Browsing by Author "Oliveira, F. A. R."
Now showing 1 - 10 of 12
Results Per Page
Sort Options
- Analysis of the inactivation kinetics of freeze-dried á-amylase from bacillus amyloliquefaciens at different moisture contentsPublication . Saraiva, J.; Oliveira, J. C.; Hendrickx, M.; Oliveira, F. A. R.; Tobback, P.The thermal inactivation kinetics of freeze-dried á-amylase in a solid matrix was studied at water contents ranging from 1.5 to 23.9 g water per 100 g dry solid. These conditions were obtained by equilibration in dry environments, with water activities ranging between 0.11 and 0.88. Isothermal inactivation experiments in the range 135 to 150 °C were performed. Results were analysed with both the Bigelow and Arrhenius models. It was concluded that there was no statistical significance to suggest that the water content influenced the kinetic parameters. An activation energy of 128 kJ/mol and pre-exponential factor with a logarithm of 33.9 described all the results well.
- Application of the acid hydrolysis of sucrose as a temperature indicator in continuous thermal processesPublication . Torres, A. Pinheiro; Oliveira, F. A. R.The hydrolysis of sucrose in an acid medium was used as a temperature indicator to measure holding temperatures in a continuous thermal-processing unit. From a theoretical analysis, target conversion (χ) and errors in both acid concentration and conversion measurements were found to affect significantly the accuracy of predicted temperatures, whereas errors in the determination of the fluid mean residence time did not show a significant effect. For different pasteurisation temperatures (70°C < T < 86°C) and flow rates (4800< Re< 11,300), the acid concentration in the medium was adjusted so that different extents of reaction could be tested, as the reaction rate was found to increase exponentially with [H+]. Nitric acid solutions were circulated through the unit, a sucrose solution was continuously fed to the entrance of the holding tube and sucrose concentration was analysed at the half way part and at the exit of the holding tube. The temperature of the holding section was then estimated from the measured conversion. These results compared well with thermocouple measurements, with deviations of less than 4°C for conversions between 0.4 and 0.7, whereas greater errors were obtained for both low and high conversions.
- Influence of selected environmental factors (water, light and oxigen) on keeping quality characteristics of freeze dried strawberriesPublication . Allegro, C.; Oliveira, F. A. R.; Poças, M. F.
- Maintaining optimal atmosphere conditions for fruits and vegetables throughout the postharvest handling chainPublication . Brech, J. K.; Chau, K. V.; Fonseca, S. C.; Oliveira, F. A. R.; Silva, F. M.; Nunes, M. C. N.; Bender, R. J.Optimal controlled and modified atmospheres (CA and MA) for fresh produce vary according to the specie, its maturity or ripeness stage, the temperature, and the duration of exposure. However, individual lots of produce are typically handled for different times and at different temperatures during storage, transportation, and retail display. In this paper, we review some of our previous work showing the potential for using different atmospheres for mangoes (Mangifera indica L.) and strawberries (Fragaria×ananassa Duchesne) depending on the anticipated storage length and temperature. Since it would be desirable, especially for produce transported over extended distances, as in marine transport, to maintain optimal atmosphere conditions throughout the postharvest handling chain, we also describe our procedure for designing a combination CA/MAP system that involves first designing the MAP for a particular commodity that will produce an optimal atmosphere for retail display conditions, then selecting a CA that will interact with the MAP to produce the optimal atmosphere within the packages during transportation at a lower temperature. An example of the design procedure is given from our work with fresh-cut kale (Brassica oleracea var. acephala DC.). Another example of this proposed MAP/CA system deals with its application to mixed load transportation of strawberries and snap beans (Phaseolus vulgaris L.).
- Modelling of dissolved oxygen concentration during storage of packaged liquid doodsPublication . Ahrné, L. M.; Oliveira, F. A. R.; Manso, M. C.; Drumond, M. C.; Osted, R.; Gekas, V.A mathematical model that combines oxygen uptake from the outside environment with oxygen consumption by oxidative reactions, in a liquid packed food during storage, was developed. The model was applied to orange juice aseptically packaged in Tetra Brik Aseptic cartons, during storage of up to 5 months at 4, 8, 20, 30, 40 and 50 °C. The parameters of the model, the oxygen mass transfer coefficient and the rate constant of consumption reactions, were estimated by fitting the model to the experimental data. The value of the rate constant estimated for the system tested in this work, was three orders of magnitude greater than the value of the oxygen mass transfer coefficient. The influence of temperature on the reaction rate was well described by an Arrhenius type equation, with an activation energy of 46 kJ/mole. This model was further tested with data reported in literature and it was found that it adequately describes the dissolved oxygen concentration changes during storage.
- Modelling optimum processing conditions for the sterilization of prepackaged foodsPublication . Silva, C. L. M.; Oliveira, F. A. R.; Hendrickx, M.Mathematical methods available to optimize heat sterilization of prepackaged foods are critically reviewed. Heat transfer models, with special relevance to those considering external resistance to heat transfer, are discussed. Different optimization methods and constraints are analysed and their applicability is evaluated. Several objective functions suggested in the literature to minimize quality degradation, energy consumption or economic costs are presented along with their impact on optimum conditions. The use of variable heating medium time temperature profile in order to improve sterilization conditions, both interms of mass average and surface quality retention, is also extensively reviewed. The review suggests that sufficient experimental evidence is lacking. Most models do not provide experimental validation and it is thought that this impairs the potential applications that would arise.
- Optimum sterilization: a comparative study between average and surface qualityPublication . Silva, C. L. M.; Oliveira, F. A. R.; Pereira, P. A. M.; Hendrickx, M.Sterilization temperatures to maximize volume average or surface quality retention were calculated for one-dimensional conduction heating foods as a function of (1) Food Properties, (2) Processing Conditions and (3) Processing Criteria. A target lethality at the least-lethality point was used as a constraint, and optimal temperatures were qualitatively and quantitatively compared for equal design variables. Average quality optimum conditions depend linearly on the inverse square of the Drefq-value for the quality factor. These conditions do not vary linearly with all the other influential variables, opposite to what had been observed for surface quality. Optimum temperature for maximum average quality is always higher than the corresponding one for surface quality, but the difference is not constant. A systematic approach to the dependence of average quality optimal conditions on all the relevant parameters was carried out and quantitative relations were obtained. Optimum average quality retention is independent of surface heat transfer resistance.
- Residence time distribution of liquids in a continuous tubular thermal processing system part I: Relating RTD to processing conditionsPublication . Torres, A. Pinheiro; Oliveira, F. A. R.A systematic experimental study on residence time distribution (RTD) in tubular flow was performed, covering a wide range of processing conditions. The flow of water in various sections of a tubular continuous thermal processing system was analysed using the classical Danckwerts approach. Methylene blue was used as tracer and different constant temperatures (25–80 °C) and flow rates (80–380 l/ h) were tested. These conditions yielded mean residence times up to 6 min and Reynolds numbers between 1350 and 9700. Various models were fitted to the experimental data, and the dispersion model showed to yield the best fit. Peak analysis led to both accurate and precise as well as conservative parameters, when compared to other methods of parameter estimation. Results revealed that fluid dispersion in tubular flow (Peclet number) can be related to processing conditions (Reynolds number) by a power law model. Results were compared to published correlations.
- Residence time distribution of liquids in a continuous tubular thermal processing system part II: Relating hold tube efficiency to processing conditionsPublication . Torres, A. Pinheiro; Oliveira, F. A. R.The concept of efficiency of a holding tube is essential to guarantee the safety of continuously processed fluid foods. A good prediction of efficiency allows a better control of the processing conditions, guaranteeing product safety while decreasing product quality losses due to overprocessing. Different published methods were compared to data obtained in this work and to reported data, to assess their ability to predict tube efficiency in a range of Reynolds number (Re) covering laminar, transient and turbulent flow. Furthermore, a model assuming dispersed plug flow and a power-law relation between Peclet number (Pe) and Re was developed and evaluated. Published models were shown to be, in general, conservative for both laminar and turbulent flow, but often overpredicted the experimental efficiency in the zone of 2100
- Residence time distribution studies in continuous thermal processing of liquid foods: a reviewPublication . Torres, A. Pinheiro; Oliveira, F. A. R.This paper presents a review on residence time distribution (RTD) studies in continuous thermal processing of liquid foods. The theoretical basis of the Danckwerts analysis is summarized, as well as the most important flow models, with special emphasis on tubular systems. Methods for experimental determination, modelling and estimation of RTD are critically described. While main design objectives in continuous thermal processes may be guaranteed by a proper minimum residence or holding time, process optimization requires the knowledge of the residence time distribution. Both concepts are reviewed and discussed. A significant scatter was noticed among published results and the need ,for a systematic work is clear It was concluded that future research should focus on studies at pasteurizationlsterilization temperatures, as well as on studies conducted with real food products or model food systems with FIO~INewtoniun flow bchaviour: Furthermore. information relating RTD to processing conditions would be a useful tool for process optimization.