G (1), Ce(IV)-H2SO4-rhodamine B (2), Ce(IV)-H2SO4-tween 20 (3), Ce(IV)-H2SO4-quinine (4) systems. Concentration of polyphenols: 1 mg L1.

In order to evaluate the possibility of application of the proposed FI-CL methods based on luminol-I2 and Mn(IV)-formaldehydehexametaphosphate chemiluminescence to the determination of the total polyphenolic content in plant-derived food samples, the influence of various substances which (apart from polyphenols) are expected to be present in wine, tea, fruit and vegetable juices, cereal coffee, herbs and spices was examined. In order to determine the selectivity of the Mn(IV)-formaldehyde-hexametaphosphate based method, the standard solutions of gallic acid (40 lg L1) were spiked with increasing amounts of the foreign substances. In the case of luminol-I2 based method interferents were added to the standard solutions of caffeic acid (2.5 lg L1). The tolerable concentration ratio of the interfering compounds (average value of three determinations) was considered to be acceptable if the relative error of determination was smaller than ±5%. The results are summarised in Table 3. Comparing the results presented in Table 3, it can be concluded that manganese(IV)-formaldehyde-hexametaphosphate based method provides greater selectivity than luminol-I2 based method. In luminol-I2 CL system many of the tested compounds (even if they are present in small concentrations) may interfere in the determination of the total polyphenolic content in real samples. Substances that cause interferences in the determination of plant polyphenols in wine are: sodium metabisulphite, citric acid, glucose, tartaric acid and ascorbic acid. In tea the most considerable interfering effect was observed for caffeine and in fruit juices – for ascorbic acid and carbohydrates. Mn(IV)-formaldehydehexametaphosphate based method exhibited much higher selectivity. The tolerable concentration ratio of the interfering compounds (except of NaCl, ethanol and MnCl2) was from 2.4 up to 68,750 times higher than in luminol-I2 based method. Comparing the obtained tolerable concentrations of the tested compounds with their contents in real samples (after an appropriate dilution) it can be concluded that none of them cause interfering effect during the determination of polyphenols by Mn(IV)-formaldehydehexametaphosphate based method.

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Luminol-I2 based method Caffeic acid Catechin Gallic acid Quercetin Rutin

Linearity range (lg L1)

Calibration equation y = (a ± SD)x + (b ± SD) (n = 3)

Correlation coefficient (r)

0.1–5 0.5–10 0.2–500 2–500 5–1000

y = (5065 ± 270)x + (427 ± 148) y = (843 ± 29)x + (266 ± 76) y = (129 ± 5)x + (1164 ± 89) y = (113 ± 6)x + (1038 ± 470) y = (70.9 ± 1.8)x + (748 ± 336)

0.999 0.998 0.999 0.998 0.999

y = (12.4 ± 0.2)x + (12.1 ± 2.2) y = (10.6 ± 0.1)x + (23.6 ± 3.5) y = (6.17 ± 0.04)x + (35.4 ± 13.6) y = (5.95 ± 0.06)x + (43.3 ± 12.2) y = (2.53 ± 0.03)x + (21.9 ± 6.9)

0.999 0.998 0.997 0.997 0.997

Mn(IV)-formaldehyde-hexametaphosphate based method Gallic acid 0.1–100 Caffeic acid 0.3–100 Catechin 0.5–200 Quercetin 1–200 Rutin 1–200

3.4. Method application Although sensitivity of Mn(IV)-formaldehyde-hexametaphosphate based method was lower than luminol-I2 based method, it exhibited significantly higher selectivity (Table 3). Therefore, Mn(IV)-based FI-CL method was applied to the determination of the total content of polyphenolic compounds in a wide range of plant-derived food products. We had already used Mn(IV)-formaldehyde-hexametaphosphate CL system for the determination of the total polyphenols content as 6-hydroxyluteolin 7-O-glucoside equivalents (by FI-CL) (Malejko et al., 2014) and individual polyphenols (by HPLC-FI-CL) (Nalewajko-Sieliwoniuk, Malejko, Mozolewska, Wołyniec, & Nazaruk, 2015) in plant extracts from leaves of Cirsium palustre (L.). The samples analysed in this study were: wines, teas, cereal coffee, fruit and vegetable juices, herbs and spices. The results were expressed as milligram of gallic acid equivalent per litre of sample, because this polyphenol exhibited the highest sensitivity in Mn(IV)-based method among all Table 3 The tolerable concentration ratio of the interfering compounds in the determination of gallic acida by the Mn(IV)-formaldehyde-hexametaphosphate based FI-CL method and caffeic acidb by the luminol-I2 based FI-CL method (relative error of determination ±5%). Interferent

NaCl Ethanol Na2S2O5 Malic acid Proline Acetic acid CaCl26 H2O Theophylline Glucose Sucrose ZnCl2 Fructose Na2B4O710H2O MgCl2 KCl Citric acid Tartaric acid Oxalic acid Theobromine FeCl3 Xanthin Caffeine Ascorbic acid MnCl2 a b

The tolerable concentration ratio Mn(IV)-formaldehydehexametaphosphate based methoda

Luminol-I2 based methodb

250,000 50,000 27,500 5000 2500 1250 1000 750 750 575 550 475 350 300 250 225 125 125 30 25 17.5 7.5 3.75 0.2

400,000 280,000 0.4 – – 28 1.4 – 40 200 4 200 – 3.6 3.2 12 2.8 – 1.2 0.036 – 0.02 0.04 0.2

Concentration of gallic acid was 40 lg L1. Concentration of caffeic acid was 2.5 lg L1.

investigated compounds. Moreover, this is the most often used standard for the determination of the total polyphenol index. The samples were prepared according to the procedures described in Sections 2.2. and 2.3. The samples were analysed in triplicate. On the basis of the obtained results (Table 4), it could be concluded that the highest total polyphenolic content was determined in: red wine (1361 ± 79 mg L1), thyme and cistus infusions (597 ± 13 mg L1 and 589 ± 42 mg L1), green and white tea infusions (from 475 ± 2 to 564 ± 6 mg L1) and blackcurrant juice (469 ± 41 mg L1). These plant-derived beverages were also identified as rich sources of polyphenols by authors of Phenol-Explorer database (PérezJiménez et al., 2010). The lowest total polyphenolic content among all investigated food samples was found in: extracts of curcuma (19.4 ± 2.1 mg L1), fruit tea (51.5 ± 0.5 and 61.8 ± 0.2 mg L1), rooibois (71.5 ± 3.6 mg L1) and cereal coffee (85.8 ± 0.8 mg L1), carrot (88.7 ± 0.7 mg L1) and red grapefruit (86.3 ± 2.2 mg L1) juices. 3.5. Correlation of FI-CL method with spectrophotometric assays (F–C, DPPH, ABTS) The values of the total content of polyphenolic compounds obtained by Mn(IV)-formaldehyde-hexametaphosphate FI-CL method were compared with those determined by Folin–Ciocalteu method (Djeridane et al., 2006) and with the values of the antioxidant activity estimated by DPPH (Miliauskas et al., 2004) and ABTS (Re et al., 1999) methods. The results of spectrophotometric assays are given in Table 4. It was found that for each group of samples (tea and cereal coffee infusions (n = 12), herb and spice infusions (n = 4), wines (n = 3), fruit and vegetable juices (n = 5)) the total content of polyphenolic compounds determined by FI-CL and F–C methods, correlate highly (correlation coefficient was equal to: 0.942, 0.920, 0.999 and 0.959, respectively). However, as it can be seen from Table 4, the total polyphenolic contents determined by F–C method were much higher than those obtained by FI-CL method. These differences are caused by very low selectivity of the F–C method, as the Folin–Ciocalteu reagent also reacts with non-phenolic food constituents (Prior et al., 2005; Pérez-Jiménez et al., 2010). A good correlation was also established between the total polyphenols determined by FI-CL method and the antioxidant activity estimated by the DPPH and ABTS methods for wines (r = 0.999 for FI-CL vs. DPPH and FI-CL vs. ABTS), tea and cereal coffee infusions (r = 0.985 for FI-CL vs. DPPH and r = 0.953 for FI-CL vs. ABTS), herb and spice infusions (r = 0.950 for FI-CL vs. DPPH and r = 0.943 for FI-CL vs. ABTS). Nevertheless, a lower correlation was found for fruit and vegetable juice samples between FI-CL and DPPH method (r = 0.830) as well as between FI-CL and ABTS method (r = 0.856). This may be due to the high heterogeneity in the composition of juice samples obtained from a variety of fruits and vegetables (Magalhães et al., 2007) as well as differences in acidity of these

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Table 4 Total polyphenolic content determined by FI-CL and F–C methods and antioxidant activity determined by ABTS and DPPH methods in teas, cereal coffee, herbs, spices, wines, fruit and vegetable juices. Type Tea and cereal coffee infusions White teas Green teas Black teas Pu-erh tea Yerba mate Cereal coffee Rooibois tea Fruit teas Herb and spice infusions Thymus vulgaris L. (dried) Cistus incanus (dried) Salvia officinalis L. (dried) Curcuma longa (dried) Wines Red wine (dry) Rosé wine (semi-dry) White wine (dry) Fruit and vegetable juices Blackcurrant juice Apple juice Tomato juice Carrot juice Red grapefruit juice a

FI-CL method (mg gallic acid L1)a

F–C method (mg gallic acid L1)a

DPPH inhibition (%)a

ABTS method (mg trolox L1)a

564 ± 6 492 ± 6 489 ± 5 475 ± 2 321 ± 11 159 ± 4 289 ± 5 206 ± 6 85.8 ± 0.8 71.5 ± 3.6 61.8 ± 0.2 51.5 ± 0.5

965 ± 15 953 ± 28 1112 ± 46 1073 ± 32 831 ± 10 353 ± 8 360 ± 8 577 ± 11 249 ± 8 319 ± 9 139 ± 6 160 ± 7

70.5 ± 2.2 66.3 ± 0.1 77.2 ± 2.5 70.6 ± 2.8 49.1 ± 1.6 31.6 ± 0.5 43.7 ± 0.5 32.4 ± 1.2 13.8 ± 0.4 17.8 ± 0.4 13.5 ± 0.3 14.0 ± 0.4

2015 ± 48 1571 ± 16 1097 ± 60 1211 ± 23 852 ± 26 557 ± 11 875 ± 6 322 ± 11 163 ± 7 244 ± 13 162 ± 3 135 ± 1

597 ± 13 589 ± 42 327 ± 6 19.4 ± 2.1

1133 ± 36 1804 ± 22 737 ± 22 66.7 ± 3.1

52.7 ± 1.4 74.6 ± 0.6 31.7 ± 1.2 3.39 ± 0.10

822 ± 40 1113 ± 61 352 ± 17 59.5 ± 2.6

1361 ± 79 389 ± 41 141 ± 8

2993 ± 93 868 ± 22 439 ± 11

74.9 ± 0.7 23.8 ± 0.4 7.66 ± 0.39

1515 ± 18 494 ± 28 176 ± 9

469 ± 41 236 ± 5 160 ± 1 88.7 ± 0.7 86.3 ± 2.2

1352 ± 26 775 ± 16 285 ± 10 387 ± 17 1029 ± 23

44.3 ± 0.7 38.8 ± 1.2 4.56 ± 0.18 15.3 ± 0.5 10.5 ± 0.2

581 ± 27 492 ± 14 99.3 ± 6.0 203 ± 9 174 ± 6

Mean of three determinations ± SD.

samples associated with the presence of organic acids which may affect the results obtained for antioxidants (Scalzo, 2008). Despite this fact, it could be concluded that there is a relationship between the total polyphenolic content in the sample determined by FI-CL method and its antioxidant activity.

4. Conclusions This paper covers the comparison of seventeen various CL systems, based on luminol, KMnO4, Mn(IV) and Ce(IV) reagents, in terms of their sensitivity and selectivity to determine polyphenolic compounds. As far as we know, this is the first time when such comparative studies have been carried out. The method based on enhancing effect of polyphenolic compounds on Mn(IV)-formaldehyde-hexametaphosphate chemiluminescence was considered to be the most suitable for polyphenols determination. Although the sensitivity of this method was lower than luminol-based method, the reproducibility, sample throughput, and above all, the selectivity were better. Therefore, the Mn(IV)-formaldehyde-hexametaphosphate FI-CL method was successfully applied for the analysis of a variety of plant-derived food products (wine, tea, fruit and vegetable juices, cereal coffee, herbs and spices). The proposed FI-CL method based on chemiluminescence of Mn(IV)-formaldehyde-hexametaphosphate system could be a good alternative to traditional spectrophotometric assays, due to its higher sample throughput, simplicity and sensitivity. Moreover, proposed FI-CL method guarantees a reliable measurement of polyphenolic content in a variety of plant-derived food samples (high dilution factor of the samples allows to eliminate interference of matrix origin). Therefore, it could be used as a routine method in a quality control of antioxidant products. Obtaining reliable data on the polyphenols content is crucial to the food industry in comparing different sources of plant polyphenols and studying the influence of food processing.

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