Scraped surface heat exchangers.

Scraped surface heat exchangers (SSHEs) are commonly used in the food, chemical, and pharmaceutical industries for heat transfer, crystallization, and other continuous processes. They are ideally suited for products that are viscous, sticky, that contain particulate matter, or that need some degree of crystallization. Since these characteristics describe a vast majority of processed foods, SSHEs are especially suited for pumpable food products. During operation, the product is brought in contact with a heat transfer surface that is rapidly and continuously scraped, thereby exposing the surface to the passage of untreated product. In addition to maintaining high and uniform heat exchange, the scraper blades also provide simultaneous mixing and agitation. Heat exchange for sticky and viscous foods such as heavy salad dressings, margarine, chocolate, peanut butter, fondant, ice cream, and shortenings is possible only by using SSHEs. High heat transfer coefficients are achieved because the boundary layer is continuously replaced by fresh material. Moreover, the product is in contact with the heating surface for only a few seconds and high temperature gradients can be used without the danger of causing undesirable reactions. SSHEs are versatile in the use of heat transfer medium and the various unit operations that can be carried out simultaneously. This article critically reviews the current understanding of the operations and applications of SSHEs.

Glycolipid transfer protein interaction with bilayer vesicles: modulation by changing lipid composition.

Glycosphingolipids (GSLs) are important constituents of lipid rafts and caveolae, are essential for the normal development of cells, and are adhesion sites for various infectious agents. One strategy for modulating GSL composition in lipid rafts is to selectively transfer GSL to or from these putative membrane microdomains. Glycolipid transfer protein (GLTP) catalyzes selective intermembrane transfer of GSLs. To enable effective use of GLTP as a tool to modify the glycolipid content of membranes, it is imperative to understand how the membrane regulates GLTP action. In this study, GLTP partitioning to membranes was analyzed by monitoring the fluorescence resonance energy transfer from tryptophans and tyrosines of GLTP to N-(5-dimethyl-aminonaphthalene-1-sulfonyl)-1,2-dihexadecanoyl-sn-glycero-3-phospho-ethanolamine present in bilayer vesicles. GLTP partitioned to POPC vesicles even when no GSL was present. GLTP interaction with model membranes was nonpenetrating, as assessed by protein-induced changes in lipid monolayer surface pressure, and nonperturbing in that neither membrane fluidity nor order were affected, as monitored by anisotropy of 1,6-diphenyl-1,3,5-hexatriene and 6-dodecanoyl-N,N-dimethyl-2-naphthylamine, even though the tryptophan anisotropy of GLTP increased in the presence of vesicles. Ionic strength, vesicle packing, and vesicle lipid composition affected GLTP partitioning to the membrane and led to the following conclusion: Conditions that increase the ratio of bound/unbound GLTP do not guarantee increased transfer activity, but conditions that decrease the ratio of bound/unbound GLTP always diminish transfer. A model of GLTP interaction with the membrane, based on the partitioning equilibrium data and consistent with the kinetics of GSL transfer, is presented and solved mathematically.

Activation of sphingomyelinase in lipid monolayer is related to interfacial water activity. Evidence from two disparate systems.

Recently it has been hypothesized that surface pressure of a lipid monolayer is a direct measure of thermodynamic activity of interfacial water and therefore surface pressure-dependent processes in lipid bilayers and monolayers are modulated essentially by interfacial water activity [S. Damodaran, Colloids Surf. B: Biointerf. 11 (1998) 231; C.S. Rao, S. Damodaran, Colloids Surf. B: Biointerf. 34 (2004) 197]. If the hypothesis is true, then it should be a general one and ought to be system independent. To further test this hypothesis, the specific activity of sphingomyelinase (SMase) was studied in two disparate systems, one involving sphingomyelin (SM) monolayer at various surface pressures at the air-water interface and the other involving a solid-state SMase-SM system exposed to various equilibrium relative humidity (ERH). The results were examined in terms of thermodynamic activity of water in the interfacial region (a(w)s) and in the hydrated solid phase (ERH). In both these physically different systems, the dependence of specific activity of SMase on ERH and a(w)s was very similar. In both cases, the specific activity exhibited a maximum at ERH (or a(w)s) approximately 0.3, which suggested that the apparent surface pressure-dependence of interfacial activation of lipolytic enzymes might be actually related to modulation of the hydration state of the enzyme through the control of thermodynamic activity of water in the lipid-water interfacial region.

Glycolipid transfer protein mediated transfer of glycosphingolipids between membranes: a model for action based on kinetic and thermodynamic analyses.

Glycolipid transfer protein (GLTP) catalyzes the intermembrane transfer of lipids that have sugars beta-linked to either diacylglycerol or ceramide backbones, including simple glycosphingolipids (GSLs) and gangliosides. The present study provides a quantitative understanding of GLTP action involving bilayer vesicles that have high and low curvature stress, i.e., small and large unilamellar vesicles (SUVs and LUVs). When the GSL intervesicular transfer was monitored in real time using an established fluorescence resonance energy approach, the initial GSL transfer rates (v(0)) and net transfer equilibrium (K(eq)) were determined for GLTP-mediated transfer from SUVs and LUVs over the temperature range of 30-44 degrees C. v(0) exhibited a linear dependence with respect to varying GLTP concentrations (0-143 nM range) in SUVs and LUVs, suggesting a first order dependence on the GLTP bulk concentration. Thermodynamic parameters associated with the GLTP-GSL transition-state complex and GSL net transfer were determined from linear Arrhenius and van't Hoff plots, respectively. Although initial transfer rates were lower for LUVs than for SUVs, the activation energy barriers were higher for LUVs, while the Gibbs's free energy of the transition states were similar. The formation of a transition-state complex was predominantly enthalpy driven, whereas the net transfer of GSLs was mainly entropy driven. The rate-limiting step for GLTP action appeared to be the surface processes leading to the GLTP-GSL complex formation and release associated with a shuttle/carrier mode of action. Because surface processes leading to the GLTP-GSL complex formation were limiting for GLTP action with SUVs and LUVs, it was concluded that GLTP is likely to be a valuable tool to probe and manipulate GSL environments in membranes.

Surface pressure dependence of phospholipase A2 activity in lipid monolayers is linked to interfacial water activity.

The specific activity of pancreatic phospholipase A2 (PLA2) was studied in two disparate systems, one involving phosphatidylcholine monolayer at various surface pressures at the air-water interface and the other involving a solid-state system exposed to various equilibrium relative humidity (ERH). The results were examined in terms of thermodynamic activity of water in the interfacial region (aws*) and in the hydrated solid phase (aw). In both these physically different systems, the specific activity versus aw and aws* profiles of PLA2 were remarkably similar. In both cases, the specific activity exhibited a maximum at aw (or aws*) approximately 0.3. These results suggested that the mechanism of control of PLA2 activity at the lipid-water interface might involve modulation of the hydration state of the enzyme through control of the thermodynamic activity of water in the interfacial region. Extension of these results to biomembranes suggests that one of the functions of lipid bilayer might be the control of local water activity at the lipid-water interface. In biological membranes, localized subtle changes in interfacial water activity may occur as a result of local stretching or compression of the membrane facilitated by conformational changes in membrane-bound receptor proteins.