SPOTLIGHT

& Facing a Multiphasic World

Reactions in multiphasic systems—such as gas–liquid–liquid—are gaining importance due to the increasednumber of applications using these systems in the bioprocessindustry. However, the operation and control of such systemsis challenging because the addition of dispersed liquidphases alters the dynamics of the system. In particular, thetransfer rate of the solute gas across the boundary layer andthe gas–liquid contact characteristics can be changed due tothe interfacial properties of the dispersed liquid. Amaral andco-workers have contributed to a better understanding ofthis multiphasic world by studying the mechanisms involvedin the oxygen transfer in a submerged aerated bioreactor inthe presence of a second liquid phase: a perfluorocarbonused as an oxygen carrier. The authors show how the masstransfer in multiphasic systems is influenced by the bio-reactor working volume, the type, and content of the organicphase and the composition of the aqueous phase. Page 588

DOI: 10.1002/bit.21742

& Flux Analysis Gets Faster, Less Expensive,and More Relevant

Metabolic flux analysis (MFA) using stable isotope tracersand mass spectrometry is a powerful platform for metabolicengineering and systems biology. Fluxes describe the ratesat which material is flowing through specific metabolicroutes and thus provide key indicators of pathway kineticbottlenecks and cellular phenotype. To date, flux determi-nation has relied on metabolic and isotopic steady statemeasurements requiring long and expensive labelingexperiments. Young and co-workers of the Bioinformatics& Metabolic Engineering Laboratory at the MassachusettsInstitute of Technology have extended the recently devel-oped elementary metabolite unit (EMU) method of pathwaydecomposition to reconstruct cell-wide fluxes from iso-topically nonstationary labeling data. Besides achievingaccurate flux determination from transient data, theirapproach gives a 5,000-fold computational speedup incomparison to previous approaches and opens the door tononstationary MFA of biologically relevant networks.Page 686

DOI: 10.1002/bit.21741

Published online in Wiley InterScience

(www.interscience.wiley.com).

� 2007 Wiley Periodicals, Inc.

& CFD Simulation of Non-Newtonian Fluid Flowin Anaerobic Digesters

Mixing in an anaerobic digester (AD) not only affects masstransfer but also bacterial retention and degree of methanesupersaturation. Mixing as a key design and operationalparameter for ADs has not been adequately studied. Manyquestions remain such as optimal mixing as a function ofsolids concentration and biological activities within thedigester. Wu and Chen’s work provides an example of theneed for further fundamental understanding of mixing-related properties of the liquid in a digester and the utility ofmixing modeling for application to ADs. For example, thevelocity field for the same mixing regime can be completelydifferent, depending on the solids concentration of a givenfluid. An optimal mixing power input can be determinedvia computational fluid dynamics (CFD) modeling and thiswork provides a base for further AD mass transfer modelingwhich is currently being undertaken by the authors’ researchgroup at Washington State University. Page 700

DOI: 10.1002/bit.21752

& Protein Engineering For BioprocessDevelopment

The vast diversity of enzymatic activities found in natureprovides fertile ground for the development of biologicalprocesses for the production of a wide range of specialtychemicals. However, obtaining sufficient biocatalyst pro-ductivity has proved challenging and often limiting formany commercial applications. Previous work described theuse of protein engineering to overcome this limitation inseveral applications that utilize the enzymatic conversion ofhydroxynitriles to hydroxycarboxylic acids (Wu et al. 2007Biotechnol Bioeng 97:689–693). The current work expandson this theme in the context of a recently developedchemoenzymatic process for the manufacture of high purityglycolic acid. A key step in this process is the enzymaticconversion of glycolonitrile to ammonium glycolate usingnitrilase. In order to achieve the required biocatalyst pro-ductivity, protein engineering and fermentation optimiza-tion were used to increase the enzyme-specific activity up to15-fold and the biocatalyst-specific activity up to 125-fold.Page 717

DOI: 10.1002/bit.21740

Biotechnology and Bioengineering, Vol. 99, No. 3, February 15, 2008