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Membrane technology for postharvest processing of fruits and vegetables Sean X Liu Department of Food Science, Rutgers University, New Brunswick, USA Abstract Purpose of the review: This article reviews current membrane technology in postharvest processing of fruits and vegetables and provides practical guidelines for selecting membrane technology for postharvest processing. An overview of the current state-of-the-art membrane technology in fruit and vegetable processing and a practical guideline for designing and scaling-up membrane systems are provided. Findings: Fruit juice clarification and concentration with membrane filtration dominate current application of membrane technology in postharvest processing; however, flavor compound retention and recovery by pervaporation and osmotic distillation has started gaining foothold in the industry. Other membrane-based concentration processes show a promising future. There is a strong interest in the food research community to utilize membrane technology to recover valuable food components from waste streams. Limitations/Implications: Wide acceptance of membrane technology in the food industry is not only limited by lack of “track record”, due to relative newness of the technology, but also limited by lack of suitable membrane systems or membrane materials. Many commercial membranes and modules are designed to accommodate the needs of water treatment comm.-unities (for example desalination) although there is evidence that the manufacturers are heeding the call from the food pro-cessing and biopharmaceutical industries. There is also a tendency for some individuals and membrane suppliers to exaggerate what membrane technology can accomplish and, as a result, the food industry as a whole is wary of any new membrane process that appears on the scene. Directions for future research: Future research should focus on the fundamental study of membrane fouling, particularly on interactions among biopolymers, inorganic matters, and the membrane surface. Manufacturers should develop a food-oriented evaluation and assessment protocol for constructing membrane property data sheets and improve the reliability and renew-ability of their membranes. The food science community needs to actively involve in basic research on membrane separations, not just run-of-the-mill type of study on concentration of yet another new exotic fruit (or vegetable) juice with a well-studied commercial membrane. Keywords: membrane technology; food processing; fruit and vegetable juices; membrane system design; membrane fouling Stewart Postharvest Review 2005, 2:1 Published online 01 August 2005 doi: 10.2212/spr.2005.2.1 Introduction Membrane technology in the fruit and vegetable industry is primarily used to concentrate or remove fluid food components by using polymeric or inorganic membranes. It is an integral part of postharvest processing of fruits and vegetables. The primary motivations for using membrane technology to replace traditional thermal evaporation are to substantially reduce the thermal damage done to thermally labile flavor compounds and improve sensory profiles of juice products. The second reason for using membrane technology is in energy saving. However, many membrane processes incur high capital and operational costs related to membrane replacement and cleaning so the overall economics should be considered when evaluating membrane technology. Additionally, some membrane processes cannot match the performance of thermal evaporation in terms of one-pass product concentration (low retention or selectivity) or have low permeation rates (low permeability). The concentration or removal of food com-ponents is achieved by enrichment of some food components of the liquid food across man-made membranes that sometimes produce chemical and physical separations at lower costs. The advantages of membrane technology are: · Most systems are simple, modular, flexible in operating mode, and compatible with the subsystems of the existing process · Membrane processes are gentle for thermally labile food components · Membrane processes are energy efficient · Membranes can be used to improve the economical value of the postharvest processing of fruits and vegetables Some noticeable disadvantages of membrane technology include: · Performance reduction over time due to concentration polarization and fouling near and on the membrane · Polymeric materials cannot maintain mechanical stability under conditions of high temperature, high pH and chlorine, and organic solvents · In some cases, sufficient separations are not always achievable It should be noted that membrane technology, though beneficial and sometimes indispensable, is just one of the many food processing technologies available, whose attractiveness must be weighed against other competing technologies. In the present paper, I attempt to critically summarize the characteristics of the main membrane processes available to postharvest processing of fruits and vegetables and of the current level of technological progress of membrane technology in the postharvest pro-cessing field. I also provide some practical guidelines for selecting membrane systems for app-lication and for dealing with operating issues such as membrane fouling and cleaning. Background Membrane technology is an emerging and evolving separation technology and, because of its multidisciplinary characters, it can be used to perform a large number of separations in post-harvest food processing. The membrane processes that are commonly found in food processing plants or research laboratories include membrane filtration (that includes microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), nanofiltration (NF)), electrodialysis (ED), membrane contactors (MCs) and pervaporation (PV) (Table 1). Membrane processes are based upon different separation principles or mechanisms and their applications in food processing range from concentration of food fluids to aromatic flavor recovery [1] . Despite these differences, all membrane processes have one thing in common – they all have a membrane that acts as a permselective barrier separating targeting food components from the feedstock and provides a large contact area in which a higher rate of mass transfer can take place. However, membrane separation can only be achieved when a driving force is applied to the underlying membrane process. A schematic diagram of a two-phase conceptual system is shown in Figure 1. The most important qualities of a membrane are high selectivity, high permeability, mechanical stability, temperature stability and chemical resistance [2]. Selectivity, the measurement of the ability of the membrane to retain or reject targeted food components, is considered as the number one criterion when considering using membrane processes to perform a given separation task. Other performance qualities of the membrane, though very important, are less critical. For example, it is possible to set up a membrane system with lesser permeation flux but high selectivity and compensate the low flux rate by using a multistage membrane process or increasing the membrane surface area. However, in doing so, one must consider the possibility of a better and more established separation technology that may exist and provide better economical and tech-nological advantages. In membrane-based fruit or vegetable juice concentration processes, one needs to pay close attention to product quality, namely flavor compound retention in concentrated juices. It is often misconceived that flavors in fruit juices will be retained entirely in a RO based concentration. In reality, flavors can be adsorbed into the membrane or penetrate through the membrane despite molecule size of the compounds and thus lost. Current polyamide RO membranes can hold on to up to 80% of flavor compounds [3]. Membrane filtration in postharvest food processing Membrane filtration in fruit and vegetable postharvest processing is often found in fruit or vegetable juice concentration and juice clarification [4]. They are also used in the treatment of food wastewater. The main drawback of membrane filtration – based on concentration of fruit and vegetable juices, mainly RO – is the low concentration it can achieve at a reasonable cost due to the steady buildup of osmotic pressure as the fruit juice concentrates. Another disadvantage of RO for concentrating fruit or vegetable juices is the loss of flavoring compounds. Microfiltration Among all membrane filtration processes, MF is the closest to an ordinary filter. It is used in a number of applications as either a pre-filtration step or as a process to separate a fluid from a process stream with a membrane pore size typically of 0.2-2µm, and is able to retain particles with molecular weights equal or larger than 200kDa. MF membranes are symmetric with A characteristic sponge-like network of inter-connecting pores. It has been successfully used in the food industry to remove bacteria or particulate substances from liquid food streams. Theoretically, microfiltration can be used to remove bac-teria from vegetable or fruit juices as in the “cold pasteurization” of draft beers. It can also be used in the treatment of food wastewater stream [5]. Due to its larger pore size, the prospect of using microfiltration may lie in the pre-treatment stage of fruit and vegetable juice processing prior to the other membrane-based concentration steps. Sometimes MF is used with ordinary bag filters or screen filters to remove particulates from the stock juice so that the severity of membrane fouling of the subsequent membrane separation step such as UF or RO will be reduced. The most successful application of UF and MF in fruit juice processing is in the pectin removal from apple or pear juice with the help of pectic enzymes in order to avoid clouding of apple or pear juice [6-9]. An excellent review of some major applications of membrane technology in food and beverage industry provides detailed information on history, applications, mechanisms, and modeling of various membrane processes involved in membrane-based food processing [10]. Ultrafiltration UF is the most common membrane process used in the dairy industry and it involves the use of membranes with a pore size ranging between 0.01-0.2µm. Applications of UF in food processing can mostly likely be found in situations that require the separation of one or more desirable food constituents that have a larger molecular weight cut-off (MWCO) of greater than 10kDa from a liquid mixture. The role of UF in post-harvest processing of fruit and vegetable juices is prominent in fruit juice clarification. As mentioned previously, an UF unit is often found in a fruit juice depectinization operation where UF is used to recover depectinization enzymes such as pectin methylesterase and poly-galacturonase that facilitate pectin removal from juices [11]. UF along with other filtration systems were also used for pre-treating fruit juices prior to RO concentration. Most UF systems used for fruit juice clarification appear to have MWCO between 50 to 100kDa [12-15]. Reverse osmosis RO will allow the removal of particles as small as ions from a solution and is capable of rejecting constituents that have a MWCO of greater than 0.15 to 0.25kDa. RO is widely used in desalination operations along with evaporation/distillation and is also used to remove water to produce concentrated fruit juices [16-20]. However, RO generally is unable to concentrate fruit juices greater than 25 – 30˚Brix with a single-stage RO system in the pressure range employed in a typical commercial RO unit due to high osmotic pressure as the juice becomes highly concentrated. Further concentration of fruit juice with RO entails high-energy costs and stringent equipment requirement. Nanofiltration NF is a newly invented term describing a class of membranes (and processes) with pore size and MWCO falling within the boundaries of UF and RO. Electrodialysis in postharvest food processing ED is a membrane process used for separating ions in the solution through selective permeable ion exchange membranes when a voltage is applied across a pair of anode and cathode that sandwich the ion exchange membranes situated in a number of compartments [21]. In addition to ions in solution, ionizable organic molecules can also be separated with ED. The most common use of electrodialysis in postharvest processing of fruits and vegetables is deacidification of fruit juices [22, 23]. Various ED systems with homopolar and bio-polar membrane for passion fruit juice deacidification were studied and the designs that consisted of more than two compartments or having both homopolar and biopolar membranes were found to work better than the traditional design of homopolar membranes in two compartments [24]. ED in deacidification of fruit juice is designed to compete with ion exchange pro-cesses. It is claimed that ED is better a method than the ion exchange because ED does not change organoleptic characteristics and gives better flavor [25]. However, ED is a rather expensive process in terms of energy and material consumption. In addition to passion fruit, other fruit juices such as orange, grape, pineapple, and lemon are also reported [26, 27]. Other uses or potential uses of ED in fruit or vegetable related areas include recovery of tartaric acid from fruit juice wastewater [28] and removal of nitrate and arsenic contaminants from vegetable juices. Membrane pervaporation in postharvest food processing PV, the separation of liquid mixtures by partial vaporization through a dense (non-porous) permselective membrane under a vacuum, has gain acceptance in food processing and food wastewater treatment [29-33]. It is best used for removing or concentrating volatile food components (flavor aromas) from liquid foods where the difference between the permeate and the background (solvent or other co-components). Unlike the other membrane processes, a phase change occurs when the permeate changes from liquid to vapor during its transport through the membrane. In fact, PV is an enrichment technique similar to distillation; however, unlike distillation, PV is not limited by the vapor-liquid equilibrium. There is a significant growth in the applications and technological developments in pervaporation; these progresses were critically reviewed by Peng, Vane, and Liu [34]. The most promising application of pervaporation in fruit juice processing is the development of simulated “freshly squeezed” juices on the industrial scale of fruit juice concentrate production. Due to the limitation of RO regarding con-centration of fruit juice, almost all industrial-scale production of fruit juice concentrate are made using the vacuum evaporation method. The significant drawback of the thermo-based fruit juice concentration method is the damage of the flavor profile of the fruit juice. Installing a pervaporation unit to remove flavor compounds prior to the evaporation step can minimize this damage of flavor profile. The flavor can later be added back to the concentrated fruit juice [33]. Pervaporation can also be implemented along with RO or other membrane-based processes (i.e. membrane contactors) to minimize flavor deterioration of fruit juices during membrane-based concentration operations. An industrial scale pervaporation unit is under testing and further fine-tuning in Europe [35]. Membrane contactors in postharvest food processing MCs are a motley group of several membrane processes that primarily use the membranes as mass transfer barriers for certain matters and interfaces between two phases. The driving force in the process is typically the difference in either vapor pressures or the osmotic pressures across the membrane barrier. The relevant membrane contactors for fruit juice processing are direct osmosis, membrane distillation, and osmotic distillation. Direct osmosis originates from an old practice for juice concentration and is caused by the difference of osmotic pressures between the dilute juice on the upstream side and the brine on the other side [36-38]. The disadvantages of DO are the high costs and low permeation rates even through DO concentration can reach a con-centration greater than those achieved by RO [3]. Membrane distillation (MD) utilizes the vapor pressure difference across the membrane resulting from temperature difference to drive the solvent molecules across the microporous hydrophobic membrane and condensed at the cold side of the membrane. MD can concentrate fruit juices up to 64ºBrix [39, 40]. Since temperature difference (subsequently vapor pressure difference) drives MD, an increase in temperature on one side of the membrane would increase permeation rate. However, temperature polarization becomes prominent at high temperatures and exerts a negative effect on permeation of MD [40]. The other problem with MD operations is that the process is limited in terms of operating tem-perature due to concerns about thermal damage to the flavor compounds in fruit juices. This limitation with MD makes osmotic distillation, also known as osmotic evaporation or isothermal membrane distillation, a better choice. In OD, a liquid mixture containing volatile components is contacted with a microporous, non-wettable membrane whose opposite surface is exposed to another liquid phase where the mass transfer takes place across the membrane [41]. This technology can selectively remove the water from fruit juices under atmospheric pressure and at room temperature hence eliminating thermal damage of flavors [42]. The OD process is actively being commercialized and incorporated into integrated postharvest processing of fruit juices [43, 44]. Like other membrane contactors, OD suffers from high costs and low permeation rate. Issues related to membrane technology for postharvest processing When contemplating the use of any particular membrane process for the separation of com-ponents in a liquid food stream, several process issues must be evaluated. The first step in doing so is to draw up the detailed requirements for the process. Accurate qualitative and, where possible, quantitative information on the following aspects should therefore be specified: · the components and range of concentrations in the feed · the intended use or fate of the treated feed liquids (i.e., final products, further processing, etc) · the intended use of or fate of the permeate (i.e., disposal, reuse, further processing, etc.); · permeation flux · the minimum properties of the treated food fluids and permeate that will make the intended use or fate possible · estimated costs and/or benefits of the outcome of application of a membrane process · information about other comparable non-membrane technologies. Any one or more of the above factors may, depending upon circumstances, influence the design of a membrane process. In many food processing applications, a membrane process can be an important intermediate operation that is vital to subsequent processing operations, and as such, an overall approach to developing a cost-effective integrated process is vital to the successful application of membrane-based technology. The next issue to be addressed is whether the membrane processes are actually capable of separating the components from liquid foods as expected. The answer to this question for the membrane filtration processes is generally affirmative provided that appropriate membranes (pore size, and for RO membrane properties such as charge, hydrophilic tendency) are used. For PV, the answer is more complicated and conditional. It is well documented that PV works well when the compound to be removed has a high vapor pressure relative to the background material and a low solubility in the background material (References here). In dilute aqueous solutions such as aromas in orange juice, it is generally the Henry’s constant that determines whether an aroma compound can effectively separated by pervaporation. The Henry’s constant represents the vapor-liquid partitioning of organic compounds in an aqueous system. The general rule of thumb is: the more dissimilar the components, the easier it will be to separate them. For ED, DO, MD and OD, the low process throughput is obviously disadvantageous for these processes to be used in the industry. There are several vendors providing these membranes [41], however, some knowledge gaps still exist in these fields thus warranting further studies by the food community. Once the potential process operator has determined that a particular membrane process will, theoretically, work, the subsequent questions to be answered are: · Does a membrane material exist which will do the job? · Is this membrane material available in a membrane module? The answer to the first question is usually positive. A great deal of membrane research has been performed on many membrane materials and feed mixtures. In addition, a wide array of membrane materials is available which may achieve the desired separation, but which have not been tested in a membrane mode of interest. The second question is about the issue of commercial availability of membrane configurations or membrane modules for particular membrane materials and the answer is usually optimistic. A module is the smallest unit into which the membrane material is packed. The reason for using modules is that although polymer membranes are made in two basic physical forms, flat sheet and tubular, many practical membrane systems that need large membrane areas can only be accommodated in membrane modules. For pressure-driven membrane filtration processes such as MD, OD, ED, DO and PV there are four primary configurations (modules), each with inherent advantages and disadvantages. These four configurations are spiral wound, hollow fiber, plate and frame, and tubular [1, 45, 46]. A thorny issue of membrane technology for food processing that has hindered the widespread use of membrane technology is the noticeable occurrence of concentration polarization/fouling in membrane processes. The detrimental effect of concentration polarization or membrane fouling added significant costs to the operator. In membrane filtration processes concentration polarization is formed as the result of the rapid accumulation of retained solutes near the membrane surface to the point that the concentration of macromolecule solute reaches the gel forming concentration and the retained molecules diffuse back into the bulk fluid. The cause of con-centration polarization in PV or ED is slightly different from that of MF in that it is triggered by the relatively slow diffusional mass transfer rates of solutes or ions from the bulk to the membrane surface. Membrane fouling is commonly observed as the membrane flux is continuously declining after a period of time of operation. This is usually an irreversible, partially concentration dependent, and time-dependent phenomenon, which distinguishes it from concentration polarization. The identification of membrane fouling often relies on the operator's experience, performing fouling tests with lab-scale static filtration experiments or silt density index (SDI) measurement, and membrane vendor's recommendations. Membrane fouling is intimately related to concentration polarization but the two are not exactly interchangeable in our description of membrane performance deterioration. The exact cause of membrane fouling is very complex and therefore difficult to depict in full confidence with available theoretical understandings. Fouling is influenced by a number of chemical and physical parameters such as concentration, size of particulates, pore size distribution, temperature, pH, ionic strength, and specific interactions (hydrophobic interaction, hydrogen bonding, dipole-dipole interactions) [45, 46]. Membrane fouling can be greatly reduced in several ways. One effective way of reducing membrane fouling is to provide pre-treatment to the feed liquids. Some simple adjustments such as varying pH values and using hydrophilic membrane materials can also provide some relief to membrane fouling. There are also persistent interests in modifying membrane properties to minimize the membrane-fouling tendency around the world. Since membrane fouling is intimately associated with concentration polarization phenomenon, any action taken to minimize concentration polarization will also help reduce membrane fouling. Fouled membranes can also be cleaned and regain some of the original performance however, frequent cleaning and washing with detergents will inevitably lead to the demise of the membrane. There are three basic types of cleaning methods currently used; hydraulic flushing (back-flushing), mechanical cleaning (only in tubular systems) with sponge balls, and chemical washing. When using chemicals to perform de-fouling, caution must be observed since many polymeric membrane materials are susceptible to chlorine, high pH solutions, organic solvents, and other chemicals [22]. Conclusion Unlike other industries, the food industry itself (including postharvest food processing operations) is seldom a driver of new processing technology. Many membrane processes (and membrane materials) in use for the food industry are adopted from other industries after years of successful practical uses in other industries. The reason for this idiosyncrasy is often attributed to low profit margin of food businesses, especially in the U.S.A. and the food industry’s aversion to risk involved in new processing technology. This reluctance of embracing new technology on the part of the food industry is really shortsighted. For example, in the case of membrane technology, the adopted membrane technology sometimes is entirely suitable for food processing since com-position of food and characteristics of food components are distinctly more different than those found in the other industries, which sometimes leads to underperformance of the membrane technology for food processing as shown in the membrane-based operations of concentrating fruit juices. This is unfortunate because the food industry is an energy intensive industry; nowhere is this more apparent than in the postharvest food processing of fruits and vegetables – current fruit or vegetable juice concentration or dehydration that requires enormous amount of thermal energy. Furthermore, the thermal labile nature of many food components clearly suggests that non-thermal membrane technology be much better. This trend can be reversed if the industry and governments are willing to invest in membrane-based food processing technology. New membrane materials and membrane processes for food processing need to be developed and tested so that the potential of membrane technology can truly be realized. Fundamental issues such as interactions between food components and the membrane materials and subsequent membrane fouling should be heavily investigated and modeling of membrane-based processes including integrated systems need to be vigorously pursued so that the industrial scale-up can be performed in a more meaningful way. The vendors of membrane materials and systems need to recognize the uniqueness of food systems and come up with a food-oriented evaluation standard. The food science community ought to be a proactive participant in basic research on membrane as well. The future of membrane technology in food processing seems good as more people move beyond the subsistence stage in the world and inevitably desire high quality products of fruits and vegetables that maintain excellent sensory characteristics, which may propel wide applications of membrane technology in postharvest food processing. Acknowledgements This work is supported financially by a Faculty Fellowship grant from United States National Aeronautics and Space Agency (NASA). References Papers of interest have been highlighted as: * Marginal importance ** Essential reading 1 Liu SX. Design of Membrane Separation Systems. In: Encyclopedia of Agricultural, Food and Biological Engi neering. Heldman DR (Ed.). New York: Marcel Dek ker; 2003: pp. 614-620. *It provides a brief introduction to membrane systems used in the food processing and a practical guideline for selecting membrane module and a cleaning procedure. 2 Rautenbach RA. Membrane Processes. 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Lancaster, Pennsylvania: Technomic Publishing Co. 1998 pp237-288. *A textbook/handbook for UF and MF. It covers all areas related to these processes. However, a lot of materials were originated from or explained by the examples from the food industry, reflecting the author’s long-time involvement in the membrane-based food processing. 46 Mulder M. Basic Principles of Membrane Technology. Dordrecht, Germany: Kluwer Academic Publishers 1991 pp 312-355. **A splendid textbook about membrane separations. New edition (2002) is published recently. |