Raquel Guiné

Pear drying process analysis: Drying rates and evolution of water and sugar concentrations in space and time.

ABSTRACT Dried pears are produced in Portugal by a traditional solar drying process and, because they represent unquestionable regional economic interest, some studies are being conducted in order to convert their production to industrial drying methods. In this work the main mass transfer phenomena occurring during drying of pears are investigated. Experiments were carried out to determine the time evolution of the radial profiles of water and sugar content. The drying rates were also studied at constant drying temperatures of 30, 40 and 50°C. The results enable to conclude that the concentration profiles of water and sugar, both in space and time, follow an expected pattern if one takes into account the rates of water removal and the diffusion mechanisms. These also allow to explain why the sugar concentration increases in the periphery of the fruit. The drying rate curves obtained are typical, with the drying process at 30°C being more gradual but also much slower than that at 50°C.

Artificial neural network modelling of the antioxidant activity and phenolic compounds of bananas submitted to different drying treatments

Bananas (cv. Musa nana and Musa cavendishii) fresh and dried by hot air at 50 and 70°C and lyophilisation were analysed for phenolic contents and antioxidant activity. All samples were subject to six extractions (three with methanol followed by three with acetone/water solution). The experimental data served to train a neural network adequate to describe the experimental observations for both output variables studied: total phenols and antioxidant activity. The results show that both bananas are similar and air drying decreased total phenols and antioxidant activity for both temperatures, whereas lyophilisation decreased the phenolic content in a lesser extent. Neural network experiments showed that antioxidant activity and phenolic compounds can be predicted accurately from the input variables: banana variety, dryness state and type and order of extract. Drying state and extract order were found to have larger impact in the values of antioxidant activity and phenolic compounds.

Analysis of Moisture Content and Density of Pears During Drying

Abstract Solar dried pears of the “S. Bartolomeu” variety are a very much appreciated and preferred dried food product in Portugal. Nevertheless, the traditional solar drying is carried out at open air during the months of 07 and 08, and this nowadays is a disadvantage for larger productions. This work is to evaluate the possibility of producing dried pears from this and other varieties, maintaining the characteristics of the traditional dried pears. In this study four different types of pears were studied, including “S. Bartolomeu” as a basis for comparison and the drying method employed was the traditional one. From the results it was concluded that, although the behaviour of the four varieties do not vary significantly, one particular variety (“D. Joaquina”) is a good alternative to the “S. Bartolomeu” pear.

Shrinkage Characteristics and Porosity of Pears during Drying

The high moisture content of pears (about 80%) results in a high degree of shrinkage when drying takes place, which strongly affects the fruit structure and quality. The aim of the present work was to study the shrinkage characteristics of pears during air drying by evaluating their dimensions and volume along the drying process and by predicting particle and bulk densities as well as porosity as a function of material moisture content. The latter properties were estimated using a mathematical model available from the literature whose parameters were either directly measured or determined by fitting some of the equations of the proposed model to the experimental data. The results predicted were subsequently validated by independent measurements.

EXPERIMENTAL DETERMINATION AND COMPUTER FITTING OF DESORPTION ISOTHERMS OF D. JOAQUINA PEARS

Desorption isotherms at different operating temperatures provide valuable information for the design and control of drying processes. In the present work the desorption isotherms of pears are determined according to two equivalent forms of the GAB model at temperatures of 20, 25 and 30°C. These cover the range of summer average daily temperatures in Portugal, when the solar drying of pears is carried out. Two different computer fitting methodologies are used, namely, Ordinary Least Squares (OLS) and Orthogonal Distance Regression (ODR), in order to evaluate the above mentioned forms of the GAB model. From the results obtained it is possible to conclude that the ODR fitting is clearly more adequate than the OLS technique for this kind of experimental problem. Moreover, it is advisable to exploit the full form of the GAB model instead of its simplified and restricted version. Finally, the desorption isotherms do not vary significantly in the temperature range under study.

Mass Transfer Properties of Pears for Different Drying Methods

In the present work, the mass transfer properties of pears were determined for three different drying systems (a solar stove, a direct solar drier, and a drying tunnel), using three calculation methodologies. The results obtained allowed concluding that the best methodology to calculate the mass transfer properties is based on Ficks equation, including shrinkage. The values of the diffusion coefficients in the solar drier and drying tunnel were similar, whereas in the solar stove the diffusivity was lower, indicating that the internal moisture transfer was less efficient. Regarding the mass transfer coefficient, this was higher in the solar drier, demonstrating a better efficiency of surface mass transfer.

Modelling and simulation of pear drying

In the present work a diffusion based model was adopted to represent the drying behaviour of pears in a continuous convective drier, taking into consideration the variation of the properties of the pears along drying. The program developed simulated a significant number of situations resultant from the variation of some of the operating conditions. The temperatures tested were 30, 40 and 50^oC, the air velocities were 0.5, 1.0 and 1.5m/s, and the relative humidity of the drying air was 40%, 50% and 60%. From the situations analyzed, and considering the specifications required for the final product, it was concluded that the drying should be carried out at 40^oC, with a drying air velocity of 1.5m/s and containing 60% RH.

Drying Kinetics and Product Quality for Convective Drying of Apples (cvs. Golden Delicious and Granny Smith)

The convective drying of apples from two different cultivars, Golden Delicious and Granny Smith, in a range of temperatures from 30 to 60°C is studied. Some chemical and physical properties were determined fresh after drying: moisture, acidity and sugar content, color, and textural attributes, in order to evaluate the effect of drying and drying temperature on these properties. Furthermore, the drying kinetics were studied in terms of thin layer models and also by means of the Fick’s equation of diffusion, and some mass transfer properties were evaluated, such as effective moisture diffusivity and convective mass transfer coefficient. It was concluded that drying decreased both acidity and sugar content for both apple cultivars. Color was significantly affected by drying, resulting in high values of the total color difference, between 19.43 and 25.04. Drying turned the products less hard and less cohesive. Furthermore, it was found that mass diffusivity increased with temperature following an Arrhenius type function, where De0 was 5.4621 × 10−4 and 1.8401 × 10−4 m2/s, and E was 35.3 and 32.8 kJ/mol, respectively, for cultivars Golden Delicious and Granny Smith.

Convective Drying of Apples: Kinetic Study, Evaluation of Mass Transfer Properties and Data Analysis using Artificial Neural Networks

Abstract In the present work, the effect of drying was evaluated on some chemical and physical properties of apples, and the functions were modelled using feed-forward artificial neural networks. The drying kinetics and the mass transfer properties were also studied. The results indicated that acidity and sugars were significantly reduced by drying. Regarding colour lightness decreases, whereas redness and yellowness increased. As for texture, the dried samples were softer and less cohesive as compared to the fresh ones. Mass diffusivity increased with temperature, from 4.4×10−10 m2/s at 30°C to 1.4×10−9 m2/s at 60°C, and so did the mass transfer coefficient, increasing from 3.7×10−10 m/s at 30°C to 7.4×10−9 m/s at 60°C. As to the activation energy, it was found to be 34 kJ/mol. Neural network modelling showed that all properties can be correctly predicted by feed-forward neural networks. The analysis of the networks’ behaviours input layer weight values also shows which properties are more affected by dehydration or more dependent on variety.

Study About the Knowledge and Attitudes of the Portuguese Population About Food Fibres

The association between dietary fiber, health benefits and healthy food has been studied in recent years. The food industry, to accompany the strong interest shown by consumers, has placed at their disposal new products, rich in dietary fiber. This work intended to make a statistical analysis of the knowledge of the Portuguese population about fibres, and for that a survey was conducted of a sample of 182 individuals. Some topics covered included consumption habits, knowledge about fibres, means of dissemination and information, food labelling or the relationship between fibres and health, among others. The most relevant results indicate that only 13% of respondents eat two meals a day with vegetables and/or salads and 9% eat at least 3 pieces of fruit. Whole grains are never consumed by only 41% and 18% do so at least once a week. Around 35% of respondents know about the origin of fibres whereas 70% say there is a greater amount of fibre in legumes, fruit with skin and whole foods. The consultation of food labels is of interest to 80% of respondents, although the fibre content is not of interest for 43% of respondents. The vast majority (90%) of respondents have the notion that fibre intake contributes to the prevention and treatment of diseases. With this work it was concluded that the individuals in the survey reveal an insufficient level of knowledge about dietary fibre and that, although they give great importance to the role of fibres in treatment and prevention of diseases, the level of intake is too low.