Introduction
1.5 State of the art
1.5.3 Applications of membrane technology
1.5.3.2 Clarification of mosambi juice Literature survey
Fruit and vegetable juices are beverages of high nutritional value. These beverages constitute several key components beneficial for human health such as minerals, vitamins and antioxidants. Traditional methods for juice processing involve filtration using fining agents such as gelatin or diatomaceous earth to remove suspended and colloidal particles and low pressure evaporation [60]. Unfortunately during these processing steps, a major portion of the compounds that contribute towards the quality of the beverage (such as aroma, flavor
compounds, sugar content and acidity) get deteriorated due to thermal and chemical treatment steps [61]. In this regard, the application of membrane technology is found to be beneficial when compared with the conventional methods due to elimination of thermal and chemical processing steps, low energy requirement, lower processing times and ease of scaling up without significant change in juice quality [2]. UF and MF of mosambi [62, 63], orange [64, 65], lemon [66], grape [67], apple [68 - 70], carrot [71], water melon [72, 73], blood orange [74] and pineapple [75] juice have been studied by many investigators. Amongst several beverage processing sectors, citrus fruits constituting orange, lemon, pineapple and mosambi are prominent due to their wider availability, low cost as well as high nutrition value to human health. Citrus fruits primarily constitute both lower molecular weight compounds (such as sugar, acid, salt, flavor, aroma compounds, etc.) as well as higher molecular weight polysaccharides (such as pectic material, cellulose, hemicellulose etc.) in addition to haze causing proteins and microorganisms. The presence of pectic material and protein in fruit juice is responsible for cloudiness and post bottling haze formation as well as their fermentation during long storage [2]. The objective of clarification of fruit juice using membrane filtration is to eliminate the high molecular weight pectic material and its derivatives and retain low molecular weight solutes (valuable for human health) such as sucrose, acid, salt, aroma and flavor compounds in the clarified juice [2].
Rai et al., [76] identified optimal conditions (pectinase concentration: 0.0004 w/v%, heating temperature: 42 oC, duration: 100 min,) for enzymatic treatment of mosambi juice. Amongst several treatment methods, the authors indicated that, enzymatic treatment followed by bentonite addition provided the highest permeate flux (23 L/m2.h at 414 kPa and 1200 rpm in a stirred cell) using 50,000 molecular weight cut-off (MWCO) polyamide (polymeric)
membranes. Cassano et al., [74] conducted UF of blood orange juice using commercial tubular polyvinylidenefluoride membranes. They observed that with an increase in operating temperature from 21 - 25 oC, 12 % increase in permeate flux occurred. Jesus et al., [64]
carried out RO of orange juice using polysulphone/polyethylene composite membrane. They observed that orange juice concentrated by reverse osmosis had a better-preserved characteristic aroma when compared to the juice concentrated by thermal evaporation.
Recently, Sarkar et al., [62] studied electric field assisted UF of mosambi juice using polyethersulfone membranes. They observed that the application of electric field reduced membrane fouling and increased permeate flux significantly. Two key factors that influence the permeation characteristics of membranes for beverage processing are the average particle size of the beverage feed and the average pore size of the membrane. The selection of appropriate average pore diameter ( ) for polymeric membranes and ceramic membranes has been investigated by several authors. Rai et al., [63] observed that polymeric membranes with higher did not influence the permeate quality to a large extent during UF of depectinized mosambi juice. However, a ten fold enhancement in membrane flux was observed when of the membrane varied from MWCO of 10 kDa to 0.2 µm. A similar observation has been reported by Youn et al., [68] after conducting UF and MF studies on reconstituted apple juice using two different membranes (MWCO 30 kDa and of 0.01 µm) and different filter-aid pre-treatment steps. Though significant quality variation was not observed, the membrane flux for the MF membrane was observed to be two to three folds higher than the flux obtained with the UF membrane. Jegatheesan et al., [77] reported the performance of zirconia ceramic membranes with of 0.02, 0.05 and 0.10 µm during UF of sugarcane juice. The authors thereby inferred that the juice quality obtained from all three membranes are comparable and
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membranes with pore sizes 0.02 - 0.05 µm provided better quality than the membrane with 0.10 µm. Also, the average permeate flux for 0.05 µm membrane was about 1.8 times higher than that obtained for 0.02 µm membrane. Barros et al., [78] reported the performance of polysulfone hollow fiber membrane (100 kDa) and alumina-titania ceramic tubular membrane (0.01 µm) for the clarification of pineapple juice. Similar juice quality was obtained for both the membranes. A higher membrane flux was observed for the ceramic membrane (90 - 150 kg/m2.h) compared to the polymeric membrane (30 - 50 kg/m2.h).
Possible scope for further research
A critical insight into the above literatures infers that mainly polymeric membranes have been studied during experimental investigations. This is due to the fact that polymeric membranes are inexpensive in comparison with the ceramic membrane and are very easy to use.
However, the major drawback of polymeric membrane is their low corrosion resistance in acidic media and hence they possess lesser life cycle times for juice processing applications.
For any industrial application the lifespan of membrane is always a major issue. Many varieties of fruit juices are acidic in nature with pH in the range of 3 - 5. Henceforth, though experimental studies indicate the successful applicability of polymeric membranes for juice processing applications, industrialization of membrane technology for juice processing has not been promising so far. For these circumstances, the usage of ceramic membranes with high corrosion resistance in both acidic as well as basic media shall be more beneficial, especially in the citrus juice sector. Vladisavljevic et al., [70] carried out UF of depectinized apple juice using commercial zirconia ceramic membranes. Jegatheesan et al., [77] reported the performance of commercial zirconia-alumina ceramic membranes with various pore sizes (200, 500 and 1000 nm) for the UF of sugarcane juice. However, the clarification of mosambi
juice using a ceramic membrane, particularly low cost ceramic membrane has not been studied and is the main objective of this study.