A Conceptual Demonstration of Decaffeination via Nanofiltration

Article pubs.acs.org/IECR

A Conceptual Demonstration of Decaffeination via Nanofiltration Yee Kang Ong, Hui Ting Ng, and Tai-Shung Chung*

Downloaded by UNIV OF SUSSEX on September 5, 2015 | http://pubs.acs.org Publication Date (Web): August 3, 2015 | doi: 10.1021/acs.iecr.5b01737

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore ABSTRACT: This study aims to evaluate the feasibility of using novel nanofiltration (NF) membranes for decaffeination of coffee wherein caffeine present in the feed solution is recovered at the permeate of the membrane. The NF hollow fiber membranes were fabricated and tested with model solutions to evaluate the recovery of caffeine as functions of molecular weight/ size of organic molecules, caffeine concentration, and feed temperature. The results show that the recovery of caffeine and the permeate flux decrease with an increase in molecular weight/size of the organic molecules and the caffeine concentration in the feed. On the other hand, both recovery and flux increase with increasing feed temperature. Decaffeination of coffee is subsequently demonstrated at 55 °C using a roasted coffee solution wherein a 25% reduction of caffeine is achieved with a volume reduction factor of ∼1.2. differences in molecular weight/size.15 The application of NF in the beverage industry has been reported by various researchers.16−22 The sugar content of grape musts prior to fermentation can be reduced by NF membranes during the production of reduced-alcohol or dealcoholized wines.17,21 On the other hand, Catarino and Mendes removed alcohol from conventional wine and concluded that NF membranes are able to produce a better quality of reduced-alcohol wine as compared with reverse osmosis (RO) membranes.18 Conidi et al. applied the NF process for recovery of phenolic compounds from orange press liquors.19 In the processing of coffee, NF has been applied to concentrate the coffee extract as well as to recover bioactive extracts from spent coffee.20,22 This study attempts to propose a new concept of decaffeination process using NF. The NF hollow fiber membrane was first fabricated and subjected to various experiments to evaluate the recovery of the caffeine as functions of (1) molecular weights of organics in the feed solution, (2) caffeine concentration in the feed solution, as well as (3) feed solution temperature. The novel concept of decaffeination process was then demonstrated using a roasted coffee solution as the NF feed solution. The outcome of this study may provide valuable insights in the exploration of an efficient and environmental friendly route in producing decaffeinated coffee.

1. INTRODUCTION Caffeine is a methylxanthine that can be commonly found in coffee and other beverages such as tea, soft drinks, and energy drinks, making it one of the world’s most widely consumed psychoactive substances.1 The United States Food and Drug Administration (U.S. FDA) has classified caffeine under the category of both drug and food additive.2 Furthermore, it is the component that is responsible for its mild stimulatory effect on the central nervous system. Studies indicate that the overdose of caffeine can lead to symptoms like anxiety, hyperventilation, vomiting, increased urinary output, etc.3,4 In general, the U.S. FDA sets the daily recommended caffeine limits for healthy adults at 400 mg day−1;5 at times of anxiety, stress, or pregnancy, the recommended maximum daily caffeine intake could be even lower. As the safe limit for individuals varies depending on how people react to the stimulant, decaffeinated beverages are therefore available for consumers who want to enjoy the taste and aroma of beverages without consuming a high amount of caffeine. Currently, decaffeinated coffee accounts for approximately 10−15% of the global coffee consumption.6 The conventional decaffeination process in the food and beverage industry uses organic solvents such as methylene chloride and ethyl acetate for the extraction of caffeine.7 The solvent decaffeination method was initially preferred before water and supercritical carbon dioxide (CO2) decaffeination methods were invented. These decaffeination methods differ from each other mainly in the process and medium used for extraction.8 The water extraction route requires a process time of ∼10 h to produce caffeine-free coffee beans, whereas extreme operating conditions are needed to produce fluid-state CO2 for the supercritical CO2 decaffeination. Nanofiltration (NF) is a pressure-driven membrane-based separation process that has been widely applied in various applications ranging from water treatment, pharmaceutical, and food and beverages industries.9−14 In general, the NF membranes possess a molecular-weight cutoff (MWCO) of around 200−1000 Da that can be very effective in separating organic molecules and pharmaceutical products based on the © 2015 American Chemical Society

2. CONCEPT The proposed membrane-based decaffeination process using a NF membrane is shown in Figure 1. As caffeine has a molecular weight of around 194 g mol−1, it will preferentially permeate through the NF membrane during the filtration process. The simulated or real coffee solution was circulated at the feed side of the membrane and a caffeine-rich solution was collected at the permeate side of the membrane. The retentate of the Received: Revised: Accepted: Published: 7737

May 10, 2015 July 18, 2015 July 28, 2015 July 28, 2015 DOI: 10.1021/acs.iecr.5b01737 Ind. Eng. Chem. Res. 2015, 54, 7737−7742

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Industrial & Engineering Chemistry Research

Table 1. Spinning Parameters for the NF Hollow Fiber Membrane


Figure 1. Schematic of the decaffeination process using NF.

membrane should consist of the decaffeinated coffee, whereas the permeate would be enriched with caffeine. In commercial practices, the decaffeinated coffee concentrate from the retentate could be either charged into a spray dryer/ freeze-dryer to produce decaffeinated coffee powder or diluted to produce bottled/boxed/canned decaffeinated coffee. On the other hand, the caffeine-rich permeate can be further purified for the manufacture of food and beverages as well as pharmaceutical products.

spinning parameters

conditions

outer-layer dope composition (wt %) inner-layer dope composition (wt %) bore fluid composition (wt %) outer layer dope flow rate (mL/min) inner layer dope flow rate (mL/min) bore fluid flow rate (mL/min) air gap (cm) take up speed (m/min) external coagulant

PAN/NMP (13/87) P84/NMP (25/75) NMP/water (95/5) 0.2 1.5 1.0 1.5 8 water

3.3.2. Pore Size Distribution, Mean Pore Size, and Molecular Weight Cutoff (MWCO). The pore size distribution, mean pore size, and molecular weight cutoff (MWCO) of the membrane were characterized by solute rejection experiments using neutral solutes such as triethylene glycol, glucose, sucrose, and raffinose based on the method described in the literature.10,15 The solute rejection (Rs) can be correlated with the Stokes radius (rs) and molecular weight of the respective neutral solute. By neglecting the influences of steric and hydrodynamic interactions between the neutral solutes and the membrane material, the effective pore radius of the membrane was assumed to be equal with the geometric mean radius of the neutral solute. Therefore, the mean pore radius (rp) of the membrane can be estimated from the aforementioned correlation with the geometric mean radius of the solute (μs) at Rs = 50%. The geometric standard deviation of the membrane (σp) was calculated from the ratio of geometric mean radius of the solute at Rs = 50% and Rs = 84.13%. 3.4. Filtration Experiments. Filtration experiments were conducted in a self-fabricated NF setup with an effective membrane area of ∼30 cm2 for each hollow fiber membrane module. A shell-feed cross-flow mode was applied in this study whereby the feed solution was circulated at the shell side of the hollow fiber membrane at a trans-membrane pressure (TMP) of 1 bar. The permeate solution was collected from the lumen side of the membrane. The NF hollow fiber modules were conditioned with DI water for at least 2 h prior to performance evaluation and sample collection. The solution flux permeating through the membrane (J) can be determined from the total volume of the permeate (Q) collected at a specific period (t) over the effective membrane area (A) applied in the filtration process:

3. EXPERIMENTAL SECTION 3.1. Materials. The dual-layer NF hollow fiber membrane was fabricated using polyacrylonitrile (PAN) and P84 copolyimide consisting of 3,3′4,4′-benzophenone tetracarboxylicdianhydride and 80% methylphenylenediamine + 20% methylenediamine as the outer and inner layer materials. PAN was provided by the R&D Center for Membrane Technology, Chung Yuan Christian University, Taiwan, while P84 was obtained from HP Polymer GmbH. N-Methyl-2pyrrolidone (NMP) from Merck was used as the solvent to prepare polymer solutions during the hollow fiber spinning. Caffeine, sucrose, raffinose, and stachyose were purchased from Sigma-Aldrich. They were employed as model solutes to evaluate membrane performance in simulated decaffeination processes. Deionized water was used to prepare the feed mixture for filtration studies. Ethyl acetate from Merck was employed as the solvent to extract caffeine from coffee solutions. All the chemicals were used as received. 3.2. Hollow Fiber Spinning. All the polymers were vacuum-dried overnight prior to dope preparation. The dope solutions were then poured into syringe pumps and degassed prior to hollow fiber spinning. The dual-layer hollow fibers were fabricated via dry-jet wet-spinning by simultaneously coextruding both outer-layer and inner-layer polymer dopes as well as bore fluid through a triple-orifice spinneret as reported in previous study,15 and the spinning parameters were tabulated in Table 1. The as-spun fibers were immersed in a water bath for 2 days to remove the residual solvent from the as-spun fibers. All the fibers were then submerged in a glycerol/water (50/50 wt %) solution for 2 days and air-dried at ambient conditions prior to module fabrication. 3.3. Characterizations. 3.3.1. Scanning Electron Microscopy (SEM). The SEM samples were prepared by fracturing the NF hollow fibers in liquid nitrogen and coated with platinum using a sputtering coater (JEOL LFC-1300). The morphology of the hollow fiber membrane was then evaluated using a field emission scanning electron microscope (FESEM JEOL JSM6700LV).

J=

Q A×t

(1)

3.5. Chemical Analyses. 3.5.1. Model Solutions. The concentrations of caffeine, sucrose, raffinose, and stachyose in the feed and permeate were measured by using a total organic carbon analyzer (TOC ASI-5000A, Shimazu) and a UV/vis spectrophotometer (Pharo 300, Merck). Because sucrose, raffinose, and stachyose were undetected via UV/vis spectrophotometer, the caffeine concentration can be solely obtained at a UV wavelength of 273 nm. Thereafter, the concentration of the saccharides (i.e., sucrose, raffinose, and stachyose) can be estimated by subtracting the total carbon concentration in the sample obtained via TOC from the carbon concentration in the caffeine determined via UV/vis spectrophotometer as follows: 7738

DOI: 10.1021/acs.iecr.5b01737 Ind. Eng. Chem. Res. 2015, 54, 7737−7742

Article

Industrial & Engineering Chemistry Research concentration of carbon (saccharide) = concentration of carbon(sample; obtained from TOC) − concentration of carbon(caffeine; obtained from UV/vis spectrophotometer)

(2)

The rejection (Rj) of each saccharide was calculated as ⎛ Cp ⎞ R j = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(3)

As caffeine was the targeted permeating component to be enriched at the permeate of the membrane, a low rejection of caffeine is highly desirable in this study. On the other hand, the rejection of other components should be as high as possible. The recovery (Rc) of caffeine was then calculated as


⎛ Cp ⎞ R c = ⎜ ⎟ × 100% ⎝ Cf ⎠

Figure 2. Morphology of the as-spun NF hollow fiber membrane.

(4)

where Cf and Cp are the solute concentrations in the feed and the permeate solutions, respectively. 3.5.2. Coffee Solution. Roasted coffee powder of 50 g was inserted into an envelope made of nonwoven fabric and stirred with 1 L of distilled water at 70 °C for 1 h to prepare the coffee solution for the decaffeination study using NF hollow fiber membranes. Because of the complexity of compounds present in the coffee solution that will affect the absorbance of UV/vis analyses, solvent extraction was performed using ethyl acetate to extract the caffeine from the coffee solution to analyze and quantify the caffeine content.23 An aliquot of 10 mL was first taken from each sample and mixed with 5 mL of ethyl acetate. The mixture was agitated and left stagnant until the formation of binary layers. The extract consisting of caffeine-rich ethyl acetate was then removed while fresh ethyl acetate was introduced into the raffinate to continue the extraction process. Three cycles of extraction were conducted for each sample, and the caffeine-rich extracts were gathered and analyzed using the UV/vis spectrophotometer (wavelength: 273 nm) to estimate the concentration.

Table 2. Pore Properties and Pure Water Permeability of the NF Hollow Fiber Membranea membrane ID NF hollow fiber

σp

rp (nm)

MWCO (Da)

pure water permeability (L m−2 h−1 bar−1)

MgSO4 rejection (%)b

1.22

0.45

∼470

3.2 ± 0.5

∼96

a

Trans-membrane pressure = 1 bar. bMgSO4 feed concentration = 1000 ppm

polysaccharide and caffeine in the simulated feed solution were maintained at a ratio of around 4:1 (polysaccharide to caffeine) throughout the experiments at ambient conditions. Figure 4 portrays the separation performance of the NF membrane as a function of molecular weight of the polysaccharide used in simulated feed solutions containing a binary mixture of the polysaccharide and caffeine. The rejection of polysaccharide increases with an increase in its molecular size. Because polysaccharides comprise neutral molecules, the separation mechanism in this study is mainly governed by the size exclusion mechanism so that the rejection trend follows the order of sucrose (MW: 342 g mol−1) < raffinose (MW: 504 g mol−1) < stachyose (MW: 667 g mol−1). Conversely, the recovery of caffeine shows a reverse trend. The simulated feed solution containing the smallest polysaccharide (sucrose) and caffeine demonstrates the highest caffeine recovery, whereas the lowest caffeine recovery is obtained from the feed solution containing stachyose and caffeine. This is due to the fact that the organic molecules may preferentially foul the membrane during the pressurized filtration and lower caffeine permeation across the membrane, which resulted in a reduction of caffeine concentration at the permeate. As a consequence, the flux decreases with an increase in the molecular weight of the polysaccharide. A similar phenomenon has been observed by Kolfschoten et al.25 4.2.2. The Effect of Caffeine Concentration. The coffee beverages may contain various concentrations of caffeine. Therefore, the effect of caffeine concentration in the feed solution on the recovery of caffeine during decaffeination via NF was investigated using sucrose as the representative organic molecule throughout the subsequent experiments. The ratio of sucrose to caffeine in the feed solution was varied by increasing

4. RESULTS AND DISCUSSION 4.1. Characterization of the NF Hollow Fiber Membrane. Figure 2 depicts the morphology of the as-spun NF hollow fiber membrane. The NF hollow fiber membrane possesses an outer layer which is full of macrovoids and an inner layer consisting of a sponge-like structure supported by a macrovoid supporting layer. These macrovoid structures may decrease the overall mass transport resistance during the filtration process while the structural integrity of the NF hollow fiber membrane is maintained by the sponge-like layer. The pure water permeability (PWP) and pore properties of the NF hollow fiber membrane are tabulated in Table 2. The NF hollow fiber membrane has an average pure water permeability of 3.2 ± 0.5 L m−2 h−1 bar−1 with a MWCO of ∼470 Da. 4.2. Performance of the NF Hollow Fiber Membrane in Decaffeination. 4.2.1. The Effect of Organic Molecular Weight. Because carbohydrates are the major chemical components in the coffee (shown in Table 3),24 polysaccharides consisting of sucrose, raffinose, and stachyose (structures shown in Figure 3) were used as representatives to probe the size effects of organic molecules on the recovery of caffeine from the simulated coffee solutions. The concentrations of 7739


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Industrial & Engineering Chemistry Research Table 3. Chemical Components of the Coffee24 caffeine carbohydrates lipids amino acids organic acids melanoidins ash (minerals) others


a

Arabica roasted (%DWa)

Robusta roasted (%DWa)

Arabica instant (%DWa)

Robusta instant (%DWa)

1.2 38 17 7.5 2.4 25.4 4.5 4.0

2.4 42 11 7.5 2.6 25.9 4.7 3.9

2.5 46.6 0.1 6.2 8.1 25.1 8.0 3.4

3.8 44.7 0.3 6.0 7.9 28.6 7.4 1.3

DW: dry weight.

Figure 3. Structures of the polysaccharides.

Figure 4. Performance of the NF membrane as a function of the molecular weight of the polysaccharide.

Figure 5. Performance of the NF membrane as a function of caffeine concentration.

the spiramycin removal from waste water where an increase in spiramycin feed concentration led to an enhanced rejection.27 4.2.3. The Effect of Feed Solution Temperature. The effect of feed temperature on the recovery of caffeine was further evaluated at 25 (ambient conditions), 40, and 55 °C using the feed solution containing a sucrose-to-caffeine ratio of 4:2. Figure 6 indicates that the recovery of caffeine increases with elevated feed temperatures. Conversely, the rejection of sucrose shows a minor reduction with an increment in feed temperature. These observations are in agreement with previous studies and can be caused by various factors.14,28−30 The free volume of the membrane would increase at elevated feed temperatures due to the increase in polymer chain mobility. Hence, the steric effect of the membrane would be reduced, which enhances the probability of molecules passing through the membrane. Doederer et al. also observed that the increment of feed temperature has greater impact to the

the concentration of caffeine while maintaining the concentration of sucrose as follows: approximately 4:1, 4:1.3, and 4:2 (sucrose:caffeine). All the experiments were conducted at ambient conditions. Figure 5 displays the performance of the NF membrane as a function of caffeine concentration in the feed mixture. The rejection of sucrose appears to be independent of caffeine concentration in the feed mixture. However, both the flux and caffeine recovery drop slightly for the feed mixture containing a high caffeine concentration (i.e., sucrose to caffeine ratio of 4:2). These slight declines are probably caused by the increment in membrane fouling at a high concentration of caffeine because caffeine molecules may adsorb or accumulate in the membrane. As a result, the mean effective pore size is reduced; not only can it increase the sieving effect of the NF membrane26 but also reduce the amount of caffeine passing through the membrane. Zhao et al. observed a similar trend for 7740


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volume reduction factor (i.e., the ratio of the initial feed volume to the final volume of the retentate) of ∼1.2, signifying the prospect of applying the newly developed NF membrane for the decaffeination process.

5. CONCLUSIONS The feasibility of decaffeination using a NF hollow fiber membrane was evaluated under various experimental conditions followed by applying the roasted coffee solution as the feed solution. The following conclusions can be drawn from this study: • The existence of neutral large molecules in the feed solution have negative effects on membrane flux and caffeine recovery. • The increment of caffeine concentration in feed mixtures marginally reduces the caffeine recovery. • An increase in feed temperature results an increment in flux and recovery of caffeine. • The newly developed NF hollow fiber membrane shows high potential to fractionate caffeine from coffee solutions and produces decaffeinated coffee. Is it worth mentioning that this research only focuses on demonstrating the concept of a novel decaffeination process via NF and detailed studies are needed to evaluate the potential loss of other components such as furans, ketones, pyrazines, carboxylic acids, etc.32 during the filtration process.


Figure 6. Performance of the NF membrane as a function of feed temperature.

substances with smaller molecular volumes as the increment of free volume and polymer chain mobility can facilitate their permeation across the membrane.29 Therefore, caffeine with a smaller molecular weight and volume would preferentially permeate through the NF membrane and increases the recovery of caffeine at elevated feed temperatures. Meanwhile, the reduction of water viscosity and the increment of mass transfer coefficient may also contribute to the flux enhancement at higher feed temperatures. 4.3. Performance of the NF Hollow Fiber Membrane in Decaffeination Using Roasted Coffee Solution. A roasted coffee solution of 300 mL containing approximately 60 mg of caffeine was applied in this experiment. The temperature of the coffee solution was maintained at 55 °C throughout the experiment. Figure 7 shows that the permeate is colorless as

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AUTHOR INFORMATION

Corresponding Author

*Phone: +65 6516 6645. Fax: +65 6779 1936. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded under the project entitled “Membrane development for osmotic power generation, Part 1, Materials development and membrane fabrication” (1102IRIS-11-01) and NUS grant no. of R-279-000-381-279. The aforementioned project is supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Programme and administered by the Environment & Water Industry Programme Office (EWI) of the PUB. Special appreciations are given to Prof. JuinYih Lai and Prof. Hui-An Tsai at R&D Center for Membrane Technology, Chung Yuan Christian University of Taiwan, for the provision of the PAN polymer.

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Figure 7. Performance of the NF membrane in decaffeination of the roasted coffee solution.

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A Conceptual Demonstration of Decaffeination via Nanofiltration

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