The effects of holding raw milk under carbon dioxide pressures of 68 to 689 kPa at temperatures of 5, 6.1, 10, and 20°C on the indigenous microbiota were investigated. These pressure-temperature combinations did not cause precipitation of proteins from the milk. Standard plate counts from treated milks demonstrated significantly lower growth rate compared with untreated controls at all temperatures, and in some cases, the treatment was microcidal. Raw milk treated with CO2 and held at 6.1°C for 4 d exhibited reduced bacterial growth rates at pressures of 68, 172, 344, and 516 kPa; and at 689 kPa, demonstrated a significant loss of viability in standard plate count assays. The 689-kPa treatment also reduced gram-negative bacteria and total Lactobacillus spp. The time required for raw milk treated at 689 kPa and held at 4°C to reach 4.30 log10 cfu/mL increased by 4 d compared with untreated controls. Total coliform counts in the treated milk were maintained at 1.95 log10 cfu/mL by d 9 of treatment, whereas counts in the control significantly increased to 2.61 log10 cfu/mL by d 4 and 2.89 log10 cfu/mL by d 9. At d 8, Escherichia coli counts had not significantly changed in treated milk, but significantly increased in the control milk. Thermoduric bacteria counts after 8 d were 1.32 log10 cfu/mL in treated milk and 1.98 log10 cfu/mL in control milk. These data indicated that holding raw milk at low CO2 pressure reduces bacterial growth rates without causing milk protein precipitation. Combining low CO2 pressure and refrigeration would improve the microbiological quality and safety of raw milk and may be an effective strategy for shipping raw single strength or concentrated milk over long distances. 相似文献
Stress corrosion cracking of a commercial 0.19 pct C steel (SA-516 Grade 70) was studied in hot (92 ‡C) caustic solutions
of NaOH and NaOH plus aluminate (AlO−2) species. Potentiostatically controlled tests were conducted near the active-passive transition, using fracture mechanics
testing techniques and fatigue precracked double cantilever beam specimens. Crack propagation rates (Ν) were determined for
a range of stress intensities (Kl). In simple NaOH solutions, Region I (K1-dependent) and Region II (K1-independent) cracking behavior were observed. Increasing the concentration of NaOH from2m to8m decreasedKISCC and displaced Region I and the onset of Region II to lowerK1 levels. The presence of AlO−2 produced a comparable effect, with Region II being extended to lowerKI-Ν levels relative to simple NaOH solutions of similar hydroxyl anion concentration. The overallKI -Νv behavior and fractography were consistent with a dissolution mechanism of crack advance based on the general principles of
the film rupture-dissolution model. The effect of environment composition uponKI— Ν behavior was attributed to changes in repassivation kinetics.
Formerly Graduate Student at the University of British Columbia 相似文献
The goal of this work was to construct Escherichia coli strains capable of enhanced arginine production. The arginine biosynthetic capacity of previously engineered E. coli strains with a derepressed arginine regulon was limited by the availability of endogenous ornithine (M. Tuchman, B. S. Rajagopal, M. T. McCann, and M. H. Malamy, Appl. Environ. Microbiol. 63:33-38, 1997). Ornithine biosynthesis is limited due to feedback inhibition by arginine of N-acetylglutamate synthetase (NAGS), the product of the argA gene and the first enzyme in the pathway of arginine biosynthesis in E. coli. To circumvent this inhibition, the argA genes from E. coli mutants with feedback-resistant (fbr) NAGS were cloned into plasmids that contain "arg boxes," which titrate the ArgR repressor protein, with or without the E. coli carAB genes encoding carbamyl phosphate synthetase and the argI gene for ornithine transcarbamylase. The free arginine production rates of "arg-derepressed" E. coli cells overexpressing plasmid-encoded carAB, argI, and fbr argA genes were 3- to 15-fold higher than that of an equivalent system overexpressing feedback-sensitive wild-type (wt) argA. The expression system with fbr argA produced 7- to 35-fold more arginine than a system overexpressing carAB and argI genes on a plasmid in a strain with a wt argA gene on the chromosome. The arginine biosynthetic capacity of arg-derepressed DH5 alpha strains with plasmids containing only the fbr argA gene was similar to that of cells with plasmids also containing the carAB and argI genes. Plasmids containing wt or fbr argA were stably maintained under normal growth conditions for at least 18 generations. DNA sequencing identified different point mutations in each of the fbr argA mutants, specifically H15Y, Y19C, S54N, R58H, G287S, and Q432R. 相似文献
We propose a modified Fitzhugh-Nagumo neuron (MFNN) model. Based on this model, an integer-order MFNN system (case A) and a fractional-order MFNN system (case B) were investigated. In the presence of electromagnetic induction and radiation, memductance and induction can show a variety of distributions. Fractional-order magnetic flux can then be considered. Indeed, a fractional-order setting can be acceptable for non-uniform diffusion. In the case of an MFNN system with integer-order discontinuous magnetic flux, the system has chaotic and non-chaotic attractors. Dynamical analysis of the system shows the birth and death of period doubling, which is a sign of antimonotonicity. Such a behavior has not been studied previously in the dynamics of neurons. In an MFNN system with fractional-order discontinuous magnetic flux, different attractors such as chaotic and periodic attractors can be observed. However, there is no sign of antimonotonicity.
In this study, a locking-free n-sided C1 polygonal finite element is presented for nonlinear analysis of laminated plates. The plate kinematics is based on Reddy's third-order shear deformation theory (TSDT). The in-plane displacements are approximated using barycentric form of Lagrange shape functions. The weak-form Galerkin formulation based on the kinematics of TSDT requires the C1 approximation of the transverse displacement over the polygonal element. This is achieved by embedding the C0 Lagrange interpolants over a cubic Bernstein-Bezier patch defined over the n-sided polygonal element. Such an approach ensures the continuity of the derivative field at the inter-element edges. In addition, Eringen's stress-gradient nonlocal constitutive equations are used in the present formulation to account for nonlocality. The effect of geometric nonlinearity is taken by considering the von Kármán geometric nonlinearity. Examples are presented to show the effect of nonlocality, geometric nonlinearity, and the lamination scheme on the bending behavior of laminated composite plates. The results are compared with analytical solutions, conventional FEM results, and with those available in the literature. Shear locking is addressed considering reduced integration and consistent interpolation techniques. The patch test is used to check the convergence of the element developed. 相似文献
Organic–inorganic hybrid perovskite multijunction solar cells have immense potential to realize power conversion efficiencies (PCEs) beyond the Shockley–Queisser limit of single‐junction solar cells; however, they are limited by large nonideal photovoltage loss (Voc,loss) in small‐ and large‐bandgap subcells. Here, an integrated approach is utilized to improve the Voc of subcells with optimized bandgaps and fabricate perovskite–perovskite tandem solar cells with small Voc,loss. A fullerene variant, Indene‐C60 bis‐adduct, is used to achieve optimized interfacial contact in a small‐bandgap (≈1.2 eV) subcell, which facilitates higher quasi‐Fermi level splitting, reduces nonradiative recombination, alleviates hysteresis instabilities, and improves Voc to 0.84 V. Compositional engineering of large‐bandgap (≈1.8 eV) perovskite is employed to realize a subcell with a transparent top electrode and photostabilized Voc of 1.22 V. The resultant monolithic perovskite–perovskite tandem solar cell shows a high Voc of 1.98 V (approaching 80% of the theoretical limit) and a stabilized PCE of 18.5%. The significantly minimized nonideal Voc,loss is better than state‐of‐the‐art silicon–perovskite tandem solar cells, which highlights the prospects of using perovskite–perovskite tandems for solar‐energy generation. It also unlocks opportunities for solar water splitting using hybrid perovskites with solar‐to‐hydrogen efficiencies beyond 15%. 相似文献
Pacific oysters, Crassostrea gigas are traditionally considered shellfish of great fishery and aquaculture value. For these reasons they are introduced worldwide. Recently there has been increasing reports about the prevalence of C. gigas as biofouling organism in cooling water systems. In the absence of relevant data on the susceptibility of oysters to commonly employed antifouling techniques such as heat treatment, it was presumed that oysters would be controlled by treatment programmes directed against other major fouling organisms. The present study was carried out to test the above hypothesis, and results showed that C. gigas has an upper temperature tolerance that is much higher than other major marine fouling animals including blue mussel Mytilus edulis. Apparently, temperature regimes presently used in heat treatment of cooling water systems fouled by mussels need to be increased, if C. gigas are to be controlled effectively. Our results also indicate that previous exposure of C. gigas to sublethal high temperatures could make them more resistant to subsequent thermal treatment, an aspect that should be taken into account when heat treatment is used as a fouling control option against oyster fouling. 相似文献
AbstractA steady state laminar natural convection flow in a trapezoidal enclosure with discretely heated bottom wall, adiabatic top wall, and constant temperature cold inclined walls is performed. The finite volume based commercial code “ANSYS-FLUENT” is used to investigate the influence of discrete heating on natural convection flows in a trapezoidal cavity. The numerical solution of the problem covers various Rayleigh numbers ranging from 103 to 106, non-dimensional heating length ranging from 0.2 to 0.8 and Prandtl number is 0.7. The performance of the present numerical approach is represented in the form of streamfunction, temperature profile and Nusselt number. Heat transfer increases with increase of Rayleigh numbers at the corners of the cavity for same heating length from center of the bottom wall. However, the heat transfer rate is less and almost constant for the Rayleigh numbers considered. It is found that the average Nusselt number monotonically increases with increase of Rayleigh number and length of heat source. The variation of local and average Nusselt numbers is more significant for larger length of heating than smaller one. The heat transfer correlations useful for practical design problems have been predicted. 相似文献