Effect of operating parameters on permeate flux and fouling behavior during clarification of pomegranate juice using ultrafiltration polymeric membrane

Introduction: Pomegranate juice is classified in the group of clarified fruit juices. Therefore, clarification operation is necessary after the extraction process of pomegranate juice. Removal of most turbidity agents and colorants by the membrane process is possible without addition of chemicals. However, membrane fouling is an important subject in the clarification of fruit juices. The objective of the study was to evaluate the efficiency of ultrafiltration polymeric membrane for clarification of pomegranate juice under the different operating parameters (different pressures and temperatures)
Materials and methods: Pomegranate Malas variety (Punica granatum L.) was supplied from local market in Saveh and after transporting to laboratory, washed and juice was extracted by juicer. The cross-flow membrane pilot system with tubular ultrafiltration (UF) polymeric membrane (polyvinylidene difluoride (PVDF)) with kDa 20 MWCO was used. Clarification process was carried out at 2 levels of temperatures (20 and 30 ° C) and 3 levels of Transmembrane pressure (TMP) (1.5, 2 and 2.5 bar) for 45 min and the permeate flux was measured. Quality indices such as tannin retention rate, clarity, titratable acidity, total soluble solids, density, pH, apparent viscosity, phenolic compounds, total anthocyanin, decomposition constant, color intensity, browning index and amount of pectin in pomegranate juice were measured before and after membrane process. The efficiency indices of ultrafiltration process including permeability of pomegranate juice, percentage of reversible and irreversible fouling of the membrane and retention rate of the compounds were measured. Total resistance, intrinsic resistance of the membrane, concentration polarization resistance or reversible resistance and irreversible fouling resistance were calculated using the resistance-in-series model. In this research, the effect of process operating parameters including temperature and TMP on ultrafiltration process efficiency indices and quality attributes of pomegranate juice using factorial experiments in a completely randomized design using SAS software was analyzed.
Results and discussion: The results showed that the permeate-time curves were divided into 3 sections: I) The rapid decrease region of the permeate flux up to the 5th minute II) the slow decrease region of the permeate flux which begins from the 6th minute up to about 22-27th minute III) the constant flow area was from the 28th minute until the end of the operation. The rapid decrease in permeate flux could be attributed to the surface adsorption of colloidal particles such as pectins, tannins, polymeric colorants, and other polymeric materials present in pomegranate juice and the formation of concentration polarization layer. Increase of the pressure at 20 and 30 °C significantly increased the permeate flux (p <0.05). This was result of the increase in the driving force of the convective flow to the surface of the membrane. The experimental results indicated that increase of TMP from 1.5 to 2 and 2.5 bar at constant temperature of 20°C led to increase of the permeate flux in the steady state by 51.33 and 58.68%, respectively. The total resistance value decreased significantly with increase of the temperature from 20 to 30 ºC at different pressures. It could be attributed to the decrease of apparent viscosity and increase of the diffusion coefficient of the compounds from the cake layer into the retentate flow. Also, the total flux increased with increase of pressure despite rise of the resistance, because the rate of deposition of colloidal particles on the surface of the membrane was enhanced at high pressures. It resulted in more compaction of the deposited components on the surface of the membrane and formation of a thicker and denser cake layer with high resistance. The results showed that the effect of different pressures and temperatures on the intrinsic resistance of the membrane was significant and Rm significantly reduced with raising the temperature from 20 to 30 ºC (p <0.05). Also, increasing the pressure from 1.5 to 2.5 bar caused a significant increase in Rm. The highest contribution in the total resistance is related to the fouling resistance (Rrf + Rif), which indicates that the membrane fouling was severe. Rif had the lowest contribution in total resistance. Furthermore, Rif was increased with raising temperature. This is due to the higher penetration of particles into the membrane pores and blockage of the membrane pores at high temperatures. Raising temperature reduced the apparent viscosity and increased the diffusion coefficient of the compounds from the cake layer to the retentate flow and reduced the fouling. The main effects of pressure and temperature on reversible fouling were significant (p <0.05). With raising pressure and temperature, it was decreased and increased respectively. In all treatments, the value of reversible fouling was about 12-31%. The pomegranate juice pectin was reduced by 99% during the ultrafiltration membrane clarification process. Anthocyanin decomposition constant increased significantly with raising temperature from 20 to 30°C (p <0.05), due to the acceleration of the decomposition reactions and polymerization of anthocyanins. The retention rate of tannins increased with increasing temperature and pressure. It should be mentioned that raising pressure had higher effect on retention rate of tannins as compared with increasing the temperature. As the surface cake layer, as a secondary membrane, became denser and thicker, the permeability of tannin compounds decreased and its retention rate was increased.
Conclusion: Finally, it was found that clarification of pomegranate juice using ultrafiltration membrane, in all treatments decreased pomegranate juice pectin content about 99%. The treatments of 20°C- 2 bar and 30 °C- 2 bar had the lowest (4.25%) and the highest (15.315) anthocyanin retention value, respectively. The lowest (42.43%) and highest (50.93%) tannin retention values were observed at treatments of 20ºC- 1.5bar and 30ºC- 2 bar, respectively. It is also suggested that the organoleptic characteristics of the pomegranate juice should be evaluated in further works.