Variation in either the physical and mechanical properties and the quality of the bean (Phaseolus vulgaris L.) during the drying process and storage / Variação das propriedades físicas e mecânicas e da qualidade do feijão (Phaseolus vulgaris L.) durante a secagem e o armazenamento

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

There is a lack of information in the available literature on the changes that may occur in the bean during the drying process and storage time. Thus, the accomplishment of studies that would generate theoretical and practical information about a better conservation of the product are really relevant. So, this work was carried out to analyze several characteristics of the bean, as well as to adjust different mathematical models to the experimental data collected during the drying process and storage period, which could be used as subsidy to estimate the changes in the quality of the product. Thus, the following hygroscopic, physical and mechanical properties of the beans during their drying and soaking processes were determined: desorption isotherms, desorption isosteric heat, porosity, both unitary and apparent specific masses, forms, size, volumetric contraction and expansion, drying curves, proportional deformity module, compression and puncture test). The technological and protein qualities of the bean were also evaluated: cooking time, color, water absorption at different temperatures, electric conductivity, content and digestibility of proteins during storage in presence of the pest-insect Acanthocelides obtectus (Coleoptera, Curculionidae). According to the results, the following conclusions were drawn: a) the hygroscopic equilibrium of the bean is directly proportional to the activity of the water, as decreasing with the increased temperature for a same activity of water, as following the same tendency of the most agricultural products already researched, and the Modified Halseys model showing to be a better adjustment to the experimental data; b) with the reduced water content, there occurs an increase in the energy required for its removal from the product, as varying the values of the isosteric integral heat of desorption for the bean grains within the water content range from 2,718 to 3,961 kJ kg-1; c) the reduction in the water content affects the physical properties of the bean grains, by providing a decrease in porosity but an increase in both the apparent specific mass and unitary specific mass that could be appropriately represented by the simple linear model; d) the values of both unitary and apparent specific masses obtained as a function of the chemical composition of the product were above the experimental ones up to the water content of 0.22 (decimal d.b.), since above this limit the values of the specific masses were underestimated; (e) the reduction in the water content affects either the unitary volumetric contraction and the grain mass of the bean, therefore causing a respective decrease of 35.8 and 46.1% in these values within the water content range under study; f) the model by Bala and Woods provides the best representation of the volumetric contraction phenomenon in bean grains during the drying process; g) the values of either sphericity and circularities are increased within this water content range during the drying process; h) the times required for drying the bean up to a water content of 0.13 (decimal d.b.) were 25, 10 and 5.5 hours at the temperatures of 35, 45 and 55 C, respectively; i) the models by Page and Midilli show the best representation for the bean drying phenomenon; j) the diffusion coefficient increases as the temperature increases, by showing values from 2.21 x 10-10 to 9.08 x 10-10 m2 s-1, whereas the introduction of the volumetric contraction of the grains in the liquid diffusion model during the drying process improves its estimate; k) during soaking, the bean grains differently expand toward the radial and axial directions, showing a higher variation in the lowest axis; l) the models by Bala and Woods, Lang and Sokhansanj, and Rahman as well as the Linear one show an adequate description for the volumetric expansion of the bean; m) the water absorption rate of the bean increases as the soaking temperature is increased, and the Pelegs model is recommended for describing the kinetics of this phenomenon at the temperature range under test; n) during the soaking process, the diffusion coefficient presents values of 1.07 x 10-9; 1.30 x 10-9; 1.58 x 10-9 and 1.41 x 10-9 m s-1 at the temperatures of 20, 30, 40 and 50 C, respectively; o) the presence of the A. obtectus insect causes intense deterioration in the bean, therefore promoting either a reduction in the apparent specific mass and increase in the electric conductivity of the grains, so resulting into alteration in the technological quality of the product; p) the cooking time of the bean grains increases during the storage period, independently from the presence of the insect-pest; q) in the absence of A. obtectus, the protein content of the bean is not changed during the storage time, but the digestibility of the bean grain proteins is not affected by the presence of this insect neither by the storage time; r) the compression strength needed to deform the bean is decreased as the water content increases, therefore presenting values between 22.3 and 551.7 N for the repose position, and 10.5 and 253.1 N for compression with the hilum at horizontal position, but 11.6 and 143 N for compression with the hilum at vertical position; s) the proportional deformity module is increased as the water content and the product deformation are reduced, so obtaining values from 4.1 to 71.3 x 107 Pa within the water content range under study, as depending on the position of the grain when compressed; t) the bean grains showed higher resistance to the compression when subjected to efforts at the natural repose position; u) the infestation by the insect-pest during storage interferes into the mechanical behavior of the bean grains, therefore resulting into lowest resistance of the product to the applied strengths ; v) the increased soaking temperature promotes an increment in the elasticity of the bean grain tegument, therefore requiring higher strength for its rupture; w) with the increase in either the cooking time and the deterioration of the grains by the insect-pest there occurs a decrease in both resistance to the compression and maximum puncture strength of the bean grains.

ASSUNTO(S)

beans hygroscopy quality engenharia de processamento de produtos agricolas qualidade thermic properties feijão propriedades físicas propriedades térmicas physical properties higroscopia

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