Fracture Induced in Al2O3 Bicrystals by Anisotropic Thermal Expansion

Bicrystals of Al₂O₃ were fabricated to study the effects of thermal expansion anisotropy on fracture in a model system containing one grain boundary. Fractures occurred perpendicular to the directions of maximum tensile stress in bicrystals with thermal expansion coefficient differences as low as 0.31×10⁻⁶⁰C⁻¹ and originated at the boundary, probably in areas of high residual stress associated with pores. Numerical stress analysis of two-dimensional model bicrystal configurations showed that stresses induced by thermal expansion anisotropy are maximal in a region localized along the bicrystal boundary.     

A BENCHMARK COMPUTATIONAL STUDY OF FINITE ELEMENT ERROR ESTIMATION

Benchmark solutions are presented for a simple linear elastic boundary value problem, as analysed using a range of finite element mesh configurations. For each configuration, various estimates of local (i.e. element) and global discretization error have been computed. These show that the optimal mesh corresponds not only to minimization of global energy (or L₂) norms of the error, but also to equalization of element errors as well. Hence, this demonstrates why element error equalization proves successful as a criterion for guiding the process of mesh refinement in mesh adaptivity. The results also demonstrate the effectiveness of the stress projection method for smoothing discontinuous stress fields which, for this investigation, are more extreme as a consequence of the assumption of nearly incompressible material behaviour. In this case, lower order smoothing produces a continuous stress field which is in close agreement with the exact solution.     

THE LAMINAR FLOW TUBULAR REACTOR WITH HOMOGENEOUS AND HETEROGENEOUS REACTIONS I. Integral Equations for Diverse Reaction Rate Regimes

Design equations for non-isothermal Laminar Flow Tubular Reactors (LFTRs) with homogeneous and heterogeneous - at the reactor wall - reactions with arbitrary kinetic equations have been satisfactorily treated transforming the original P.D.E. problem into a system of integral equations. The kernels of the integral operators are related to an eigenvalue problem which does not depend on the kinetic equations; this avoids repetitive computational effort in the treating of different reaction kinetics.

To render a more efficient numerical treatment and according to the governing reaction rate regime, modified expressions of the general solution were obtained was follows: (i) a solution with kernels depending only on the diffusion and convective times was obtained for a low reaction regime; (ii) another solution with kernels including the reaction lime, besides the diffusion and convective ones, was necessary for a fast reaction regime and (iii) the local quasi-steady-state approximation was obtained as limiting case of solution (ii) for a instantaneous reaction regime.     

RHEOLOGY OF SUCROSE-CMC MODEL SOLUTION

The rheological behavior of an aqueous solution composed of 65% sucrose and 0.1% CMC was experimentally determined from -10C to 50C using a rotational viscometer equipped with coaxial cylinders. The model solution behaved as pseudoplastic fluid, represented by the power-law model. Flow behavior and consistency indices determined at -10.1, 0.5, 9.8, 19.0, 30.1 and 50. OC, showed values between 0.79 to 0.86 and 0.15 to 10.22 Pa. sⁿ, respectively. The effect of temperature on the rheological parameters was represented by Arrhenius-type relationships, with R²≥ 0.97. They were compared to the rheological behavior of a concentrated orange juice with 65° Brix and 7% pulp content. The comparison of separate rheograms for both fluids showed that the model solution can be used to represent concentrated orange juice.     

ION LEVELS OF FRESH AND PROCESSED SPINACH USING ION CHROMATOGRAPHY

The anion (Cl^?, NO^?₃, So^2-₄, PO^3-₄, OX^2-) and cation (Na⁺, K⁺, NH4⁺) composition of raw, frozen and canned spinach samples was determined by Ion Chromatography (IC). In general, the levels of these ions were determined after typical food processing operations. Important for its health-related and nutritional point of view is the observation that nitrate levels decreased by about 50% with losses of 15% in total organic nitrogen after typical commercial processing methods.     

STARCH MOBILIZATION AND SUCROSE ACCUMULATION IN THE PULP OF KEITT MANGOES DURING POSTHARVEST RIPENING

Sugar is a determinant for the quality of mangoes, but information about its accumulation is scarce. Although starch can contribute to sugar production during ripening, not much is known about the enzymes involved. This work presents the changes in carbohydrate and enzymes during the development and ripening of Keitt mangoes. Starch disappearance was concomitant to a fivefold increase of sucrose, the most abundant sugar of the ripe fruits. The activities of α-amylase, β-amylase, phosphorylase and isoamylase were detected in the pulp, and while α-amylase increased parallel to the starch content, β-amylase presented a 20-fold increase during ripening. On the other hand, high phosphorylase activity was observed when fruits were still accumulating starch, and lowered during ripening. Isoamylase was detected during development and increased slightly during ripening, which would be in agreement to the expected role for isoamylases as acting on both subproduct of starch synthesis and degradation.

PRACTICAL APPLICATIONS

The present work reinforces our previous works that Keitt mangoes do not ripen when attached to the tree. This fact allowed us to study all the starch degradation after mango harvesting which does not occur with the other cultivars. Data obtained in this work reinforce the role of α-amylase, β-amylase and isoamylase rather than the starch phosphorylases on starch granule degradation in mangoes, and the subsequent soluble sugar accumulation.     

OSMOTIC DEHYDRATION OF KIWIFRUIT (ACTINIDIA CHINENSIS): FLUXES AND MASS TRANSFER KINETICS

Pulsed‐Vacuum‐Osmotic‐Dehydration of kiwifruit (Actinidia chinensis) was studied using two kinds of osmotic solution: sucrose (65 Brix) and concentrated grape juice (63 Brix), three temperatures (25, 35 and 45C) and four vacuum pulse times (0, 5, 10 and 15 min). Experimental results enabled the mass transfer kinetics for water and solutes to be studied (compositional changes of the liquid fraction and changes in the liquid fraction/solid matrix ratio). The impregnation of samples, because of the vacuum pulse, was higher when concentrated grape juice was used as the osmotic solution, probably due to its lower viscosity. The effective diffusivity (D_e) was obtained for each experimental condition and the results show higher diffusivities for vacuum‐pulsed treatments, although differences between treatments with different vacuum‐pulse time could not be observed. D_e values were slightly greater for treatments with concentrated grape juice. The activation energies (E_o) were obtained by fitting the data to the Arrhenius equation.     

Brief Note : Improved <i>in vitro</i> embryo development of stenospermic grape by putrescine

The goal of this study was to determine the effect of putrescine, added to the culture medium, on the in vitro development of stenospermic grape (Vitis vinifera L) embryos. The cross breedings of Perlón x G.C88552 and Perlón x Argentina were used. 0 (control), 2 and 4 mM of putrescine were added to the immature seed’s culture medium. In Perlón x Argentina, 2mM of putrescine statistically increased the percentage of total embryos, direct germination, polyembryos and normal plants. In Perlón x G.C88552, only 2 mM of putrescine increased all the variables considered, eventually tripling the percentage of normal plants obtained. The results suggest that the endogenous concentration of putrescine may be a growth limiting factor. Adding putrescine to the culture medium of immature grape seeds is a legitimate resource to significantly increase the results of this technique.