Browsing by Author "Silva, Vitor"
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- Comparação de secagens convectivas em túnel, em regime contínuo e descontínuo.Publication . Silva, Vitor; Figueiredo, Rui; Costa, J; Guiné, Raquel; Ferreira Wessel, Dulcineia; Gonçalves, FernandoA secagem convectiva, em túnel, de pêra São Bartolomeu, pode ser efectuada em modo contínuo com equipamentos que imponham valores pré- escolhidos para a temperatura e velocidade do ar de secagem, tornando o processo independente das condições meteorológicas, modo este que não é análogo ao verificado numa secagem solar tradicional, inerentemente descontínua (suspensão do processo nos períodos nocturnos). O presente estudo comparou as eventuais vantagens e desvantagens da utilização de uma metodologia de secagem descontínua, convectiva e forçada em túnel, mais próxima das condições habituais de secagem por via solar, quando comparada com uma secagem contínua, para valores análogos da temperatura e velocidade do ar de secagem. Os resultados demonstram que o tempo total de secagem em modo contínuo é mais curto (um decréscimo de cerca de 28 horas). Contudo, em modo descontínuo, a ventilação e o aquecimento estiveram desligados 67,5 horas, correspondentes a 55,8% do tempo total do ensaio. Foi também verificado que a remoção de água no modo descontínuo foi sempre superior em cada reactivação do sistema de termo ventilação quando comparada com o instante imediatamente anterior à pausa nocturna. Os resultados obtidos evidenciam que o modo descontínuo apresenta reduções substanciais da energia gasta, se bem que à custa de um aumento, em todo o caso relativamente pouco significativo, do tempo total de secagem, o que é um resultado muito interessante do ponto de vista da poupança energética em processos desta natureza.
- Comparação entre as Propriedades das Peras Secadas sob Diferentes MétodosPublication . Guiné, Raquel; Barroca, Maria João; Lopes, Paulo; Silva, Vitor; Santos, Monica; Ferreira, DulcineiaNo primeiro ano de secagem das peras, 2007, teve-se por objectivo o estudo da secagem da pêra de S. Bartolomeu por dois métodos, secagem em estufa realizada na Escola Superior Agrária de Viseu e secagem em túnel realizada na Universidade de Coimbra. Este estudo foi efectuado para três estados de maturação diferentes visando o estabelecimento da melhor altura para a realização da colheita dos frutos. No ano de 2008 as pêras, de colheita única, foram secadas na ESAV, em estufa solar com aquecimento directo e ventilação forçada, na Universidade de Coimbra, em túnel com aquecimento solar indirecto e na Escola Superior de Tecnologia de Viseu, também em estufa solar com incidência directa da luz, e convecção natural. Da comparação verifica-se que as pêras secadas na ESAV e na ESTV (com incidência directa do sol) assemelham-se mais às tradicionais, enquanto as secadas na UC (sem incidência directa da luz solar) se apresentam com características distintas.
- Comparative study of pear drying using solar stove and tunnel dryingPublication . Guiné, Raquel; Barroca, Maria João; Lopes, Paulo; Silva, Vitor; Lima, Maria João; Ferreira, DulcineiaSun-dried pears of Portuguese S. Bartolomeu variety are relatively small fruits with peculiar organolephtic characteristics. The open sun drying has some disadvantages related with drying efficiency and food safety. However the drying with an economic and friendly energy is an important characteristic of the drying methods and therefore several attempts have been made to develop processes using solar drying. The present work studies two types of pears drying, namely solar stove and tunnel drying, using the solar energy and simultaneously reducing the exposure of fruits to contamination agents. Both processes were conducted in two consecutive years to analyse if the behaviour of each drying method shows evidence of a similar pattern. The results enable us to conclude that the variation of normalized moisture of pears with the drying time follows a linear relationship while an exponential decay is observed in the tunnel drying. Despite the different climatic conditions, it is possible still to observe that the behavior of each process is quite similar in both consecutive years. However, in the year of 2008 the drying tends to be faster in both processes. In addition, it is also possible to conclude that the Total Soluble Solids/Acidity ratio has a similar behavior along the drying in the two years for each process. However, the decrease of acidity of pears in solar stove is more pronounced than the TSS and consequently a higher ratio of Total Soluble Solids/Acidity is observed along the solar stove drying when compared with the tunnel drying.
- Estimation of the diffusivities and mass transfer coefficients for pears dried under different methods.Publication . Guiné, Raquel; Barroca, Maria João; Lopes, Paulo; Silva, Vitor; Santos, Mónica; Ferreira, DulcineiaTraditionally, in Portugal pears of the variety S. Bartolomeu are dried through an open-air sun exposure. However, this method of drying has some disadvantages, among which not being able to fully guarantee the necessary quality and sanitary standards for commercialization. Hence, the development of alternative drying methods to replace the traditional drying is of great importance nowadays, and the knowledge of the transfer phenomena happening during drying helps to understand these processes. The present work aimed at determining the mass transfer properties of pears for air drying performed in three drying systems: solar stove with forced air convection (STFC), solar stove with natural air convection (STNC) and drying tunnel with hot air from a solar collector (DT). The methodology used to determine the mass transfer properties was a diffusion model, which accounted for shrinkage. The values of the diffusion and mass transfer coefficients for the drying in the three systems were estimated, and from them was possible to conclude that the values of D e in the STNC and DT systems are similar to each other, whereas the STFC system gives a lower value of diffusivity, indicating that in this system the internal moisture transfer is less efficient. As to h m , the system that presents the higher values is STNC, demonstrating a higher efficiency of surface mass transfer.
- Evaluation of the colour of pears dried using different methods.Publication . Guiné, Raquel; Barroca, Maria João; Lopes, Paulo; Silva, Vitor; Lima, Maria João; Ferreira, DulcineiaPears of the variety S. Bartolomeu (Pyrus communis L.) have been used over the years in Portugal to produce a traditional dried pear known as “pera passa”. It is a very much appreciated dried fruit, despite the drop in the production that has occurred during the past years. The processing comprehends a solar drying performed at open air, with obvious disadvantages, either concerning the drying efficiency or the sanitary quality of the final product, taking into account that the products are exposed to multiple contamination agents. Obviously, these handicaps do not guarantee the necessary quality and sanitary standards for commercialization. Therefore, in the past years some work has been done in order to encounter alternative reliable drying methods that can replace the traditional drying. These include the use of solar stoves, either with forced air convection or with natural convection, and a drying tunnel with air heated though a solar collector. However, not only the drying efficiency and economic or sanitary aspects matter in the present case, since this particular product is very much appreciated for its unique colour. In fact, these pears in particular, when dried by the traditional method, develop a reddish colour that is not found in other varieties of pears, and this aspect constitutes one of the quality parameters for their acceptability by the consumers. The present work aims at comparing the colour of the pears dried by different methods, in order to evaluate which method allows the obtaining of a final product as much similar as possible to the traditional one.
- Properties of pears dried with different drying processes.Publication . Guiné, Raquel; Barroca, Maria João; Lopes, Paulo; Silva, Vitor; Lima, Maria João; Ferreira, DulcineiaPears of S. Bartolomeu variety have been used over the years in Portugal to produce a traditional dried pear using the cheapest practice with environmental friendly characteristics: open-sun drying. However, the obvious disadvantages concerning the drying efficiency and safety of the dried product have been an incentive to design or improve solar drying methods as an alternative to the traditional one. The present work has tested three types of drying processes, namely solar stove, solar drying and tunnel drying. In addition, the efficiency of drying and the properties of the dried pears were compared with the traditional dried fruits. The results of the tests have allowed us to conclude that all methods have lead to a considerable reduction in drying time as compared to open-sun drying. Indeed, the drying time for the solar stove and the solar dryer showed a reduction of 40% while in the drying tunnel this reduction increased to 60%. In addition, it was also possible to conclude that the properties of the dried pears obtained by the three processes correspond to a dried fruit similar to the traditional dried pears. Thus, based on these results, it can be inferred that the traditional sun drying at open-air can be replaced by either of the processes tested in the present study, which, allow a higher drying efficiency associated to a higher safety of the dried food, while maintains the properties of the traditional food product.