Chapter 4 Ultrafiltration M.J. LEWIS, Department of Food Science and Technology, The University of Reading RG6 6AP 4.1 INTRODUCTION Ultrafiltration offers the opportunity to concentrate large molecular weight components without the application of heat or a change of phase. Such components are rejected by the membrane, whereas the permeate produced will contain the low molecular weight components present in the food, at a concentration similar to that in the feed. This results an increase in their concentration both on a wet weight and dry weight basis in the olution. It is a pressure-activated process, with pressures in the range of 1-15 bar; these pressures are considerably lower than those used in reverse osmosis. For many heat labile macromolecules,e.g proteins and starches, concentration by UF at ambient temperature will minimise heat-induced reactions which may adversely influence their functional behaviour in foods. Some important functional properties are solubility, foaming capacity, gelation, emulsification capacity, fat and water binding properties. These are discussed in more detail in section 4.5 In the case of enzymes or pharmaceutical agents, their biological activity needs to be conserved. It also affords the opportunity to separate small molecular weight components from lex mixtures, containing components with a wide range of molecular weights There have also been investigations into using UF for protein fractionation, but this is not straightforward due to the diffuse nature of the membranes and their selectivity UF is also very useful for recovering valuable components from food processing waste treams and fermentation broths, Probably the greatest impetus has come from the dairy industry and dairying applications. However, in all applications, flux decline due oncentration polarisation and fouling are probably the two most important practical
Chapter 4 Ultrafiltration M. J. LEWIS, Department of Food Science and Technology, The University of Reading, RG6 6AP. 4.1 INTRODUCTION Ultrafiltration offers the opportunity to concentrate large molecular weight components without the application of heat or a change of phase. Such components are rejected by the membrane, whereas the permeate produced will contain the low molecular weight components present in the food, at a concentration similar to that in the feed. This results in an increase in their concentration both on a wet weight and dry weight basis in the solution. It is a pressure-activated process, with pressures in the range of 1-15 bar; these pressures are considerably lower than those used in reverse osmosis. For many heat labile macromolecules, e.g. proteins and starches, concentration by UF at ambient temperature will minimise heat-induced reactions which may adversely influence their functional behaviour in foods. Some important functional properties are solubility, foaming capacity, gelation, emulsification capacity, fat and water binding properties. These are discussed in more detail in Section 4.5. In the case of enzymes or pharmaceutical agents, their biological activity needs to be conserved. It also affords the opportunity to separate small molecular weight components from complex mixtures, containing components with a wide range of molecular weights. There have also been investigations into using UF for protein fractionation, but this is not straightforward due to the diffuse nature of the membranes and their selectivity. UF is also very useful for recovering valuable components from food processing waste streams and fermentation broths. Probably the greatest impetus has come from the dairy industry and dairying applications. However, in all applications, flux decline due to concentration polarisation and fouling are probably the two most important practical aspects
98 M.J. Lewis 4.2 PROCESSING CHARACTERISTICS This section will deal with some of the important processing parameters encountered in ultrafiltration. There are various factors which will influence the outcome of the process, such as the concentration factor and rejection. See Section 3.3 The extent of the concentration is defined by the concentration factor (), defined as VE/V(see eq.(3.5). Usually the permeate is the biggest fraction by volume. Milk for heese making is concentrated by UF fivefold, whereas cheese whey is concentrated twentyfold for the production of protein concentrates. Sometimes the resulting permeates are further concentrated by reverse osmosis 4.2.1 Rejection or retention factors The rejection or retention factor(R)of any component is defined as where ce is the concentration of component in the feed and cp is the concentration in the permeate. The rejection is determined experimentally for each component in the feed, by sampling the feed and permeate at the same time and analysing that component. It is very important and will influence the extent(quality)of the separation achievable Rejection values normally range between 0 and 1; sometimes they are expressed as percentages(0 to 100%) when Cp=0; R=l; all the component is retained in the feed when cp = c R=0; the component is freely permeating In ultrafiltration experiments, some workers have measured negative rejection,i.e Cp>CE, particularly for minerals. It is not immediately obvious why this should have occurred. Possible explanations for this are higher concentrations at the membrane Irface than in the bulk, due to concentration polarisation. However, this is unlikely to be the case for freely permeating species. Another explanation is the basis on which concen- tration is measured (Glover, 1985). This may arise when there is substantial fat in the eed which is rejected by the material. It is suggested that concentrations be expressed in the aqueous portion. A third explanation lies in the Donnan effect; Donnan predicted and later demonstrated that concentration of electrolyte in the solutions on either side of a alysis membrane were unequal when the colloid on one side was electrically charged (see later). For example, at low pH values, where proteins are likely to be positively charged, this could lead to higher concentrations of cations in the permeate membrane for a particular application. Rejection values may also be influenced by operating conditions An idealultrafiltration membrane would have a rejection value of 1.0 for high molecular weight components and zero for low molecular weight components. However, typical values observed for real membranes are between 0.9 and 1.0 for high molecular weights and between 0 and 0. 1 for low molecular weight components. values for
98 M.J.Lewis 4.2 PROCESSING CHARACTERISTICS This section will deal with some of the important processing parameters encountered in ultrafiltration. There are various factors which will influence the outcome of the process, such as the concentration factor and rejection. See Section 3.3. The extent of the concentration is defined by the concentration factor cf), defined as VF/Vc (see eq. (3.5)). Usually the permeate is the biggest fraction by volume. Milk for cheese making is concentrated by UF fivefold, whereas cheese whey is concentrated twentyfold for the production of protein concentrates. Sometimes the resulting permeates are further concentrated by reverse osmosis. 4.2.1 Rejection or retention factors The rejection or retention factor (R) of any component is defined as R = (CF - cp )/cF (4.1) where cF is the concentration of component in the feed and cp is the concentration in the permeate. The rejection is determined experimentally for each component in the feed, by sampling the feed and permeate at the same time and analysing that component. It is very important and will influence the extent (quality) of the separation achievable. Rejection values normally range between 0 and 1; sometimes they are expressed as percentages (0 to 100%). when cp =O; when cp = cF R = 1; all the component is retained in the feed R = 0; the component is freely permeating. In ultrafiltration experiments, some workers have measured negative rejection, Le. cp > cF, particularly for minerals. It is not immediately obvious why this should have occurred. Possible explanations for this are higher concentrations at the membrane surface than in the bulk, due to concentration polarisation. However, this is unlikely to be the case for freely permeating species. Another explanation is the basis on which concentration is measured (Glover, 1985). This may arise when there is substantial fat in the feed which is rejected by the material. It is suggested that concentrations be expressed in the aqueous portion. A third explanation lies in the Donnan effect; Donnan predicted and later demonstrated that concentration of electrolyte in the solutions on either side of a dialysis membrane were unequal when the colloid on one side was electrically charged (see later). For example, at low pH values, where proteins are likely to be positively charged, this could lead to higher concentrations of cations in the permeate. Rejection characteristics can readily be determined for different substances using different membranes. This is one practical way of selecting the most appropriate membrane for a particular application. Rejection values may also be influenced by operating conditions. An ‘ideal’ ultrafiltration membrane would have a rejection value of 1.0 for high molecular weight components and zero for low molecular weight components. However, typical values observed for real membranes are between 0.9 and 1.0 for high molecular weights and between 0 and 0.1 for low molecular weight components. Values for
Ultrafiltration 99 minerals often are usually in the region of 0.1, but may be as high as 0.5, if the mineral binds to macromolecules. It is important to appreciate that any component with a rejection value greater than 0 will increase in concentration during the course of an ultrafiltration process. Rejection values can be used to check the integrity and performance of a membrane. Some values for components in dairy processing are given in Table 4. 1. Note the relatively high values for minerals, which suggests some binding to the proteins, particularly for calcium and magnesium. Membrane manufacturers some times present performance data in terms of rejection values of a range of components of different molecular weights(see Table 4.2). This will give some guidelines in terms of selection. However, very rarely are those components selected that one is interested in An alternative form of representation widely used is the molecular weight cut-off value. Table 4. 1. Rejection characteristics obtained during ultra filtration of dairy products Product Protein Lactose Ash Sweet whey 0.85-1.0 00.2 0-0.5 Ac 0.85-1.0 00.2 Skim milk 0.965-1.0 Whole milk 0.965-0.999 00.03 00.1 a Based on Kjeldahl nitrogen x 6.38 Taken from Lewis(1982) Table 4. 2. Some cited rejection characteristics for different components MW 3000 1000030000 100000 Insulin 600 >0.98 Cytochrome C 12400 045 >0.98 095 0.75 0.20 67000>0.980.98 0.95 Adapted from data from Amicon(1992) The molecular weight cut-off values for UF membranes range betv and 300 000. At values of about 2000, it overlaps with nanofiltration or loose reverse osmosis, whereas at 30 000 it overlaps with microfiltration. Generally the applied pressure required will decrease with increasing cut-off value and pressures in the range 1-15 bar are used It is implied that a membrane with a molecular weight cut-off of 5000, would reject all omponents with that molecular weight value or higher (R= 1)and allow components below that molecular weight to permeate freely. Often dextrins have been used for esti mating molecular weight cut-off, but these are linear molecules. However, due to the
Ultrafiltration 99 minerals often are usually in the region of 0.1, but may be as high as 0.5, if the mineral binds to macromolecules. It is important to appreciate that any component with a rejection value greater than 0 will increase in concentration during the course of an ultrafiltration process. Rejection values can be used to check the integrity and performance of a membrane. Some values for components in dairy processing are given in Table 4.1. Note the relatively high values for minerals, which suggests some binding to the proteins, particularly for calcium and magnesium. Membrane manufacturers sometimes present performance data in terms of rejection values of a range of components of different molecular weights (see Table 4.2). This will give some guidelines in terms of selection. However, very rarely are those components selected that one is interested in. An alternative form of representation widely used is the molecular weight cut-off value. Table 4.1. Rejection characteristics obtained during ultrafiltration of dairy products Product Proteina Lactose Ash Sweet whey 0.85-1.0 0-0. 2 0-0.5 Acid whey 0.85-1.0 0-0.2 0-0.5 Skim milk 0.965-1 .O 0-0.2 0-0.5 Whole milk 0.965-0.999 0-0.03 0-0.1 a Taken from Lewis (1982). Based on Kjeldahl nitrogen x 6.38. Table 4.2. Some cited rejection characteristics for different components MW 3000 10 000 30 000 100 000 Insulin 6 000 >0.98 - - - Cytochrome C 12 400 >0.98 0.85 0.45 - a-Ch ymotrypsinogen 24 500 >0.98 0.95 0.75 0.20 Albumin 67 000 >0.98 >0.98 0.95 0.30 Adapted from data from Amicon (1992). The molecular weight cut-off values for UF membranes range between about 2000 and 300 000. At values of about 2000, it overlaps with nanofiltration or ‘loose reverse osmosis’, whereas at 30 000 it overlaps with microfiltration. Generally the applied pressure required will decrease with increasing cut-off value and pressures in the range 1-15 bar are used. It is implied that a membrane with a molecular weight cut-off of 5000, would reject all components with that molecular weight value or higher (R = 1) and allow components below that molecular weight to permeate freely. Often dextrins have been used for estimating molecular weight cut-off, but these are linear molecules. However, due to the
100 M.J. Lewis diffuse nature of the membrane, this is not so. This approach ignores molecular shape face and within the membrane itself, A nponents in the feed, and at the membrane sur- vertheless it is useful for a preliminary(initial) selection of a suitable membrane. However, it tells you nothing about the rejection value of a component below the molecular weight cut-off, say 500 or 1000. In fact, it rather implies that such components would be freely permeating. In reality, this is not the case as most membranes are diffuse in their separation ability. The concept of a sharp and diffuse membrane is useful in this respect(see data from Table 4.2) Figure 4. I shows the rejection characteristics of two such membranes. The sharp membrane is an ideal situation, offering the perfect separation. Real membranes offe quite diffuse rejection characteristics, requiring a molecular weight difference of about tenfold to provide an effective separation. Therefore they would give a poor separation of components with slight differences in molecular weights, even components with differences up to two times would not necessarily be well separated. For example it ould not be easy to fractionate the proteins in cheese whey or to separate mono saccharides from disaccharides. McGregor (1986)has undertaken some interesting xperiments, using electrophoresis to examine the sharpness of separations performed on mixtures of protein of different molecular weights. His results showed considerable differences in the sharpness of the separation between different membranes with the same nominal molecular weight cut-off value. Gekas et al.(1990)found that experimental flux and rejection data correlated better with porosimetric data(pore size and pore size distri bution as measured by bubble pressure and solvent permeabilities)than molecular weight cut-off value These types of observation illustrate that although some physicochemical measure- ments might be useful, the selection of the best membrane is best done experimentally, by measuring the rejection characteristics of the components to be separated at the selected There is also evidence that the rejection value for most components increases during the course of an ultrafiltration process. Some of the experimental work on rejection measurement and practical problems involved are described in Section 4.2.4 Fig. 4. 1. Characteristics of a sharp and diffuse membrane: Ij, ideal, 10 000 molecular weight cut- off: 12. ideal, 100 000 molecular weight cut-off: S, sharp membrane: D, diffuse mem
1.0 c 0 8 0.5 '6 a: .- c 0 - - -4' I
Ultrafiltration 101 4.2.2 Yield Ultrafiltration is now being used to concentrate and recover some very valuable com- pounds. The yield or recovery of a component is a very important variable, as it will strongly influence the economics of the process. present in the feed, which is retained in the concentrate. For recovery of compollco. a The yield of a component is defined as the fraction of that component, origina mportant to have a high yield. However, when washing out components, such as toxins, the yield should be lov For a batch process, it can be shown that the yield of any component depends upon the concentration factor and rejection Concentration factor and the yield (n is given by Y= mass component in final concentrate=vacc (4.3) mass substance in feed where Vc and Ve are the volumes of feed and concentrate and cc and cF are the concen trations in the concentrate and feed If we now consider a batch concentration process depicted by Fig. 4.2, where permeate is removed and the retentate is recycled At any instance let the volume of the concentrate V and the concentration of the component of interest =c Permeate Fig. 4.2. Batch concentration process for yield calculation Let the removal of a small volume of permeate(dv), result in a change of concentration A mass balance on the component will give the following equation (feed)(concentrate) rmeate Rejection (R)=(c-Cp)
Ultrafiltration 101 4.2.2 Yield Ultrafiltration is now being used to concentrate and recover some very valuable compounds. The yield or recovery of a component is a very important variable, as it will strongly influence the economics of the process. The yield of a component is defined as the fraction of that component, originally present in the feed, which is retained in the concentrate. For recovery of components it is important to have a high yield. However, when washing out components, such as toxins, the yield should be low. For a batch process, it can be shown that the yield of any component depends upon the concentration factor and rejection. Concentration factor cf) = vF/vC (4.2) and the yield (Y) is given by mass component in final concentrate - VCCC Y= -- mass substance in feed VFCF (4.3) where V, and VF are the volumes of feed and concentrate and cc and CF are the concentrations in the concentrate and feed. If we now consider a batch concentration process depicted by Fig. 4.2, where permeate is removed and the retentate is recycled: At any instance let the volume of the concentrate = V and the concentration of the component of interest = c in" f" Permeate CP Fig. 4.2. Batch concentration process for yield calculation. Let the removal of a small volume of permeate (dV), result in a change of concentration (dc) * A mass balance on the component will give the following equation: VC = (V-dV)(c-dc) + cpdV (feed) (concentrate) (permeate) Rejection (R) = (c - cp )/c