Journal of Food Engineering

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1 Journal of Food Engineering 89 (8) 7 77 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: Analyses of hydrodynamic resistances and operating parameters in the ultrafiltration of grape must A. Cassano *, A. Mecchia, E. Drioli Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, via P. Bucci, Cubo 7/C, I-87 Rende, Cosenza, Italy article info abstract Article history: Received January 8 Received in revised form April 8 Accepted April 8 Available online 8 April 8 Keywords: Ultrafiltration Fouling Must clarification Resistances Grape must destined to the production of white wine was clarified by cross-flow ultrafiltration (UF), on laboratory scale, by using polysulphone hollow fibre membranes with a molecular weight cut-off of kda. The effects of transmembrane pressure (DP), feed flow rate (Q f ) and temperature (T) on permeation flux and resistances were studied. Results showed that permeate fluxes decreased with time until a steady-state was established. A maximum steady-state permeate flux was reached at a DP of about kpa. Steady-state permeate fluxes increased with temperature in the range 9 C. It was also observed that flux increased by increasing bulk flow rates according to the concentration polarization model. The permeate flux decay was analysed through the resistance-in-series model. Analyses of results showed that the membrane resistance (R m ) was constant, while both cake layer resistance (R c ) and fouling resistance (R f ) increased with DP and decreased with Q f. An increasing of temperature determined a significant decreasing of the cake layer resistance in the range C, while the fouling resistance showed a little increase in all the range of the investigated temperatures. Observed rejections of the UF membrane towards total soluble solids (TSS) and total phenolics increased by increasing DP in the range kpa. A decreasing of the color intensity associated to an increasing of the clarity of the clarified must was observed when the pressure is raised. ph and acidity of the clarified must were not affected by the operating pressure. Ó 8 Elsevier Ltd. All rights reserved.. Introduction * Corresponding author. Tel.: ; fax: address: a.cassano@itm.cnr.it (A. Cassano). In wine making grape must characteristics affect remarkably the final quality of the product. In addition, wine making is subject to numerous rules and restrictions dictated by local, national and international regulations. Must clarification to remove suspended solids and undesirable substances is one of the most important preliminary steps of the oenological cycle, especially in the white wine production. However, due to the seasonality of the grapes harvest, working times, also for big productions, are very short. Consequently conventional separation techniques such as static defecation, centrifugation and depth-filtration are often unsuccessful and difficult to realise. These processes, being often discontinuous, require extensive spaces, due to the intrinsic limitations imposed by the dimensions of vats, high energy consumptions for the temperature control and high quantities of coadiuvants and additives (bentonite, gelatines, activated carbon, casein, silica sol, diatomaceous earth, etc.) with consequent problems of disposal and environmental impact (Drioli and Todisco, 999; Drioli and Molinari, 99). Membrane processes play today an important role in the treatment of musts and wines. In particular, cross-flow microfiltration (MF) and ultrafiltration (UF) represent a valid answer to the problem of the must clarification (Güell, 999; Rektor et al., ; Salazar et al., 7). These processes can be operated at low temperatures, do not require a high energy consumption and permit to separate bacteria cells and spores, to reduce the SO addition, to remove totally the suspended solids, to reduce remarkably the pectin content, proteins and colloids giving a sterilised and clear product (without modifications of the sugar content) in one single continuous operation (Czekaj et al., ). However, the main limiting factor of these processes is the permeate flux decay over time caused by the accumulation of feed components in the pores (membrane fouling) and on the membrane surface (concentration polarization and gel formation) (Czekaj et al., ). Membrane fouling has been investigated by numerous membrane researchers since it reduces productivity and membrane life (Nilsson, 99; Mulder, 99; de Bruijn et al., ; Jiraratananon and Chanachai, 996; Jiraratananon et al., 998; de Barros et al., ). In most studies UF has been modelled using hydrodynamic theories based on the cake formation on the membrane surface which 6-877/$ - see front matter Ó 8 Elsevier Ltd. All rights reserved. doi:.6/j.jfoodeng.8..

2 7 A. Cassano et al. / Journal of Food Engineering 89 (8) 7 77 offers a hydrodynamic resistance to permeate flow (Fane and Fell, 987). This concept can be also applied to the UF of grape must since a cake or a gel layer is formed (Vladisavljević et al., ). In this work, the effect of operating conditions such as temperature, feed flow rate and transmembrane pressure on membrane fouling and permeate fluxes of polysulphone hollow fibre membranes, during the UF of grape must destined to the production of white wine, was investigated. Experimental results were analysed in terms of resistances to permeation flux by using the resistance-in-series model. The influence of the UF treatment and DP on juice composition was also evaluated.. Materials and methods.. Must All experiments were performed on white must obtained from grape cultivar grown in Southern Italy (varietal Verdeca) and supplied by ITEST Srl (Corato, Bari, Italy). Must was refrigerated and decanted at low temperature ( C). Potassium meta-bisulphite ( mg/l SO ) was added to sterilize and to preserve the must before use... UF experimental setup and procedure UF experiments were performed using the laboratory unit supplied by Verind SpA (Milan, Italy) shown in Fig.. The equipment consists of a litres stainless steel feed tank, a feed pressure pump, two manometers ( kpa) located at the inlet (P in ) and at the outlet (P out ) of the membrane module and a magnetic flow meter for the measure of the axial feed flow rate (Q f ). Transmembrane pressure (DP) was calculated as DP =(P in + P out )/. A tube and shell heat exchanger, placed after the feed pump, was used to maintain the temperature of the grape must constant. A data acquisition system, permitting the continuous monitoring of the DP and of the axial feed flow rate, was connected to the UF plant. A digital balance, connected to the system, was used to measure the permeate fluxes Fig.. Scheme of the experimental setup: () feed tank; () feed pump; (,6) manometers; () membrane module; () thermometer; (7) heat exchanger; (8) pressure valve; (9) flowmeter; () permeate; () digital balance. 6 Table Characteristics of UF membrane module Type DCQ III-6C Configuration Hollow fibre Membrane polymer Polysulphone Nominal molecular weight cut-off (kda) Membrane surface area (m ). Dimension (mm) 9 Inner fibre diameter (mm). ph operating range Temperature operating range ( C) Typical operating pressure (bar). The UF plant was equipped with a hollow fibre membrane module (Table ) supplied by China Blue Star Membrane Technology Co., Ltd. (Beijing, China). UF experiments were performed according to the total recycle configuration in which the permeate and retentate streams were continuously recycled to the feed tank to ensure a steady-state in the volume and the composition of the feed. Experimental trials were devoted to the determination of the optimal operating and fluid-dynamic conditions (DP, axial feed flow rate and temperature) for the clarification process. In particular, DP was modified in the range kpa. The flow rate through the membrane module was in the range of 6 l/h, which was equivalent to a mean velocity of.8. m/s. The effect of the temperature was investigated in the range 9 C. For each set of experiments, two parameters were fixed and the other one was varied to cover the ranges indicated. The permeate flux was measured every min for min... Membrane cleaning and hydraulic permeability The water flux was determined by feeding tap water to the membrane module and by measuring the volume of permeate (V permeate ) collected in a certain time t through the membrane surface area A according to the equation: J ¼ V permeate t A where A is the inner surface area, since it is usually denser and is supposed to offer the main resistance. The hydraulic permeability measured for the new clean membrane in fixed conditions of temperature ( C) at different DP values was indicated as L p. It was given by the slope of the straight line obtained by plotting J vs. DP. The hydraulic permeability measured after the treatment with grape must was indicated as L p. After the experiments with must, the membrane module was cleaned in two steps. The first cleaning step was performed by recirculating tap water for min at kpa and 6 l/h through the membrane module in order to remove the reversible polarized layer. The hydraulic permeability measured afterwards was L p.in the second step the membrane module was submitted to a cleaning procedure using an alkaline solution (Ultraclean, WA, %, 6 min, C). Then the membrane module was submitted to a final rinsing with tap water... Analyses of resistances The decline of the permeate flux was analysed through the resistance-in-series model (Cheryan, 986; Mulder, 99; de Bruijn et al., ; Jiraratananon, and Chanachai, 996). The permeate flux for UF is usually written in terms of DP and total resistance, as reported in the following equation: J p ¼ DP ðþ lr t ðþ

3 A. Cassano et al. / Journal of Food Engineering 89 (8) where J p is the permeate flux (m/s), DP is the transmembrane pressure (kpa), R t is the total resistance (m ) and l is the permeate viscosity (Pa s). R t is given by R t ¼ R m þ R c þ R f where R m is the intrinsic membrane resistance; R c is the cake layer resistance due to the concentration polarization and the deposition of solids on the membrane surface; and R f is the fouling resistance due to the internal fouling inside the pores. Experimentally the resistances defined in Eq. () can be determined from the values of the hydraulic permeability after the cleaning procedures described earlier. In particular, R m was calculated by measuring the hydraulic permeability of the new or clean membrane as: R m ¼ ðþ l w L p where l w is the viscosity of water (Pa s) and L p ¼ J w=dp is the hydraulic permeability (m s Pa ) of the new membrane. R t was calculated by using the following equation: R t ¼ l w L p in which L p ¼ J w =DP is the hydraulic permeability of the membrane after the treatment with the grape must. The cleaning procedure described in the previous section allows the evaluation of R c and R f. R c is removed by cleaning the membrane with water. The hydraulic permeability measured after such cleaning is L p, therefore: R m þ R f ¼ l w L p Each resistance can be readily calculated using the experimental data and Eqs. () (6)... Grape must characterization Feed and clarified must samples were analysed for color, clarity, total soluble solids (TSS), ph, acidity and total phenolics. Color, clarity, total soluble solids (TSS) and ph were determined according to official methods (GUCE, 99). The color (optical density) was measured by absorbance at nm, clarity by transmittance at 6 nm using a Shimadzu UV vis Recording Spectrophotometer (UV-6A, Shimadzu Scientific Instruments Inc., Japan). TSS measurements were carried out by using an Abbe refractometer Atago NAR-T (Atago Co., Ltd., Tokyo, Japan). Prior to each set of measurements the instrument was calibrated at Brix by using deionised water. Measurements were made at the standard temperature, C. ph was measured by an Orion expandable ion analyzer EA 9 ph meter (Allometrics Inc., LA, USA). The acidity was determined by titration with. N NaOH according to Office International de la Vigne et du Vin methods (OIV, 99). Results were expressed as percent tartaric acid. Viscosity was measured by using a Brookfield DV-III Ultra viscometer (Brookfield Engineering Laboratories Inc., MA, USA). Total polyphenols were estimated colorimetrically using the Folin Ciocalteau method (Singleton and Rossi, 96) and the results were calculated as mg/l gallic acid. The rejection (R) of the UF membrane towards a specific compound was calculated as: R ð%þ ¼ C p ð7þ C f where C f and C p are the feed and permeate concentration, respectively. ðþ ðþ ð6þ. Results and discussion.. Hydraulic permeability and membrane resistance The hydraulic permeability of the new clean membrane was 9.6 l/m h bar at C. Membrane resistance, R m, was calculated from Eq. () to be.87 m. After the cleaning procedure the membrane resistance was reported to its original value as observed from the hydraulic permeability data... Effect of operating conditions on permeate flux Fig. shows the time course of permeate flux in the range kpa at constant values of feed flow rate ( l/h) and temperature ( C). Each curve can be divided into two domains. Domain corresponds to a rapid decay of the permeate flux which can be attributed to the deposition and growth of a polarized layer formed by high molecular weight compounds present in the grape must (Rai et al., 7). Domain is characterized by a less pronounced flux decay followed by the achievement of a steady-state value. The initial flux decline in the first domain was % of the total flux decline. The steady-state was established after 8 min of operation and steady-state permeate fluxes were % of their initial values. The permeate flux-pressure behaviour is in agreement with results obtained by Rektor et al. (). Steady-state permeate flux values increased by increasing the applied pressure. It is also clear that the flux decay is more pronounced at higher DP values. Fig. shows the effect of DP on the steady-state permeate flux: the permeate fluxes show a linear increase with DP at lower pressures, while at higher pressures the permeate fluxes approach a limiting value (J lim ) independent of further increases in pressure. The point at which the pressure independence is evident is considered the optimum DP ( kpa). Fig. shows the time course of the permeate flux at different feed flow rates in the range 6 l/h at constant values of DP ( kpa) and temperature ( C). It may be noted that permeate flux profiles show similar trends. According to the film model theory (Fane and Fell, 987) the steady-state permeate flux increased by increasing the feed flow rate (Fig. ). In particular, an increase in recirculation reduced concentration polarization, enhanced the mass transfer coefficient and increased permeation flux. The variation of permeate flux with time at different temperatures (range 9 C) and at constant values of DP ( kpa) and feed flow rate (7 l/h) is illustrated in Fig. 6. At higher tempera- J p (l/m h) kpa kpa kpa kpa 6 8 Operating time (min) Fig.. Time course of permeate flux at different DP values (T = C; Q f = l/h).

4 7 A. Cassano et al. / Journal of Food Engineering 89 (8) 7 77 Steady-state permeate flux (l/m h) J p (l/m h) C C C 9 C TMP (kpa) Operating time (min) Fig.. Effect of DP on permeate flux (T = C; Q f = l/h). Fig. 6. Variation of permeate flux with time at different temperature values (D- P =. bar; Q f = 7 l/h). J p (l/m h) l/h l/h l/h 7 l/h Steady-state permeate flux (l/m h) 6 8 Operating time (min) Fig.. Variation of permeate flux with time at different feed flow rate values (T = C; DP =. bar). Temperature ( C) Fig. 7. Effect of temperature on permeate flux (DP =. bar; Q f = 7 l/h). Steady-state permeate flux (l/m h) Q f (l/h) Fig.. Effect of flow rate on permeate flux (T = C; DP =. bar). tures ( and 9 C) a more rapid decline of permeate flux was observed in the first stage of the process. The steady-state permeate flux increases linearly in the range of the investigated temperature (Fig. 7). In particular, an increase in the flux of %, at steady-state, was observed when the temperature was raised from to 9 C. This phenomenon can be attributed to the reduction of the feed viscosity (Fig. 8) and to the increase of the diffusion coefficient of macromolecules. The effect of these two factors is to enhance the mass transfer coefficient and to increase the permeation rate, according to the film model (Fane and Fell, 987)... Effect of operating conditions on resistances Fig. 9 shows the influence of DP on the total, fouling and cake layer resistance. As shown in the figure, R t increased in the range kpa following the same trend as R c. This phenomenon can be explained by assuming that an increase of pressure enhanced flux and convective flow of the solute towards the membrane. Consequently the concentration polarization is more pronounced, determining an increase of R c. In particular, for pressure values higher than kpa the permeate flux was controlled by the cake layer resistance. As reported by Labbe et al. (99) at higher pressures, more solutes such as sugars and acids passed through the membrane pores determining an increase of fouling resistance. In Table the percentages of each resistance to that of the total resistance, at different DP, are reported: at kpa R m gave the

5 A. Cassano et al. / Journal of Food Engineering 89 (8) Viscosity (mpa s) Resistance x - (m - ) R t R f R c R m 6 Temperature ( C) Fig. 8. Grape must viscosity vs. temperature Q f (l/h) Fig.. The effect of flow rate on resistances (T = C; DP =. bar). Resistance x - (m - ) R t R f R c R m 7 ΔP (kpa) Fig. 9. The effect of TMP on resistances (T = C; Q f = l/h). Table Contribution of membrane, fouling and cake layer resistances to the total resistance at different transmembrane pressures (T = C; Q f = l/h) DP (kpa) R m /R t (%) R c /R t (%) R f /R t (%) R m + R f /R t (%) R c + R f /R t (%) Table Contribution of membrane, fouling and cake layer resistances to the total resistance at different feed flow rates (T = C; DP =. bar) Flow rate (l/h) R m /R t (%) R c /R t (%) R f /R t (%) R m + R f /R t (%) R c + R f /R t (%) Temperature plays an important role in membrane fouling because of changes to the fluid characteristics which produce deposits (Watkinson and Wilson, 997). It is known that the bulk temperature and viscosity are inversely related; therefore, the effect of temperature on viscosity, and hence flow regime through the membrane, is very important (Saleh et al., 6). According to results reported by Jiraratananon et al. (996), the increase of the temperature enhanced back diffusion of solutes into the bulk solution, reducing consequently the thickness of the polarized layer: therefore R c decreased with temperature in the range C (Fig. ). This is consistent also with the findings of Goosen et al. () which report as the porosity of polysulphone membranes can be very sensitive to changes of feed temperature. Lower temperatures could also increase the formation of highest contribution to R t (about 7.6%) while the contribute of R c is modest (.6%); starting from kpa the contribution of R c is higher than the contribution given by R f. In Fig. the influence of Q f on the total, cake layer and fouling resistance is shown. It can be seen that R t decreased as Q f increased, showing the same trend of R c. R f remained nearly constant up to a Q f value of l/h and it decreased when Q f was raised. This phenomenon can be attributed to the enhancement of the mass transfer coefficient and to the reduction of the concentration polarization when Q f is raised. R m was the controlling resistance for the permeation over the whole range of Q f investigated. Data reported in Table show that the contribution of R c to R t decreased progressively up to reach a minimal value in correspondence of the higher Q f investigated. Resistance x - (m - ) R t R f R c R m T ( C) Fig.. The effect of temperature on resistances (DP =. bar; Q f = 7 l/h).

6 76 A. Cassano et al. / Journal of Food Engineering 89 (8) 7 77 Table Contribution of membrane, fouling and cake layer resistances to the total resistance at different temperatures (DP =. bar; Q f = 7 l/h) T ( C) R m /R t (%) R c /R t (%) R f /R t (%) R m + R f /R t (%) R c + R f /R t (%) R (%) TSS Total phenolics insoluble aggregates, increasing the secondary membrane that restricted the flow and reduced the pore size of the membrane. As showed in Fig., a little increase of both R c and R f, in the range 9 C, was observed. However, this weak increase did not affect the performance of the membrane in terms of permeate fluxes which increased by increasing the temperature. For low temperatures ( C) R f accounted only for 9 % of the total resistance and it was lower than R c.at9 C R f was.8% of R t and it was higher than R c (Table )... Effect of operating pressure on physiochemical composition of grape must 6 8 ΔP (kpa) Fig.. Observed rejections of total soluble solids (TSS) and total phenolics at different transmembrane pressures (Q f = l/h; T = C). In Table physiochemical properties of the clarified must (permeate) at different transmembrane pressures are reported and compared with the untreated must (feed). As showed in the table, the content of soluble solids and polyphenols decreased by increasing the applied pressure: consequently the observed rejections of the UF membrane towards these parameters increased by increasing the pressure (Fig. ). This phenomenon can be explained assuming that fouling increases with pressure (see Fig. 9) determining a reduction of the membrane pore size. The average rejection of the total soluble solids (8%) was lower than that of total phenolics (6.%): this phenomenon can be explained assuming that the soluble solids are mostly sugars of a molecular weight which is lower than that of total phenolics. Polyphenols are smaller than the molecular weight cut-off of the UF membrane used in this study ( kda): so the observed rejection can be justified assuming the formation of macromolecular aggregates between these compounds and proteins (Siebert et al., 996). A moderate rejection ( %) towards phenolics was also observed by Peri et al. (988) in the clarification of white and red wines by using a polysulphone UF membrane having a nominal molecular weight cut-off of kda. The absorbance at nm is a measurement of yellow-brown color in wine and must, it being used by many authors and wine making industries as an index of browning. Table shows a decreasing of the color intensity associated to an increasing of the clarity when the pressure is raised. This phenomenon could be linked to the removal of phenolic compounds responsible of the color. The clarified must presented acidity values higher than the original untreated must (and consequently lower ph values); however, the titratable acids and ph of the clarified must were not affected by the transmembrane pressure.. Conclusions Hollow fibre UF membranes with kda molecular weight cut-off were used to clarify grape must destined to the production of white wine. The steady-state permeate flux increased with DP until it reached a maximum value of about 7 l/m h at about kpa and then remained almost constant. An increasing of steady-state permeate fluxes was observed by increasing both temperature and feed flow rate. Under all conditions studied R f was..6 lower than R m and it was 9.7.8% of total resistance, R t. R m controlled the permeation flux over the whole range of temperatures ( 9 C) and feed flow rates ( 7 l/h) investigated. R c was the controlling resistance for the permeation flux at values higher than kpa. The best conditions for processing the grape must in order to obtain the maximum permeation flux should be at C, kpa of pressure and 7 l/h of flow rate. If the minimum fouling is the requirement, the best conditions should be at C, kpa and 7 l/h. The rejection of the UF membrane towards total soluble solids and total phenolics increased by increasing DP due to an increasing of fouling which determines a reduction of the membrane pore size. Acknowledgments The authors gratefully acknowledge Mr. Giuseppe Cicco and IT- EST Srl (Corato, Bari, Italy) which supported this work within a research Collaboration between ITEST and ITM-CNR. Table Physiochemical properties of untreated and clarified must at different transmembrane pressures (T = C; Q f = l/h) Sample Color (%A ) Clarity (%T 6 ) TSS ( Brix) ph Total phenolics (mg/l) Acidity (% tartaric acid) Feed Clarified must ( kpa) Clarified must ( kpa) Clarified must (6 kpa) Clarified must ( kpa) Clarified must ( kpa)

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