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Influence of process temperature and microfiltration pre-treatment on flux and fouling intensity during cross-flow ultrafiltration of sweet whey using ceramic membranes
International Dairy Journal 51 (2015) 1e7
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Influence of process temperature and microfiltration pre-treatment on flux and fouling intensity during cross-flow ultrafiltration of sweet whey using ceramic membranes Irena Baruk ci c a, *, Rajka Bo zani c a, Ulrich Kulozik b a Laboratory for Technology of Milk and Dairy Products, Department of Food Process Engineering, Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia b €t Chair for Food Process Engineering and Dairy Technology, Technology Unit, Research Centre for Nutrition and Food Sciences, Technische Universita München, 85354 Freising-Weihenstaphan, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 April 2015 Received in revised form 9 July 2015 Accepted 9 July 2015 Available online 18 July 2015
The use of ultrafiltration in whey processing is limited by reduced flux and separation originating from fouling. Since the processing conditions applied determine ultrafiltration performance, this study focused on investigating the influence of temperature (20 or 50 C) and the pre-treatment applied (pasteurisation, cross-flow microfiltration) on the ultrafiltration of sweet whey using a 20 kDa ceramic membrane. The highest flux and the lowest total filtration resistance were recorded at 20 C, with predominantly reversible fouling, most probably arising from protein deposition, while at 50 C, calcium could have played an important role. Whey microfiltration using a 0.5 mm ceramic membrane enhanced ultrafiltration, giving significant flux increase and fouling reduction, while the obtained microbial reduction was almost equal to pasteurisation. Ultrafiltration of pasteurised whey at 50 C proved to be least suitable, while ultrafiltration of fresh or microfiltered whey at 20 C provided the most desirable conditions. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Membrane processes are well implemented in the dairy industry for numerous applications. Application of the common range of processing parameters (membranes with an approximate cut off of 1e100 kDa and working pressures 0.1e1 MPa) ultrafiltration (UF) provides for excellent separation of milk constituents. The main use of UF in the dairy industry is concentrating milk or whey to obtain total milk protein concentrates, whey protein concentrates (WPCs) and whey protein isolates (WPIs), or concentrates of specific whey protein fractions, such as b-lactoglobulin (b-Lg), a-lactalbumin (a-La) or glycomacropeptide (GMP) (Kumar et al., 2013). UF is used to reduce the lactose and ash content and to concentrate proteins from whey using membranes with approximate cut offs between 10 and 30 kDa (Saxena, Tripathi, Kumar, & Shahi, 2009). WPCs and WPIs are characterised by preserved functional properties and nutritional quality, due to limited * Corresponding author. Tel.: þ385 1 4605 039. E-mail address: [email protected] (I. Baruk ci c). http://dx.doi.org/10.1016/j.idairyj.2015.07.002 0958-6946/© 2015 Elsevier Ltd. All rights reserved.
heat load, and are used in infant formula, sports nutrition and formulas for weight control (Kumar et al., 2013). However, the use of UF in whey processing is hindered by membrane fouling due to deposition and accumulation of microorganisms, residual fat, proteins and minerals from whey at the membrane surface. Interactions between the accumulated components result in one of several possible membrane fouling mechanisms, such as adsorption to membrane pores, cake formation, pore blocking, and depth fouling (Kühnl et al., 2010; Kumar et al., 2013; Piry et al., 2012). Consequently, adverse effects such as flux reduction and poor protein separation usually occur, causing diminished UF performance. Numerous studies suggest whey protein aggregates to cause considerable fouling and negatively influence UF efficiency (Kelly & Zydney, 1995; MourouzidisMourouzis, & Karabelas, 2006; Musale & Kulkarni, 1998), so whey pasteurisation might be rated as undesirable prior to UF. Hence, other treatments, such as microfiltration (MF), should be considered to reduce microbial load, while minimising fouling and its adverse effects on UF performance (Steinhauer, Schwing, Krauß, & Kulozik, 2015).
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In our previous study (Barukci c, Bo zani c, & Kulozik, 2014) we examined optimal conditions for MF of sweet whey with ceramic membranes with different average pore size (0.1, 0.5 and 0.8 mm) to find an alternative to pasteurisation of whey. Membranes with nominal pore size of 0.5 mm showed the highest potential for further use by combining high microbiological quality of whey with the lowest fouling intensity. The objectives of the present study were to investigate the UF of sweet whey by ceramic membranes from the perspective of whey protein and calcium retention, flux, fouling type and fouling intensity in relation to the process temperature. Special emphasis was put on comparing the influence of whey microfiltration using a 0.5 mm ceramic membrane and the influence of commonly used whey pasteurisation on a subsequent UF. 2. Materials and methods 2.1. Whey samples Whey was produced from pasteurised skimmed milk and analysed for chemical composition (content of native a-La, b-LgB, bLgA, BSA as well as calcium content, pH value, and dynamic viscosity) as previously described by Barukcic et al. (2014). Particle size distribution of fresh and pasteurised whey samples was determined using a Zetasizer Nano ZS (Malvern Instruments Ltd., Malvern, UK). Microbiological properties (total bacteria count, coliform bacteria) of fresh, pasteurised and MF whey were determined by a plate pouring method as described (Baruk ci c et al., 2014). 2.2. Whey pre-treatment prior to ultrafiltration Approximately 30 L of whey was subjected to pasteurisation at 73 C for 20 s in an indirect UHT pilot plant as described by Tolkach, Steinle, and Kulozik (2005). Pasteurised whey was cooled, and cool stored for no more than 24 h at 6 C prior to UF. Another portion of ~30 L of whey was subjected to MF at 20 or 50 C using an aalumina multichannel ceramic membrane (ZrO2, length of 1.020 m, the effective filtration area 0.24 m2, Membralox series, Pall GmbH, Dreieich, Germany) with nominal pore size of 0.5 mm (Barukcic et al., 2014). The MF permeate was subsequently ultra-filtered at the same processing temperature, as described in Section 2.3. 2.3. Ultrafiltration procedure and calculations UF was performed at 20 and 50 C using the mini pilot plant (SIMA-tec, Hurth, Germany) as described by Piry et al. (2008). A multichannel a-alumina tubular ceramic TiO2 membrane (length 1.020 m, 7 channels, internal diameter of each channel 6 mm, membrane internal diameter 24.5 mm; Type 7/6, A-tech innovations GmbH, Gladbeck, Germany) with a cut off of 20 kDa was used. The effective filtration area was 0.16 m2. The membrane was cut into 4 equally long (29.5 cm) sections (S1eS4), the two neighbouring sections were 0.05 cm away from one another, while the membrane diameter did not change (Piry et al., 2008). Each membrane section was equipped with custom valves, which enabled separate measurement of permeate/retentate flow and permeate/retentate sampling, but also allowed flow diversion into a joint bypass for purposes of average flow measurements. The whey and the UF unit were equilibrated at the required temperature (20 or 50 C) prior to each experiment. The measurements of water flux and membrane fouling resistance calculations were performed as described by Piry et al. (2008). To examine the length dependency of flux and fouling intensity, permeate fluxes and whey retentate/permeate samples were taken
separately from each section (S1eS4). The sampling was performed immediately after the start, after 90 min and after 120 min (at the end) of UF by activating custom valves, so that the permeate flow of a target section (S1eS4) was redirected to a flow indicator, while permeates of the remaining three sections was directly recirculated into the feed tank. Samples were analysed for the content of native a-La, b-LgB b-LgA, calcium, and viscosity. Permeate flux (J) was measured separately in each section (S1eS4), as previously described. The flux measurements were performed gravimetrically after 5, 10, 15, 30, 45, 60, 75, 90 and 120 min of filtration. After each UF experiment with whey, the plant was cleaned as described by Baruk ci c et al. (2014). For all filtrations the inlet pressure (p1) was adjusted to 2.1 105 Pa and the outlet (p2) pressure to 0.9 105 Pa, which corresponded to a pressure drop (Dp) of 1.2 105 Pa along the membrane. In each experimental filtration approximately 20 L of whey was recirculated for 120 min at 20 or 50 C, with approximate cross flow velocity of 6 m s1 and wall shear stress of 120 Pa. The mean transmembrane pressure was calculated according to Eq. (1):
DpTMðPaÞ ¼
p1 þ p2 2
(1)
Since the membrane was cut down into 4 sections (S1eS4), the inlet and outlet pressure as well as transmembrane pressure for each section was calculated according to Piry et al. (2008). To check the agreement between the calculated and real pressures in each section, a pressure transducer (model Eco-1, Wika, Klingenberg, Germany) was used as described by Piry et al. (2008). Filtration parameters, i.e., inlet and outlet pressures were constant, while the calculated and measured values in separate membrane sections were almost equal (data not shown). Such results were crucial for relevance of calculations related to filtration resistances. The total filtration resistance (RT) and the membrane resistance (RM) were calculated as described previously (Baruk ci c et al., 2014). For UF permeates average viscosities were measured and were used for reversible (Rrev) and irreversible fouling resistance (Rirev) calculations as previously described (Baruk ci c et al., 2014). 2.4. Calcium and whey protein retention Calcium and native whey protein (a-La, b-LgB, b-LgA) retention (R) was calculated according to a general expression from Eq. (2):
Ri ð%Þ ¼
1
Cip Cir
100
(2)
with i being a specific component (i.e., Ca, whey protein fractions), Cip concentration (g L1) of the component in permeate and Cir concentration (g L1) of the component in retentate. 2.5. Statistical analysis Statistical processing (one-way analysis of variance, ANOVA; p < 0.05) of the obtained results was performed by using Microsoft Office Excel 2010 (Microsoft Corporation, Redmond, WA, USA). 3. Results and discussion 3.1. Whey composition and filtration parameters Composition and properties of the whey samples are presented in Table 1 and were in good agreement with results of our previous study (Barukci c et al., 2014). When considering the contents of native a-La, b-LgB, and b-LgA and BSA, it is apparent that ~40% of proteins were denatured as a result of pasteurisation. As previously
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Table 1 Native whey protein content, calcium content and microbiological quality in samples of fresh, pasteurised and microfiltered whey.a Parameter
Fresh whey
Pasteurised whey
MF at 20 C
MF at 50 C
Ca (mg L1) Native a-La (g L1) Native b-Lg B (g L1) Native b-LgA (g L1) Native BSA (g L1) Total bacteria count (cfu mL1) Coliform bacteria (cfu mL1)
448 ± 41 0.83 ± 0.03 1.14 ± 0.11 2.66 ± 0.16 0.17 ± 0.01 5.3 107 1.2 104
440 ± 20 0.69 ± 0.01 0.69 ± 0.01 1.616 ± 0.00 0.103 ± 0.01 8.3 102 1.5 101
e e e e e 1.0 104 9.0 101
e e e e e 1.4 104 <10
a Pasteurisation was at 73 C for 20 s; microfiltration (MF) was at 20 C or 50 C using a 0.5 mm membrane. Abbreviations are: a-La, a-lactalbumin; b-Lg, b-lactoglobulin; BSA, bovine serum albumin. The data presented are the mean values ± standard deviations obtained by analysing 10 samples in duplicate, a dash indicates not applicable.
suggested (Law & Leaver, 1997), BSA was the most susceptible to denaturation. Dynamic light scattering (DLS) data of pasteurised whey showed two peaks, most probably indicating the presence of protein aggregates. In contrast, the highest light scattering intensity in fresh whey samples was observed at a particle size below 10 nm, which could have originated from individual whey proteins (Mourouzidis-Mourouzis & Karabelas, 2008) or dimers of b-Lg (Kulozik & Ripperger, 2008). The other two peaks in the particle size distribution of fresh most probably originated from larger particles. Dynamic viscosity of pasteurised whey at 50 C was somewhat higher (around 0.806 mPa s) compared with fresh whey, most probably due to the presence of protein aggregates (Ryan et al., 2012) which can negatively influence UF performance of whey by favouring gel/cake formation. Microbiological parameters indicated that MF of whey using a 0.5 mm a-alumina membrane at 20 C or 50 C provided acceptable reduction rates in comparison with conventionally used pasteurisation. Such results are in good agreement with data of Baruk ci c et al. (2014) and Steinhauer et al. (2015), who also applied similar processing parameters during sweet whey microfiltration. 3.2. Whey permeate flux during ultrafiltration of fresh, microfiltered and pasteurised whey Flux decline as a result of membrane fouling reduces UF efficiency. UF of fresh whey at 20 C resulted in a 2-fold higher flux values in comparison with UF of fresh whey at 50 C (Fig. 2A). Even more intense flux differences were observed after MF pretreatment of whey using a 0.5 mm membrane; UF of MF whey at 20 C resulted in up to 3 and 4.5 times higher flux values compared with UF of fresh whey at 20 C and pasteurised whey at 50 C (Fig. 2A). Steinhauer et al. (2015) also achieved a twofold higher UF
Average light scattering intensity (%)
25 20 15 10 5 0 0
1
10
d (nm)
100
1,000
10,000
Fig. 1. Intensity-based particle size distribution of fresh ( ) and pasteurised ( ; 73 C for 20 s) whey.
Fig. 2. The average permeate flux (J) of fresh ( , ) or pasteurised ( ; 73 C for 20 s) whey and whey previously subjected to microfiltration ( , ; 0.5 mm membrane) across the membrane during 120 min of ultrafiltration at 20 C ( , ) and 50 C ( , ) using a ceramic 20 kDa membrane (A) and the average permeate fluxes in separate membrane sections (S1eS4) at the beginning (B) and at the end (C) of ultrafiltration at the same filtration parameters.
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flux of sweet whey by applying MF pre-treatment using 0.5 and 0.8 mm ceramic membranes on UF performance. Considering the lower whey viscosity (Table 1), opposite flux trends were expected, i.e., higher flux values during UF of fresh whey at 50 C. The whey contained a considerable amount of calcium (~448 mg L1; Table 1), which might have started to precipitate at the process temperature of 50 C (Koutsoukos, 2007) and contributed more intensively to fouling mechanisms, thereby causing lower UF flux values. Such assumptions, however, were not in complete agreement with results of some previous studies that did not confirm a relation between calcium salts and flux decrease during UF of whey (Musale & Kulkarni, 1998; Ramachandra Rao, 2002). However, Konrad, Kleinschmidt, and Faber (2012) found that removal of Ca-salts resulted in a considerable flux increase during UF of acid whey. Furthermore, the overall flux decrease during UF of fresh (48%) or MF whey (around 30%) was reasonably lower in comparison with the overall flux decline (about 60%) observed during UF of pasteurised whey, regardless of the applied process temperature. When taking a closer look at length dependency of permeate flux at the beginning (Fig. 2B) and at the end (Fig. 2C) of UF, somewhat different trends could be observed in relation to the applied process temperature. The highest initial flux (Fig. 2A) was again recorded at 20 C, regardless of the membrane section, whereas the values tended to decrease almost linearly with a DpTM decrease along the membrane and were then maintained throughout the entire filtration (Fig. 2C). In contrast, during UF at 50 C (Fig. 2B), a considerable decrease was observed at the membrane inlet (between Sections 1 and 2), while almost equal values were recorded in the remaining part (sections 2, 3 and 4); i.e., the flux was constant regardless of the DpTM level. This trend was maintained until the end of the filtration (Fig. 2C). Similar trends were observed also during UF of pasteurised whey at 50 C, but the recorded flux values were considerably lower and the highest decrease in flux was observed in Section 2 of the membrane (Fig. 2C). As Kulozik and Ripperger (2008) previously explained, according to models established by Altmann and Ripperger (1997a, 1997b) the deposition of particles during cross flow MF was affected by several forces, such as the hydrodynamic, adhesive or friction forces. Thereby the balance between the lift and the drag force of the filtrate mostly determined the boundary layer formation. Deposition of particles below 20 nm is diminished by diffusive forces (Piry et al., 2008), which increase with decreasing particle size and reduce the concentration polarisation at the membrane surface. In contrast, the sum of lift and diffusive forces reaches a minimum in particle size range from 20 to 200 nm causing a maximum of deposition and a minimum of the filtration speed. Whey protein fractions with average size up to 10 nm generated the majority of the light scattering intensity in samples of fresh whey (Fig. 1). Consequently, during filtration of fresh or MF whey, especially at 20 C, diffusive forces obviously promoted reduced deposition of present components and less intensive concentration polarisation in the vicinity of membrane surface, with higher flux values as a result. In contrast, larger particles were present in pasteurised whey (Fig. 1) lead to high particle deposition and lower flux values (Fig. 2). Interestingly, the flux values observed during UF of fresh or MF whey at 50 C (Fig. 2) were somewhat lower than at 20 C, while the observed flux trends were similar to those recorded during UF of pasteurised whey (Fig. 2B and C). Such results might be correlated to similar fouling mechanisms occurring during filtrations preformed at 50 C, i.e., more intense involvement of precipitated calcium salts in filtration resistances formation causing thereat lower flux values. Considering all of the data obtained, regardless of the applied process temperature, MF of sweet whey by a 0.5 mm ceramic
membrane proved to be an effective pre-treatment for purposes of flux enhancement during the subsequent UF. Also, it is obvious that performing UF of fresh or MF whey at 20 C or, for microbiological reasons, even lower temperatures, might be more effective solution instead of applying commonly used higher filtration temperatures (50e55 C) irrespective of the applied pre-treatment. 3.3. Filtration resistances with respect to calcium and whey protein retention, the applied whey pre-treatment and ultrafiltration temperature The highest total filtration resistance (Rt) was observed for UF of pasteurised whey at 50 C (Fig. 4A), with values 2e4 times higher compared with values for UF of fresh whey and even 8e10 times higher than values for MF whey (Fig. 4A). During UF of pasteurised or fresh whey at 50 C, the Rt was higher at the membrane inlet (S1
Fig. 3. Retention (R) of native b-lactoglobulin B (A), a-lactalbumin (B) and calcium (C) in different membrane sections (S1eS4) after 120 min of ultrafiltration of fresh ( , ), pasteurised ( ; 73 C for 20 s) or microfiltered ( , ; 0.5 mm membrane) whey at 20 C ( , ) and 50 C ( , , ) using a ceramic 20 kDa membrane.
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Fig. 4. Total filtration resistance (Rt) formed after 120 min of ultrafiltration of fresh ( , ), pasteurised ( ; 73 C for 20 s) or microfiltered ( , ; 0.5 mm membrane at 20 C ( , ) or 50 C ( , , ) and comparison with the initial membrane (Rm, ) resistance (A); and comparison of the initial membrane (Rm, ) resistance with reversible (Rrev, ) and irreversible (Rirev, ) fouling resistances formed in different sections (1e4) across the membrane after 120 min ultrafiltration of fresh whey and MF whey at 20 C (B and C, respectively) and at 50 C (D and E, respectively).
and S2) and decreased gradually towards the membrane outlet (Fig. 4A). In contrast, the Rt for UF fresh and MF whey at 20 C and MF whey at 50 C was more or less equal along the membrane (Fig. 4A). Since UF membranes almost entirely retain the present protein components, many authors have suggested whey proteins, especially their aggregates, to be the main foulants (Baldasso, Barros, & Tessaro, 2011; Jonsson, Pradanos, & Hernandez, 1996; Song, 1998). Among the four possible mechanisms of membrane fouling (pore narrowing, pore plugging/blockage, gel formation, selective plugging of larger pores) suggested by Saxena et al. (2009), cake/gel formation and classical pore plugging were claimed to predominate n-Ba guena, Alvarez-Blanco, during whey UF (Corbato & VincentVela, 2015). Indeed, as presented in Fig. 3, retention of b-Lg fractions was above 85% (Fig. 3A) and for a-La above 80% (Fig. 3B),
regardless of the applied temperature and/or whey pre-treatment. The BSA fraction was entirely rejected, since it was not detected in any of the analysed whey permeates (data not shown). Reversible fouling (Rrev), generally related to cake/gel formation, predominated in almost all of the investigated filtrations and contributed the most to Rt (Fig. 4BeE). The highest values of Rrev were recorded after UF of pasteurised whey (data not shown) following by values recorded after UF of fresh whey at 50 C (Fig. 4D). In contrast, significantly lower Rrev values were observed after UF of fresh whey at 20 C (Fig. 4B), and especially after UF of MF whey (Fig. 4C and E). Thus, particle deposition was obviously reduced either when lower process temperature was applied or when whey was previously subjected to MF. By applying MF prior to UF, considerable removal of microbial cells was achieved (Table 1), but most probably also a great part of large particles,
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which resulted in considerable decrease of total fouling resistance (Rt) recorded during the subsequent UF (Fig. 4). According to models suggested by Altmann and Ripperger (1997a, 1997b) and the findings of Piry et al. (2008), decreases in friction forces of filtrate flow along the membrane determine the deposit layer formation during UF of solutions containing proteins. Under such conditions, higher deposition and, consequently, more intense fouling mechanisms occurred at the membrane inlet. However, due to the relatively high transmembrane pressures (S1 ¼ 1.95 105 Pa; S2 ¼ 1.60 105 Pa) and wall shear stress (tw ¼ 120 Pa), the reached flux values were proportionally high (Fig. 2). Accordingly, during UF of fresh or MF whey at 20 C the observed retention of b-Lg and a-La fractions slightly decreased over the first three sections of the membrane, while an apparent drop (Rß-Lg ~ 95%) was observed at the membrane outlet (Fig. 3A and B). Hence, the recorded filtration resistances obviously relied on protein deposition solely since the recorded retention of whey proteins (Fig. 3A and B) was among the highest, while calcium retention was moderate to low (Fig. 3C) at 20 C. Higher viscosity (Table 1) as well as high wall shear stress apparently created an equilibrium, resulting in almost equal deposit and filtration resistance formation across the membrane (Fig. 4A and B). b-Lg showed to have the greatest influence on fouling since it had high retention rates (Fig. 3A) and was the dominant fraction in whey (Table 1). aLa obviously played an important role too, especially during UF of fresh and pasteurised whey at 50 C where the highest retention was observed at the membrane inlet (S1 and S2; Fig. 3B) and was accompanied with the highest formation of the deposit (Fig. 4D). Pasteurised whey did contain notable amounts of protein aggregates (Fig. 1) that were responsible for extremely higher filtration resistance and deposit formation. Such results fully support findings of some former studies proposing that even in exceptionally small quantities, protein aggregates might cause considerable fouling, most probably by serving as “anchors” that promote further interactions and crosslinking of proteins and/or minerals (Kelly & Zydney, 1995; Marshall, Munro, & Tr€ agårdh, 1997, 2003; Steinhauer et al., 2015). Calcium did not play a significant role during UF of pasteurised whey due to low retention (Fig. 3C). In contrast, the highest retention of calcium was recorded at UF of fresh or MF whey (Fig. 3C), thus indicating notable involvement of minerals in fouling mechanisms, which probably relied on interactions between the deposited proteins and calcium. Hence, deposit layer formation most probably occurred according to some earlier established models (Bacchin, 2004; Song, 1998), which proposed higher deposit formation at the membrane outlet due to the decrease in the diffusive back transport across the membrane. However this appears to be contrary to expectation; the reason why these authors came to their conclusion could be that they worked with small pieces of membrane with little difference in DpTM between inlet and outlet. Several studies also suggested the possible involvement of calcium in fouling mechanisms during filtrations of milk or whey, but did not confirm whether it considerably influenced permeate flux (DuclosOrselloa, Lib, & Ho, 2006; Konrad et al., 2012; Marshall, Munro, €gårdh, 2003; Musale & Kulkarni, 1998). According to the & Tra results of present study, the influence of calcium was obviously crucial for permeate flux decrease during UF of fresh or MF whey at 50 C. 4. Conclusions According to the results obtained with regard to the application of 20 kDa ceramic membranes, UF of pasteurised whey at 50 C
provided the least effective filtration conditions, despite being commonly used at industrial scale. The obtained flux values were the lowest (2e5 times lower) while the recorded total filtration resistance (Rt) was extraordinary high (up to 10 times higher) in comparison with all other tested regimes. In contrast, UF of fresh whey, especially at 20 C, proved to be much more effective providing a twofold higher flux and a 2e4 times lower total filtration resistance. Applying MF with a 0.5 mm ceramic membrane prior to UF resulted in a significant flux increase (4.5 times higher), while the recorded total filtration resistance was up to 10 times lower in comparison with conventionally applied UF conditions. MF also resulted in almost equal total bacteria number reduction (around 3.8 log cycles) to that given by pasteurisation (4.81 log cycles), thus delivering proper microbiological quality of whey. Accordingly, MF pre-treatment using a 0.5 mm ceramic membrane considerably enhanced UF performance, and applying the lower process temperature (20 C) appeared to be the most convenient due to the highest flux and the lowest fouling intensity observed. Acknowledgement The data presented in this study are based on work co-financed by the Croatian Science Foundation (Project Nr. 03.01./77). References Altmann, J., & Ripperger, S. (1997a). Particle deposition and layer formation at the cross flow filtration. Journal of Membrane Science, 124, 119e128. Altmann, J., & Ripperger, S. (1997b). Beitrag zur Modellierung der Deckschichtbildung bei der Querstrom-Mikrofiltration [Contribution to modelling the boundary layer formation during cross-flow microfiltration]. Chemie Ingenieur Technik, 69, 468e472. Bacchin, P. (2004). A possible link between critical and limiting flux for colloidal systems: consideration of critical deposit formation along a membrane. Journal of Membrane Science, 228, 237e241. Baldasso, C., Barros, T. C., & Tessaro, I. C. (2011). Concentration and purification of whey proteins by ultrafiltration. Desalination, 278, 381e386. Baruk ci c, I., Bo zani c, R., & Kulozik, U. (2014). Effect of pore size and process temperature on flux, microbial reduction and fouling mechanisms during sweet whey cross-flow microfiltration by ceramic membranes. International Dairy Journal, 39, 8e15. n-B Corbato aguena, M. J., Alvarez-Blanco, S., & Vincent-Vela, M. C. (2015). Fouling mechanisms of ultrafiltration membranes fouled with whey model solutions. Desalination, 360, 87e96. Duclos-Orselloa, C., Lib, W., & Ho, C. C. (2006). A three mechanism model to describe fouling of microfiltration membranes. Journal of Membrane Science, 280, 856e866. Jonsson, G., Pradanos, P., & Hernandez, A. (1996). Fouling phenomena in microporous membranes. Flux decline kinetics and structural modifications. Journal of Membrane Science, 112, 171e183. Kelly, S. T., & Zydney, A. L. (1995). Mechanisms for BSA fouling during microfiltration. Journal of Membrane Science, 107, 115e127. Konrad, G., Kleinschmidt, T., & Faber, W. (2012). Ultrafiltration flux of acid whey obtained by lactic acid fermentation. International Dairy Journal, 22, 73e77. Koutsoukos, P. G. (2007). Current knowledge of calcium phosphate chemistry and in particular solid surface-water interface interactions. Available at http://www. nhm.ac.uk/research-curation/research/projects/phosphate-recovery/ Nordwijkerhout/Koutsoukos.pdf. last accessed: 22.01. 15. Kühnl, W., Piry, A., Kaufmann, V., Grein, T., Ripperger, S., & Kulozik, U. (2010). Impact of colloidal interactions on the flux in crossflow microfiltration of milk at different pH: a surface energy approach. Journal of Membrane Science, 352, 107e115. Kulozik, U., & Ripperger, S. (2008). Innovative Eins€ atze der Membrantrenntechnik in der Lebensmitteltechnologie [Innovative Applications of membrane processes in Food Technology]. Chemie Ingenieur Technik, 80, 1045e1058. Kumar, P., Sharma, N., Ranjan, R., Kumar, S., Bhat, Z. F., & Kee Jeong, D. (2013). Perspective of membrane technology in dairy industry: a review. Asian-Australasian Journal of Animal Sciences, 26, 1347e1358. Law, A. J. R., & Leaver, J. (1997). Effect of protein concentration on rates of thermal denaturation of whey proteins in milk. Journal of Agricultural and Food Chemistry, 45, 4255e4261. €gårdh, G. (1997). Influence of permeate flux on Marshall, A. D., Munro, P. A., & Tra fouling during the microfiltration of b-lactoglobulin solutions under cross-flow conditions. Journal of Membrane Science, 130, 23e30.
I. Barukcic et al. / International Dairy Journal 51 (2015) 1e7 €gårdh, G. (2003). Influence of ionic calcium on Marshall, A. D., Munro, P. A., & Tra fouling during the cross-flow microfiltration of b-lactoglobulin solutions. Journal of Membrane Science, 217, 131e140. Mourouzidis-Mourouzis, S., & Karabelas, A. (2006). Whey protein fouling of micro filtration ceramic membranesepressure effects. Journal of Membrane Science, 282, 124e132. Mourouzidis-Mourouzis, S. A., & Karabelas, A. J. (2008). Whey protein fouling of large pore-size ceramic microfiltration membranes at small cross-flow velocity. Journal of Membrane Science, 323, 17e27. Musale, D. A., & Kulkarni, S. S. (1998). Effect of whey composition on ultrafiltration performance. Journal of Agricultural and Food Chemistry, 46, 4717e4722. Piry, A., Heino, A., Kühnl, W., Tolkach, A., Grein, T., Ripperger, S., et al. (2012). Effect of membrane length, membrane resistances and filtration conditions on the fractionation of milk proteins by microfiltration. Journal of Dairy Science, 95, 1590e1602. Piry, A., Kühnl, W., Grein, T., Tolkach, A., Ripperger, S., & Kulozik, U. (2008). Length dependency of flux and protein permeation in crossflow microfiltration of skimmed milk. Journal of Membrane Science, 325, 887e894.
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Ramachandra Rao, H. G. (2002). Mechanisms of flux decline during ultrafiltration of dairy products and influence of pH on flux rates of whey and buttermilk. Desalination, 144, 319e324. Ryan, K. N., Vardhanabhuti, B., Jaramillo, D. P., van Zanten, J. H., Coupland, J. N., & Foegeding, E. A. (2012). Stability and mechanism of whey protein soluble aggregates thermally treated with salts. Food Hydrocolloids, 27, 411e420. Saxena, A., Tripathi, B. P., Kumar, M., & Shahi, V. K. (2009). Membrane-based techniques for the separation and purification of proteins: an overview. Advances in Colloid and Interface Science, 145, 1e22. Song, L. (1998). Flux decline in crossflow microfiltration and ultrafiltration: mechanisms and modelling of membrane fouling. Journal of Membrane Science, 139, 183e200. Steinhauer, T., Schwing, J., Krauß, S., & Kulozik, U. (2015). Enhancement of ultrafiltration-performance and improvement of hygienic quality during the production of whey concentrates. International Dairy Journal, 45, 8e14. Tolkach, A., Steinle, S., & Kulozik, U. (2005). Optimization of thermal pretreatment conditions for the separation of native a-lactalbumin from whey protein concentrates by means of selective denaturation of b-lactoglobulin. Journal of Food Science, 70, 557e566.
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International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj
Influence of process temperature and microfiltration pre-treatment on flux and fouling intensity during cross-flow ultrafiltration of sweet whey using ceramic membranes Irena Baruk ci c a, *, Rajka Bo zani c a, Ulrich Kulozik b a Laboratory for Technology of Milk and Dairy Products, Department of Food Process Engineering, Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia b €t Chair for Food Process Engineering and Dairy Technology, Technology Unit, Research Centre for Nutrition and Food Sciences, Technische Universita München, 85354 Freising-Weihenstaphan, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 April 2015 Received in revised form 9 July 2015 Accepted 9 July 2015 Available online 18 July 2015
The use of ultrafiltration in whey processing is limited by reduced flux and separation originating from fouling. Since the processing conditions applied determine ultrafiltration performance, this study focused on investigating the influence of temperature (20 or 50 C) and the pre-treatment applied (pasteurisation, cross-flow microfiltration) on the ultrafiltration of sweet whey using a 20 kDa ceramic membrane. The highest flux and the lowest total filtration resistance were recorded at 20 C, with predominantly reversible fouling, most probably arising from protein deposition, while at 50 C, calcium could have played an important role. Whey microfiltration using a 0.5 mm ceramic membrane enhanced ultrafiltration, giving significant flux increase and fouling reduction, while the obtained microbial reduction was almost equal to pasteurisation. Ultrafiltration of pasteurised whey at 50 C proved to be least suitable, while ultrafiltration of fresh or microfiltered whey at 20 C provided the most desirable conditions. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Membrane processes are well implemented in the dairy industry for numerous applications. Application of the common range of processing parameters (membranes with an approximate cut off of 1e100 kDa and working pressures 0.1e1 MPa) ultrafiltration (UF) provides for excellent separation of milk constituents. The main use of UF in the dairy industry is concentrating milk or whey to obtain total milk protein concentrates, whey protein concentrates (WPCs) and whey protein isolates (WPIs), or concentrates of specific whey protein fractions, such as b-lactoglobulin (b-Lg), a-lactalbumin (a-La) or glycomacropeptide (GMP) (Kumar et al., 2013). UF is used to reduce the lactose and ash content and to concentrate proteins from whey using membranes with approximate cut offs between 10 and 30 kDa (Saxena, Tripathi, Kumar, & Shahi, 2009). WPCs and WPIs are characterised by preserved functional properties and nutritional quality, due to limited * Corresponding author. Tel.: þ385 1 4605 039. E-mail address: [email protected] (I. Baruk ci c). http://dx.doi.org/10.1016/j.idairyj.2015.07.002 0958-6946/© 2015 Elsevier Ltd. All rights reserved.
heat load, and are used in infant formula, sports nutrition and formulas for weight control (Kumar et al., 2013). However, the use of UF in whey processing is hindered by membrane fouling due to deposition and accumulation of microorganisms, residual fat, proteins and minerals from whey at the membrane surface. Interactions between the accumulated components result in one of several possible membrane fouling mechanisms, such as adsorption to membrane pores, cake formation, pore blocking, and depth fouling (Kühnl et al., 2010; Kumar et al., 2013; Piry et al., 2012). Consequently, adverse effects such as flux reduction and poor protein separation usually occur, causing diminished UF performance. Numerous studies suggest whey protein aggregates to cause considerable fouling and negatively influence UF efficiency (Kelly & Zydney, 1995; MourouzidisMourouzis, & Karabelas, 2006; Musale & Kulkarni, 1998), so whey pasteurisation might be rated as undesirable prior to UF. Hence, other treatments, such as microfiltration (MF), should be considered to reduce microbial load, while minimising fouling and its adverse effects on UF performance (Steinhauer, Schwing, Krauß, & Kulozik, 2015).
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In our previous study (Barukci c, Bo zani c, & Kulozik, 2014) we examined optimal conditions for MF of sweet whey with ceramic membranes with different average pore size (0.1, 0.5 and 0.8 mm) to find an alternative to pasteurisation of whey. Membranes with nominal pore size of 0.5 mm showed the highest potential for further use by combining high microbiological quality of whey with the lowest fouling intensity. The objectives of the present study were to investigate the UF of sweet whey by ceramic membranes from the perspective of whey protein and calcium retention, flux, fouling type and fouling intensity in relation to the process temperature. Special emphasis was put on comparing the influence of whey microfiltration using a 0.5 mm ceramic membrane and the influence of commonly used whey pasteurisation on a subsequent UF. 2. Materials and methods 2.1. Whey samples Whey was produced from pasteurised skimmed milk and analysed for chemical composition (content of native a-La, b-LgB, bLgA, BSA as well as calcium content, pH value, and dynamic viscosity) as previously described by Barukcic et al. (2014). Particle size distribution of fresh and pasteurised whey samples was determined using a Zetasizer Nano ZS (Malvern Instruments Ltd., Malvern, UK). Microbiological properties (total bacteria count, coliform bacteria) of fresh, pasteurised and MF whey were determined by a plate pouring method as described (Baruk ci c et al., 2014). 2.2. Whey pre-treatment prior to ultrafiltration Approximately 30 L of whey was subjected to pasteurisation at 73 C for 20 s in an indirect UHT pilot plant as described by Tolkach, Steinle, and Kulozik (2005). Pasteurised whey was cooled, and cool stored for no more than 24 h at 6 C prior to UF. Another portion of ~30 L of whey was subjected to MF at 20 or 50 C using an aalumina multichannel ceramic membrane (ZrO2, length of 1.020 m, the effective filtration area 0.24 m2, Membralox series, Pall GmbH, Dreieich, Germany) with nominal pore size of 0.5 mm (Barukcic et al., 2014). The MF permeate was subsequently ultra-filtered at the same processing temperature, as described in Section 2.3. 2.3. Ultrafiltration procedure and calculations UF was performed at 20 and 50 C using the mini pilot plant (SIMA-tec, Hurth, Germany) as described by Piry et al. (2008). A multichannel a-alumina tubular ceramic TiO2 membrane (length 1.020 m, 7 channels, internal diameter of each channel 6 mm, membrane internal diameter 24.5 mm; Type 7/6, A-tech innovations GmbH, Gladbeck, Germany) with a cut off of 20 kDa was used. The effective filtration area was 0.16 m2. The membrane was cut into 4 equally long (29.5 cm) sections (S1eS4), the two neighbouring sections were 0.05 cm away from one another, while the membrane diameter did not change (Piry et al., 2008). Each membrane section was equipped with custom valves, which enabled separate measurement of permeate/retentate flow and permeate/retentate sampling, but also allowed flow diversion into a joint bypass for purposes of average flow measurements. The whey and the UF unit were equilibrated at the required temperature (20 or 50 C) prior to each experiment. The measurements of water flux and membrane fouling resistance calculations were performed as described by Piry et al. (2008). To examine the length dependency of flux and fouling intensity, permeate fluxes and whey retentate/permeate samples were taken
separately from each section (S1eS4). The sampling was performed immediately after the start, after 90 min and after 120 min (at the end) of UF by activating custom valves, so that the permeate flow of a target section (S1eS4) was redirected to a flow indicator, while permeates of the remaining three sections was directly recirculated into the feed tank. Samples were analysed for the content of native a-La, b-LgB b-LgA, calcium, and viscosity. Permeate flux (J) was measured separately in each section (S1eS4), as previously described. The flux measurements were performed gravimetrically after 5, 10, 15, 30, 45, 60, 75, 90 and 120 min of filtration. After each UF experiment with whey, the plant was cleaned as described by Baruk ci c et al. (2014). For all filtrations the inlet pressure (p1) was adjusted to 2.1 105 Pa and the outlet (p2) pressure to 0.9 105 Pa, which corresponded to a pressure drop (Dp) of 1.2 105 Pa along the membrane. In each experimental filtration approximately 20 L of whey was recirculated for 120 min at 20 or 50 C, with approximate cross flow velocity of 6 m s1 and wall shear stress of 120 Pa. The mean transmembrane pressure was calculated according to Eq. (1):
DpTMðPaÞ ¼
p1 þ p2 2
(1)
Since the membrane was cut down into 4 sections (S1eS4), the inlet and outlet pressure as well as transmembrane pressure for each section was calculated according to Piry et al. (2008). To check the agreement between the calculated and real pressures in each section, a pressure transducer (model Eco-1, Wika, Klingenberg, Germany) was used as described by Piry et al. (2008). Filtration parameters, i.e., inlet and outlet pressures were constant, while the calculated and measured values in separate membrane sections were almost equal (data not shown). Such results were crucial for relevance of calculations related to filtration resistances. The total filtration resistance (RT) and the membrane resistance (RM) were calculated as described previously (Baruk ci c et al., 2014). For UF permeates average viscosities were measured and were used for reversible (Rrev) and irreversible fouling resistance (Rirev) calculations as previously described (Baruk ci c et al., 2014). 2.4. Calcium and whey protein retention Calcium and native whey protein (a-La, b-LgB, b-LgA) retention (R) was calculated according to a general expression from Eq. (2):
Ri ð%Þ ¼
1
Cip Cir
100
(2)
with i being a specific component (i.e., Ca, whey protein fractions), Cip concentration (g L1) of the component in permeate and Cir concentration (g L1) of the component in retentate. 2.5. Statistical analysis Statistical processing (one-way analysis of variance, ANOVA; p < 0.05) of the obtained results was performed by using Microsoft Office Excel 2010 (Microsoft Corporation, Redmond, WA, USA). 3. Results and discussion 3.1. Whey composition and filtration parameters Composition and properties of the whey samples are presented in Table 1 and were in good agreement with results of our previous study (Barukci c et al., 2014). When considering the contents of native a-La, b-LgB, and b-LgA and BSA, it is apparent that ~40% of proteins were denatured as a result of pasteurisation. As previously
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Table 1 Native whey protein content, calcium content and microbiological quality in samples of fresh, pasteurised and microfiltered whey.a Parameter
Fresh whey
Pasteurised whey
MF at 20 C
MF at 50 C
Ca (mg L1) Native a-La (g L1) Native b-Lg B (g L1) Native b-LgA (g L1) Native BSA (g L1) Total bacteria count (cfu mL1) Coliform bacteria (cfu mL1)
448 ± 41 0.83 ± 0.03 1.14 ± 0.11 2.66 ± 0.16 0.17 ± 0.01 5.3 107 1.2 104
440 ± 20 0.69 ± 0.01 0.69 ± 0.01 1.616 ± 0.00 0.103 ± 0.01 8.3 102 1.5 101
e e e e e 1.0 104 9.0 101
e e e e e 1.4 104 <10
a Pasteurisation was at 73 C for 20 s; microfiltration (MF) was at 20 C or 50 C using a 0.5 mm membrane. Abbreviations are: a-La, a-lactalbumin; b-Lg, b-lactoglobulin; BSA, bovine serum albumin. The data presented are the mean values ± standard deviations obtained by analysing 10 samples in duplicate, a dash indicates not applicable.
suggested (Law & Leaver, 1997), BSA was the most susceptible to denaturation. Dynamic light scattering (DLS) data of pasteurised whey showed two peaks, most probably indicating the presence of protein aggregates. In contrast, the highest light scattering intensity in fresh whey samples was observed at a particle size below 10 nm, which could have originated from individual whey proteins (Mourouzidis-Mourouzis & Karabelas, 2008) or dimers of b-Lg (Kulozik & Ripperger, 2008). The other two peaks in the particle size distribution of fresh most probably originated from larger particles. Dynamic viscosity of pasteurised whey at 50 C was somewhat higher (around 0.806 mPa s) compared with fresh whey, most probably due to the presence of protein aggregates (Ryan et al., 2012) which can negatively influence UF performance of whey by favouring gel/cake formation. Microbiological parameters indicated that MF of whey using a 0.5 mm a-alumina membrane at 20 C or 50 C provided acceptable reduction rates in comparison with conventionally used pasteurisation. Such results are in good agreement with data of Baruk ci c et al. (2014) and Steinhauer et al. (2015), who also applied similar processing parameters during sweet whey microfiltration. 3.2. Whey permeate flux during ultrafiltration of fresh, microfiltered and pasteurised whey Flux decline as a result of membrane fouling reduces UF efficiency. UF of fresh whey at 20 C resulted in a 2-fold higher flux values in comparison with UF of fresh whey at 50 C (Fig. 2A). Even more intense flux differences were observed after MF pretreatment of whey using a 0.5 mm membrane; UF of MF whey at 20 C resulted in up to 3 and 4.5 times higher flux values compared with UF of fresh whey at 20 C and pasteurised whey at 50 C (Fig. 2A). Steinhauer et al. (2015) also achieved a twofold higher UF
Average light scattering intensity (%)
25 20 15 10 5 0 0
1
10
d (nm)
100
1,000
10,000
Fig. 1. Intensity-based particle size distribution of fresh ( ) and pasteurised ( ; 73 C for 20 s) whey.
Fig. 2. The average permeate flux (J) of fresh ( , ) or pasteurised ( ; 73 C for 20 s) whey and whey previously subjected to microfiltration ( , ; 0.5 mm membrane) across the membrane during 120 min of ultrafiltration at 20 C ( , ) and 50 C ( , ) using a ceramic 20 kDa membrane (A) and the average permeate fluxes in separate membrane sections (S1eS4) at the beginning (B) and at the end (C) of ultrafiltration at the same filtration parameters.
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flux of sweet whey by applying MF pre-treatment using 0.5 and 0.8 mm ceramic membranes on UF performance. Considering the lower whey viscosity (Table 1), opposite flux trends were expected, i.e., higher flux values during UF of fresh whey at 50 C. The whey contained a considerable amount of calcium (~448 mg L1; Table 1), which might have started to precipitate at the process temperature of 50 C (Koutsoukos, 2007) and contributed more intensively to fouling mechanisms, thereby causing lower UF flux values. Such assumptions, however, were not in complete agreement with results of some previous studies that did not confirm a relation between calcium salts and flux decrease during UF of whey (Musale & Kulkarni, 1998; Ramachandra Rao, 2002). However, Konrad, Kleinschmidt, and Faber (2012) found that removal of Ca-salts resulted in a considerable flux increase during UF of acid whey. Furthermore, the overall flux decrease during UF of fresh (48%) or MF whey (around 30%) was reasonably lower in comparison with the overall flux decline (about 60%) observed during UF of pasteurised whey, regardless of the applied process temperature. When taking a closer look at length dependency of permeate flux at the beginning (Fig. 2B) and at the end (Fig. 2C) of UF, somewhat different trends could be observed in relation to the applied process temperature. The highest initial flux (Fig. 2A) was again recorded at 20 C, regardless of the membrane section, whereas the values tended to decrease almost linearly with a DpTM decrease along the membrane and were then maintained throughout the entire filtration (Fig. 2C). In contrast, during UF at 50 C (Fig. 2B), a considerable decrease was observed at the membrane inlet (between Sections 1 and 2), while almost equal values were recorded in the remaining part (sections 2, 3 and 4); i.e., the flux was constant regardless of the DpTM level. This trend was maintained until the end of the filtration (Fig. 2C). Similar trends were observed also during UF of pasteurised whey at 50 C, but the recorded flux values were considerably lower and the highest decrease in flux was observed in Section 2 of the membrane (Fig. 2C). As Kulozik and Ripperger (2008) previously explained, according to models established by Altmann and Ripperger (1997a, 1997b) the deposition of particles during cross flow MF was affected by several forces, such as the hydrodynamic, adhesive or friction forces. Thereby the balance between the lift and the drag force of the filtrate mostly determined the boundary layer formation. Deposition of particles below 20 nm is diminished by diffusive forces (Piry et al., 2008), which increase with decreasing particle size and reduce the concentration polarisation at the membrane surface. In contrast, the sum of lift and diffusive forces reaches a minimum in particle size range from 20 to 200 nm causing a maximum of deposition and a minimum of the filtration speed. Whey protein fractions with average size up to 10 nm generated the majority of the light scattering intensity in samples of fresh whey (Fig. 1). Consequently, during filtration of fresh or MF whey, especially at 20 C, diffusive forces obviously promoted reduced deposition of present components and less intensive concentration polarisation in the vicinity of membrane surface, with higher flux values as a result. In contrast, larger particles were present in pasteurised whey (Fig. 1) lead to high particle deposition and lower flux values (Fig. 2). Interestingly, the flux values observed during UF of fresh or MF whey at 50 C (Fig. 2) were somewhat lower than at 20 C, while the observed flux trends were similar to those recorded during UF of pasteurised whey (Fig. 2B and C). Such results might be correlated to similar fouling mechanisms occurring during filtrations preformed at 50 C, i.e., more intense involvement of precipitated calcium salts in filtration resistances formation causing thereat lower flux values. Considering all of the data obtained, regardless of the applied process temperature, MF of sweet whey by a 0.5 mm ceramic
membrane proved to be an effective pre-treatment for purposes of flux enhancement during the subsequent UF. Also, it is obvious that performing UF of fresh or MF whey at 20 C or, for microbiological reasons, even lower temperatures, might be more effective solution instead of applying commonly used higher filtration temperatures (50e55 C) irrespective of the applied pre-treatment. 3.3. Filtration resistances with respect to calcium and whey protein retention, the applied whey pre-treatment and ultrafiltration temperature The highest total filtration resistance (Rt) was observed for UF of pasteurised whey at 50 C (Fig. 4A), with values 2e4 times higher compared with values for UF of fresh whey and even 8e10 times higher than values for MF whey (Fig. 4A). During UF of pasteurised or fresh whey at 50 C, the Rt was higher at the membrane inlet (S1
Fig. 3. Retention (R) of native b-lactoglobulin B (A), a-lactalbumin (B) and calcium (C) in different membrane sections (S1eS4) after 120 min of ultrafiltration of fresh ( , ), pasteurised ( ; 73 C for 20 s) or microfiltered ( , ; 0.5 mm membrane) whey at 20 C ( , ) and 50 C ( , , ) using a ceramic 20 kDa membrane.
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Fig. 4. Total filtration resistance (Rt) formed after 120 min of ultrafiltration of fresh ( , ), pasteurised ( ; 73 C for 20 s) or microfiltered ( , ; 0.5 mm membrane at 20 C ( , ) or 50 C ( , , ) and comparison with the initial membrane (Rm, ) resistance (A); and comparison of the initial membrane (Rm, ) resistance with reversible (Rrev, ) and irreversible (Rirev, ) fouling resistances formed in different sections (1e4) across the membrane after 120 min ultrafiltration of fresh whey and MF whey at 20 C (B and C, respectively) and at 50 C (D and E, respectively).
and S2) and decreased gradually towards the membrane outlet (Fig. 4A). In contrast, the Rt for UF fresh and MF whey at 20 C and MF whey at 50 C was more or less equal along the membrane (Fig. 4A). Since UF membranes almost entirely retain the present protein components, many authors have suggested whey proteins, especially their aggregates, to be the main foulants (Baldasso, Barros, & Tessaro, 2011; Jonsson, Pradanos, & Hernandez, 1996; Song, 1998). Among the four possible mechanisms of membrane fouling (pore narrowing, pore plugging/blockage, gel formation, selective plugging of larger pores) suggested by Saxena et al. (2009), cake/gel formation and classical pore plugging were claimed to predominate n-Ba guena, Alvarez-Blanco, during whey UF (Corbato & VincentVela, 2015). Indeed, as presented in Fig. 3, retention of b-Lg fractions was above 85% (Fig. 3A) and for a-La above 80% (Fig. 3B),
regardless of the applied temperature and/or whey pre-treatment. The BSA fraction was entirely rejected, since it was not detected in any of the analysed whey permeates (data not shown). Reversible fouling (Rrev), generally related to cake/gel formation, predominated in almost all of the investigated filtrations and contributed the most to Rt (Fig. 4BeE). The highest values of Rrev were recorded after UF of pasteurised whey (data not shown) following by values recorded after UF of fresh whey at 50 C (Fig. 4D). In contrast, significantly lower Rrev values were observed after UF of fresh whey at 20 C (Fig. 4B), and especially after UF of MF whey (Fig. 4C and E). Thus, particle deposition was obviously reduced either when lower process temperature was applied or when whey was previously subjected to MF. By applying MF prior to UF, considerable removal of microbial cells was achieved (Table 1), but most probably also a great part of large particles,
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which resulted in considerable decrease of total fouling resistance (Rt) recorded during the subsequent UF (Fig. 4). According to models suggested by Altmann and Ripperger (1997a, 1997b) and the findings of Piry et al. (2008), decreases in friction forces of filtrate flow along the membrane determine the deposit layer formation during UF of solutions containing proteins. Under such conditions, higher deposition and, consequently, more intense fouling mechanisms occurred at the membrane inlet. However, due to the relatively high transmembrane pressures (S1 ¼ 1.95 105 Pa; S2 ¼ 1.60 105 Pa) and wall shear stress (tw ¼ 120 Pa), the reached flux values were proportionally high (Fig. 2). Accordingly, during UF of fresh or MF whey at 20 C the observed retention of b-Lg and a-La fractions slightly decreased over the first three sections of the membrane, while an apparent drop (Rß-Lg ~ 95%) was observed at the membrane outlet (Fig. 3A and B). Hence, the recorded filtration resistances obviously relied on protein deposition solely since the recorded retention of whey proteins (Fig. 3A and B) was among the highest, while calcium retention was moderate to low (Fig. 3C) at 20 C. Higher viscosity (Table 1) as well as high wall shear stress apparently created an equilibrium, resulting in almost equal deposit and filtration resistance formation across the membrane (Fig. 4A and B). b-Lg showed to have the greatest influence on fouling since it had high retention rates (Fig. 3A) and was the dominant fraction in whey (Table 1). aLa obviously played an important role too, especially during UF of fresh and pasteurised whey at 50 C where the highest retention was observed at the membrane inlet (S1 and S2; Fig. 3B) and was accompanied with the highest formation of the deposit (Fig. 4D). Pasteurised whey did contain notable amounts of protein aggregates (Fig. 1) that were responsible for extremely higher filtration resistance and deposit formation. Such results fully support findings of some former studies proposing that even in exceptionally small quantities, protein aggregates might cause considerable fouling, most probably by serving as “anchors” that promote further interactions and crosslinking of proteins and/or minerals (Kelly & Zydney, 1995; Marshall, Munro, & Tr€ agårdh, 1997, 2003; Steinhauer et al., 2015). Calcium did not play a significant role during UF of pasteurised whey due to low retention (Fig. 3C). In contrast, the highest retention of calcium was recorded at UF of fresh or MF whey (Fig. 3C), thus indicating notable involvement of minerals in fouling mechanisms, which probably relied on interactions between the deposited proteins and calcium. Hence, deposit layer formation most probably occurred according to some earlier established models (Bacchin, 2004; Song, 1998), which proposed higher deposit formation at the membrane outlet due to the decrease in the diffusive back transport across the membrane. However this appears to be contrary to expectation; the reason why these authors came to their conclusion could be that they worked with small pieces of membrane with little difference in DpTM between inlet and outlet. Several studies also suggested the possible involvement of calcium in fouling mechanisms during filtrations of milk or whey, but did not confirm whether it considerably influenced permeate flux (DuclosOrselloa, Lib, & Ho, 2006; Konrad et al., 2012; Marshall, Munro, €gårdh, 2003; Musale & Kulkarni, 1998). According to the & Tra results of present study, the influence of calcium was obviously crucial for permeate flux decrease during UF of fresh or MF whey at 50 C. 4. Conclusions According to the results obtained with regard to the application of 20 kDa ceramic membranes, UF of pasteurised whey at 50 C
provided the least effective filtration conditions, despite being commonly used at industrial scale. The obtained flux values were the lowest (2e5 times lower) while the recorded total filtration resistance (Rt) was extraordinary high (up to 10 times higher) in comparison with all other tested regimes. In contrast, UF of fresh whey, especially at 20 C, proved to be much more effective providing a twofold higher flux and a 2e4 times lower total filtration resistance. Applying MF with a 0.5 mm ceramic membrane prior to UF resulted in a significant flux increase (4.5 times higher), while the recorded total filtration resistance was up to 10 times lower in comparison with conventionally applied UF conditions. MF also resulted in almost equal total bacteria number reduction (around 3.8 log cycles) to that given by pasteurisation (4.81 log cycles), thus delivering proper microbiological quality of whey. Accordingly, MF pre-treatment using a 0.5 mm ceramic membrane considerably enhanced UF performance, and applying the lower process temperature (20 C) appeared to be the most convenient due to the highest flux and the lowest fouling intensity observed. Acknowledgement The data presented in this study are based on work co-financed by the Croatian Science Foundation (Project Nr. 03.01./77). References Altmann, J., & Ripperger, S. (1997a). Particle deposition and layer formation at the cross flow filtration. Journal of Membrane Science, 124, 119e128. Altmann, J., & Ripperger, S. (1997b). Beitrag zur Modellierung der Deckschichtbildung bei der Querstrom-Mikrofiltration [Contribution to modelling the boundary layer formation during cross-flow microfiltration]. Chemie Ingenieur Technik, 69, 468e472. Bacchin, P. (2004). A possible link between critical and limiting flux for colloidal systems: consideration of critical deposit formation along a membrane. Journal of Membrane Science, 228, 237e241. Baldasso, C., Barros, T. C., & Tessaro, I. C. (2011). Concentration and purification of whey proteins by ultrafiltration. Desalination, 278, 381e386. Baruk ci c, I., Bo zani c, R., & Kulozik, U. (2014). Effect of pore size and process temperature on flux, microbial reduction and fouling mechanisms during sweet whey cross-flow microfiltration by ceramic membranes. International Dairy Journal, 39, 8e15. n-B Corbato aguena, M. J., Alvarez-Blanco, S., & Vincent-Vela, M. C. (2015). Fouling mechanisms of ultrafiltration membranes fouled with whey model solutions. Desalination, 360, 87e96. Duclos-Orselloa, C., Lib, W., & Ho, C. C. (2006). A three mechanism model to describe fouling of microfiltration membranes. Journal of Membrane Science, 280, 856e866. Jonsson, G., Pradanos, P., & Hernandez, A. (1996). Fouling phenomena in microporous membranes. Flux decline kinetics and structural modifications. Journal of Membrane Science, 112, 171e183. Kelly, S. T., & Zydney, A. L. (1995). Mechanisms for BSA fouling during microfiltration. Journal of Membrane Science, 107, 115e127. Konrad, G., Kleinschmidt, T., & Faber, W. (2012). Ultrafiltration flux of acid whey obtained by lactic acid fermentation. International Dairy Journal, 22, 73e77. Koutsoukos, P. G. (2007). Current knowledge of calcium phosphate chemistry and in particular solid surface-water interface interactions. Available at http://www. nhm.ac.uk/research-curation/research/projects/phosphate-recovery/ Nordwijkerhout/Koutsoukos.pdf. last accessed: 22.01. 15. Kühnl, W., Piry, A., Kaufmann, V., Grein, T., Ripperger, S., & Kulozik, U. (2010). Impact of colloidal interactions on the flux in crossflow microfiltration of milk at different pH: a surface energy approach. Journal of Membrane Science, 352, 107e115. Kulozik, U., & Ripperger, S. (2008). Innovative Eins€ atze der Membrantrenntechnik in der Lebensmitteltechnologie [Innovative Applications of membrane processes in Food Technology]. Chemie Ingenieur Technik, 80, 1045e1058. Kumar, P., Sharma, N., Ranjan, R., Kumar, S., Bhat, Z. F., & Kee Jeong, D. (2013). Perspective of membrane technology in dairy industry: a review. Asian-Australasian Journal of Animal Sciences, 26, 1347e1358. Law, A. J. R., & Leaver, J. (1997). Effect of protein concentration on rates of thermal denaturation of whey proteins in milk. Journal of Agricultural and Food Chemistry, 45, 4255e4261. €gårdh, G. (1997). Influence of permeate flux on Marshall, A. D., Munro, P. A., & Tra fouling during the microfiltration of b-lactoglobulin solutions under cross-flow conditions. Journal of Membrane Science, 130, 23e30.
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