P1: SFK/UKS BLBS102-c40 P2: SFK BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come 40 Separation Technology in Food Processing 775 membrane, as long as the applied pressure is greater than the osmotic pressure of the feed solution Reverse osmosis membranes have been widely applied in water treatment, such as desalting, pollution control, pure water treatment, and wastewater treatment Reverse osmosis operates by diffusion from a solution Under the high differential pressure across the membrane, the solvent from the solution actually dissolves in the material of the membrane, diffuses across it, and transfers out into the clean solvent on the other side It is not a perfect separation, because the dissolved species from the feed solution have definite abilities to diffuse through it as well, but the diffusion coefficient for the solvent is so much higher than that for the solute that the separation is virtually complete Reverse osmosis is often used to remove dissolved organics and metals Thus, reverse osmosis, as a device solely required to separate sodium chloride from water in the early days, is now being used to reject a wider range of solutes Electrodialysis Electrodialysis, an electrically driven membrane process, is commonly used to desalt solutions Ions in aqueous solution can be separated using a direct current electrical driving force on an ion-selective membrane Electrodialysis usually uses many thin compartments of solution separated by membranes that permit passage of either positive ions (cations) or negative ions (anions) and block passage of the oppositely charged ion Cation-exchange membranes are alternatively stacked with anion-exchange membranes placed between two electrodes The solution to be treated is circulated through the compartments and a direct current power source is applied All cations gravitate toward the cathode (negatively charged terminal) and transfer through one membrane, while anions move in the opposite direction, thereby concentrating in alternating compartments Electrodialysis is commonly used to recover spent acid and metal salts from plating rinse It obviously is not effective for nonpolar solutions Membrane-Based Separation in Industrial Applications Membrane-based processes offer many advantages in terms of lower energy consumption, higher selectivity, faster separation rate, and lower recovery costs (Cassano et al 2008) There has been increasing interest in the use of membrane-based technologies in the food industry Various designs of membrane-based separation equipment are used in the food industry (Figure 40.6) In the dairy industry, ultrafiltration is widely used to fractionate cheese whey and pre-concentrate milk for cheese making (Pouliot 2008) In sequential membrane processes, ultrafiltration may also be used as a membrane reactor to produce protein hydrolysate from protein isolate (Feins and Sirkar 2005, Charoenvuttitham et al 2006, Sarkar et al 2007, Annathur et al 2010) and as a membrane fermenter to produce fuels and chemicals from reverse osmosis concentrates (Greenlee et al 2009, Qu et al 2009, Saxena et al 2009) In sugar and corn syrup process- Figure 40.6 The pilot scale membrane separation system ing, microfiltration and ultrafiltration are used for clarification to remove impurities (Singh and Cheryan 1994, Jaeger de Carvalho et al 2008) In juice, wine, and beer processes, microfiltration and ultrafiltration not only remove insoluble solids to produce sparkling clear product, but also remove spoilage microorganisms, eliminating the need for pasteurization (Massot et al 2008, Vaillant et al 2008, Cassano et al 2010) Ultrafiltration is also used to concentrate natural colorants and flavors (Nawaz et al 2005, Nawaz et al 2006, Shi et al 2007d, Galaverna et al 2008, Ren et al 2008a, 2008b, 2008c) The main operating problem of all membrane separation processes is that the membrane material eventually plugs, called fouling, causing the resistance to flow to increase The plugging process is accentuated by the concentration polarization that occurs in the relatively quiescent fluid zone close to the membrane surface, as the species separated from previously processed fluids build up in this zone and interfere with fresh material trying to get to the surface Membrane-based separation is still evolving and finding more and more applications in a broad range of fields, and the P1: SFK/UKS BLBS102-c40 P2: SFK BLBS102-Simpson 776 March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come Part 7: Food Processing development of new membrane materials will strongly influence separation processes in the future Because of the great advantages of membrane separation over conventional separation practices, the availability of the choice of membrane size (microfiltration, ultrafiltration, nanofiltration, reverse osmosis, preevaporation membrane, and distillation membrane), materials (polymeric and ceramic, hydrophilic and hydrophobic, symmetric and asymmetric), configurations (spiral wound, hollow fiber, and plate and frame), operation modes (dead-end and cross-flow, batch, semi-batch, and continuous), and membrane technology offers more selective, flexible, and efficient separations over a wide range of compounds This technology will continue to gain recognition and acceptance in the food industry Pressurized Low-Polarity Water Extraction Pressurized low-polarity water extraction, also known as subcritical water extraction (or hot water extraction, pressurized hot water extraction, superheated water extraction, or high-temperature water extraction), that is, extraction using hot water under pressure, has recently become a popular green processing technology and emerges as a promising extraction and fractionation technique for replacing the traditional extraction methods The pressurized low-polarity water extraction is also used in sample preparation to extract organic contaminants from foodstuff for food safety analysis and solids/sediments for environmental monitoring purpose The pressurized low-polarity water extraction process is an environmentally friendly technique that can provide higher extraction yields from solid plant materials (Luque de Castro and Jim´enez-Carmona 1998) Pressurized low-polarity water extraction is based on the use of water as an extractant in a dynamic mode, at temperatures between 100◦ C and 374◦ C (critical point of water, 221 bar and 374◦ C) and under pressure high enough to maintain the liquid state The critical temperature and pressure of water are shown as a phase diagram in Figure 40.5 (T c = 374◦ C, Pc = 221 bar or 22 MPa) The pressurized low-polarity water extraction process can maintain the water in the liquid form up to a temperature of 374◦ C and a pressure of 22.1 MPa (221 bar) (Haar et al 1984, Hawthorne et al 2000) A pressure of MPa would be high enough to prevent the water from vaporizing at temperatures from 100◦ C to 250◦ C Once pressure is high enough to keep water in a liquid state, additional pressure is not necessary as it has limited influence on the solvent characteristics of water Increasing the water temperature from 25◦ C to 250◦ C causes similar changes in dielectric constant, surface tension, and viscosity (Kronholm et al 2007, Brunner 2009) Pressurized low-polarity water extraction can easily solublize organic compounds such as phytochemicals, which are normally insoluble in ambient water Pressurized low-polarity water extraction has the ability to selectively extract different classes of compounds, depending on the temperature used The selectivity of subcritical water extraction allows for manipulation of the composition of the extracts by changing the operating parameters, with the more polar ones extracted at lower temperatures and the less polar compounds extracted at higher temperatures (Basile et al 1998, Ammann et al 1999, Clifford et al 1999, Miki et al 1999, Kubatova et al 2001, Soto Ayala and Luque de Castro 2001) Process System As shown in Figure 40.7, the instrumentation consists of a water reservoir coupled to a high-pressure pump to introduce the pressurized low-polarity water into the system, an oven, where the extraction cell is placed and extraction takes place, and a restrictor or valve to maintain the pressure Extracts are collected in a vial placed at the end of the extraction system In addition, the system can be equipped with a coolant device for rapid cooling of the resultant extract As the unique properties of pressurized low-polarity water, the pressurized low-polarity water extraction has a disproportionately high boiling point for its mass, a high dielectric constant, and high polarity As the temperature rises, there is a marked and systematic decrease in permittivity, an increase in the diffusion rate, and a decrease in the viscosity and surface tension In consequence, more polar target materials with high solubilities in water at ambient conditions are extracted most efficiently at lower temperatures, whereas moderately polar and nonpolar targets require a less-polar medium, induced by elevated temperature Water changes dramatically when its temperature rises, because of the breakdown in its hydrogen-bonded structure with temperature The high degree of association in the liquid causes its relative permittivity (more commonly called its dielectric constant) to be very high at ca 80 under ambient conditions But as the temperature rises, the hydrogen bonding breaks down and the dielectric constant falls, as shown in Figure 40.5 The most outstanding feature of this leaching agent is the easy manipulation of its dielectric constant (ε) In fact, this parameter can be changed within a wide range just by changing the temperature under moderate pressure Thus, at ambient temperature and pressure, water has a dielectric constant of ca 80, making it an extremely polar solvent This parameter is drastically lowered by raising the temperature under moderate pressure For example, subcritical water at 250◦ C and a pressure over 40 bar has ε = 37, which is similar to that of ethanol and allows for the leaching of low-polarity compounds By 250◦ C, its dielectric constant has fallen so that it is equal to that for methanol (i.e., 33) at ambient temperature Thus, between 100◦ C and 200◦ C, superheated water is behaving like a water–methanol mixture Partly because of its fall in polarity with temperature, superheated water can dissolve organic compounds to some extent, especially if they are slightly polar or polarizable like aromatic compounds Therefore, water can be used as extraction solvent to extract the polar, the moderately polar, and the nonpolar compounds by adjusting the extraction temperature from range of 50◦ C to 275◦ C The solubility of an organic compound is often many orders of magnitude higher than its solubility in water at ambient temperature for two reasons One is the polarity change and the other is that of a compound with low solubility at ambient temperature Pressurized low-polarity water will have a high positive enthalpy of solution and thus a large increase in solubility with temperature Because of the greater solubility of some P2: SFK BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come 777 Water reservoir Supply valve (1) Pump Extraction cell (3) 40 Separation Technology in Food Processing Heater (2) P1: SFK/UKS BLBS102-c40 Eluent valve (4) Micro Micro filters filters Oven Pressure regulator pump and valve controller Temperature controller Sample collector (5) Figure 40.7 Diagram of pressurized low-polarity water extractor The electrical connections are marked by dashed lines, while the path of subcritical water is shown by solid line with arrow The high-pressured water passes through a supply vale (1) into a heating coil (2) and into an extraction cell (3) The microfilters are placed before and after an eluent valve (4) The extract is collected in a sample collector (5) organic compounds in superheated water, this medium can be considered for the extraction and other processes, to replace conventional organic solvents But some additional reactions of the compounds being processed may also occur, by hydrolysis, oxidation, etc Industrial Applications Using pressurized low-polarity water provides a number of advantages over traditional extraction techniques (i.e., hydrodistillation, organic solvents, solid–liquid extraction) These are mainly shorter extraction times, higher quality of the extracts (mostly for essential oils), lower costs of the extracting agent, and an environmentally compatible technique Since water is perhaps the most environmentally friendly solvent available in high purity and at low cost, it has been exploited for the extraction of avoparcin in animal tissue (Curren and King 2001), fungicides in agricultural commodities (Pawlowski and Poole 1998), fragrances from cloves (Rovio et al 1999), antioxidative components from sage (Ollanketo et al 2002), anthocyanins and total phenolics from dried red grape skin (Ju and Howard 2003), ă undag et al 2007), saponins from cow cockle seed (Găucálău-Ustă and other bioactive components from plant materials (Ong and Len 2003) Some additional successful applications of this technique are for the extraction of essential oils from various plant materials (Khajenoori et al 2009, Mortazavi et al 2010), extraction of sweet components from Siraitia grosvernorii, extraction of lactones from kava roots, extraction of antioxidant compounds from microalgae S platensis (Ib´an˜ ez et al 1999, Ib´an˜ ez et al 2003, Herrero et al 2004), extraction of Ginkgo biloba, and of biophenols from olive leaves (Jap´on-Luj´ana and Luque de Castro 2006) The quality of the oil obtained is therefore better than that from steam distillation, as it contains more of the oxygenated compounds and lower terpene content The yield is also slightly higher than from steam distillation, in spite of the fact that all the terpenes are not extracted This may be because, at the higher temperatures and under pressure, the plant material is more effectively penetrated However, about twice the amount of water is required than for steam distillation Energy costs are much less than for steam distillation The energy required to heat a given mass of water from 30◦ C to 150◦ C under pressure is one-fifth of that needed to boil water at atmospheric pressure from 30◦ C Furthermore, it is possible to recycle most (three-quarters) of the heat, whereas it is difficult to recycle heat in steam distillation Thus, in spite of the fact that twice as much water is needed, only one-tenth of the energy of a steam distillation is required Pressurized low-polarity water extraction has been suggested as a method to extract valuable health-promoting compounds from plant materials Molecular Distillation Molecular distillation is a unit operation that is a peculiar case of evaporation, which happens under extremely low pressures and low temperatures and is used for the separation of constituents from mixtures by partial evaporation It is based on the fact that the vapor is relatively richer in the component with the highest vapor pressure, that is, the more volatile component Distillation is a process of heating a substance until the most volatile constituents change into the vapor phase, and then cooling the vapors to recover the constituents in liquid form by condensation The main purpose of distillation is to separate a mixture into individual components by taking advantage of their different level of volatilities Distillation is one of the main methods of extracting essential oils from plants It can be carried out either as simple distillation or fractional distillation (rectification) In P1: SFK/UKS BLBS102-c40 P2: SFK BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm 778 Printer Name: Yet to Come Part 7: Food Processing simple distillation, the vapors are recovered by condensation In rectification, successive vaporization and condensation are carried out simultaneously and a part of the condensed liquid, called the reflux, flows down the column countercurrent to the flow of vapors for isolating components from a mixture based on differences in boiling points The percentage of each constituent in the vapor phase usually depends on its vapor pressure at a certain temperature The principle of vacuum distillation may be applied to substances, such as oils, that would be damaged by overheating by the conventional method (Liu et al 2008) Several new methods have been developed for the separation and recovery of minor components from vegetable oils such as palm oil (Rodr´ıguez et al 2007, Shi et al 2007c) Distillation and its companion processes, azeotropic and extractive distillations, are by far the most widely used separation processes for mixtures that can be vaporized Vapors are generated from liquids or solids by heating and are then condensed into liquid products However, many mixtures exhibit special states, known as azeotropes, at which the composition, temperature, and pressure of the liquid phase become equal to those of the vapor phase Thus, further separation by conventional distillation is no longer possible By adding a carefully selected other component as an entrainer to the mixture, it is often possible to “break” the azeotrope and thereby achieve the desired separation In azeotropic distillation, a compound is added to form an azeotrope with at least one of the components of the mixture That component can then be more readily separated from the mixture because of the increased difference between the volatilities of the components Extractive distillation combines continuous fractional distillation with absorption A relatively high-boiling solvent is used to selectively scrub one or more of the components from a mixture of components with similar vapor pressures Distillation processes are also widely used for the separation of organic chemicals, usually at cryogenic temperatures Separation Processes in Distillation When vacuum is applied, there are three major reduced pressure ranges that can be utilized for distillation: (a) distillation at moderate vacuum or equilibrium distillation, (b) unobstructed path distillation, and (c) molecular distillation Distillation at moderate vacuum is characterized by the use of conventional distillation equipment as shown in Figure 40.8 Its lowest pressure limit is of the order of Torr, that is, mm Hg Unobstructed path distillation is defined as distillation in which the path between the evaporator and the condenser is not blocked, in other words there is a free transfer of molecules (Eckles and Benz 1992) When the transfer distance is comparable with the mean free path of the vapor molecules, the distillation is known as molecular distillation Mean free path is defined as the average distance a molecule will travel in the vapor phase without colliding with another vapor molecule (Eckles et al 1991) This implies that, in molecular distillation, the vapor molecules can reach the condenser without intermolecular collisions A dynamic equilibrium can not be established between the vapor and the liquid An individual Figure 40.8 Molecular distillation system molecule that has evaporated will be able to travel any distance without a collision Molecular distillation occurs at low temperatures and, therefore, reduces the problem of thermal decomposition High vacuum also eliminates oxidation that might otherwise occur in the presence of air Unobstructed path and molecular distillations are often classified together as short-path or high-vacuum distillation The difference is in the dimensions and operating conditions Unobstructed path distillation is carried out at pressures as low as 10−2 Torr, while in molecular distillation pressures of 10−3 Torr, that is, a mTorr, are used Another useful distinction between the methods of vacuum distillation can be made with reference to the way in which the vapor phase is formed: (a) Ebullition, (b) evaporative distillation, and (c) molecular distillation Ebullition is accompanied by the formation of bubbles when the saturated vapor pressure exceeds the pressure of the surrounding gas The only limit to the rate of evaporation in this case is the rate at which heat can be transferred to the liquid Evaporative distillation occurs when the pressure of the surrounding gas is higher than the vapor pressure of the liquid at a given temperature, so that bubbles are not formed The rate of evaporation is controlled by the temperature of the liquid and the conditions above the liquid surface In molecular distillation, the rate of evaporation is controlled by the rate at which the molecules escape from the free surface of the liquid and P1: SFK/UKS BLBS102-c40 P2: SFK BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come 779 40 Separation Technology in Food Processing condense on the condenser The condenser should be in the immediate vicinity of the evaporating surface When an appreciable number of collisions can occur in the vapor space, some of the molecules will return to the liquid This leads to a decrease in the number of molecules that reach the condensation surface High-vacuum distillation may be used for certain classes of chemical compounds that decompose, polymerize, react, or are destroyed by conventional distillation methods Low cost per pound and high throughput may be obtained on certain groups of compounds such as vitamins, epoxy resins, highly concentrated pure fatty acids, plasticizers, fatty acid nitrogen compounds, and a host of other heat-sensitive materials, which may require only deodorizing and decolorizing (Spychaj 1986, Batistella et al 2002a,b, Cermak et al 2007, Shao et al 2007, Compton et al 2008) High-vacuum distillation is a safe process to separate mixtures of organic or silicon compounds, most of which can not withstand prolonged heating without excessive structural change or decomposition With short residence times and lower distilling temperatures, thermal hazards to the organic materials are greatly reduced Purity of the distillate also depends on the film thickness Controlling positive pressure and supply to the heated evaporator surface will usually provide a uniform film throughout the distillation The absence of air molecules in the high-vacuum distillation column permits most of the distilling molecules to reach the condenser with relatively few molecules returning to the liquid film surface in the evaporator Experimental results show a relationship between the molecular weight and distillation temperature for a broad range of different materials There are several basic design variations with short-path evaporators These are the short-path falling film evaporator, the centrifugal molecular still, and the short-path, wiped-film evaporator They operate at the lowest pressure of any system and are capable of high throughput per unit size, due to their continuous nature They also offer the shortest thermal exposure to any process (Eckles and Benz 1992) Short-path, falling film evaporators can handle materials with viscosities up to 5000 cP, and the residence time, temperature, and pressure can be controlled This is the simplest of the short-path stills, but more current designs use either centrifugal force or roller-wipers to spread the feed material on the evaporator surface In the centrifugal molecular still, feed material is fed into the center of a heated spinning rotor The material is evenly spread towards the edge and condensed in front of the rotor Molecular Distillation Applications Molecular distillation, with its important characteristics of low pressure and low temperature, gives a high potential for this process in the separation, purification, and concentration of natural products, which usually consist of complex and thermally sensitive molecules Especially under high vacuum for short operating times, while using no solvents, it avoids any toxicity problems The effects of feed flow rate and distillation temperature on the extraction of minor components are related to the yield, purity, and rate of evaporation in terms of concentrations, distribution coefficients, and relative volatilities Molecular distillation is a valuable processing method that is often used to separate or purify high-boiling materials that decompose, oxidize, or polymerize at elevated temperatures This process can be considered for industrial uses if both distillate and residue become high value-added products Some molecular distillation processes are used in the production of biologically active substances such as vitamins, sterols, and antioxidants from natural oils and fats such as the extraction of vitamin A from fish liver and whale oil and carotenoid recovery from esterified palm oil (Batistella and Wolf Maciel 1998, Mutalib et al 2003, Martins et al 2006, Bettini 2007, Fregolente et al 2007, Shi et al 2007c), separation of mono and diglycerides in partially saponified fats (Holl´o and Kurucz 1968), and the refining of crude animal and vegetable oils (Martins et al 2006, Behrenbruch and Dedigama 2007, Posada et al 2007) For instance, a Malaysian R company produces Carotino , an edible red palm oil, using a process that involves a pretreatment of crude palm oil (i.e., degumming with phosphoric acid and treatment with bleaching earth) followed by deacidification and deodorization using molecular distillation (Ooi et al 1996; Fig 40.9) Recovery of special components used in the nutritional, pharmaceutical, and cosmetic areas from natural products by molecular distillation Water phase + Oil phase recovery Condenser Vent Juice 86°F Scrubber Condenser Juice 122°F Chilled (30% Alcohol) water 32°F IV Liquid seal pump W phase + O phase Aroma column Chilled water 32°F Condensed water Pump Condensed Variable water + pump Residual aroma Chiller Legend Vapour Condensed water Steam Juice W phase + O phase Chilled water Figure 40.9 Recovery system of water phase and oil phase in the orange juice industry (Bettini 2007) ... membrane, and distillation membrane), materials (polymeric and ceramic, hydrophilic and hydrophobic, symmetric and asymmetric), configurations (spiral wound, hollow fiber, and plate and frame),... separation of mono and diglycerides in partially saponified fats (Holl´o and Kurucz 19 68) , and the refining of crude animal and vegetable oils (Martins et al 2006, Behrenbruch and Dedigama 2007,... antioxidants from natural oils and fats such as the extraction of vitamin A from fish liver and whale oil and carotenoid recovery from esterified palm oil (Batistella and Wolf Maciel 19 98, Mutalib et al 2003,