Rheological and thermophysical properties of blackberry juice

Abstract

Rheological and thermophysical properties were determined for blackberry juice, which was produced from blackberry fruit at 9.1 0.5°Brix and density of 1.0334 0.0043 g cm -3 . The concentration process was performed using a roto evaporator, under vacuum, to obtain concentrated juice at about 60 °Brix. In order to obtain different concentrations, concentrated juice was diluted with distilled water. Rheological measurements were carried out using a Rheotest 2.1 Searle type rheometer. In the tested ranges, the samples behaved as pseudoplastic fluids, and the PowerLaw model was satisfactorily fitted to the experimental data. The friction factor was measured for blackberry juice in laminar flow conditions of pseudoplastic behavior. Thermal conductivity, thermal diffusivity and density of blackberry juice at 9.4 to 58.4 °Brix were determined, in triplicate, at 0.5 to 80.8 °C. Polynomial regression was performed to fit experimental data obtaining a good fit. Both temperature and concentration showed a strong influence on thermophysical properties of blackberry juice. Calculated apparent specific heat values varied from 2.416 to 4.300 kJ.kg -1 °C in the studied interval. Keywords: fruit juice; rheology; thermal conductivity; thermal properties. ResumoAs propriedades reológicas e termofísicas foram determinadas para suco de amora produzido a partir da fruta com 9.1 0.5 °Brix e densidade de 1,0334 0,0043 g cm -3 . O processo de concentração foi realizado utilizando-se um roto evaporador, sob vácuo, obtendo-se o suco concentrado em aproximadamente 60 °Brix. As diferentes concentrações foram obtidas a partir da diluição do suco concentrado em água destilada. As medidas reológicas foram conduzidas utilizando-se o reômetro Rheotest 2.1 do tipo Searle. Em todos os experimentos as amostras apresentaram um comportamento pseudoplástico, e o modelo da Lei da Potência foi ajustado satisfatoriamente aos dados experimentais. O fator de atrito foi medido para o suco em regime laminar com comportamento pseudoplástico. A condutividade térmica, a difusividade térmica e a densidade foram determinadas em triplicata para o suco de amora nas faixas de 9,4 a 58,4 °Brix e de 0,5 a 80,8 °C.A temperatura e a concentração mostraram forte influência nas propriedades termofísicas do suco de amora. Os valores do calor específico aparente variaram de 2,416 a 4,300 kJ.kg -1 °C. Palavras-chave: condutividade térmica; propriedades térmicas; reologia; suco de fruta.

Determination and Modeling of Thermophysical and Transport Properties of Tropical Pulps

“…According to the results presented on Figure 1, in all cases, the density of the pulps increases with ºBrix from 10 to about 60 and decreases with the temperature from 10 to 70°C. This behavior is also coherent with literature [32,33]. From the results presented in Figure 1, the experimental data were fitted to a polynomial equation of second degree for each pulp.…”

“…At the same time, the ºBrix of these pulps were B mangaba < B pitanga < B graviola . The same effect of ºBrix on thermal conductivity has been observed for other juices and pulps [32][33][34][35]. Cabral et al [32] in the evaluation of thermophysical properties from blackberry juice observed that the thermal conductivity and thermal diffusivity decreased with increasing ºBrix, however in different proportions, being the thermal conductivity more sensitive of ºBrix changes then the thermal diffusivity.…”

“…Even comparing different pulps, it's verified that the effect of ºBrix on the thermal conductivity is coherent with literature: this property decreases with increasing ºBrix. For thermal diffusivity, according to literature [32], this property decreases with increasing ºBrix. From Table 2, this behavior occurred from mangaba to graviola.…”

“…Cabral et al [32] in the evaluation of thermophysical properties from blackberry juice observed that the thermal conductivity and thermal diffusivity decreased with increasing ºBrix, however in different proportions, being the thermal conductivity more sensitive of ºBrix changes then the thermal diffusivity. From experimental data of Cabral et al [32], it is possible to observe that the thermal conductivity decreased around 31% while the diffusivity only 4% with increasing of ºBrix, in the same range of ºBrix. Here, in our work, the effect of ºBrix on the thermal conductivity is not being evaluated for a specific pulp.…”

Propiedades termofísicas del jugo concentrado de lulo a temperaturas por encima del punto de congelación

“…Las propiedades termofisicas del jugo de mora fueran recientemente obtenidas por Cabral et al (2007). Estos autores obtuvieron valores semejantes de densidad, calor especifico aparente, conductividad y difusividad, en función de la temperatura e concentración del jugo de mora.…”