Integration of bioconversion and downstream processing: Starch hydrolysis in an aqueous two-phase system

Enzymatic conversions of starch

This article surveys methods for the enzymatic conversion of starch, involving hydrolases and nonhydrolyzing enzymes, as well as the role of microorganisms producing such enzymes. The sources of the most common enzymes are listed. These starch conversions are also presented in relation to their applications in the food, pharmaceutical, pulp, textile, and other branches of industry. Some sections are devoted to the fermentation of starch to ethanol and other products, and to the production of cyclodextrins, along with the properties of these products. Light is also shed on the enzymes involved in the digestion of starch in human and animal organisms. Enzymatic processes acting on starch are useful in structural studies of the substrates and in understanding the characteristics of digesting enzymes. One section presents the application of enzymes to these problems. The information that is included covers the period from the early 19th century up to 2009.

Aqueous two-phase systems for biomolecule separation

Over the past thirty years, aqueous polymer two-phase technology has evolved, both experimentally and theoretically, into a separation science with many useful applications in biomolecule purification and bioconversion. This paper summarizes the developments in the applications of aqueous two-phase systems to biotechnology. The main topics to be considered are the phase diagram and its characteristics, fundamentals of biomolecule partition, large-scale and multi-stage aqueous two-phase biomolecule purification, and extractive bioconversions. The first topic involves a discussion of the thermodynamics of aqueous polymer two-phase formation and how it is influenced by such factors as polymer molecular weight and concentration, temperature, and salt type and concentration. Next, the theoretical and experimental aspects of biomolecule partition in aqueous two-phase systems will be discussed in light of the factors which influence biomolecule partition: polymer concentration and molecular weight; temperature; salt type and concentration; the addition of charged, hydrophobic and affinity derivatives. Having reviewed the fundamentals of phase diagram formation and biomolecule partition, the next two topics are applications of aqueous two-phase technology. The first set of applications involve the large-scale extraction of proteins using one to three equilibrium stages and multi-stage purifications using countercurrent distribution, liquid-liquid partition chromatography and continuous countercurrent chromatography. The second application, and very promising area for future aqueous two-phase technology, is the extractive bioconversion which permits the simultaneous production and purification of a biomolecule.

Multistage magnetic and electrophoretic extraction of cells, particles and macromolecules

Improved techniques for separating cells, particles, and macromolecules (proteins) are increasingly important to biotechnology because separation is frequently the limiting factor for many biological processes. Manufacturers of new enzymes and pharmaceutical products require improved methods for recovering intact cells and intracellular products. Similarly isolation, purification, and concentration of many biomolecules produced in fermentation processes is extremely important. Often such downstream processing contributes a large portion of the product cost. In conventional methods like centrifugation and even modern methods like chromatography, scale-up problems are enormous, making them uneconomical and prohibitively expensive unless the product is of very high value. Therefore there has been a need for efficient and economical alternative approaches to bioseparation processes to eliminate, reduce, or facilitate solids handling. Magnetic and electric field assisted separations may hold considerable potential for providing a future major improvement in bioseparation technology. In the present review the merits and demerits of the existing methods are discussed. We present mainly our own research on the development of unified multistage extraction processes that are versatile enough to handle cells and particles as well as macromolecules as described below. We describe multistage methods, namely ADSEP (Advanced Separator), MAGSEP (Magnetic Separator), and ELECSEP (Electrophoretic Separator), for quantitatively separating cells, particles, and solutes by using magnetically and electrophoretically assisted extraction processes. To the best of our knowledge, multistage magnetic and electrophoretic separations have not been reported in the earlier literature. The theoretical underpinnings of these separations are crucial to their success and to the identification of their advantages over other separation processes in particular applications. Hence mathematical modeling is stressed here, presenting our own models while also reviewing models reported in the literature. We also present suggestions for future work while analyzing the scale-up and economic aspects of these extraction processes. Commercial uses of the magnetic and electrophoretic processes, having both ground- and space-based research elements, also are presented in this review.

Enhancement of mass transfer using colloidal liquid aphrons: measurement of mass transfer coefficients in liquid-liquid extraction

Interphase mass transfer of a sparingly soluble solute is often the rate-limiting step in multiphase biocatalytic processes. Colloidal liquid aphrons (CLA) provide very large interfacial areas, and thus could enhance mass transfer in such processes. The aim of this study was to characterize mass transfer properties of CLA dispersions during transfer of heptanoic acid from water to limonene. The interfacial area per unit volume (a), film mass transfer coefficient (K(L)), and volumetric mass transfer coefficient (K(L)a) values were determined in a stirred-tank reactor. These results were used, along with a literature correlation, to estimate the mass transfer coefficient of the surfactant-stabilized shell surrounding the CLA. The very large increase in a provided by the CLA was only partially offset by a slight increase in the mass transfer resistance of the shell. As a result, the range of K(L)a values obtained using CLA was about an order of magnitude greater than that obtained using a conventional dispersion. The concentration of the aqueous-phase surfactant used to form the CLA strongly affected the Sauter mean diameter of the CLA; however, the concentration of the nonpolar-phase surfactant had little effect. These results suggest that CLA have considerable potential for multiphase biocatalytic applications.

Compartmentalization of Enzymes and Distribution of Products in Aqueous Two-Phase Systems

Phase separation is a common phenomenon in water solutions of polymers due to "polymer incompatibility." Polymeric aqueous two-phase systems are much used for separations in biochemistry and cell biology. When macromolecules are included in a phase system, it is often possible to obtain a one-sided distribution to one of the phases, i.e., the macromolecule is compartmentalized within one aqueous phase. This chapter describes the thermodynamic forces which govern the partitioning of molecules in aqueous two-phase systems. For a high molecular weight macromolecule, e.g., an enzyme, both enthalpic and entropic effects contribute to a one-sided partitioning. Molecules of low molecular weight will be more evenly distributed between the phases. These mechanisms are significant in biological systems and can be used for enzyme reactors in bioconversions. Enzymatic reactions can take place with enzyme and substrate compartmentalized in one of the phases. A low-molecular weight product which is evenly partitioned between the phases can be continuously removed from the enzyme-substrate compartment. These principles are described in the enzymatic conversion of cellulose in an aqueous two-phase system.

Extractive bioconversions in aqueous two-phase systems

Although extractive bioconversions in aqueous two-phase systems (ATPSs) have been studied for over a decade, this has not yet resulted in widespread industrial application. The main reasons are the cost of the phase-forming polymers and the complexity of ATPS behavior. A number of recent developments may give a new impetus to this technology. First of all, the use of extractive bioconversions in ATPSs has recently been extended to high-value protein products, while in the meantime the development of low-cost ATPSs is ongoing. Furthermore, novel chromatographic methods enable the analysis of polymer and metabolite compositions in complex ATPS mixtures, and recently employed statistical experimental designs provide a tool for efficient data gathering, while they also reveal synergistic effects between process parameters. Together, these developments open the way towards improved modeling of partitioning behavior in ATPSs.

Simultaneous production and purification of Bacillus subtilis alpha-amylase

Production of alpha-amylase with B. subtilis CCM 2722 in an aqueous two-phase polyethylene glycol/dextran system integrated with product purification by affinity chromatography on crosslinked starch during cultivation was studied. The medium was drawn from the bioreactor to the external settler during fermentation. After phase separation in the settler the dextran-rich bottom phase with cells was returned to the bioreactor. The PEG-rich top phase was pumped to the column with crosslinked starch and returned to the bioreactor after alpha-amylase adsorption. The same volumetric productivities, 0.53 U/mL/h, were reached in both batch and described process, but total productivity of the latter method was much higher owing to shortening upstream and downstream processing time. The enzyme of 98% homogenity in 95% yield was obtained after its elution from the column.