Ultrafiltration selectivity

Researchers have recently developed four main types of processes to separate and purify whey proteins in an attempt to leave them in their more biologically active native forms. The types are selective precipitation, membrane filtration (ultrafiltration), selective adsorption, and selective elution (ion exchange). ... 

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

A second factor determining module selection is resistance to fouling. Membrane fouling is a particularly important problem in Hquid separations such as reverse osmosis and ultrafiltration. In gas separation appHcations, fouling is more easily controlled. Hollow-fine fibers are notoriously prone to fouling and can only be used in reverse osmosis appHcations if extensive, costiy feed-solution pretreatment is used to remove ah. particulates. These fibers caimot be used in ultrafiltration appHcations at ah. 

Membrane Sep r tion. The separation of components ofhquid milk products can be accompHshed with semipermeable membranes by either ultrafiltration (qv) or hyperfiltration, also called reverse osmosis (qv) (30). With ultrafiltration (UF) the membrane selectively prevents the passage of large molecules such as protein. In reverse osmosis (RO) different small, low molecular weight molecules are separated. Both procedures require that pressure be maintained and that the energy needed is a cost item. The materials from which the membranes are made are similar for both processes and include cellulose acetate, poly(vinyl chloride), poly(vinyHdene diduoride), nylon, and polyamide (see AFembrane technology). Membranes are commonly used for the concentration of whey and milk for cheesemaking (31). For example, membranes with 100 and 200 p.m are used to obtain a 4 1 reduction of skimmed milk. 

Ultrafiltration. Membranes are used that are capable of selectively passing large molecules (>500 daltons). Pressures of 0.1—1.4 MPa (<200 psi) are exerted over the solution to overcome the osmotic pressure, while providing an adequate dow through the membrane for use. Ultrafiltration (qv) has been particulady successhil for the separation of whey from cheese. It separates protein from lactose and mineral salts, protein being the concentrate. Ultrafiltration is also used to obtain a protein-rich concentrate of skimmed milk from which cheese is made. The whey protein obtained by ultrafiltration is 50—80% protein which can be spray dried. 

Fouling is controlled by selection of proper membrane materials, pretreatment of feed and membrane, and operating conditions. Control and removal of fouling films is essential for industrial ultrafiltration processes. 

Ultra filtration. This process removes macromolecules, microorganisms, particulate matter, and pyrogens using a thin, selectively permeable membrane. Ultrafiltration caimot remove ions from water and is generally employed as a polishing process. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Membrane Processes Membrane processes are also used diafiltration is convenient for the removal of small contaminating species such as salts and smaller proteins, and can be combined with subsequent steps to concentrate the protein. Provided that proper membrane materials have been selected to avoid protein-membrane interactions, diafiltration using ultrafiltration membranes is typically straightforward, high-yielding and capital-sparing. These operations can often tolerate the concentration or the desired protein to its solu-bihty limit, maximizing process efficiency. 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

Ultrafiltration is an improvement on the dialysis principle. Filters having pore sizes over the range of biomolecular dimensions are used to filter solutions to select for molecules in a particular size range. Because the pore sizes in these filters are microscopic, high pressures are often required to force the solution through the filter. This technique is useful for concentrating dilute solutions of macromolecules. The concentrated protein can then be diluted into the solution of choice. 

A different approach is the use of an ultrafiltration membrane with an immobilized chiral component [31]. The transport mechanism for the separation of d,l-phenylalanine by an enantioselective ultrafiltration membrane is shown schematically in Fig. 5-4a. Depending on the trans-membrane pressure, selectivities were found to be between 1.25 and 4.1, at permeabilities between 10 and 10 m s respectively (Fig. 5-4b). 

Two other major factors determining module selection are concentration polarisation control and resistance to fouling. Concentration polarisation control is a particularly important issue in liquid separations such as reverse osmosis and ultrafiltration. Hollow-fine-fibre modules are notoriously prone to fouling and concentration polarisation and can be used in reverse osmosis applications only when extensive, costly feed solution pretreatment removes all particulates. These fibres cannot be used in ultrafiltration applications at all. 

Cell-free translation system, used for the identification of cloned genes and gene expression, has been investigated extensively as a preparative production system of commercially interesting proteins after the development of continuous-flow cell-free translation system. Many efforts have been devoted to improve the productivity of cell-free system [1], but the relatively low productivity of cell-free translation system still limits its potential as an alternative to the protein production using recombinant cells. One approach to enhance the translational efficiency is to use a condensed cell-free translation extract. However, simple addition of a condensed extract to a continuous-flow cell-free system equipped with an ultrafiltration membrane can cause fouling. Therefore, it needs to be developed a selective condensation of cell-free extract for the improvement of translational efficiency without fouling problem. 

The purification of a product p from an impurity i by an ultrafiltration process is shown in Fig. 20-59 and Eq. (20-72), where Cjo and C o are the initial concentrations (g/L) of the two components, Y is the yield of product in the retentate, and r / = selectivity = S,/Sp, the ratio of passages. High yields are obtained in purifying out small solutes (high selectivity) but are compromised in removing larger impurities with similar passages to the product. 

Generally, the major adverse effects associated with colloids are fluid overload, dilutional coagulopathy, and anaphy-lactoid/anaphylactic reactions.24,32 Although derived from pooled human plasma, there is no risk of disease transmission from commercially available albumin or PPF products since they are heated and sterilized by ultrafiltration prior to distribution.24 Because of direct effects on the coagulation system with the hydroxyethyl starch and dextran products, they should be used cautiously in hemorrhagic shock patients. This is another reason why crystalloids maybe preferred in hemorrhagic shock. Furthermore, hetastarch can result in an increase in amylase not associated with pancreatitis. As such, the adverse-effect profiles of the various fluid types should also be considered when selecting a resuscitation fluid. 

Attached to the periphery of a lst-generation dendrimer, Ti(OCHMe2)2-com-plexes of the six TADDOL moieties in 84 catalyze - in homogeneous solution -the enantioselective addition of diethylzinc to benzaldehyde with about the same selectivity ((S) (R) 97 3) as do six monomeric TADDOL units [105],but, with a molecular weight of only 3833 Da, dendrimer 84 had to be separated by column chromatography rather than by ultrafiltration methods. 

Membranes. Photopolymer chemistry is being applied to the design and manufacture of a variety of membrane materials. In these applications, photopolymer technology is used to precisely define the microscopic openings in the membrane as it is being formed or to modify an existing membrane. Some of the applications of photopolymer chemistry to membranes include the modification of ultrafiltration membranes (78) and the manufacture of amphiphilic (79), gas permeable (80), untrafiltration (81), ion-selective electrode (82) and reverse osmosis membranes. 

Several strategies have been described for the preconcentration of sample components present at low concentrations. These techniques include zone sharpening,28-29 on-line packed columns,30 and transient capillary isotachophoresis (cITP).31-32 Other standard laboratory techniques are often used, including solid-phase extraction, protein precipitation, ultrafiltration, etc. Two important points to keep in mind when selecting a concentration protocol are the sample requirements of the method and the potential selectivity on relative concentrations of sample components. The latter point applies to purity and concentration analysis. 

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