Cell membranes, limiting mass transfer

The drying of solid food matrices depends heavily on diffusion and mass transfer through the cells and tissue. The presence of intact cell membranes in the food raw material limits mass transfer processes due to their barrier function. Furthermore, the tissue structure and the network of intercellular air spaces affect mass transfer during drying. 

The actual processes of uptake of chemical species by an organism typically encompass transport in the medium, adsorption at extracellular cell wall components, and internalisation by transfer through the cell membrane. Each of these steps constitutes a broad spectrum of physicochemical aspects, including chemical interactions between relevant components, electrostatic interactions, elementary chemical kinetics (in this volume, as pertains to the interface), diffusion limitations of mass transfer processes, etc. 

The membrane system considered here is composed of two aqueous solutions wd and w2, separated by a liquid membrane M, and it involves two aqueous solution/ membrane interfaces WifM (outer interface) and M/w2 (inner interface). If the different ohmic drops (and the potentials caused by mass transfers within w1 M, and w2) can be neglected, the membrane potential, EM, defined as the potential difference between wd and w2, is caused by ion transfers taking place at both L/L interfaces. The current associated with the ion transfer across the L/L interfaces is governed by the same mass transport limitations as redox processes on a metal electrode/solution interface. Provided that the ion transport is fast, it can be considered that it is governed by the same diffusion equations, and the electrochemical methodology can be transposed en bloc [18, 24]. With respect to the experimental cell used for electrochemical studies with these systems, it is necessary to consider three sources of resistance, i.e., both the two aqueous and the nonaqueous solutions, with both ITIES sandwiched between them. Therefore, a potentiostat with two reference electrodes is usually used. 

Although a preindustrial project of ethanol production with immobilized cells goes back to the 1980s (8), its application is still limited because of technical problems related to the gel stability and even more to the mass-transfer resistance of the gel membrane (9). Specifically, both substrate and product counterdiffusion (in addition to the presence of cells inside the beads) could reduce the diffusion coefficients of glucose and ethanol up to 13.7 and 28.1%, respectively (10). 

It is important to minimize the overpotential and ohmic components of cell voltage in order to maximize energy efficiency. This can be done by using conductive, catalytic electrodes and membranes, small interelectrode (or membrane-electrode) gaps, and by careful choice of the counterelectrode chemistry to minimize the equilibrium cell voltage. Costs at the optimum usually become higher in the case of more dilute solutions as the mass-transfer-imposed limiting current is lower. 

An important point to consider about the stack management, with reference to an electric power train operating in dynamic conditions, as determined by road requirements, is the regulation of the stack temperature together with the other control parameters of water and reactants to avoid mass transfer limitations and membrane drying out or flooding. Moreover, the interaction between stack and auxiliaries has to be balanced taking into account the optimization of fuel cell system efficiency and reliability (see Sect. 4.6). 

Whole-cell, hollow-fiber MBR are still under development. Despite their significant potential they have, so far, found only limited application for biochemicals production. One of the reasons is that cleaning of the hollow-fiber membranes is difficult, especially when whole-cell biocatalysts are immobilized in the small fibers. The mass transfer between the nutrients and cells has also to be taken into consideration and enhanced. Immobilizing the biocatalysts in porous beads, instead of directly on the membrane, may tend to avoid some of these problems, and to simplify membrane cleaning. The concept of using MBR as bioartificial organs is technically very attractive the various MBR under development, however, must still be validated with clinical results. One can expect, however, that their development will follow the success of artificial kidneys, which are currently employed worldwide. 

For solutions of polymers and proteins up to about 300,000 in molecular weight, some tests with tubular membranes and thin-channel cells gave limiting fluxes in reasonable agreement ( 30 percent) with those predicted using standard correlations for mass transfer, such as Eq. (21.50) for laminar flow and Eq. 

Most of the membrane segregated enzyme systems previously examined suffer some constitutive drawbacks which limit their yield and area of application. When enzymes are entrapped within the sponge of asymmetric membranes, product and substrate mass transfer occur mainly by a diffusive mechanism reactor performance is then controlled only by means of the amount and kind of charged enzyme, and the fluid dynamics of the solution in the core of the fibers. UF or RO fluxes, moreover, result in enzyme losses. Enzyme crosslinking in the membrane pores can reduce these losses, but it can lead to an initial activity loss, as compared to that of the native enzyme. Of course, once the enzyme is deactivated, it makes the reactor useless for further operation. Such immobilization techniques are seldom useful for microbial cells due to their large size. 

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