Browsing by Author "Assis, Fernanda R."
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- Dehydration of cheese by hot air, microwave and freeze-dryingPublication . Pinho, Ana Rita C.; Assis, Fernanda R.; Peres, Ana Paula; Pintado, Manuela E.; Morais, Alcina M. M. B.The objective of this work was to study the dehydration of skim cheese through different methods, in particular by hot air, microwave and freeze-drying, in order to assess which of these methods would be more suitable for the development of a new product (cheese snack). For the three processes of dehydration, several temperatures, powers and times were used, respectively. The drying time was optimized to allow the water activity of the final product to be between 0.3 and 0.4. The color and texture of the product obtained by the three processes were evaluated, and the nutritional analysis (protein, lipids, ash) of the product dried by hot air at 52 degrees C and by microwave at 750 W and 850 W was performed. The sensory analysis of the microwave dehydrated products was also carried out. The results obtained revealed that the temperature played a relevant role in the drying time and the hardness of the product. In the dehydration by microwave, the power of 850 W resulted in a lower drying time and a better color preservation, but in a high hardness of the samples. Among the three processes studied, the microwave drying was the fastest for the water removal from the cheese.
- Desidratação de queijo por ar quente, liofilização e micro-ondasPublication . Pinho, Ana Rita C.; Assis, Fernanda R.; Peres, Ana Paula; Pintado, M. E.; Morais, Alcina M. M. B.O presente trabalho teve como objetivo o estudo da desidratação de queijo magro através de vários métodos, nomeadamente por ar quente, liofilização e micro-ondas, no sentido de avaliar qual dos métodos de secagem será mais vantajoso para a criação de um novo produto (snack de queijo). Para os três processos de desidratação referidos foram utilizadas temperaturas, potências e tempos diferentes. O tempo de secagem foi otimizado para que a atividade da água final do produto se encontrasse entre 0,3 e 0,4. Procedeu-se também à avaliação da cor e da textura do produto obtido pelos três processos e uma análise nutricional (proteína, gordura, cinzas) do produto seco por ar quente a 52ºC e por micro-ondas com as potências de 750 W e 850 W. Foi ainda realizada uma prova sensorial. Os resultados obtidos permitiram constatar que a temperatura desempenha um papel fundamental no tempo de secagem e na dureza do produto obtido. Na desidratação por microondas, a potência de 850 W resultou num menor tempo de secagem e numa melhor conservação da cor. De entre os três processos estudados, a desidratação por micro-ondas é o mais rápido para a redução da água do queijo.
- Desidratação de queijo por ar quente, liofilização e micro-ondasPublication . Pinho, Ana Rita C.; Assis, Fernanda R.; Peres, Ana Paula; Pintado, M. E.; Morais, Alcina M. M. B.
- Desidratação osmótica de cubos de maçã e fisális com soluções de sorbitolPublication . Assis, Fernanda R.; Morais, Rui M. S. C.; Morais, Alcina M. M. B.Este trabalho teve como objetivo estudar a utilização potencial do sorbitol como agente no processo de desidratação osmótica (DO). Além de apresentar baixas calorias e ser menos doce e menos cariogénico que a sacarose, o sorbitol é um prebiótico com propriedades de saúde comprovados. Cubos de maçã foram desidratados osmoticamente a temperaturas entre 25 e 60 ºC, utilizando solução de sacarose ou sorbitol a 60 ºBrix e razão mássica de fruto/solução de 1:4 ou 1:10, sob pressão atmosférica ou vácuo de 150 mbar. A DO de fisális foi realizada nas condições otimizadas da DO de cubos de maçã (60 ºC, 1:4), relativamente às cinéticas de perda de água (PA). A utilização do sorbitol como agente osmótico resultou num aumento das cinéticas, a ambas as pressões, pelo que é uma boa alternativa para substituir a sacarose. A razão de 1:4 é também uma melhor opção que a razão de 1:10. A utilização de vácuo na DO de maçãs não produziu diferenças significativas, mas revelou tendência para aumentar a velocidade inicial de PA. À pressão atmosférica, a fisális apresentou o mesmo comportamento dos cubos de maçã. Porém, a vácuo, não se notou diferença entre as cinéticas dos solutos. A modelagem matemática das cinéticas de PA e ganho de soluto (GS) foi efetuada. Em geral, os modelos de Azuara, Peleg, Page e Weibull ajustaram-se bem aos dados experimentais; porém, o modelo de Penetração apresentou reduzida capacidade de ajuste.
- Mass transfer in osmotic dehydration of food products: comparison between mathematical modelsPublication . Assis, Fernanda R.; Morais, R. M. S. C.; Morais, Alcina M. M. B.Osmotic dehydration (OD) is a method to partially reduce water of fruits, vegetables, meat or fish aiming to increase the shelf life or as a pre-treatment in the processing of dehydrated foods. The aim of this work is to present the models most used in mathematical modelling of experimental data obtained from the OD processes, correlating the water loss and solid gain of the food product with process variables, and to perform a comparative and critical analyses of the different models and ability to fit data. The osmotic solution concentration, the temperature, the level of agitation and the geometry of the product are some of the operating parameters that will be mainly focused. Azuara’s, Peleg’s, Page’s, the Penetration, Magee’s, Weibull’s, Toupin et al.’s, Marcotte et al.’s, the Hydrodynamic mechanism, Spiazzi and Mascheroni’s, Seguı´ et al.’s, Crank’s, Hough et al.’s were the models approached. These were classified into empirical and semi-empirical, phenomenological and mechanistic, and the advantages and disadvantages were presented. An extensive list of applications of the different models to the osmotic dehydration, in variable ranges of operating conditions, of fruits, vegetables, meat and fish is provided in this work. Furthermore, equivalences between parameters of different models were established, based on the affinity of the functions used in the equations of the models, these equivalences allowing a better understanding of the adequacy of the different models to fit the same experimental data. A decision tree is provided in order to allow the selection of the most adequate model(s) to fit and predict experimental data from OD processes. All this information could assist and be helpful to researchers in the choice of the most adequate model(s) to fit experimental data, as well as to predict the water loss and solid gain of food products during OD processes.
- Mathematical modelling of osmotic dehydration kinetics of apple cubesPublication . Assis, Fernanda R.; Morais, Rui M. S. C.; Morais, Alcina M. M. B.Apple cubes were osmotically dehydrated at 25, 40 and 60C, using sucrose or sorbitol, and the mass ratio of sample to solution of 1: 4 and 1: 10, at atmospheric pressure or vacuum pressure of 150 mbar. Six mathematical models were tested to describe the mass transfer kinetics of water loss (WL) and sugar gain (SG). Crank's, Azuara's, Peleg's, Page's and Weibull's models could fit well the experimental data, but the Penetration model resulted in a poor fit. The mass ratio of sample to solution did not have an influence on the mass transfer kinetics at the atmospheric pressure. The increase of temperature and the use of sorbitol as the osmotic agent resulted in an increase of the osmotic process rate at both pressures used. Therefore, sorbitol is a good alternative to sucrose. The vacuum presented a tendency to increase the initial rate of WL.
- Osmotic dehydration with sorbitol combined with hot air convective drying of apple cubesPublication . Assis, Fernanda R.; Morais, Rui M. S. C.; Morais, Alcina M. M. B.The aim of the present work was to study the effect of the osmotic dehydration (OD) pre-treatment on the mass transfer kinetics and water activity (aw) of apple cubes during hot air drying. The adequacy of different mathematical models to describe the moisture content of the product during this process was also evaluated. Apple cubes were osmotically dehydrated with sucrose or sorbitol solutions at 60 °C, and then dried by air at 25-80 °C. Overall, the OD and rise of the air temperature resulted in an increased water loss rate and a reduction of the aw. The osmotic agent used in the OD was not relevant to the air drying kinetics, but the pre-treatment with sorbitol solutions produced dried samples with lower aw. Newton's, Page's, modified Page's, Henderson and Pabis', Two-term, Two-term exponential, Logarithmic, Midilli et al.'s models could describe the moisture content well during the air drying process.