Desalination 267 (2011) 217–221
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
A study of demineralization of whey by nanofiltration membrane Kai Pan, Qi Song, Lei Wang, Bing Cao ⁎ College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
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Article history: Received 17 May 2010 Received in revised form 16 September 2010 Accepted 16 September 2010 Available online 30 October 2010 Keywords: Whey Nanofiltration pH Demineralization
a b s t r a c t Whey is the main byproduct obtained from cheese production, and it contains a high concentration of valuable organic matter, but demineralization is needed before it can be used. Nanofiltration (NF) membranes have high permeability for monovalent salts (e.g. NaCl, KCl) and organic compounds with low molecular weight, and very low permeability for organic compounds of molecular weight higher than 300 Da, so it is a good alternative for simultaneous concentration and demineralization of whey. In this paper, whey was acidified at different pH to measure the desalination. The results indicated that the best condition for demineralization was at the isoelectric point of whey (pH = 4.60). On the other hand, when the pH was at the isoelectric point of β-lactoglobulin (5.18, the main protein of whey), a good effect of demineralization could be obtained as well, and when diafiltration was applied, the demineralization effect was even higher at the pH of 4.60, and the demineralization rate was 72%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanofiltration (NF) is a pressure-driven membrane technology that is intermediate between ultrafiltration and reverse osmosis, and it can be used as an alternative to traditional water treatment methods. NF membranes can reject some salts, and it is highly efficient in retarding organic compounds with molecular weights in the 300–1000 Da range. This process is one of the simplest low energy requirement methods of water softening for industrial production of drinking water. The NF membranes are also highly permeable for monovalent salts (as NaCl and KCl) and low molecular weight organic compounds, but very lowly permeable for organic compounds with high molecular weight such as lactose, protein, etc. [1]. This selective separation performance to multivalent electrolytes with univalent electrolytes makes NF favorable in the cases where selective removal of electrolytes is required [2]. So, NF becomes a new technology for the desalination and concentration of whey [3], an especially good alternative to the combined process of evaporation (EV) and electrodialysis (ED) [4]. Whey is the main byproduct obtained from cheese production and is used mainly as animal feed or released into the waste water treatment process, although it is rich in valuable components, such as protein and lactose [5]. However, it shows a very high mineral salt content, especially NaCl, which causes a drop in the quality of whey [6]. Moreover, β-lactoglobulin and α-lactoalbumin compose 70% of the total protein content of whey and are responsible for hydration, gelation, emulsifying and foaming process of whey solutions, which qualify whey as good raw material for ice cream manufacture or the production of yoghurt with different fruits [7]. To keep the valuable components and to remove salts, NF process can be effective in this field. ⁎ Corresponding author. Tel.: + 86 10 64413857; fax: + 86 10 64436876. E-mail address:
[email protected] (B. Cao). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.029
This paper focuses on the study of the concentration and diafiltration process applying NF membranes. Diafiltration is a technique used for achieving high purification rates of macromicrosolutes with an economically acceptable permeation flux [8]. E. Suárez used NF membrane to carry out partial demineralization of whey and milk ultrafiltration permeation through the concentration and changing of pressure and VCR. The degree of mineral salt removal was 27–36%. Losses of lactose and protein in the permeate stream are negligible [9]. B. Cuartas Uribe used NF membrane to treat whey ultrafiltration permeate through diafiltration mode with water addition. They tested different trans-membrane pressures ranging between 0.5 and 2.5 MPa to find the best operating conditions. The results were 2 MPa and a volume dilution factor (VDF) of around 2 [8]. A. Román et al. took the measure of continuous variable volume diafiltration with the semi-batch mode technique to treat with acid whey. They used a flat sheet NF membrane and in the case of α = 0.75, the effect of desalination is better [6]. However, the effect of demineralization with adjusting feeding pH has not been mentioned in the past. In this paper, in order to change the combination between whey protein and metal ions, whey was acidified at different pH. The purpose of this study is to find the best demineralization conditions through adjusting the feed's pH. In addition, diafiltration process will be used to raise the rate of demineralization. 2. Materials and methods 2.1. Whey composition The whey powder is manufactured in Poland. Its composition is shown in Table 1. Before use, the whey powder is dissolved in pure water at the concentration of around 6%.
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Table 1 Characteristic of the whey powder. Value
pH Protein Lactose Ash Total plate count
≥6.0 ≥11% ≥70% ≤8.5% ≤30,000 g
4
Ash(%)
Parameters
3
2
2.2. Equipment The equipment were carried out in an NF pilot plant specially designed for this work. It was equipped with a temperature and pressure control system. Fig. 1 shows a schematic diagram of the experiment set-up. The system consisted of a 40 L feed tank, pump, stainless steel membrane housing, and plate heat exchanger. The feed was pumped to the membrane model with pressure adjusting to create sufficient cross-flow velocity through the membrane. The trans-membrane pressure was adjusted by throttling valve. The NF membrane module (TFC 2540 SR2) was supplied by Koch Company. It consisted of a spiral-wound aromatic polyamide membrane with a total effective area of 2.3 m2. 2.3. Analytical For conductivity analysis, a conductivity meter (DDSJ-308A, Shanghai Leici Co.) was used. The pH was measured by a pH meter (Phs-3c, Shanghai Leici Co.). For ions analysis, the following methods were applied: Sodium (Na+), Calcium (Ca2+), Potassium (K+), and Magnesium (Mg2+) were analyzed by CIP(SPS800, Seiko Instrument Inc.). For lactose, the method followed was that found in China National Standards (GB5413–85, 327–329). Ash was measured by incineration in a Muffle furnace (SX2-4-10, Tianjin Zhonghuan Test Furnace Co.) at 550 °C. 2.4. Experimental procedure 2.4.1. Protein desorption In this procedure, pH of whey samples was adjusted gradually from below isoelectric point to the natural state of the whey. Each sample's protein was separated by heating at 80–90 °C to cause denaturation, and then the filtrated solution was removed. The ash content was measured by incineration at 550 °C. 2.4.2. NF experiment The membrane was characterized by measuring the permeate flux of whey at different trans-membrane pressures from 0.5 MPa to 2.4 MPa. Both permeate and retentate were recycled back to the feed tank so that concentration remained constant.
1
0
4.5
5.0
5.5
6.0
pH Fig. 2. Ash attaching on protein with different solution pH.
In the next set of experiments, permeate was continuously removed while retentate was recycled to the feed tank. Therefore, feed concentration increased, and these runs were performed at a trans-membrane pressure of 1.3 MPa. The pH of feed was adjusted by HCl every time. The pure water flux was measured before each filtration. Diafiltration process was taken through adding the pure water (adjusting the same pH with feed) and the adjusted water apart at the same flow rate as the permeate flow rate. Thus, the feed volume was constant during the process. The experiments ended when the feed solution conductivity was constant. After that, post-concentration was performed with removing the permeation. The feeding pH was 4.60. This operation mode is used when it is needed to increase the removal efficiency of a component that is only partially retained by the membrane. Solute removal efficiencies can be calculated considering the concentration values in the diafiltration process (Eq. (1)). Removal% = ðCi –Cf Þ = Ci × 100%
ð1Þ
where Ci and Cf are the solute concentrations in the initial and the final feed volume respectively. In both the concentration and diafiltration experiments, the degree of demineralization was measured in terms of total ash as well as individual ion removal. Membrane cleaning was carried out after each experiment using the cleaning solution and pure water at low operating pressure.
28
J(L/m2h)
24
20
16
12 0.5
1.0
1.5
2.0
2.5
P(MPa) Fig. 1. Diagram of the NF progress. 1, Feed tank; 2, low pressure pump; 3, high pressure pump; 4, NF membrane; 5, plate heat exchanger.
Fig. 3. The whey permeate flux with trans-membrane pressure.
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5.5 10
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a
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5.0 5
4.5 0
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Time(min)
4.5 0
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Fig. 4. Permeate flux and conductivity with operating time at different pH of feed (ΔP = 1.3 MPa). (a) is 4.34, (b) is 4.60, (c) is 4.76, (d) is 5.03, (e) is 5.18, and (f) is 5.31.
3. Results and discussion 3.1. Protein desorption Salts dissolved in milk exist only in whey and will affect some properties of milk (such as the stability of milk protein). The composition of salts in milk is rather complicated, and their changes are also interrelated [10]. The isoelectric point of whey is 4.6. Whey is positively charged when pH is lower than 4.6, otherwise whey is
negatively charged. The solubility of protein reached the lowest point at its isoelectric point, because protein will aggregate and lose solubility at this point. The pH will affect the protein's net charge and the electrostatic interaction between proteins [11]. β-Lactoglobulin is the major component of milk, about 50 wt.% of whey and 12 wt.% of total protein content, and its isoelectric point is 5.18. The β-lactoglobulin can combine with monovalent and divalent metal ions. When pH is above isoelectric point, Na+ can combine with the carboxyl and imidazolyl of β-lactoglobulin [10].
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In this experiment, by adjusting the pH of whey, an obvious difference of protein-attached ash between natural whey and adjusted whey detected and was shown in the Fig. 2. It can be observed that when the whey's pH was adjusted to 4.60, the least ash was obtained. The fact can be explained by the isoelectric point theory of whey protein. At the isoelectric point of whey protein, the combination between protein and ions is weak. Thus, more ions can be released from protein. Therefore, in this condition, the salts can be removed by NF process more easily.
17
16
J(L/m2h)
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3.2. Total recycle mode
15 14
13
Previous to NF experiments, the permeate flux of whey at different pressures were measured to find an appropriate pressure. In the experiment with total recycle, as the pressure from 0.5 MPa to 1.3 MPa, a linear increase of the permeate flux was observed. At transmembrane pressures higher than 1.3 MPa, the flux tended to decrease instead of increase continuously as shown in Fig. 3. This fact can be explained as a consequence of the protein concentration polarization layer formation on the membrane surface. 3.3. Concentration process Operating conditions at different pH of feed are shown in Fig. 4. It can be observed from (a) to (f) that at each situation, the tendency of permeate flux and of conductivity was almost the same. In this process, when the concentration of the feeding whey was constant, a gradual decrease of the permeate flux and an increase of the permeate conductivity were observed as expected. Fig. 4 (b) illustrates the permeate flux and conductivity at feeding pH of 4.60 (isoelectric point of whey protein). The beginning flux was higher than the other situations. The phenomenon can be explained by the whey isoelectric point theory. At the isoelectric point, protein will aggregate. However, with the constant concentration, the permeate flux was still decreased. In Fig. 5 the ash of dry concentrated production at different feeding pH is presented. The degree of demineralization changes to different values at different feeding pH. The lowest ash value was obtained when the feeding pH was 4.60 which is the isoelectric point of whey protein, and at each process, whey was concentrated at around 4 times.
12 4
2
6
8
12
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18
20
V(L) Fig. 6. Permeate flux with volume of dialyzation water at 1.3 MPa.
experiment ended when the feed solution conductivity was constant. Post-concentration was carried out after that. Fig. 6 shows the permeate flux with volume of dialyzation water. The decreasing value of permeate flux was not so distinguished during the diafiltration process. The feed solution conductivity was gradually decreased and can be seen in Fig. 7. During this process, 60% of sodium and potassium ions were removed. The removals of calcium and magnesium were 19% and 10% respectively. No lactose was detected in the permeate water. Concerning the dry production ash, a removal of 72% was reached. 4. Conclusions NF process was effective for mineral salt removal from whey in order to reuse lactose and concentrate whey protein. The protein desorption experiment proved the whey protein isoelectric theory. So, the best effect of demineralization was achieved when the feeding pH was adjusted to 4.60, which is the whey protein isoelectric point. Through the diafiltration process, the removal of ash was approximately 72%, and there was almost no lactose loss. Compared with the results reported in former studies (27–36%) [9], the removal rate of this method is much higher.
3.4. Continuous diafiltration process Acknowledgements The continuous diafiltration process was carried out with water addition (at the same pH and temperature of the whey) at the same flow rate as the permeate flow rate. The feed was adjusted to pH 4.60 by HCl solution. The feed volume was constant during the process. The
The project is supported by the Major Project of Science and Technology Research from the Ministry of Education of China (308003), the National Natural Science Foundation of China 7.0 6.5
Conductivity F(ms/cm)
5
Ash(%)
4
3
2
1
0
6.0 5.5 5.0 4.5 4.0
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
pH Fig. 5. Dry concentration ash with different feeding pH.
6.0
3.5
0
5
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V(L) Fig. 7. Feed solution conductivity with volume of dialyzation water at 1.3 MPa.
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