Effects of Interlayer Ion in Montmorillonite on Appearance of Decaffeinated Tea Beverage

Abstract

We examined changes in the appearance of green tea decaffeinated by montmorillonite (MMT) and the effects of interlayer ion in MMT on the appearance of decaffeinated green tea. After MMT treatment, turbidity was generated in decaffeinated green tea over time and was suggested to be due to precipitation of calcium oxalate generated by a reaction between Ca ion released from the interlayer of MMT and oxalate ion in green tea. Exchanging the interlayer ion in MMT with Na, K, Mg or NH₄ ion suppressed the precipitation of calcium oxalate in decaffeinated green tea, such that turbidity after decaffeination was not observed. The ion-exchanged MMTs (K, Mg or NH₄-MMT) had similar caffeine adsorption abilities on the decaffeination of green tea. Our findings suggest that ion-exchanged MMT can remove caffeine in green tea and maintain the appearance quality.

Introduction

Tea is a beverage produced using the leaves of Camellia sinensis L. and is one of the most popular beverages in the world. The world consumption of tea products reached 5 million tons in 2013 (Chang, 2015). In Japan, the consumption of tea products, mainly as green tea, has reached about 100 thousand tons. Green tea contains catechins (epicatechin, EC; epigallocatechin, EGC; epicatechin gallate, ECg; epigallocatechin gallate, EGCg) as the major polyphenols (Perva-Uzunalić et al., 2006). Recently, various health benefits of catechins have been reported, such as hypoglycemic effect (Matsumoto et al., 1993), anti-allergic effect (Suzuki et al., 2000) and anti-obesity effect (Rains et al., 2011) and the health benefits of green tea are attracting worldwide attention.

Tea leaves contain about 2 to 5% of caffeine (Naik and Nagalakshmi, 1997). Caffeine, 1,3,7-trimethylxanthine, has a bitter taste, and various physiological effects, some of which are beneficial; however, excessive ingestion may have negative effects, such as risk of fetal growth disorder (Fernandes et al., 1998), blood pressure elevating effect (Nurminen et al., 1999), and sleep disorder (Hindmarch et al., 2000). Therefore, restricted caffeine intake, according to the constitution and situation, is recommended. Tea leaves also contain about 0.7 to 1.3% of oxalic acid (Horie and Kohata, 2000), a dicarboxylic acid contained in many plants and vegetables, which has an acidic, harsh taste. Calcium oxalate, a calcium salt of oxalic acid, is thought to cause scale in the production process of sugar and beer; however, precipitation suppression effects of polymeric additives have been reported (Doherty et al., 1995; Demadis and Leonard, 2011). In tea, when extracting with hard water, calcium oxalate causes the generation of a white precipitate (Horie et al., 1998; Xu et al., 2013) and tea incrustation (Inagaki and Nishikawa, 2010). Thus, calcium oxalate is an important compound in the appearance quality of tea beverage.

We previously reported on the use of montmorillonite (MMT) as a selective decaffeination technology in tea (Shiono et al., 2017a) and coffee (Shiono et al., 2017b). MMT, a layered clay mineral of the smectite group, mined in many places of the world, has swelling, thickening, adsorptivity and plasticity properties. Furthermore, because of its low cost and high stability, MMT is used in various industries including agriculture, civil engineering, pharmaceutical, cosmetics and food (Tajchakavit, 2001; Lee and Hu, 2003). MMT has a three-layer structure, composed mainly of silica and aluminum (Brigatti et al., 2006). Isomorphous replacement occurs between Si and Al in the silica layers at both ends of the structure and between Al and Mg or Fe in the middle alumina layer, and a permanent charge, called layer charge, occurs. Exchangeable cations also exist between the layers. The swelling property of MMT is dependent upon the interlayer ions (Norrish, 1954; Sato et al., 1992). Studies on the properties of ion-exchanged MMT have been reported, such as changes in the structure and gas adsorption properties after inorganic ion exchange (Volzone and Ortiga, 2004) or organic ion exchange (Okada et al., 2014; Okada et al., 2015). There have been no studies investigating the properties of ion-exchanged MMT in tea extract.

We analyzed changes in the appearance of a tea beverage after decaffeination treatment by MMT and investigated the effects of interlayer ion in MMT on caffeine adsorption and the appearance of a decaffeinated tea beverage.

Materials and Methods

Decaffeination of tea extract    MMT (MIZULITE, produced in Yamagata Prefecture) was obtained from Mizusawa Industrial Chemicals, Ltd. (Japan) and was used as an adsorbent and in ion-exchange, as described below. Green tea leaves (Sencha, produced in Shizuoka Prefecture) were obtained from a specialized tea shop in Japan. Tea leaves (100 g) were extracted with 3000 mL of hot water at 70°C for 6 min: the caffeine concentration in the green tea extract was 3.3 mmol/L. To obtain a decaffeinated green tea beverage, MMT (800 mg) was added to 40 mL of the green tea extract. After incubation at 25°C for 1 min, the suspension was centrifuged at 1920 × g for 5 min (KUBOTA5930 Kubota Corp., Japan), and the supernatant was collected. Ultrapure water (24 mL) was added to 36 mL of the supernatant to obtain the decaffeinated green tea beverage. To obtain a Ca added green tea beverage, Ca ion solutions (24 mL) were added to 36 mL of the green tea extract, which were then centrifuged at 1920 × g for 5 min to collect the supernatant, so that the concentrations of added Ca ion were 0.1 to 1.0 mmol/L. The Ca ion solutions were prepared using calcium chloride (CaCl₂·2H₂O, Wako Pure Chemical Industries, Ltd., Japan).

The decaffeinated green tea beverage was filtered using a membrane filter (0.2 µm, PTFE), and the concentration of caffeine was measured using HPLC (LC-2000Plus, JASCO, Corp., Japan). Column: CAPCELL PAK C18 UG120 (4.6 nm × 150 mm, 3 µm, Shiseido Co., Ltd., Japan); column temperature: 40°C; flow rate: 0.9 mL/min; mobile phase A: water/ acetonitrile/ phosphoric acid (1000/ 25/ 1.0, v); mobile phase B: water/ methanol/ acetonitrile/ phosphoric acid (600/ 300/ 15/ 0.6, v); mobile phase C: methanol/ acetonitrile/ phosphoric acid (800/ 200/ 1.0, v); gradient conditions: 0-3 min: 94% A and 6% B, 3-8 min: from 94% A and 6% B to 50% A and 50% B, 8-22 min: from 50% A and 50% B to 0% A and 100% B, 22-24 min: from 100% B to 0% B and 100% C, and 22-27 min: 100% C; detector: UV (275 nm). Caffeine quantification was performed using a calibration curve method. A caffeine standard was purchased from Wako Pure Chemical Industries, Ltd. (Japan). The amount of adsorbed caffeine per unit weight of MMT (mmol/g) was calculated from the measured caffeine concentration.

Precipitation test of tea beverage    Precipitation tests for the decaffeinated green tea and Ca added green tea beverages were performed. The decaffeinated green tea beverage was agitated at 4°C using a reciprocal shaker (shaking speed, 60 rpm), and the turbidity was measured over time (0.5 to 5 h). Turbidity measurement was performed as reported in a previous study (Kovacevic et al., 2013): optical density was measured at 660 nm using a spectrophotometer (U-3900H, Hitachi High-Tech Science Corp., Japan) and a quartz cell with an optical path length of 10 mm. The decaffeinated green tea beverage was then filtered using a membrane filter (0.45 µm, PTFE) before the concentrations of Ca and oxalate ions were measured using an ion chromatograph (pump: LC-20AD; auto-sampler: SIL-10Ai; column oven: CTO-20AC; system controller: CBM-20A; Shimadzu Corp., Japan) after dilution. The analytical conditions for ion chromatography were: for cation analysis, column: Shim-pack IC-C4 (Shimadzu Corp., Japan); column temperature: 30°C; flow rate: 1.2 mL/min; mobile phase: oxalic acid (3.0 mmol/L); for anion analysis, column: Shim-pack IC-SA2 (Shimadzu Corp., Japan); column temperature: 30°C; flow rate: 1.0 mL/min; mobile phase: NaHCO₃ (12 mmol/L)/Na₂CO₃ (0.6 mmol/L); detector: electrical conductivity (CDD-10A VP, Shimadzu Corp., Japan). Ca and oxalate ion quantifications were performed using a calibration curve method.

The Ca added green tea beverages were agitated for 2 h using a reciprocal shaker (temperature, 4°C; shaking speed, 60 rpm), and turbidity was measured. The Ca added green tea beverages were then filtered, and the concentrations of oxalate were measured using the same method for the decaffeinated green tea beverage.

To analyze precipitates generated in the decaffeinated green tea and the Ca added green tea (1.0 mmol/L added), these beverages were agitated for 2 h using a reciprocal shaker (temperature, 4°C; shaking speed, 60 rpm) and centrifuged at 1920 × g for 5 min to collect the precipitates. The collected precipitates were dried at 80°C overnight, before analysis by FT-IR (IRTracer-100, Shimadzu Corp., Japan) using ATR with a Ge prism (MIRacle10, Shimadzu, Corp., Japan). The analytical conditions were: temperature: 20°C; measurement mode: absorbance; number of scans: 20 times; resolution: 4 cm⁻¹; measured wavelength: 750–4000 cm⁻¹. Precipitates collected from untreated green tea beverage and MMT were used as references, and calcium oxalate (CaC₂O₄·H₂O, Wako Pure Chemical Industries, Ltd., Japan) was used as a standard.

Cation exchange of MMT    Cation exchange treatments of MMT with Na, K, Mg, Ca or NH₄ ions were performed. The cation exchange treatments were conducted by allowing contact between MMT and the cation solutions, which were prepared with chloride salts (Wako Pure Chemical Industries, Ltd., Japan). MMT (4000 mg) was added to the cation solution (1 mol/L) and agitated for 30 min using a reciprocal shaker (temperature, 20°C; shaking speed, 60 rpm), before centrifugation at 3420 × g for 10 min (KUBOTA5930 Kubota Corp., Japan) to collect the MMT pellet. The same procedure was repeated 3 times. The collected pellet was washed at least twice by resuspending in 40 mL of ultrapure water and further centrifugation. The pellet was then washed once with ethanol and then once with acetone using the same method as for the ultrapure water. Ethanol and acetone were purchased from Wako Pure Chemical Industries, Ltd. (Japan). After washing, the pellet was dried at 40°C for 12 h and crushed using a ceramic mill to obtain ion-exchanged MMT (Na-MMT, K-MMT, Mg-MMT, Ca-MMT or NH₄-MMT). The ion-exchanged MMTs were used in the decaffeination of tea extract using the same method for raw MMT.

Analyses of ion-exchanged MMT    Elemental and structural analyses of the ion-exchanged MMT were conducted. For the elemental analysis, the amounts of SiO₂, Al₂O₃, Fe₂O₃, Na₂O, K₂O, MgO and CaO were measured using a scanning fluorescent X-ray spectrometer (ZSX Primus II, Rigaku Corp., Japan). Samples were prepared using a pressure forming method; element quantifications were performed using a calibration curve obtained from the wet chemical analysis data of raw MMT, as previously reported (Toda, 1996). Structural analysis was conducted using an X-ray diffractometer (RINT-Ultima PC, Rigaku Corp., Japan) and CuKα as the X-ray source. Measurements were conducted using the continuous scanning method, and the analysis was conducted within the scan range of 5° to 90°. An interlayer distance was calculated based on the diffraction peak angle of the d001 plane, in accordance with Bragg's law (Bragg, 1913).

Results and Discussion

Changes in appearance of decaffeinated tea    The caffeine concentration of a green tea beverage decaffeinated by MMT was 0.30 mmol/L, whereas that of the untreated green tea beverage was 2.0 mmol/L: the caffeine removal rate was 84.8%. Turbidity generation during the precipitation test was observed after decaffeination treatment (Figure 1a). Turbidity was observed immediately after the decaffeination treatment and increased gradually over time (Figure 1b). It was previously reported that calcium oxalate precipitation was generated in concentrated green tea stored at a low temperature (Xu et al., 2014). In addition, MMT was found to elute Ca and Mg ions by ion-exchange (Ogawa, 2002). In this study, it was confirmed that MMT treatment increased Ca ions in the green tea from 0.12 to 0.73 mmol/L within 0.5 h while oxalate ions decreased from 1.43 to 0.68 mmol/L. This is because the calcium oxalate precipitate was mostly removed by centrifugation, a step necessary to remove MMT from green tea. Furthermore, Ca and oxalate ions decreased over time, along with a successive increase in turbidity (Figures 1b and 2). It was considered that a precipitate of calcium oxalate was gradually generated by a reaction between the Ca and oxalate ions in decaffeinated green tea during the precipitation test.

Fig. 1.

Change in the appearance of green tea decaffeinated by MMT. (a) Appearance of untreated green tea and decaffeinated green tea after agitation for 5 h at 4°C; (b) time course of turbidity after decaffeination treatment. Closed circle, decaffeinated green tea; open circle, untreated green tea. The bars indicate standard deviation (n=5).

Fig. 2.

Time courses of concentration of Ca and oxalate ion in decaffeinated green tea after MMT treatment. (a) Ca ion; (b) oxalate ion. Closed circle, decaffeinated green tea; open circle, untreated green tea. The bars indicate standard deviation (n=5).

Turbidity generation was also confirmed in the green tea beverages when 0.3 mmol/L or more of Ca ions were added. Figure 3 shows the increase in turbidity and decrease in oxalate ions by the addition of Ca ions. Previous reports stated that calcium oxalate precipitation and turbidity generation occurred in green tea extracts which were either extracted with hard water or had Ca ions added (Horie et al., 1998; Xu et al., 2013). It was suggested that the calcium oxalate precipitation and turbidity generation were caused by the addition of Ca ions to the green tea beverage.

Fig. 3.

Effect of Ca ion addition on turbidity and concentration of oxalate ion in green tea. Closed circle, turbidity (OD 660 nm); open circle, oxalate ion. The bars indicate standard deviation (n=5).

The precipitates causing the turbidity generated in the green tea beverage decaffeinated by MMT and Ca added green tea beverage were collected by centrifugation; the collected solids were then analyzed by FT-IR. A comparison with untreated green tea identified specific peaks around 1635 cm⁻¹ and 1320 cm⁻¹ in the spectrums of decaffeinated green tea and Ca added green tea (Figure 4b, c and d). In addition, specific peaks were also detected around 1608 cm⁻¹ and 1315 cm⁻¹ in the spectrum of calcium oxalate (Figure 4e), however, these were not confirmed in the spectrum of MMT (Figure 4a). It was previously reported that calcium oxalate has specific peaks around 1619 cm⁻¹ and 1313 cm⁻¹, which are attributable to the stretching vibration of carboxylate (Kitade et al., 2010). The precipitate generated in the green tea beverage decaffeinated by MMT was similar to the precipitate generated in the Ca added green tea beverage and a standard of calcium oxalate.

Fig. 4.

Infrared spectrums of precipitates in green tea beverages, MMT and calcium oxalate. (a) MMT; (b) untreated green tea; (c) green tea decaffeinated by MMT; (d) Ca added green tea; (e) calcium oxalate.

These results suggest that a precipitation of calcium oxalate is generated in green tea beverage during and after decaffeination using MMT, and the precipitation generates turbidity. Because the appearance of green tea beverage is a critical factor, reflecting its quality, it is essential to avoid generating turbidity during and after decaffeination treatment. Thus, to prevent the elution of Ca ions from MMT and generating turbidity, we investigated the effects of the interlayer ions in MMT on the appearance of a decaffeinated tea beverage.

Element composition and structure of ion-exchanged MMT    During MMT treatment of green tea, Ca ions are released from the interlayer of MMT due to ion exchange with cations such as K ions in the green tea extract. To investigate the effects of the interlayer ions in MMT, ion-exchanged MMTs were prepared by ion exchange treatment using various cations (Na, K, Mg or NH₄). To confirm that ion exchange in MMT was successfully advanced, elemental and structural analyses of the ion-exchanged MMT were carried out. The element composition ratios of ion-exchanged MMTs are shown in Table 1. The amounts of CaO in MMT exchanged with Na, K, Mg or NH₄ ions were decreased. In addition, increases in the amounts of Na₂O, K₂O and MgO were observed in Na-MMT, K-MMT and Mg-MMT, respectively. Hence, it was suggested that Ca ions in the MMT interlayer were replaced with one of the corresponding cations (Na, K, Mg or NH₄). In addition, the amount of CaO increased in Ca-MMT. Based on these results, it was suggested that the occupancy ratio of Ca ions in the interlayer space increased in Ca-MMT.

Table 1. Elemental composition and interlayer distance of ion-exchanged MMT.

	raw MMT	Na-MMT	K-MMT	Mg-MMT	Ca-MMT	NH4-MMT
SiO2 (%)	75.4	74.9	75.1	73.8	74.6	75.7
Al2O3 (%)	12.0	11.4	11.8	11.3	11.6	12.1
Fe2O3 (%)	2.5	2.3	2.4	2.4	2.2	2.8
Na2O (%)	0.2	2.5	0.1	0.1	0.1	0.1
K2O (%)	0.3	0.2	2.9	0.2	0.2	0.2
MgO (%)	3.0	2.3	2.4	4.6	2.3	2.4
CaO (%)	1.1	0.3	0.3	0.3	2.9	0.3
Interlayer distance (nm)	1.59	1.51	1.24	1.60	1.60	1.26

XRD patterns and interlayer distances of ion-exchanged MMTs are shown in Figure 5 and Table 1. The XRD patterns and the interlayer distances differed according to the interlayer ions present. The interlayer distances in Na-MMT, K-MMT and NH₄-MMT were 1.51 nm, 1.24 nm and 1.26 nm, respectively. In Mg-MMT and Ca-MMT, the interlayer distances were 1.60 nm and 1.60 nm, respectively, which were similar to the interlayer distance of raw MMT, which was 1.59 nm. It has been reported that the interlayer distance changed according to the type of interlayer ions and hydration state (Yamanaka et al., 1975; Yoshida, 1979). Distances for the hydration state of anhydrous, one layer and two layers of water molecules, when the interlayer ion was monovalent, such as Na, were 1.00 nm, 1.24 nm and 1.56 nm, respectively (Sato et al., 1992), and when the interlayer ion was divalent, such as Ca, the value were 1.03 nm, 1.17 nm and 1.50 nm, respectively (Bray et al., 1998). It was considered that the interlayers contain one layer of water molecules in K-MMT and NH₄-MMT and two layers of water molecules in Na-MMT, Mg-MMT and Ca-MMT. These results suggest that the structural properties, including interlayer distance in ion-exchanged MMT, are changed by replacement of the interlayer Ca ion in MMT with other cations.

Fig. 5.

XRD patterns of ion-exchanged MMT. (a) raw MMT; (b) Na-MMT; (c) K-MMT; (d) Mg-MMT; (e) Ca-MMT; (f) NH₄-MMT.

Effects of the interlayer ions in MMT on turbidity in decaffeinated tea    The appearance of the decaffeinated green tea beverages after ion-exchanged MMT treatment was observed after the precipitation test (agitation for 2 h at 4°C). Turbidity was significantly generated in green tea decaffeinated by raw MMT and Ca-MMT (Figure 6a). In addition, oxalate ions were decreased by 53.9% and 82.4%, and Ca ions were increased by about 7 times and 28 times in decaffeinated green tea treated with raw MMT and Ca-MMT, respectively (Figure 6b). In contrast, no turbidity and no decreases in oxalate ions were observed in green tea decaffeinated by Na-MMT, K-MMT, Mg-MMT and NH₄-MMT (Figure 6). These results indicate that precipitation of calcium oxalate in decaffeinated green tea during and after decaffeination by MMT is suppressed by the replacement of interlayer Ca ions with Na, K, Mg or NH₄ ions.

Fig. 6.

Effects of interlayer ion in ion-exchanged MMT on appearance and concentrations of Ca ion and oxalate ion in decaffeinated green tea after agitation for 2h at 4°C. (a) Turbidity (OD 660 nm); (b) concentrations of Ca ion and oxalate ion. The bars indicate standard deviation (n=5).

Caffeine adsorption from green tea to ion-exchanged MMT is shown in Figure 7. The amounts of adsorbed caffeine were 0.10–0.14 mmol/g in the ion-exchanged MMTs, and 0.13 mmol/g in raw MMT: K-MMT, Mg-MMT, Ca-MMT and NH₄-MMT retained more than 90% of caffeine adsorptivity of the raw MMT. We suggest that ion-exchange with K, Mg, Ca and NH₄ does not affect the caffeine adsorptivity of MMT. The amount of adsorbed caffeine in Na-MMT was 75.4% of the raw MMT. We previously reported that water molecules are exchanged with caffeine in the MMT interlayer (Okada et al., 2015); we suggest that caffeine adsorption in Na-MMT was partially suppressed by swelling and water molecules held in the interlayers.

Fig. 7.

Effects of interlayer ion in ion-exchanged MMT on caffeine adsorption in green tea. The bars indicate standard deviation (n=5).

Our findings suggest that the turbidity generated in a green tea beverage decaffeinated by MMT is suppressed by replacement of the interlayer Ca ions in MMT with Na, K, Mg or NH₄ ions. Furthermore, the caffeine adsorptivity of MMT is retained, even after ion-exchange with K, Mg or NH₄ ions, indicating that adsorbing caffeine and retaining the appearance quality of green tea beverages can be achieved by the use of ion-exchanged MMTs.

Conclusions

The appearance of green tea beverages, including decaffeinated green tea beverage, is an important factor in its quality. MMT treatment caused turbidity during and after decaffeination, which may have been due to precipitation of calcium oxalate generated by a reaction between Ca ions eluted from the interlayer of MMT and oxalate ions in the green tea. It was suggested that the precipitation of calcium oxalate was suppressed by the use of MMT ion-exchanged with Na, K, Mg or NH₄ ions, which maintained the appearance quality of the decaffeinated green tea beverage. The caffeine adsorptivities of K-MMT, Mg-MMT and NH₄-MMT were equivalent to that of raw MMT. Therefore, the use of ion-exchanged MMTs maintained caffeine adsorption and the appearance quality of a green tea beverage.

Acknowledgements    We thank all the members of our research group for valuable discussions, especially Hideyuki Wakabayashi, Josuke Yamamoto, Nanami Imada, Akiko Kinoshita, Ai Tokura, and Yurika Yajima.

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