P1: SFK/UKS BLBS102-c40 P2: SFK BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm 770 Printer Name: Yet to Come Part 7: Food Processing Table 40.1 Comparison of Physical Properties of Supercritical CO2 at 20 MPa and 55◦ C with Some Selected Liquid Solvent at 25◦ C Properties Density (g/mL) Kinematic viscosity (m2 /s) Diffusivity (m2 /s) Cohesive energy density (δ (cal/cm3 )) CO2 n-Hexane Methylene Chloride Methanol 0.75 1.0 6.0 × 109 10.8 0.66 4.45 4.0 × 109 7.24 1.33 3.09 2.9 × 109 9.93 0.79 6.91 1.8 × 109 14.28 Source: Modified from King et al 1993 other raw or waste agricultural materials (Kasamma et al 2008, Shi et al 2009a, Yi et al 2009, Huang et al 2010, Shi et al 2010a, 2010b, Xiao et al 2010) The scale up of some supercritical extraction processes have already proven to be possible and are in industrial use The process requires intimate contact between the packed beds formed by a ground solid substratum (fixed-bed of extractable material) with supercritical CO2 fluid During the supercritical extraction process, the solid phase comprising of the solute and the insoluble residuum (matrix) is brought into contact with the fluid phase, which is the solution of the solute in the supercritical CO2 fluid (solvent) The extracted material is then conveyed to a separation unit Process Concept Schemes The physicochemical properties of the supercritical fluids are crucial to the understanding of the process design calculation and modeling of the extraction process Therefore, selectivity of solvents to discriminate solutes is a key property for the process engineer Physical characteristics such as density and interfacial tension are important for separation to proceed; the density of the extract phase must be different from that of the raffinate phase and the interfacial properties influence coalescence, a step that must occur if the extract and raffinate phase are to separate The supercritical state of the fluid is influenced by temperature and pressure above the critical point The critical point is the end of the vapor–liquid coexistence curve as shown on the pressure–temperature curve in Figure 40.1, where a single gaseous phase is generated When pressure and temperature are further increased beyond this critical point, it enters a supercritical state At this state, no phase transition will occur, regardless of any increase in pressure or temperature, nor will it transit to a liquid phase Hence, diffusion and mass transfer rates during supercritical extraction are about two orders of magnitude greater than those with solvents in the liquid state Substances that have similar polarities will be soluble in each other, but increasing deviations in polarity will make solubility increasingly difficult Intermolecular polarities exist as a result of van der Waals forces, and although solubility behaviors depend on the degree of intermolecular attraction between molecules, the discriminations between different types of polarities are also important Substances dissolve in each other if their intermolecular forces are similar or if the composite forces are made up in the same way Properties such as the density, diffusivity, dielectrical constant, viscosity, and solubility are paramount to supercritical extraction process design The dissolving power of SCF depends on its density and the mass transfer characteristic, and is superior due to its higher diffusivity and lower viscosity and interfacial tension than liquid solvents Although many different types of supercritical fluids exist and have many industrial applications, CO2 is the most desirable for SCE of bioactive components Table 40.1 shows some physical properties of compressed (20 MPa) supercritical CO2 at 55◦ C as compared to some condensed liquids that are commonly used as extraction solvents at 25◦ C It should be noticed that supercritical CO2 exhibits similar density as those of the liquid solvents, while being less viscous and more highly diffusive This fluid-like attribute of CO2 coupled with its ideal transport properties and other quality attributes outlined above make it a better choice over other solvents The specific heat capacity (Cp ) of CO2 rapidly increases as the critical point (31.1◦ C temperature, 7.37 MPa pressure, and 467.7 g/L flow rate) is approached Like enthalpy and entropy, the heat capacity is a function of temperature, pressure, and density (Mukhopadhyay 2000) Under constant temperature, both the enthalpy and entropy of supercritical CO2 decreases with increased pressure and increases with temperature at constant pressure The change in specific heat as a result of varying the pressure and temperature is also dependent on density For example, under constant temperature, specific heat increases with increasing density up to a certain critical level Above this critical level, any further increase in density reduces the specific heat Process System The supercritical CO2 fluid extraction process is governed by four key steps: extraction, expansion, separation, and solvent conditioning The steps are accompanied by four generic primary components: extractor (high pressure vessel), pressure and temperature control system, separator, and pressure intensifier (pump; Fig 40.2) Raw materials are usually ground and charged into a temperature-controlled extractor forming a fixed bed, which is usually the case for a batch and single-stage mode (Shi et al 2007a, 2007b, Kasamma et al 2008) The supercritical CO2 fluid is fed at high pressure by means of a pump, which pressurizes the extraction tank and also P2: SFK BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come 771 40 Separation Technology in Food Processing Back pressure valve CO2 outlet Separator Heat exchanger Cooling bath Extractor column CO2 pump Heat exchanger Pressure valve CO2 source Extracts outlet Figure 40.2 Schematic diagram of a typical single-stage supercritical fluid extraction system with CO2 Pressure regulator The processes described above are semi-batch continuous processes where the supercritical CO2 flows in a continuous mode while the extractable solid feed is charged into the extraction vessel in batches In commercial-scale processing plants, multiple extraction vessels are sequentially used to enhance process performance and output Although the system is interrupted at the end of the extraction period, when the process is switched to another vessel prepared for extraction, the unloading and/or loading of the spent vessels can be carried out while extraction is in progress, reducing the downtime and improving the production efficiency A semi-continuous approach on a commercial scale uses a multiple stage extraction processes that involve running the system concurrently by harnessing a series of extraction vessels in tandem, as shown in Figure 40.4 In this system, the process is not interrupted at the end of extraction period for each 3-stage separator columns Dry test meter Extracts circulates the supercritical medium throughout the system Figure 40.2 shows an example of a typical single-stage supercritical CO2 fluid extraction system Once the supercritical CO2 and the feed reach equilibrium in the extraction vessel, through the manipulation of pressure and temperature to achieve the ideal operating conditions, the extraction process proceeds The mobile phase consisting of the supercritical CO2 fluid and the solubilized components is transferred to the separator where the solvating power of the fluid is reduced by raising the temperature and/or decreasing the pressure of the system The extract precipitates in the separator while the supercritical CO2 fluid is either released to the atmosphere or recycled back to the extractor In the case where highly volatile components are being extracted, a multistage configuration may have to be employed as shown in Figure 40.3 (Shi et al 2007a, 2007b, Kasamma et al 2008) Extractor column P1: SFK/UKS BLBS102-c40 Ice water trap CO2 inlet CO2 pump Mixer Co-solvent inlet CO-solvent pump Figure 40.3 Schematic diagram of supercritical fluid extraction system used to fractionate bioactive components from plant matrix using supercritical carbon dioxide P2: SFK BLBS102-Simpson March 21, 2012 772 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come Part 7: Food Processing 3-stage extraction columns 2-stage separation vessels Storage tank P1: SFK/UKS BLBS102-c40 Co-solvent inlet CO2 inlet Pumps Figure 40.4 Schematic diagram of commercial scale multistage supercritical fluid extraction system used to fractionate bioactive components The symbol “ ” is pressure valves and “ ” is heat exchangers vessel, because the process is switched to the next prepared vessel by control valves for extraction while unloading and/or loading the spent vessels Thus, supercritical CO2 technology is available in the form of single-stage batch that could be upgraded to multistage semi-continuous batch operations coupled with a multi-separation process The need to improve the design into truly continuous modes is growing Supercritical CO2 fluid extraction could be cost-effective under large-scale production Industrial Applications Large-scale supercritical CO2 fluid extraction has become a practical process for the extraction of high-value products from natural materials The solvating power of supercritical CO2 fluids is sensitive to temperature and pressure changes; thus, the extraction parameters may be optimized to provide the highest possible extraction yields with maximum antioxidant activity for health-promoting components in bioactive extraction production (Kasamma et al 2008, Chen et al 2009, Yi et al 2009b) A supercritical CO2 fluid extraction process offers the unique advantage of adding value to agricultural waste by extracting bioactives from agricultural by-products, which are then used for the fortification of foods and other applications Its drawbacks are the difficulties in extracting polar compounds and its difficulty of extracting compounds from a complex matrix where the phase interaction with the intrinsic properties of the product inhibits its effectiveness Some drawbacks can be ameliorated by using small amounts of food-grade co-solvents (less than 10%) to approach the high extraction efficiency (Shi et al 2009a) However, much investigation is required to understand the solvation effects on the targeted bioactive components being extracted The CO2 density, pressure, and temperature have been noted to have great impacts on the results of the extraction process By understanding the effect of the parameters that influence the extraction process, the conditions may be set to optimize yield and cost efficiency When determining the parameters that should be used to maximize yields and solubility of the targeted components, many researchers attempted to use conditions that may be applicable in large-scale applications (Shi et al 2007f, Kasamma et al 2008) For example, nontoxic co-solvents and modifiers could be acceptable for food processing; therefore, a number of researchers have opted to use food-grade co-solvents and modifiers in extraction processes (Shi et al 2009a) The nature of the material used as a source of high-value components, such as health-promoting components, governs the availability of the compounds for the extraction process The presence of other components such as lipids may impede the process or elevate costs due to an elongated extraction time Although a high temperature in the extraction process generally increases the solubility of components in supercritical CO2 fluids, the conditions under which thermally labile targeted compounds are negatively affected should be considered (Shi et al 2007a, 2007b, 2007e) The intensity and the length of heat processing affect the health-promoting properties of bioactives Therefore, ideally, the extraction time and temperature should be minimized Minimizing such conditions also leads to a more economically viable process (Shi et al 2007f, Kasamma et al 2008) Excessively high flow rates may reduce the contact time between the solute and the solvent and restrict the fluid flow in the sample if it becomes compacted The optimal flow rate appears to vary with the targeted molecule, relatively high flow rates having a negative effect on some components Raising the pressure increases extraction yields Sample matrix is an important parameter that influences the solubility and mass transfer process during SCE Properties such P1: SFK/UKS BLBS102-c40 P2: SFK BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come 773 40 Separation Technology in Food Processing as particle shape and size distribution, porosity and pore size distributions, surface area, and moisture content influence solubility and mass transfer The presence of water (moisture content) in the sample matrix during supercritical extraction also has an effect on the extraction outcome In order to improve the yield and quality of the extracted high-value food components from raw material, a pretreatment of the raw material is an essential process (Yang et al 2008, Zheng et al 2009, Nagendra et al 2010) Cell disruption is the most important pretreatment, and this procedure can be conducted by several processes such as mechanical, ultrasonic, high electronic field pulse, and nonmechanical treatments With improved processing conditions and reduced cost, high-value components extracted from natural materials by supercritical CO2 extraction process will become even more economical at high throughput Membrane-Based Separation Technology Membrane-based technology is a rising separation technology At present, more and more fields use membrane technology to separate the fluids and get good results With the principal development of membrane technology, the applications are expanding much wider and the technology brings remarkable economic benefits Now, many countries in the world have noticed the importance of membrane technology in the food processing field, especially since we are short of energy and resources and the deteriorating environment all exist in our lifetime So, industries regard membrane-based separation technologies as important technologies in food processing areas The common membrane-based separation process is usually run under pressure A membrane can be described as a thin barrier between the two bulk phases, and it is either a homogeneous phase or a heterogeneous collection of phases In membrane separations, each membrane has the ability to transport one component more readily than the others because of differences in physical and chemical properties between the membrane and the permeating components Furthermore, some components can freely permeate through the membrane, while others will be retained The stream containing the components that pass through the membrane is called the permeate, and the stream containing the retained components is called the retentate The flow of material across a membrane is kinetically driven by the application of pressure concentration, vapor pressure, hydrostatic pressure, or electrical potential (Mulder 1996, Cheryan 1998) Membrane separation technology adopts a selective multihole membrane as the separation medium The separated fluid, driven by the outside pressure, goes through the surface of the membrane with different pore sizes The molecules will pass the membrane but larger molecules will be rejected Therefore, the fluid can be separated into different molecular weights with high efficiency (Cheryan 1998) Compared with the traditional separation process technologies, membrane-based separation processes can run under normal temperatures, so are especially good for separating and concentrating heat-sensitive materials such as juices, enzymes, or phytochemicals (Vaillant et al 2001, Enevoldsen et al 2007, Pouliot 2008) Moreover, membrane-based processes are energy-efficient processes that not involve phase change or heat input; thus, the processes not require ancillary equipment such as heat generator, evaporator, and condenser They offer ease of operation and great flexibility, and not require the addition of any chemical agents The process also provides for minimal thermal degradation, occurs at ambient temperatures, and is used both to filter molecular-sized particulates and also to concentrate an isolate of interest, such as lycopene Determination of the optimum operating conditions is vital importance Membrane-based processes are usually more energy efficient than distillation, adsorption, and chromatography Furthermore, membrane-based separation has the advantage of compatibility with a wide range of solvents and chemical products, an ability to process thermally sensitive compounds and easy amenability to automation These advantages open up several possibilities of membrane application in the production of bioactive compounds in the areas of energy-efficient pre-concentration of dilute solutions, fractionation of diverse classes of compounds from complex mixtures, recovery of intermediates, and recycling of solvents The challenges posed by a membrane-based separation process are limited selectivity, fouling leading to performance decline, and a necessary and occasionally difficult periodic cleaning process The performance of a membrane can be distinguished by two simple factors: flux (or product rate) and selectivity through the membrane Flux is defined as the permeation capacity that refers to the quantity of fluid permeating per unit area of membrane per unit time Flux depends linearly on both the permeability and the driving force The flux also depends inversely upon the thickness of the membrane (Yang et al 2001, Bhanusali and Bhattacharyya 2003, Kert´esz et al 2005) Figure 40.5 shows a classification of various separation-based processes that are based upon particle or molecular size The five major membrane separation processes, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis, cover a wide range of particle sizes according to the Membrane separation Microfiltration (MF) Ultrafiltration (UF) Nano filtration (NF) Reverse osmosis (RO) Water Figure 40.5 Schematic diagram of membrane molecular cut property P1: SFK/UKS BLBS102-c40 P2: SFK BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm 774 Printer Name: Yet to Come Part 7: Food Processing Table 40.2 Size of Materials Retained, Driving Force, and Type of Membrane Process Microfiltration Ultrafiltration Nanofiltration Reverse osmosis Dialysis Electrodialysis Size of Materials Retained 0.2–10 µm (microparticles) 1–100 nm MWCO 103 –106 Da (macromolecules) 0.5–5 nm (molecules)