Browsing by Author "Oliveira, Fernanda A.R."
Now showing 1 - 10 of 10
Results Per Page
Sort Options
- Application of image analysis to the study of water losses from potato slices during fryingPublication . Costa, Rui M.; Oliveira, Fernanda A.R.; Gekas, Vassilis
- 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.
- Experimental validation of models for predicting optimal surface quality sterilization temperaturesPublication . Silva, Cristina L. M.; Oliveira, Fernanda A.R.; Lamb, Jack; Torres, Andreia Pinheiro; Hendrick, MarcAn experimental procedure was developed to validate experimentally the theoretical optimal process temperatures giving the maximum quality at the surface of conduction heating products for specified levels of sterility. the method involved filling a can with a 10% bentonite solution, placing glass capillaries containing a food quality indicator at the surface, and processing it at different temperatures for the necessary time in order to attain a specified sterility value at the least-lethality point. the acid catalysed sucrose hydrolysis reaction was used as a chemical time-temperature-integrator for a quality index of heat sterilized products. At a pH of 3.8, the kinetic parameters D121.15°C ard z values of this reaction, were 214min and 31°C, respectively. Final surface retentions of sucrose compared well with theoretically predicted values from simulation modelling. Optimal temperatures calculated using either numerical computer optimization or generalized regression equations were in the range of experimentally determined optimal conditions.
- Influence of the variability of processing factors on the F-value distribution in batch retortsPublication . Varga, Szabolcs; Oliveira, Jorge C.; Oliveira, Fernanda A.R.The F-value distribution in a batch retort processing of in-pack conduction heating foods resulting from the variability of four processing factors (initial temperature, heating time, headspace and external heat transfer coe cient) was determined by mathematical modelling. Three hundred combinations were generated from normally distributed values for each factor, following the Monte Carlo approach. The retort temperature was assumed the same for all containers and the F-value at the geometrical centre of containers of di erent dimensions was calculated by solving the governing heat transfer equations using ®nite elements. It was found that in the range tested the variability of the heating time and of the external heat transfer coe cient had the biggest impact on the F-value distribution and that the head space variability had a negligible impact. It was also found that although each in¯uencing factor had a normal distribution, the F-value distribution was not normal, showing a signi®cant tailing, being well described by a gamma function
- Modelling ascorbic acid thermal degradation and browning in orange juice under aerobic conditionsPublication . Manso, Maria C.; Oliveira, Fernanda A.R.; Oliveira, Jorge C.; Frías, Jesús M.The thermal degradation of ascorbic acid (AA) in orange juice was analysed over in a 20–45 °C temperature range. Dehydroascorbic acid (DA), pH and browning were also monitored. Small amounts of AA degradation could be described by first order kinetics, but when only low amounts of AA were retained sigmoidal kinetics were clearly appropriate. The Weibull model was used to describe this pattern (R2adj > 0.995). The rate constant increased with temperature according to an Arrhenius-type relationship. The activation energy was 38.6 kJ/mol and at the average temperature of the range tested, 32.5 °C, the rate constant was 64.4 × 10−3 h−1. The shape constant decreased linearly with temperature, from 2.17 to 1.13. Before the time when the maximum degradation rate occurred, pH, DA concentration and browning remained fairly constant, and then increased. It was found that this behaviour, as well as the dependence of the shape constant on temperature, might be explained by (i) the reconversion of DA into AA, following first order kinetics in relation to DA and second order kinetics in relation to AA, and by (ii) different sensitivities of the reaction rate constants to temperature. Browning was also well described by the Weibull model with a temperature independent shape constant.
- Modelling O2 and CO2 exchange for development of perforation mediated modifed atmosphere packagingPublication . Fonseca, Susana C.; Oliveira, Fernanda A.R.; Lino, Isabel B.M.; Brecht, Jeffrey K.; Chau, Khe V.Perforation-mediated modi®ed atmosphere packaging relies on the use of macro perforations or tubes to control the O2 and CO2 exchange and create the desired atmosphere inside an otherwise gas-tight package. In this work, the O2 and CO2 exchange through a single tube was studied. Di erent temperatures (5±20°C) and tube dimensions (length from 9 to 17 mm and diameter from 6 to 30 mm) were tested. O2 and CO2 mass transfer coe cients were determined according to a lumped mass transfer capacitance model that yielded a good description of the gas transfer. Temperature in the range tested did not show a signi®cant e ect on the mass transfer coe cients. A multiplicative non-linear equation was found to yield a good prediction of the dependence of the O2 mass transfer coe cient on tube diameter and length. The ratio between the CO2 and O2 mass transfer coe cients, an important parameter in the design of MAP, was 0.81 and none of the factors tested in¯uenced its value.
- Modelling respiration rate of fresh fruits and vegetables for modified atmosphere packages: a reviewPublication . Fonseca, Susana C.; Oliveira, Fernanda A.R.; Brecht, Jeffrey K.Respiration rate and gas exchange through the package material are the processes involved in creating a modified atmosphere inside a package that will extend shelf life of fresh fruits and vegetables. Thus, modelling respiration rate of the selected produce is crucial to the design of a successful modified atmosphere packaging (MAP) system. In this paper, general aspects of the respiration process are presented. The major methods for measuring respiration rates, along with their advantages and limitations are discussed. Factors affecting the respiration rate and respiratory quotient are outlined, stressing the importance of temperature, O2 and CO2 concentrations, and storage time. Respiration rate models in the literature are also reviewed. 2002 Elsevier Science Ltd. All rights reserved.
- Modelling respiration rate of shredded Galega kale for development of modified atmosphere packagingPublication . Fonseca, Susana C.; Oliveira, Fernanda A.R.; Frias, Jesus M.; Brecht, Jeffrey K.The design of modified atmosphere packaging (MAP) for fresh-cut produce requires an adequate model for prediction of respiration rate as a function of both temperature and gas composition. In this work, the O2 consumption and CO2 production rates of shredded Galega kale were studied. The storage temperatures used were 1, 5, 10, 15 and 20 C. The atmospheres tested were all combinations of 1, 5 and 10% v/v O2 plus 0, 10 and 20% v/v CO2 with the balance being N2, as well as ambient air. Temperature was the variable with the greatest influence on respiration rate and the effect of gas composition increased with temperature. The dependence of respiration rate on gas composition was well described by a Michaelis–Menten type equation with uncompetitive CO2 inhibition. The respiratory quotient (RQ) was found to be constant for the range of temperatures and gas compositions tested and was equal to 0:93 0:01. The constants of the Michaelis–Menten equation increased exponentially with temperature. The change over time of respiration rate of leaves exposed to air at 20 C was also analysed. It was observed that respiration rate decreased with time and that the ratio between the respiration rate of shredded and intact leaves was approximately constant in the period tested and equal to 2.8. 2002 Elsevier Science Ltd. All rights reserved.
- Quality optimization of conduction heating foods sterilized in different packagesPublication . Silva, Cristina L.M.; Oliveira, Fernanda A.R.; Hendrickx, MarcHeat transfer during the sterilization of conduction heating foods packaged in cylindrical, retort pouch and rectangular containers, was modelled using a finite difference numerical method. Finite surface heat transfer coefficients were considered. Optimal temperatures, defined as the processing conditions that maximize surface or volume average quality retention with the constraint of a target sterility value at the coldest point, were calculated for several types of container. Optimal conditions for several case studies were compared with temperatures estimated by generalized formulae available in literature for the prediction of optimal temperatures as a function of all relevant variables. For all case studies, optimal temperatures predicted in both ways were very similar, with deviations often below ± 1°C. For most packages, the simple-to-use generalized equations, developed for one-dimensional geometries, can therefore be used instead of lengthy numerical calculations involving two- and three-dimensional geometries.