Percorrer por autor "Oliveira, Jorge C."
A mostrar 1 - 10 de 27
Resultados por página
Opções de ordenação
- Análise da mobilidade molecular em matrizes sólidas utilizando conceitos de ciências de biomateriais: aplicação na optimização da qualidade de produtos congeladosPublication . Oliveira, Jorge C.; Cruz, Isabel B.; Pereira, Pedro M.; Frias, Jesus M.O estudo de matrizes sólidas bioquímicas utilizando as noções de transição de estado vítreo desenvolvidos na ciência dos materiais (poliméricos) é um assunto que tem recebido crescente atenção na biotecnologia e ciência e engenharia de processos alimentares, sobretudo pela sua aplicação na melhoria de produtos desidratados, congelados preparações enzimáticas e culturas celulares. Esta comunicação apresenta uma revisão sucinta destes conceitos, aplicados em particular à optimização da qualidade de alimentos congelados. Esta teoria permite desenvolver linhas de investigação sobre alteração da formulação dos produtos no sentido de aumentar a sua capacidade de manter a qualidade obtida no processo de congelação durante o armazenamento e distribuição.
- Analysis of mathematical models to describe the migration of additives from packaging plastics to foodsPublication . Poças, Maria F.; Oliveira, Jorge C.; Brandsch, Rainer; Hogg, TimothyThe mathematical modeling of migration of additives from plastics to food simulants was studied using experimental data published in the literature, following two routes: the conventional approach using the solution of Fick’s 2nd law, and a kinetic model based on the Weibull distribution function. The objective of this comparison was to permit using a mathematically simpler model equally able to describe migration data, and that could have a generally wider applicability by describing situations more complex than those that simple diffusional phenomena can describe. The relationship between the parameters of the two models was analyzed by regression of data generated by Fick’s law using the Weibull model. The results show that the time constant parameter is related to the diffusion coefficient and the material thickness.It depends on temperature and on the molecular weight of the migrant in a similar manner to the diffusion coefficient. The activation energy presented values from 72 to 125 kJ/mol. The shape parameter is a function of the contribution of the mass transfer resistance at the interface. It was independent of temperature and of the migrant, assuming a global constant value of 0.67, for the systems and conditions studied. The results indicate that the Weibull model can be used to describe and analyze the migration of additives from plastics to foods with a meaning of the parameters in terms of the underlying physical phenomena.
- Analysis of the kinetic patterns of horseradish peroxidase thermal inactivation in sodium phosphate buffer solutions of different ionic strengthPublication . Saraiva, Jorge; Oliveira, Jorge C.; Lemos, Adília; Hendrickx, MarcThe thermal inactivation of horseradish peroxidase was studied in sodium phosphate buffer solutions and in pure water at pH 7 in the temperature range of 70–95°C. The sodium phosphate ions concentration affected both the thermostability and the kinetic patterns and had a stabilizing effect. The gradual change observed at low concentrations made a series-type mechanism theoretically more coherent with the experimental observations than the conventionally applied two-fraction model. In water the kinetics is apparently First order at high temperatures, while the results obtained at 25°C support the occurrence of a series-type inactivation mechanism. The pH and enzyme concentration also affect the inactivation proFile, supporting the conclusion that the thermal inactivation is not a monomolecular process with respect to protein concentration.
- Application of D-optimal design for determination of the influence of water content on the thermal degradation kinetics of ascorbic acid at low water contentsPublication . Frias, Jesus M.; Oliveira, Jorge C.; Cunha, Luís M.The kinetics of thermal degradation of ascorbic acid was studied in a Maltodextrin matrix at different temperatures between 7.5 and 140°C. Isothermal experiments were performed with samples earlier equilibrated in environments of known a, at 4°C enclosed in hermetically sealed vials, with water contents (w) between 0.17 and 168g water/g solids. Sampling times were selected according to an optimal experimental design in terms of minimum confidence regions of the parameters estimated, using the Bigelow model and estimates from preliminary experiments. The results showed a quadratic dependence of the reference D-value and linear dependence of the z-value with moisture content: D r(1400C, = 150*07-290*73w+-269w2 (min); z = 12*14+22.99w (“C) and showed the applicability of D-optimal designs for determining kinetic parameters in complex situations, with limited experimental requirements. The dependence of the rate constants with water content could not be described by the WLF model in qualitative terms, although in absolute values this model could be used with constants similar to those expected from glass transition theory. The thermodynamic analysis of the results showed a good application of the compensation theory in the whole range of water contents.
- Determination of the heat transfer coefficient between bulk medium and packed containers in a batch retortPublication . Varga, Szabolcs; Oliveira, Jorge C.The external heat transfer coefficient in steam retort processing was determined experimentally in a pilot scale retort. The heat transfer equations were solved applying finite elements and using the actual retort temperature profile as boundary condition. The instantaneous values of the heat transfer coe cient were determined, to analyse its time-variability along a retort cycle. It was found that reliable results for the external heat transfer coefficient at time t could be obtained from the derivatives of the average heat transfer coefficient calculated between time zero and time t. The results showed a sharp increase of the heat transfer coe cient in the earlier times of heating (up to 4±5 min of processing), followed by a slightly increasing pattern during the remaining heating period. Using average heat transfer coe cients for the heating phase (and for the cooling phase) also resulted in quite accurate estimates of the temperature at the geometric centre of a can. The influence of the two average heat transfer coe cients, heating and cooling, on the lethality was studied by building a response surface. It was concluded that the variability of the coe cient during heating has a greater impact, especially in the range 150±260 W/m2 K.
- Development of a non-isothermal method for determination of diffusional parametersPublication . Moreira, Lília A.; Oliveira, Fernanda A. R.; Silva, Teresa R.; Oliveira, Jorge C.A non-isothermal method for determination of diffusional parameters was developed. The dependence of diffusivity on temperature was described by an Arrhenius type equation, 'with the pre-exponential factor and the,activation energy as parameters. These parameters were obtained by optimization of the fit between experimental and theoretical values, using the Simplex method of minimization. Equilibrium conditions were described with a constant unit partition coefficient. This model was applied to the diffusion of acetic acid into turnips. Results compared well with values obtained with the conventional method, using a set of experiments at isothermal conditions. The non-isothermal method has significant advantages, nameiy the need for much less experimental data and better insight into the statistical significance of the results.
- Dimensionless analysis of fluid-to-particle heat transfer coefficientsPublication . Baptista, Paulo N.; Oliveira, Fernanda A.R.; Oliveira, Jorge C.; Sastry, Sudhir K.Average fluid-to-particle heat transfer coefficients were experimentally determined for spherical aluminium particles heated in car☐ymethylcellulose solutions. Two situations were considered: a still panicle immersed in a moving fluid, and a particle rotating in an otherwise stagnant fluid. Fluid flow rate, rotating particle velocity, particle diameter and fluid rheological properties were varied, covering a large range of the generalized Reynolds (0 to 801) and Prandtl (69 to 5358) numbers. Average heat transfer coefficients ranged between 56 and 2612 W/m2K. The results were compared with values predicted by published dimensionless correlations, showing that correlations based on a Fro¨szling-type equation were more adequate. It was found that the contribution due to natural convection should be considered for proper correlation of the results at low Reynolds numbers. The results also show the importance of the fluid velocity profile.
- Dimensionless analysis of the flow of spherical particles in two-phase flow in straight tubesPublication . Baptista, Paulo N.; Oliveira, Fernanda A.R.; Oliveira, Jorge C.; Sastry, SudhirIn the aseptic processing of particulate fluid foods, the residence time distribution of particles and the fluid-to-particle heat transfer are intimately related to the linear and rotational velocities of the particles. The development of models correlating these velocities with fluid and particle characteristics and with operating conditions is therefore very useful, as it provides a fast method to estimate fluid-to-particle heat transfer coefficients without measuring the actual particle velocities. The models can further be used to estimate the average residence time of the particles. Data for modelling particle linear and rotational velocities were obtained by videotaping the flow, along the wall, of individual spherical particles in transparent 2-m long straight tubes. Linear and rotational velocities were correlated with generalized Reynolds, Archimedes and Froude numbers, to the particle-to-fluid density simplex and to the particle/tube diameter ratio. The models obtained had correlation coefficients of 0.89 and 0.86 for the particle linear and rotational velocities, respectively.
- Dynamic mechanical thermal analysis of aqueous sugar solutions containing fructose, glucose, sucrose, maltose and lactosePublication . Cruz, Isabel B.; Oliveira, Jorge C.; MacInnes, William M.The glass transition of glucose, fructose, lactose, maltose and sucrose solutions at maximum cryo-concentration was studied by Dynamic Mechanical Thermal Analysis (DMTA), using the disc bending technique. The glass transition temperatures were determined from the peaks in the loss modulus E′′, which corresponds theoretically to the resonance point (Maxwell model) for several input frequencies. The frequency dependence was well described by both an Arrhenius-type model and by the WLF (Williams, Landel and Ferry) equation, yielding glass transition temperatures for an average molecular vibration time of 100 s, which were similar to published midpoint temperatures determined by DSC scans. Some sugar mixtures were studied, yielding results that were well described by the Gordon–Taylor equation, using literature data. The frequency dependence of the viscoelastic ratio was also well approximated by an Arrhenius-type equation, with activation energies similar to those of the glass transition temperature and corresponded well to published values of the endset of glass transition.
- Effect of operating conditions on physical characteristics of extruded rice starchPublication . Tomás, Rui; Oliveira, Jorge C.; Akdogan, Hulya; Mccarthy, KathrynThe effect of screw speed, flow rate, barrel temperature and added moisture on physical characteristics of high moisture extruded rice starch was studied. the product physical characterization was made by measuring density, final moisture content and the maximum load force in a puncture test. Rice flour containing approximately 1.5% protein and 6% moisture was mixed with 55–65% (w/w) water and extruded in a co-rotating twin screw extruder with a slit die. A two level factorial design was used to analyse each parameter. It was found that added moisture was the most important variable affecting the force-deformation curve maxima, with the interaction between added moisture and barrel temperature exhibiting a less significant effect. Added moisture and flow rate each affected density, with the interaction between added moisture and screw speed showing a less significant effect. Temperature and flow rate affected final moisture interactively with added moisture, which was evidently the most important variable.
- «
- 1 (current)
- 2
- 3
- »