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- 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
- 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.
- 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.
- The influence of water activity on thermal stability of horseradish peroxidasePublication . Hendrickx, M.; Saraiva, J.; Lyssens, J.; Oliveira, J.; Tobback, P.The thermal stability of horseradish peroxidase in the solid state was studied as a function of water activity, from 0.11 to 0.88. At all activities the enzyme was found to be much more stable in the solid state than in solution. Inactivation temperatures were in the range of 140–160°C. Inactivation curves show a biphasic behaviour which can be described by a model assuming two fractions (heat labile and heat stable) with independent first order inactivation kinetics. The labile fraction represents approximately 30% of the total activity. The z-value for both stable and labile fractions depends on water activity (moisture content) and has a maximum at aw= 0.76 (44.4°C and 43.8°C, 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.
- Influence of large solid spherical particles on the residence time distribution of the fluid in two-phase tubular flowPublication . Baptista, Paulo N.; Oliveira, Fernanda A. R.; Cunha, Luís M.; Oliveira, Jorge C.The effect of large solid particles on the residence time distribution of the fluid in two-phase flow was studied in a pilot scale continuous tubular system. The residence time distribution was characterized with four parameters: the normalized minimum and mean residence time, the normalized standard deviation and the skewness. The effect of particle concentration (1 and 3%) and particle diameter (diameters approximately 0.3 and 0.4 of the tube diameter) were analysed for fluids with different viscosities and at different flow rates. It was observed that the presence of particles, even at very low concentration, has a significant effect, increasing the mean residence time and decreasing the standardized dispersion. This effect was attributed to particles breaking the velocity profile of the fluid and causing some localized mixing due to their rotational velocity. Although the presence of particles affected the results, their concentration (or number) was not relevant, in the range tested, which is very low for particulate foods.
- Selection of low swelling materials for protein adsorption from white winesPublication . Sarmento, Miguel R.; Oliveira, Jorge C.; Boulton, Roger B.The protein adsorption capacity of different materials was evaluated to assess their potential efficacy as alternative adsorbents for the removal of proteins from wines, with the purpose of finding suitable low-swelling materials that could be used in a percolated bed. The adsorbents tested were thermally treated sodium bentonite, low-swelling adsorbing clays, ion exchange resins and other protein adsorbents (silica gel, hydroxyapatite and alumina). The materials were evaluated by analysing both the capacity to stabilize untreated white wines, according to a heat test, and by characterizing the adsorption isotherms of bovine serum albumin (BSA) in a model wine (water, ethanol and K-bitartrate). Breakthrough curves in a packed bed were determined experimentally for some materials, showing the influence of the adsorption isotherm shape. Some ionexchange resins showed a favourable behaviour and have good potential as alternative adsorbents.
- 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.
- Rheological behavior of heated starch dispersions in excess water: role of starch granulePublication . Rao, M. A.; Okechukwu, P. E.; Silva, P. M. S. Da; Oliveira, J. C.Granule size and size distribution, measured by laser diffraction, affected the flow behavior at 20 °C of (2.6% w/w) corn and cowpea starch dispersions heated for various time intervals above their gelatinization temperatures. The standard deviation of the granules' size described the transition of flow behavior from shear thickening in the early stages of gelatinization to shear thinning in the latter stages and influenced the critical shear rate, yc, for the onset of shear thickening in starch dispersions. The granules swelled to a maximum of about 3.5 times the raw starch granule mean diameter and 65% granule mass fraction. The consistency index of the dispersions increased with granule mean diameter. Modified waxy maize starch dispersions heated at 80 °C exhibited antithixotropic behavior at a shear rate of 200s−1; both dynamic frequency data and Cox-Merz plots revealed their gel-like behavior.
- Migration of two antioxidants from packaging into a solid food and into TenaxPublication . Reinas, I.; Oliveira, J.; Pereira, J.; Machado, P.; Poças, M. F.The migration of chemicals from packaging materials into foods is an important issue in food safety and quality. European legislation sets specific migration limits which compliance must be assessed, surveyed and controlled by industry and authorities. Recently, the new Regulation UE 10/2011 included a new simulant - the Modified Polyphenylene Oxide (Tenax ) for solid dry foods. The objective of this work was to compare the migration kinetics of two antioxidants into Tenax as compared to rice at 3 temperatures: 23, 40 and 70 C. The application of two different solutions of the 2nd Fick’s law to describe and simulate the migration of the migrants to the present systems was studied. Diffusion coefficients ranged between 4.80E-13 and 2.84E-11 cm2/s for the migration into Tenax and between 6.90E-18 and 4.33E-17 cm2/s for the migration into rice. The partition coefficients ranged between 6 and 29 for Tenax and were over 1000 for rice. The activation energy for the migration into rice was half of that for Tenax . The models described relatively well the experimental data (ε < 12% and < 30% for rice and Tenax , respectively). Results indicate that the food simulant tends to overestimate migration values and thus can be safely used to assess materials compliance when materials are intended to contact with rice. However, results also indicate that Tenax is a much more severe simulant in representing rice.