V. Silveira

Enthalpy-entropy compensation based on isotherms of mango

SUMMARY ENTHALPY-ENTROPY COMPENSATION BASED ON ISOTHERMS OF MANGO. Moisture equilibrium data of mango pulp were determined using the static gravimetric method. Adsorption and desorption isotherms were obtained in the range of 30-70 C, to water activities (a ) from 0.02 to 0.97. The application of the GAB model to the experimental results, using direct nonlinear regression analysis, provided agreement between experimental and calculated values. The net isosteric heat of sorption was estimated from equilibrium sorption data, using the Clausius-Clapeyron equation. Isosteric heats of sorption were found to increase with increasing temperature and could be well adjusted by an exponential relationship. The enthalpy-entropy compensation theory was applied to sorption isotherms and plots of H versus S provided the isokinetic temperatures, indicating an enthalpy controlled sorption process. moisture content; thermodynamic properties; mango. o w ∆∆

Influence of Drying Conditions on Ascorbic Acid during Convective Drying of Whole Persimmons

Degradation kinetics of food constituents may be related to the matrix molecular mobility by glass transition temperature. Our objective was to test this approach to describe ascorbic acid degradation during drying of persimmons in an automatically controlled tray dryer with temperatures (40 to 70°C) and air velocities (0.8 to 2.0 m/s) varying according a second order central composite design. The Williams-Landel-Ferry model was satisfactorily adjusted to degradation curves for both control strategies adopted—constant air temperature and temperature fixed inside the fruit. Degradation rates were higher at higher drying temperatures, independent of the necessary time to attain the desired moisture content.

Medidas da condutividade térmica efetiva de modelos de polpas de frutas no estado congelado

As propriedades termofisicas de alimentos, necessarias nas simulacoes e calculos do processo de congelamento, incluem principalmente a densidade, condutividade termica e calor especifico. Neste trabalho, as difusividades e condutividades termicas da solucao, usadas como modelo para o congelamento de polpas de frutas, foram medidas pelo metodo da sonda com aquecimento. Os experimentos foram conduzidos na faixa de -25 a 0oC com modelos alimenticios constituidos de 0,5% de K- carrageenan + 10% de sacarose (massa/volume de agua). Modelos estruturais foram usados para as avaliacoes da conducao de calor, combinada com a fracao de gelo predita para as amostras a partir dos modelos de Heldman e foram comparados com os valores das condutividades termicas efetivas medidas. Os modelos estruturais empregados foram: em serie, paralelo e Maxwell-Eucken, com o gelo considerado como a fase dispersa. Em todos os ensaios, o modelo de Maxwell-Eucken apresentou os melhores resultados (erro maximo de 6,13% quando comparado com os valores experimentais medidos) e foi escolhido para a predicao da condutividade termica efetiva de solucoes-modelo de polpas de frutas congeladas. Os valores calculados da condutividade termica foram ajustados em termos de funcoes polinomiais, divididas em quatro faixas de temperatura e podem ser usadas na resolucao dos problemas de transferencia de calor, nos processos de congelamento.

Escoamento de ar através de embalagens de polapa de frutas em caixas comerciais: efeito sobre os perfis de velocidade em túneis de congelamento

The changes in air velocity, as a function of stacking of fruit pulp packages (100g), offering a resistance to air flow in commercial boxes were evaluated and the air flow rates estimated during blast freezing operations. Velocities were initially measured prior to passing the air through the multi-layer stacks of boxes in the chamber. Processing of the data was accomplished by applying a non-linear regression routine. Air flow rates were determined by numerical integration from adjusted air flow velocity profiles and appeared to be 10% higher than those obtained by the traditional procedure. Under constant operating conditions of the fan, the results for the 7-layer arrays of boxed products exhibited a 62% reduction in the flux of chilling air, in relation to the 3-layer arrays, and 50.9% decrease with respect to the 5-layer array. These values were directly proportional to the void space available for air flow.

Forced convection to laminar flow of liquid egg yolk in circular and annular ducts

The steady-state heat transfer in laminar flow of liquid egg yolk - an important pseudoplastic fluid food - in circular and concentric annular ducts was experimentally investigated. The average convection heat transfer coefficients, determined by measuring temperatures before and after heating sections with constant temperatures at the tube wall, were used to obtain simple new empirical expressions to estimate the Nusselt numbers for fully established flows at the thermal entrance of the considered geometries. The comparisons with existing correlations for Newtonian and non-Newtonian fluids resulted in excellent agreement. The main contribution of this work is to supply practical and easily applicable correlations, which are, especially for the case of annulus, rather scarce and extensively required in the design of heat transfer operations dealing with similar shear-thinning products. In addition, the experimental results may support existing theoretical analyses.

Solar-Powered Adsorption Icemaker With Double-Stage Mass Recovery Cycle

The performance of an adsorption icemaker prototype operating in a double-stage mass recovery cycle was experimentally investigated under the generation temperature of 75°C. The prototype was heated in the regeneration phase by an electric heater, and an analytical model was employed to assess the daily number of cycles and the ice production, if evacuated tube solar collectors were used to supply the regeneration heat. It was found that the parasitic heat consumed by the metallic part of the reactor greatly reduced the coefficient of performance (COP) and ice production per unit area of solar collector. A reduction of the parasitic heat by 60% would increase the COP in 54%, and the daily ice production per unit area of solar collector in 50 to 54%, depending on the insolation. The performance of the studied prototype was inferior to the performance of some prototypes presented in the literature, which were directly heated by solar energy and used methanol as refrigerant, but this type of prototype could be an alternative to avoid subatmospheric working pressures and to provide ice on rainy and low insolation days, if waste heat or another heat source is available.

Air blast freezing of fruit pulp models in commercial boxes: influence of preferential channels in the bed on freezing times estimating

Os tempos de congelamento da solucao modelo de polpas de frutas embaladas e acondicionadas em diferentes arranjos de camadas no interior das caixas do empilhamento foram avaliados sob varias condicoes de operacao encontradas nas praticas comerciais. Nestes tipos de processos, as caixas contem uma porosidade significativa e metodos para avaliar como esta afeta o tempo de congelamento, torna o problema de dificil resolucao. Procedimentos para as estimativas dos tempos de congelamento, usando modelos encontrados na literatura, foram comparados com medidas experimentais atraves da aquisicao de dados de tempo/temperatura. Os resultados mostraram que o escoamento do ar entre as embalagens e um parâmetro significativo nas estimativas dos tempos de congelamento. Quando e considerada a presenca de canais preferenciais, dependendo do metodo utilizado, estes valores sao subestimados em cerca de 10% quando comparados com os valores experimentais. As isotermas tracadas em funcao da localizacao das amostras no interior das caixas comprovam o deslocamento do centro termico, em relacao ao centro geometrico do produto.