Membrane Separation Technology in the Food Industry
By Jayne Stratton
Membrane technology has been used in specialized applications in the food industry for more than 30 years. The technology can be applied to several production methods, including milk-solids separations in the dairy industry, juice clarification and concentration, concentration of whey protein, sugar and water purification, and waste management. Several filtration mediums exist as well as many types of membrane configurations. Knowledge of the various membrane technologies and how they are used in the food industry can enhance overall production and offers cost-cutting options for a variety of separations.
Membrane basics
Separation is based on principles that rely on the chemical and physical properties of particles and molecules. For example, centrifugation uses the physical property of weight to separate solids from liquids. Another example, ion exchange, relies on the principle of charge to separate different species from one another. Other principles such as vapor pressure, solubility and diffusion also can perform separations. Membranes use the principle of size to separate different materials.[1]
Membrane filters are very thin microporous sheets of film attached to a thicker porous support structure. At its most basic, a membrane serves as a sieve, separating solids from liquids forced through it. Not only can membranes separate solids from liquids, they can separate soluble molecules and ionic particles of different sizes from each other.
Membranes in the process industry use tangential flow filtration, or crossflow filtration. In tangential flow filtration (TFF) the flow of the feed stream runs parallel to the surface of the membrane at high velocities as illustrated in figure 1.
As the fluid passes across the membrane surface it acts to sweep the retained components, or retentate, from the membrane surface and back into the bulk solution to avoid plugging the pores[2]. This is opposed to traditional through-flow or dead-end filtration, in which the feed stream flows perpendicular to the surface of the membrane, the object of which is to form a dry cake. In TFF, the fraction that contains the solutes and lower molecular weight components that can pass through the membrane is known as permeate. The retentate is continuously recirculated, passing over the membrane surface until the desired effect—such as the concentration or clarification of a desired product—is achieved. The rate of permeation is known as flux and gives an indication of membrane performance.[3]
TFF operations can perform both concentration and clarification applications. In concentration, the membrane retains the desired product, and liquid is removed as permeate. The retentate becomes more and more concentrated as permeate is removed. In clarification applications, the desired product passes through the membrane and is collected as permeate, perhaps leaving insoluble materials or other undesired compounds in the retentate. Both concentration and clarification operations are used extensively in the food industry, primarily to process juice and other beverages.[4]
Membrane construction
Membranes are fabricated from many different types of materials. Initially, reverse osmosis and ultrafiltration membranes were cellulose-based, but are now made from polymers based on engineering plastics.[5] Typical materials have included polyacrylonitrile (PAN) and blends of PAN with polyvinyl chloride (PVC), aromatic polyamid, and polyvinylidene fluoride (PVDF).[5,6] Today, most polymeric membranes used in the food industry are made of polysulfone (PS) or polyethersulfone (PES).[5,6] As required by other processing equipment in the food industry, all membrane surfaces, backings, spacers, and support structures that make contact with food products must meet Title 21, Part 177, of the Code of Federal Regulations, generally recognized as safe (GRAS), or otherwise approved by the FDA for food contact.[5] Materials also are chosen for their cleanability and ability to withstand a variety of conditions under which it might perform.
Membranes can be divided into four basic categories: reverse osmosis (RO), nanofiltration (NO), ultrafiltration (UF), and microfiltration (MF).[5] Each of these categories is distinguished by the size of the species they retain. Retention is based on the pore size of the membrane. A range of particle sizes at which each of these operates is illustrated in figure 2.
Reverse osmosis has the tightest pore construction, and can separate in the ionic range. Nanofiltration, a newer category of membranes, operates similar to reverse osmosis but has a somewhat looser construction, allowing monovalent ions and some divalent ions to pass. Ultrafiltration is used to separate different-size molecules such as proteins and other macromolecules. Microfiltration membranes have the largest pore sizes of all categories, and is primarily used for removing suspended solids and bacteria.[6] Another difference between these types of filtration is the pressures at which they operate. RO membranes operate up to 1,500 psi, NO membranes operate up to 300 psi, UF membranes operate from 10 to 200 psi, while MF membranes operate in the range of 1 to 25 psi.[4] Pore size differences between the membranes determine the operating pressure. Higher pressures are necessary to force liquid through membranes with smaller pore sizes.
Filtration foundation
The module design—or support structure—of a membrane is critical to its performance. Some factors to consider include flux (the rate of permeation), the solids content of the process fluid, cost, cleanability and scalability. Food industry applications make use of four basic module designs: spiral-wound, tubular, hollow-fiber, and plate-and-frame styled systems.
Spiral-wound membranes cover more than 60% of food-industry applications, mainly for dairy and other soluble protein processing, polysaccharide gum concentration, and in most RO and NO applications.[5] Figure 3 shows a schematic of the spiral-wound design. Fluid is pumped into the spacer channel parallel to the membrane surface and permeate passes through the membrane and into a porous permeate channel until it reaches a perforated tube at the center which acts as a permeate carrier.
Although resistance to fouling is good, these membranes have difficulty handling viscous material, or material with a high solids content.
Tubular systems account for about 10% to 15% of food industry applications.[5] Tubular designs have a porous outer structure with a semi-permeable membrane coating on the inside of the tube. As seen in figure 4, the module consists of a collection of tubes fastened together at each end and encased in a module.[6]
The shell of the module collects permeate while retentate discharges at the end. Tubular designs are easy to clean and can be visually inspected.[5] They can handle liquids with high solids content and larger suspended particulates better than some of the other membrane designs. The membrane area, however, is typically small. Tubular membranes are suitable for beverage clarification or the reverse osmosis of pulp containing juices.[5]
Plate-and-frame and hollow-fiber systems are among the miscellaneous configurations that make up the remaining percentage of designs used in food industry applications.[5] In plate-and-frame styles, flat-sheet membranes are affixed to both sides of a porous plate and sandwiched in a holder. As indicated in figure 5, the feed stream enters the system and is directed via several channels to sweep over the surface of the membrane.
The fluid flows from these channels into the same outlet line and leaves as retentate. The permeate passes through the membrane into channels separate from the retentate and leaves as permeate. Several membranes and their supports can be stacked together in the holder to increase the overall membrane surface area. The advantage of this system is that if one membrane fails, it typically can be replaced at low cost. Also, plate-and-frame systems offer increased diversity. Once the initial capital cost to acquire the hardware is accomplished, a variety of membranes may be used in them. For example, both ultrafiltration and microfiltration processes may be done on the same unit merely by swapping out the proper membranes. Plate-and-frame systems have been used for the dealcoholization of beer in Europe and Australia, and also have been used for some high-viscosity concentration applications in the dairy industry.[5]
Hollow-fiber membranes are similar to tubular membranes except that the hollow fibers are much smaller. The inside diameter of the fiber may range from 0.5 to 1.1 mm as opposed to 12.5 to 25 mm for the tubular design.[3] The feed stream flows through the inside of the fibers and the permeate is collected in the containment shell. Hollow-fiber elements may have of hundreds of fibers all oriented in parallel. This allows for a high packing density and resistance to blockage of the flow channels. Also, this membrane can be backwashed to aid in cleaning. The strength of the fibers, however, is a limitation and low transmembrane pressures must be used to avoid bursting the fibers. The entire element must be discarded if even one fiber breaks.[3]
Reverse Osmosis
Reverse osmosis has become a standard process in the food industry. It is used to purify water for plant operations, to concentrate cheese whey proteins or milk in the dairy industry, for sugar concentration in the cereal processing industry, for concentration of juices, and for wastewater treatment in meat and fish processing industries.[3,4,5,7] Reverse osmosis deserves a special look because of its suitability for a wide variety of applications.
Natural osmosis is a phenomenon in which a liquid passes through a semi-permeable membrane from a dilute solution to a more concentrated one. In reverse osmosis, pressure is applied to the more concentrated solution forcing water to flow towards the dilute solution as shown in figure 6.
In this process, the reject water is typically 30% to 50% of the feed flow.[8] Thus, at maximum efficiency, every 100 gallons of water entering the system would produce 50 gallons of purified water. RO is helping to save companies both energy and water by helping to limit evaporation steps or by treatment of wastewater streams.
Many foods require the removal of large amounts of water to concentrate the product for more efficient packaging or shipping.[3] Although evaporation is common, it requires substantial amounts of energy requirements compared with RO. In the U.S. the corn wet-milling industry alone consumes about 93.7 trillion Btu/year, which is equal to 90% of the energy consumed in grain milling operations.[7] The amount of water evaporated in this industry is around 35 billion lb/year for steep water, and around 12 billion lb/year for sweet water.[7] The energy required for RO is approximately 110 kJ/kg water versus 700 kJ/kg for the most efficient evaporator, resulting in substantial savings.[7]
Wastewater is a problem in any industry. Reducing the level of the dissolved solids and biological oxygen demand (BOD) is sometimes the only way processing plants can discharge water safely. Also, removing dissolved solids from water allows it to be reused, which not only cuts down on water usage but on the amount that is discharged.[4] The collected solids also may be recovered if they are of value. For example, an RO system can recover proteins, sugars, and enzymes from wastewater that may be reused within plant operations.
Membrane technology has made a tremendous impact on the food industry over the last several years. The separation of materials for different applications has become an important industrial operation. Considerable progress continues to be made in membrane technology, and newer applications for existing systems are being discovered as the trend is to create integrated systems which utilize several different membrane types within a process.
Jayne E. Stratton, M.S., is the fermentation facility manager with the Dept. of Food Science and Technology, University of Nebraska-Lincoln.
References
- 1.Crueger, W. and A. Crueger. 1982. Biotechnology: A Textbook of Industrial Microbiology. Thomas Brock, (ed). Science Tech., Inc., Madison, WI. pp. 98-102.
2. Rudolf, E.A. and J.H. MacDonald. 1994. Tangential flow filtration systems for clarification and concentration. In Bioprocess Engineering: Systems, Equipment, and Facilities, B.K. Lydersen, N.A. Delia, and K.L. Nelson (eds). John Wiley & Sons, Inc. New York, NY, pp.121-157.
3. Paulson, D.J., R.L. Wilson, and D. Dean Spatz. 1984. Crossflow membrane technology and its applications. Food Technology 38(12): 77-87.
4. Mans, Jack. 1991. Save bucks and BTUs with membranes. Prepared Foods 160(11): 94-98.
5. Short, J. 1995. Membrane separation in food processing. In Bioseparation Processes in Foods, Rakesh K. Singh, Syed H. Rizvi (eds), Marcel Dekker, Inc. New York, pp. 333-350.
6. Elias, B. and J. Van Cleef. 1998. High-shear membrane separation for process and wastewater treatment. Chem. Eng. News 105(10):94-104.
7. Koseoglu, S.S, K.C. Rhee, E.W. Lusas. 1991. Membrane separations and applications in cereal processing. Cereal Foods World 377: 376-383.
8. Slabicky, R. 1994. Pharmaceutical water systems: Design and validation. In Bioprocess Engineering: Systems, Equipment and Facilities, B.K. Lydersen, N. A. Delia, K.L. Nelson, (eds), John Wiley & Sons, Inc., New York, NY, pp. 525-573.