Leaf protein concentration of alfalfa juice by membrane technology

Leaf protein concentration of alfalfa juice by membrane technology

Journal of Membrane Science 489 (2015) 183–193 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

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Journal of Membrane Science 489 (2015) 183–193

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Leaf protein concentration of alfalfa juice by membrane technology Wenxiang Zhang a, Nabil Grimi a, Michel Y. Jaffrin b, Luhui Ding a,n a b

EA 4297 TIMR, University of Technology of Compiegne, 60205 Compiegne Cedex, France UMR 7338, Technological University of Compiegne, 60205 Compiegne Cedex, France

art ic l e i nf o

a b s t r a c t

Article history: Received 28 February 2015 Received in revised form 26 March 2015 Accepted 27 March 2015 Available online 20 April 2015

Membrane technology (microfiltration (MF) and ultrafiltration (UF)) of alfalfa juice was studied as an alternative method to conventional leaf protein concentration. Three types of filtration modules (dead end filtration using laboratory Amicon cell (DA), dynamic cross filtration using rotating disk module (CRDM) and dead end filtration using rotating disk module (DRDM)) were used to investigate concentration efficiency of MF and UF with full recycling tests and concentration tests. Rotating speed and transmembrane pressure (TMP) improved flux behavior, but higher permeate flux caused by higher rotating speed reduced leaf protein rejection of MF. The strong rotating shear effect and open flow channel structure of CRDM could control concentration polarization and membrane fouling, therefore, it gave the best flux behavior, least flux decline, best clarification effect, smallest irreversible fouling and highest permeability recovery in membrane cleaning. However, the best leaf protein rejection was obtained by DRDM, because of high shear rate and “secondary filtration” of fouling layer created by closed flow channel structure. Besides, CRDM showed the highest productivity and best potential for industrial application. These results from laboratory-scale tests can be very useful for concentrating leaf protein from alfalfa juice and serve as a valuable guide for process design in industrial scale. & 2015 Elsevier B.V. All rights reserved.

Keywords: Alfalfa juice Leaf protein concentration Dead-end filtration Dynamic cross-flow filtration Flux behavior

1. Introduction As a common perennial vegetable, alfalfa has been widely cultivated as a forage crop in Europe and North America and represents about 32 million hectares in the worldwide [1]. After drying green crop, alfalfa is utilized as raw material for the production of fodder pellets for cattle, due to its high feed value and high crude proteins content (about 2600 kg/ha) [2]. During the pellet production process, the green crop of alfalfa is chopped and pressed before drying, while much alfalfa juice is generated. This green extracted juice containing high proteins content has been recognized as an effective source of high quality proteins for animals and human consumption, because of abundant sources, high nutritive value and absence of animal cholesterol [3]. Alfalfa leaf protein includes about 50% hydrophilic proteins and 50% lipophilic proteins [2]. Hydrophilic proteins have high digestibility and a balanced aminogram, which possesses significant functional properties, such as emulsifying, jellifying and foaming agents [4–6]. In order to recover protein in alfalfa juice, numerous protein separations and concentration technologies have been used to concentrate and produce leaf proteins for food industry of alfalfa

n

Corresponding author. Tel.: þ 33 3 4423 4634; fax: þ 33 3 4423 7942. E-mail address: [email protected] (L. Ding).

http://dx.doi.org/10.1016/j.memsci.2015.03.092 0376-7388/& 2015 Elsevier B.V. All rights reserved.

juice. According to different separation mechanisms, they can be divided into three categories: (1) difference of solubility [7–9]: salting, organic solvent fractionation, chromatography, crystallization, heating and centrifugation; (2) differences of molecular size and shape [2,10]: molecular sieve chromatography and membrane; (3) difference of charge: ion exchange [11]. However, most conventional separation and concentration methods have various intrinsic disadvantages, such as high energy cost, low separation efficiency, damage of nutritive proteins, complex operation and high investment, which limit their industrial applications. As a promising separation method, membrane technology has wide applications in food industry and water treatment, including manufacturing of vegetal extracts and juices, meat and fish products, sugar, alcoholic and non-alcoholic beverages, dairy effluent treatment and dairy products [7]. A few previous studies [7,10,12,13] used UF to separate and concentrate crude protein from waste leaf extraction juice. However, during the concentration process, serious flux decline caused by membrane fouling and concentration polarization occurred, increasing the operation cost and restricting its sustainable operation and industrial application. For the purpose of controlling flux decline, various strategies have been utilized to eliminate membrane fouling and decrease concentration polarization, such as the modification of feed characteristic and membrane surface [14–16], optimization of operation [17], choice of filtration modules [18] and membrane cleaning [19].

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At the same time, different strategies may also affect separation efficiency, production quality and membrane cleaning and reuse [17,20]. Therefore, a suitable flux decline control strategy cannot only sustain the flux behavior of whole filtration process at a high level, but also improve separation efficiency and permeability recovery in subsequent membrane cleaning. The configuration of membrane modules (i.e., dead-end, crossflow and shear-enhanced) strongly influences the flux behavior and fouling evolution [21]. The shear rates of dead-end and crossflow modules are produced, respectively, by a stirring effect and an increase the tangential fluid velocity along the membrane. Shearenhanced modules can create a high shear rate on the membrane surface by a moving part such as a rotating membrane, or a disk rotating near a fixed circular membrane or by vibrating the membrane either longitudinally or torsionally around a perpendicular axis, inducing dispersion of solutes on the membrane surface and elimination of membrane fouling [22]. Therefore, it outweighs the conventional cross-flow membrane filtration process in excellent effluent quality, stable permeate flux, low concentration polarization and high retention and has been successfully implemented in many fields of research and engineering such as wastewater treatment [18], medical engineering [22] and food engineering [23] as well as biotechnological separations [24]. The objective of the present work is to investigate the application of membrane technology (MF and UF) to separate and concentrate leaf protein from alfalfa juice. In order to control flux decline, three types of filtration modules (DA, DRDM and CRDM) were chosen and compared. The focus of this work is to (1) discuss the effect of TMP and rotating speed on flux behavior and

Table 1 Main characteristics of alfalfa juice. Index

Alfalfa juice

Crude protein (g L  1) Chlorophyll a (mg L  1) Chlorophyll b (mg L  1) Dry matter (g L  1) Ash (g L  1) Turbidity (NTU) Conductivity (ms cm  1) pH Soluble matter (1Brix) Density, ρ (g ml  1) Protein purity (%)

21 12.38 20.82 86 21 600 9.29 5.8 8.1 1.20 24.4

separation efficiency in full recycling tests; (2) investigate concentration efficiency, flux behavior and membrane cleaning in concentration tests. The experiments should be useful for evaluating the membrane performance of concentrating leaf protein from alfalfa juice and understanding the process efficiency, separation performance, flux behavior, and membrane fouling in various filtration modules. 2. Materials and methods 2.1. Test fluid Alfalfa juice provided by Luzéal, Pauvres, France, was prefiltered by a mesh of 0.4 mm pore size and mixed, then stored at the temperature of  20 1C until further use. In order to prevent serious membrane fouling, before experiment the juice was centrifuged at 4000 rpm for 10 min using a Sigma 3-16P device for separating the insoluble materials. The main characteristics of alfalfa juice are shown in Table 1. 2.2. Filtration modules 2.2.1. Dead end filtration using Amicon cell (DA) The dead-end filtration Amicon 8200 cell (Millipore, Billaica, USA) was used for alfalfa juice filtration. As shown in Fig. 1, the internal diameter of the cell is 6.35 cm and maximum volume is 180 mL. The membrane was located at the bottom of the cell. The effective membrane area is 3.17  10  3 m2. A constant pressure was provided by filling the cell with nitrogen gas and maximal pressure could reach 6 bar, while permeate was collected in a tube placed on an electronic scale in order to calculate the permeate flux. 2.2.2. Dynamic cross filtration using rotating disk module (CRDM) A rotating disk module (RDM), shown in Fig. 2, was used for alfalfa juice filtration. A flat membrane, with an effective area of 176 cm2 (outer radius R1 ¼7.72 cm, and inner radius R2 ¼1.88 cm), was fixed on the cover of the cylindrical housing in front of the disk. The disk equipped with 6 mm-high vanes, which can generate very high shear rates on the membrane, at rotation speeds up to 2500 rpm. The module was fed from a thermostatic and stirred tank containing 12 L of fluid by a volumetric diaphragm pump (Hydra-cell, Wanner, USA). The peripheral pressure (Pc) was adjusted by a valve on outlet tubing and monitored at the top of the cylindrical housing by a pressure sensor (DP 15–40, Validyne,

Fig. 1. (a) Photo of dead end filtration cell and (b) schematic representation of DA.

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Fig. 2. Schematic diagrams of (a) RDM and (b) CRDM experiment set-up.

USA), and the data was collected automatically by computer. Permeate was collected in a beaker placed on an electronic scale (B3100P, Sartorius, Germany) connected to a computer in order to measure the permeate flux.

2.2.3. Dead end filtration using rotating disk module (DRDM) The RDM, described in Section 2.2.2, was fed from a 1 L reservoir connected to compressed air (with a maximum pressure of 6 bar). As shown in Fig. 3, permeate was collected in a beaker placed on an electronic scale (B3100P, Sartorius, Germany) connected to a computer in order to measure the permeate flux.

2.3. UF and MF membranes UF and MF membranes fabricated by MICRODYN-NADIR GmbH were tested in the present study. According to manufacturer's information, their properties are summarized in Table 2.

2.4. Experimental procedure A new membrane was used for each series of experiments to ensure the same initial membrane conditions for the entire study. The membranes were soaked in deionized water for at least 24 h before use, and pre-pressured with deionized water for 1 h under a pressure of 2 bar. After stabilization, the pure water flux of membranes was measured to calculate water permeability (Lp). Before the experiments started, the feed was heated to 35 1C, and was fully recycled in the system at zero TMP, and this process lasted about 10 min for each test. Then experiments were performed in two modes: full recycling tests and concentration tests. Series 1: full recycling tests: in order to estimate the membrane performance rapidly in alfalfa juice filtration, these tests were performed with permeate and retentate recycling to limit the change of feed concentration to less than 10%. Feed volumes were 100 ml, 0.6 L and 3 L for DA, DRDM and CRDM, respectively. A prefiltration was carried out for 10 min at lowest tested TMP and a rotating speed of 200 rpm, to ensure membranes stabilization.

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Fig. 3. Scheme of DRDM experiment set-up.

2.6. Calculated parameters

Table 2 Properties of membranes tested. Membrane

Manufacturer Surface material

P020F (UF 20 K) Nadir MV020T (MF 0.2 μm) Nadir

PH-PES PVDF

Water permeability (L m  2 h  1 bar  1) 43 710

Afterward, the TMP was increased in steps at 500 rpm. Then for DRDM and CRDM, TMP was fixed, while rotating speed was decreased in steps from 2500 rpm to 500 rpm. The filtration was periodically stopped by suddenly releasing the pressure (0.2 bar) in order to mimic back flushing and minimize the fouling accumulation from the last TMP step. Samples were collected in permeate 5 min after the beginning of each TMP increment or each rotating speed decrement in order to obtain stabilized flux and transmission conditions. Series 2: concentration tests: in this series, permeate was not returned to feed tank. Feed volumes of 180 ml, 1.2 L and 6 L were concentrated to 30 ml, 0.2 L and 1 L for DA, DRDM and CRDM at constant conditions, respectively. All volume reduction ratios (VRR) were 6. Samples were collected in permeate at every half unit of VRR. After each series of tests, at a rotating speed of 500 rpm, the filtration system was flushed by deionized water for 10 min at 1000 rpm. Then alkaline cleaning was carried out by using a P3ultrasil 10 (Ecolab, cleaning USA) detergent to remove protein and colloid residues, at 0.25% concentration and 1000 rpm, and Lp was measured to determine the permeability recovery.

The permeate flux (J) is calculated as follows: J¼

1 dV A dt

ð1Þ

where A is the effective membrane area (m2), V is the total volume of permeate (m3), and t is the filtration time (h). Volume reduction ratio (VRR) is defined as follows: VVR ¼

V0 VR

ð2Þ

where V0 and VR are initial feed volume and retentate volume, respectively. The mean TMP is obtained by integrating the local pressure pc (Pa) over the membrane area as follow: 1 TMP ¼ pc  ρκ2 ωR2 4

ð3Þ

where ρ is the density of the fluid (g L  1), κ is the velocity factor (0.89) for this RDM system and R is the housing inner diameter (m). Protein purity (PP, %), which can be used to estimate clarification effect, is calculated as follows: PP ¼

Cp Cd

ð4Þ

where Cp (g/L) and Cd (g/L) are, respectively, crude protein and dry matter concentrations. Productivity (Pr, L h  1 m  2 bar  1) is defined as follows: Pr ¼

Vc T o  A  TMP

ð5Þ

where Vc is concentrated volume (L) and To is operation time (h). 2.5. Analytical methods Turbidities of permeate were measured with a Ratio Turbidimeter (Hach, USA). Conductivity was measured with a MultiRange Conductivity Meter (HI 9033, Hanna, Italy) and pH was measured with a pH Meter (MP 125, Mettler Toledo, Switzerland). Dry matter was determined by measuring the weight loss after drying samples at 10572 1C for 5 h in an oven. Soluble matter measurements (1Brix) were done, at room temperature, by means of a digital refractometer PR-32α (ATAGO Co., LTD, Japan). The crude protein concentration in solution was determined using the Kjeldahl method. To convert organic nitrogen, except ammonium, the factor 6.25 was used [7]. Chlorophyll A and B were measured by a spectrophotometric method (Biochrom Ltd., Cambridge Science Park, Cambridge, Angleterre).

3. Results and discussion 3.1. Full recycling tests Full recycling tests were performed to investigate the effect of TMP and rotating speed on separation performance and flux behavior for different types of filtration modules. 3.1.1. Effect of TMP on separation performance and flux behavior Fig. 4 shows the effect of TMP on performance flux and separation performance (crude protein concentration in permeate and 1Brix of permeate) for DA, DRDM and CRDM at rotating speed of 500 rpm. As expected, higher TMP offered larger driving force

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Fig. 4. Flux behavior and separation performance at various TMP for MF and UF.

and led to higher permeate flux, however, when TMP exceeded 2 bar for MF and 3 bar for UF, the flux reached a plateau. This could be explained by the threshold flux regime [21]: below a certain value, fouling rate remained at a low and nearly constant level and flux increased linearly with increasing TMP. But above it, a fouling layer of higher thickness or density on membrane surface was formed, and fouling rate was high and flux dependent, so the growth rate of flux decreased. The flux increased in the following order: CRDM 4DRDM4 DA. Comparing with DA, because of the disk equipped with vanes, higher shear rate at the membrane surface for CRDM and DRDM could eliminate concentration polarization and prevent membrane fouling effectively, thus improving flux behavior greatly. Besides, the open flow channel

structure of CRDM enhanced the mobility of feed on membrane surface, which improved shear rate and decreased concentration polarization, so CRDM had the best flux behavior. Due to bigger pore size and higher permeability, MF gave a higher permeate flux than UF, despite its lower TMP. In a filtration process, the permeate flux and fouling layer are two main factors that affect separation efficiency [25,26]. When flux is low, its increase caused more solvent to pass through membrane and reduced solute transmission by “dilution effect” [27]. But at high fluxes, a further increase improved concentration polarization and solute concentration on membrane surface increased, then enhancing the concentration gradient and diffusive effect transfer through the membrane, thus solute transmission improved [27].

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The fouling layer dominated by solutes accumulation on membrane surface narrowed membrane pores, played a “secondary filtration” role and improved solute rejection [28]. As shown in Fig. 4(c)–(f), with the elevation of TMP, permeate flux increases, more solutes accumulate on membrane surface and the “diffusion effect” improves, so solute rejection reduces. Due to more serious concentration polarization, higher TMP enhanced fouling layer on membrane surface, which may reduce solute rejection. But this effect was less significant than permeate flux during a TMP-increasing process, because at rotating speed of 500 rpm, concentration polarization could be controlled at a low level. Furthermore, solute rejection was in the sequence DRDM4CRDM4DA. Comparing with DA, CRDM and DRDM had much higher Reynolds number Re

than DA at the same rotating speed due to their significantly different structures, bigger agitator diameter and higher shear rate [23], producing more intense hydrodynamics and greatly decreasing concentration polarization. Therefore, for CRDM and DRDM, the effect of permeate flux on separation efficiency was significantly reduced and their main factor was fouling layer, but for DA, the effect of permeate flux was dominant. Moreover, because of the open flow channel structure, CRDM had a thinner fouling layer, thus its “secondary filtration” effect was lower than for DRDM. As for DA, its shear rate created by the stirring effect on membrane surface was least, coupling with its closed flow channel structure [23], thus DA had a most serious concentration polarization and “diffusion effect”. In addition, due to larger membrane pores and higher

Fig. 5. Flux behavior and separation performance at various rotating speed for MF and UF.

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permeate flux, MF had a higher transmission for crude proteins and soluble organic matters and less separation performance than UF. 3.1.2. Effect of rotating speed on separation performance and flux behavior Fig. 5 shows the effect of rotating speed on permeate flux and separation performance for MF and UF of alfalfa juice in full recycling tests for CRDM and DRDM. The TMPs were set to 3 and 4 bar for MF and UF and permeate flux, crude protein concentration and 1Brix were measured at rotating speeds of 500, 1000, 1500, 2000 and 2500 rpm, respectively. It is obvious that increment of rotating speed improved flux for both MF and UF, due to lower concentration polarization and membrane fouling at higher shear rate. However, MF and UF had different growth tendencies: for MF, the flux of CRDM and DRDM increased linearly with rotating speed, implying that elevating rotating speed could decrease concentration polarization effectively. But for UF, when rotating speed reached 1500 rpm, the flux reached a steady-state and improving rotating speed could not elevate flux significantly, because the concentration polarization was already minimized for UF at a TMP of 4 bar. Permeate flux of MF was much higher than that of UF, resulting in the different flux behavior with variation of rotating speed, which was reasonable. According to the studies of Bacchin et al. [29] and Luo et al. [17], there were two anti-fouling mechanisms: Brownian diffusion (back diffusion of rejected particles into the bulk, due to concentration gradient) and shear-enhanced back transport. The shear-induced back diffusion is proportional to the shear rate and square of the particle size, while the Brownian diffusion coefficient is inversely proportional to particle size and independent of shear rate [30]. The elevation of rotating speed could improve shear-enhanced back transport, and also facilitate the recombination of some macroparticles, including leaf proteins and colloids, especially for UF with higher solute rejection [23], thus decreasing Brownian diffusion. The compromise of Brownian diffusion elimination and shear-enhanced back transport enhancement probably made the flux improvement less significant when rotating speed rose from 1500 to 2500 rpm. Furthermore, CRDM and DRDM had similar variation tendencies, because they had the same rotating equipment and similar hydrodynamics on membrane surface. In addition, because of the open flow channel structure, CRDM had a higher flux. Fig. 5(c)–(f) describes the effect of rotating speed on crude protein and 1Brix in permeate. With increase of rotating speed, the crude protein concentration and 1Brix of MF in permeate increased, whereas these of UF decreased. These different mechanisms of separation efficiency could be explained as follow: because of much higher permeate flux, the main factor of MF for separation efficiency was permeate flux, so, as mentioned in Section 3.1.1, the increase of

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permeate flux improved solute transmission; for UF, fouling layer was dominant, higher permeate flux led to thicker fouling layer, greater “secondary filtration” effect and better separation performance. 3.2. Concentration tests Based on previous tests in full recycling mode, moderate operation conditions (rotating speed: 1000 rpm for CRDM and DRDM and 500 rpm for DA, TMP: 3 bar for MF and 4 bar for UF) were selected for concentration tests. 3.2.1. Flux behavior As shown in Fig. 6, the flux decreases rapidly when VRR varies from 1 to 2, during which leaf protein foulants deposited and adsorbed at membrane promptly and foulant–cleaning membrane interaction was the main fouling mechanism. Then the flux reduced slightly with increasing VRR in semi-log coordinates and fluctuated a little when VRR increased from 2 to 6, due to mass transfer limited regime [17]. With the “self-cleaning” effect of shear rate, membrane fouling did not accelerate and fouling mechanism was foulant–deposited foulant interaction. It could be observed that compared with DA, CRDM and DRDM not only had much higher permeate flux, but also presented much less flux decline. This implied that with respect to stirring effect of DA, RDM could control solute accumulation at membrane surface [23] and total filtration resistance, and maintain permeate flux at a high level during the whole concentration process, although solution concentration and concentration polarization layer kept increasing with VRR. Due to closed flow channel structure and smaller shear rate on membrane surface, DA had the least flux and biggest flux decline, even close to 95% for MF. Besides, because of its open flow channel structure, CRDM had a slightly higher flux and smaller flux decline than DRDM. Compared with UF, permeate flux of MF was much higher, but its flux decline was also higher, which corresponded to a relationship between membrane pore and foulant [31]: as the main foulants, leaf proteins had a size similar to MF pores and caused pore blocking easily, therefore, MF had a more pore blocking and greater flux decline than UF. 3.2.2. Separation and concentration performance The variation of crude protein concentration and 1Brix in permeate at various VRR is illustrated in Fig. 7. The crude protein concentration and 1Brix in permeate follow a similar trend: from VRR 1 to 2, they kept at a stable value, however, when VRR exceeded 2, they kept increasing with VRR. This phenomenon corresponded to the variation of flux behavior and membrane fouling mechanisms [32]. As mentioned in Section 3.2.1, when VRR

Fig. 6. Flux behaviors vs VRR during alfalfa juice concentration process by (a) MF at 3 bar and (b) UF at 4 bar.

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Fig. 7. Separation performances vs VRR during concentration of alfalfa juice by (a) crude protein in permeate and (b) 1Brix of permeate.

Fig. 8. Characteristics (crude protein and 1Brix) of permeate and retentate for MF and UF.

increased from 1 to 2, a large number of leaf proteins deposited, were absorbed at membrane and formed foulant–cleaning membrane interaction fouling, thus feed leaf proteins and soluble matters remained stable and did not affect the separation performance. When VRR varied from 2 to 6, the main fouling mechanism was foulant–deposited foulant interaction and concentration polarization increased, thus the greater concentration gradient made more crude proteins and soluble matters pass through the membrane. The concentration efficiency of crude protein, 1Brix and chlorophyll (A and B) for various types of filtration modules are shown

in Figs. 7–9. As shown in Fig. 7, with increase of VRR, more solutes were rejected and feed concentration increased, causing higher solute transmission, while flux decline strengthened this process. However, fouling layer also aggravated and higher “secondary filtration” formed, decreasing the solute transmission. At the beginning of concentration test (VRR from 1 to 2), there was a tradeoff between these two mentioned effects, so crude protein concentration and 1Brix in permeate almost kept stable; while in the latter stage, fouling layer was stable, but permeate flux was decreasing and the feed concentration was increasing, so the solute transmission increased slightly. Fig. 8 illustrates that CRDM

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Fig. 9. Chlorophyll A and B in permeate and retentate for MF and UF.

Table 3 Protein purity for various types of filtration modules. VRR ¼ 6

Protein purity in permeate (%)

Protein purity in retentate (%)

MF

14 4.6 5.5 9.3 4.8 5.3

44 73 93 39 41 45

UF

DA DRDM CRDM DA DRDM CRDM

and DRDM had a much higher crude protein concentration in retentate than DA, implying that the RDM with a high shear rate could enhance the concentration capacity of leaf protein in alfalfa juice significantly. Compared with CRDM, DRDM had better concentration efficiency, which was probably due to its better “secondary filtration” effect caused by closed flow channel structure. As for 1Brix and chlorophyll, as shown in Figs. 8(c) and (d), and 9, DA and DRDM had the highest concentration in retentate for 1Brix and chlorophyll, respectively. Table 3 presents that retentates for all these filtration modules had much higher protein purities than feed alfalfa juice, indicating that membrane technology could not only concentrate leaf protein, but also improve protein purity, because a membrane separation process could reject large solutes, such as leaf protein, while many small solutes passed through membrane and solute matter concentration reduced. In comparison with DA and DRDM, CRDM had higher protein purity and best clarification effect, due to its higher shear rate and lower “secondary filtration” effect. Besides, UF had an obvious superiority for concentration capacity than MF, owing to its small pore size and

Table 4 Operation time, concentrated volume and productivity for various types of filtration modules. VRR ¼ 6

Operation time (h)

Concentrated volume (L)

Productivity (L m  2 h  1 bar  1)

MF DA DRDM CRDM UF DA DRDM CRDM

6.5 0.81 3.11 5.42 1.95 8.33

0.03 0.2 1 0.03 0.2 1

0.48 4.73 6.06 0.43 1.42 1.70

higher rejection, but its permeate flux and protein purity were obviously lower than MF. This implied than UF gave higher concentration capacity but lower efficiency than MF, which was contradictory. In Section 3.2.3, a new productivity concept is first proposed to evaluate operation efficiency.

3.2.3. Production efficiency In membrane filtration, there are many factors [26,33], such as flux behavior, separation performance, concentration efficiency, membrane fouling and membrane cleaning, affecting its operation and industrial application. However, some factors are interacting or conflicting [34], for example flux behavior and separation performance are related in membrane filtration, and membrane fouling has an influence on concentration efficiency and membrane cleaning during concentration process. In order to study operation efficiency and evaluate its potential for industrial application,

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90 Fouled membrane After cleaning

Permeability recovery (%)

80 70 60 50 40 30 20 10 0

DA DRDM CRDM

MF

DA DRDM CRDM

Fig. 11. Schematic of filtration behavior for various types of filtration modules.

UF

Fig. 10. Membrane cleaning for concentration tests.

productivity was first proposed to estimate the production efficiencies of various types of filtration modules. As shown in Table 4, at VRR¼6, CRDM uses 3.11 h to generate 1 L of concentrated alfalfa juice, and its flux per bar is 6.06 L h  1 m  2 bar  1, which is highest among all filtration modules. Compared with UF, MF needs less operating time to generate the same concentrated volume and has higher productivity. Therefore, the CRDM with MF was the most efficient for industrial application. 3.2.4. Membrane fouling and cleaning Membrane cleaning using P3-ultrasil 10 (Ecolab, USA) detergent was utilized to investigate the permeability recovery and efficiency of membrane cleaning. As shown in Fig. 10, permeability recovery defined as the ratio of pure water flux before concentration and being fouled or after cleaning [24], is used to estimate membrane fouling and cleaning efficiency for different types of filtration modules. MF had a larger membrane fouling and lower permeability recovery than UF, because the more serious pore blocking of MF produced greater irreversible fouling. It is evident that for both MF and UF, the optimal permeability recovery increases in the order as follows: CRDM 4DRDM 4DA. This obviously indicated that CRDM had the lowest irreversible fouling and recovered more easily its membrane permeability, due to its excellent “self-cleaning” capacity induced by high shear-effect [35] in the concentration process. The high recovery of membrane permeability confirms a high potential application for alfalfa juice concentration. 3.3. Discussion of various types of filtration modules When alfalfa juice is concentrated by MF and UF, flux behavior, separation performance and membrane cleaning are three main indexes to estimate the process efficiency for various types of membrane filtration modules. In this study, three types of filtration modules (DA, DRDM and CRDM) were utilized to concentrate leaf protein from alfalfa juice. DA used a conventional and simple stirring effect to produce shear rate on membrane surface and control concentration polarization and membrane fouling. CRDM and DRDM with a rotating-disk equipped with 6 mm-high vanes can create a much higher shear rate than DA for improving the elimination of serious flux decline (seen in Fig. 6). At the same time, the high shear rate of CRDM and DRDM also enhances leaf protein rejection and decreases impurities in concentration process (seen in Figs. 7–9). Furthermore, it reduces membrane pore blocking caused

by leaf protein and irreversible fouling and enhances permeate recovery in membrane cleaning, permitting membrane sustainable utilization. Besides, by calculating productivities, Table 4 shows that CRDM has the best production efficiency. Due to its open flow channel structure, the CRDM has a better mobility for feed flow and reduces concentration polarization. In fact, dynamic filtration systems can operate at low feed flow rates, which are just slightly greater than permeate flow rate (about 10–13% for nanofiltration and reverse osmosis and 3–5% for UF and MF) [22,24], thus they do not need powerful and large pumps as in traditional dead-end and cross-flow filtration and have lower energy consumption for feed pump. In summary, compared with other filtration modules, as shown in Fig. 11, the CRDM owns many advantages and is most suitable for the industrial application of alfalfa juice concentration.

4. Conclusions DA, DRDM and CRDM were tested to concentrate leaf protein from alfalfa juice. MF and UF permitted filtration to obtain better separation and concentration efficiency. Full recycling tests demonstrated that rotating speed reduced concentration polarization and enhanced higher flux behavior, but limited the separation effect in MF. TMP improved permeate flux significantly and reduced leaf protein rejection slightly. MF had a better flux behavior and less leaf protein rejection. Then concentrations tests indicated that due to shear effect and open flow channel structure, CRDM had the best flux behavior, least flux decline, smallest irreversible fouling and highest permeability recovery in membrane cleaning, while its lowest fouling layer led to high protein purity in retentate. Because of high shear rate and “secondary filtration” effect of fouling layer created by closed flow channel structure, DRDM had highest crude protein concentration in retentate. Through comprehensive calculation and consideration, CRDM with MF had the highest productivity for leaf protein concentration from alfalfa juice and best potential for industrial application. This investigation could serve as valuable information for industry by extrapolating this process to industrial production.

Acknowledgments The authors would like to acknowledge Luzéal company (Pauvres, France) for the kind supply of alfalfa juice and also would like to acknowledge the financial support of China Scholarship Council for Wenxiang Zhang's thesis fellowship.

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