Browsing by Author "Cruz, Rui M. S."
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- Efeito dos abusos de temperatura no teor de ácido ascórbico em agrião congelado ao longo da cadeia de distribuiçãoPublication . Cruz, Rui M. S.; Vieira, Margarida C.; Silva, Cristina L. M.Este trabalho consistiu no estudo do efeito dos abusos de temperatura no teor de ácido ascórbico (AA) em agrião congelado ao longo da cadeia de distribuição. Para tal, simulou-se um conjunto de condições a que os produtos congelados são sujeitos desde a saída da fábrica até ao consumidor. O agrião, antes do processo de congelação, foi branqueado a uma temperatura de 95 ºC durante 20 s, para uma redução de 90% na actividade específica da enzima peroxidase. O teor de ácido ascórbico do agrião congelado foi determinado ao longo do plano de abusos de temperatura, baseado numa situação real e comparado com um modelo de previsão, usando os parâmetros cinéticos determinados em condições isotérmicas. Verificou-se que durante os abusos de temperatura o teor de AA sofreu oscilações, mas não apresentou grandes alterações no final do plano a que foi submetido. Alguns pontos experimentais e o valor final do teor de ácido ascórbico do agrião congelado abusado foram semelhantes aos valores previstos, apesar de na maior parte dos casos os valores experimentais terem sido superiores aos previstos. O processo de branqueamento, apesar de ter sido responsável por degradar o ácido ascórbico, contribuiu para a estabilidade do agrião ao longo do armazenamento. Este tipo de estudos são uma ferramenta importante, pois permitem a previsão de resultados de diferentes cenários de armazenamento e condições de distribuição, e assim poderão contribuir para a melhoria da qualidade dos produtos congelados e para o estabelecimento de melhores condições da cadeia de frio.
- Effect of heat and thermosonication treatments on peroxidise inactivation kinetics in watercress (nasturtium officinale)Publication . Cruz, Rui M. S.; Vieira, Margarida C.; Silva, Cristina L. M.The effect of heat and the combined heat/ultrasound (thermosonication) treatment on the inactivation kinetics of peroxidase in watercress (Nasturtium officinale) was studied in the temperature range of 40-92.5 degrees C. In the heat blanching processes, the enzyme kinetics showed a first-order biphasic inactivation model. The activation energies and the rates of the reaction at a reference temperature for both the heat-labile and heat-resistant fractions were, respectively, E-a1 = 421 +/- 115 kJmol(-1) and E-a2 = 352 +/- 81 kJmol(-1), k(184.6 degrees C) = 18 +/- 14min(-1) and k(284.6 degrees C) = 0.24 +/- 0.14min(-1). The initial relative specific activity for both isoenzyme fractions were also estimated, being C-01 = 0.5 +/- 0.08 mu mol min(-1)mg protein(-1) and C-02 = 0.5 +/- 0.06 mu mol min(-1)mg protein(-1), respectively. The application of thermosonication was studied to enable less severe thermal treatments and, therefore, improving the quality of the blanched product. In this treatment the enzyme kinetics showed a first-order model. The activation energy, the rate of reaction at a reference temperature and the initial relative specific activity were, respectively, E-a3 = 496 +/- 65 kJmol(-1), k(387.5 degrees C) = 10 +/- 2min(-1) and C-03 = 1 +/- 0.05 mu mol min(-1)mg protein(-1), proving that the enzyme became more heat labile. The present findings will help to design the blanching conditions for the production of a new and healthy frozen product, watercress (Nasturtium officinale), with minimized colour or flavour changes along its shelf life.
- Effect of heat and thermosonication treatments on watercress (Nasturtium officinale) vitamin C degradation kineticsPublication . Cruz, Rui M. S.; Vieira, Margarida C.; Silva, Cristina L.M.The use of ultrasound in food processing creates novel and interesting methodologies, which are often complementary to classical techniques. In this work, the effect of heat and the combined treatment heat/ultrasound (thermosonication) on the thermal degradation kinetics of vitamin C in watercress (Nasturtium officinale) was studied in the temperature range of 82.5 to 92.5 °C. First order reaction kinetics adequately described the vitamin C losses during both blanching processes. The activation energies and the reaction rates at 87.5 °C for heat (H) and thermosonication (Ts) treatmentswere, respectively, EavitCH =150.47± 42.81 kJ mol⁻¹ and EavitCTs =136.20±60.97 kJ mol⁻¹, and K₈₇.₅ °CvitCH=0.75±0.10 min−1 and K₈₇.₅°CvitCTs=0.58±0.11 min−1. No significant differences (P>0.05) were detected between both treatments. The thermosonication treatment was found to be a better blanching process, since it inactivates watercress peroxidase at less severe blanching conditions and consequently retains vitamin C content at higher levels. The present findings will help to optimise the blanching conditions for the production of a new and healthy frozen product, watercress, with heat and a new blanching process methodology.
- Impact of thermal blanching and thermosonication treatments on watercress (Nasturtium officinale) quality: thermosonication process optimisation and microstructure evaluationPublication . Cruz, Rui M. S.; Vieira, Margarida C.; Fonseca, Susana C.; Silva, Cristina L. M.The objectives of the present work were to optimise watercress heat and thermosonication blanching conditions, in order to obtain a product with better quality for further freezing, and to evaluate the effects of thermosonication on the microstructure of watercress leaves. In a chart of optimal time-temperature conditions for a 90% peroxidase inactivation (imposed constraint), vitamin C (objective function) and a-value (improvement toward green) were mathematically predicted for both heat and thermosonication blanching treatments. Two optimal thermosonication combinations were selected: 92 A degrees C and 2 s, retaining 95% of vitamin C content and 5% a-value improvement, and a better condition in terms of practical feasibility, 86 A degrees C and 30 s, allowing a 75% vitamin C retention and 8% a-value improvement. The experimental values, for each thermosonication optimal time-temperature zone, were in good agreement with the models' predicted responses. In terms of microstructure, thermosonicated watercress at 86 and 92 A degrees C showed similar loss of turgor and release of chloroplasts. The proposed optimal thermosonication blanching conditions allow the improvement of the blanched watercress quality and consequently contribute for the development of a high-quality new frozen product. However, a suitable scale-up is mandatory for industrial implementation.
- The impact of cold chain temperature abuses on the quality of frozen strawberries (Fragaria x ananassa)Publication . Cruz, Rui M. S.; Vieira, Margarida C.; Silva, Cristina L. M.The quality of frozen foods can be negatively affected if improper storage and distribution temperatures are allowed. The objective of this study was to investigate the effect of freeze-thaw cycles, which may occur in the cold chain, on colour (Lab, Total Colour Differences (TCD), chroma and hue angle) and vitamin C (ascorbic and dehydroascorbic acids) content of frozen strawberries (Fragaria × ananassa, Duschesne, cv. Selva). A plan of temperature abuses (TAs) was established, based on a real situation, and applied to frozen strawberries during a four month frozen storage period. The results showed that the lightness (L) was the only parameter that was not significantly affected by range of TAs studied. The colour showed some variation on the parameters a, b, TCD, chroma and hue angle. During TAs, ascorbic acid decreased about 75% and dehydroascorbic acid increased 73%. The non-abused strawberry samples showed better overall appearance than the abused samples.