Percorrer por autor "Frias, Jesus M."
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- 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.
- 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.
- 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.