Percorrer por autor "Oliveira, F.A.R."
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- Development of perforation-mediated modified atmosphere packaging to preserve fresh fruit and vegetable quality after harvestPublication . Oliveira, F.A.R.; Fonseca, S.C.; Oliveira, J.C.; Brecht, J.K.; Chau, K.V.The use of perforations as a means of obtaining large size containers suitable for modified atmosphere packaging (MAP) would greatly enhance the applicability of this technology for storage and distribution of fresh and minimally processed products. General concepts of MAP, and advantages and drawbacks of using perforations to achieve it are discussed. Products for which perforated packages can be used are listed. The variables that affect this type of package are presented and the methodology for designing an adequate package is described and illustrated with the case study of shredded cabbage.
- Modelling the kinetics of water loss during potato frying with a compartmental dynamic modelPublication . Costa, R. M.; Oliveira, F.A.R.The complexity of the frying process impairs the development of mechanistic models to describe water losses. Empirical models, though lacking a theoretical basis, may provide a suitable and simpler alternative. In this work it was found that a two parallel compartments model (core and edge compartments), with a time-dependent boundary condition, could be successfully applied for this purpose. The model showed a good fit to the experimental data for a variety of experimental conditions tested: potato geometry and size, temperature, oil type and oil use. The core compartment was found to contain about 63% of the initial potato water content. The rate constants of two-compartments were very sensitive to slice thickness, whereas temperature affected only the rate constant of the core compartment. No significant effects were detected in terms of oil type or oil use.
- A study on the accuracy and precision of external mass transfer and diffusion coefficients jointly estimated from pseudo-experimental simulated dataPublication . Azevedo, I.C.A.; Oliveira, F.A.R.; Drumond, M.C.Optimal experimental designs for maximum precision in the estimation of diffusivities (D) and mass transfer coefficients (Kc) for solute transport from/to a solid immersed in a fluid were determined. Diffusion in the solid was considered to take place according to Fick's second law. It was found that the optimal design was dependent on the Biot number. In the range of Biot numbers tested (0.1±200), the first sampling time corresponded to values of fractional loss/uptake between 0.10 and 0.32, and the second sampling time corresponded to values of fractional loss/uptake between 0.67 and 0.82. Pseudo-experimental data were simulated by applying randomly generated sets of errors, taken from a normal distribution with 5% standard deviation, to data calculated using given values of the model parameters. Both optimal and heuristic designs (for which the sampling times corresponded to values of fractional loss/uptake from 0.30 to 0.95) were analyzed. The accuracy and precision of the estimates obtained by non-linear regression were compared. It was confirmed that optimal designs yield best results in terms of precision, although it was concluded that the joint estimation of D and Kc should, in general, be avoided. For intermediate values of the Biot number, reasonably precise and accurate estimates can however be obtained if the experimental error is small
- The influence of ph on the kinetics of acid hydrolysis of sucrosePublication . Torres, A. Pinheiro; Oliveira, F.A.R.; Silva, C. L. M.; Fortuna, S.P.Sucrose acid hydrolysis was studied as a potential chemical time-temperature integrator to use under pasteurization conditions. A nonisothermal method was used to determine the kinetic parameters of this reaction at different pH values in the range of 0.8 to 2.5 and covering the range of temperatures from 50 to 90G. the nonisothermal method was first validated with the classical two-step isothermal method at pH 2.5. Kinetic parameters showed to be highly collinear (correlation of 0.99), but it was concluded that the activation energy can be assumed independent of pH and equal to 99 kJ/mole with the preexponential factor being proportional to the H+ concentration. Results are favorable for the future application of this system in the evaluation of pasteurization processes. Since the activation energy was found to be independent of the pH, this system is useful as a TTI for validation of mathematical models, but not so much for monitoring quality factors, except those with an equal activation energy.