Laura A. Ramallo

Simultaneous determination of specific heat capacity and thermal conductivity using the finite-difference method

Thermal conductivity and specific heat capacity and their variation with moisture content were determined for twigs of yerba mate (Ilex paraguariensis Saint Hilaire). For the simultaneous determination of these two properties, transient heating data and the finite-difference method were used to determine the specific heat capacity and steady-state data to determine the thermal conductivity. The values of thermal conductivity varied between 0.137 and 0.519 W/m K and those of specific heat capacity between 1.63 and 3.22 kJ/kg K for moisture contents that varied between approximately 4 and 60 wt% (wet basis). The values obtained were fitted to theoretical models and other empirical equations.

Rate of water loss and sugar uptake during the osmotic dehydration of pineapple

Water loss, sucrose gain and the variation in concentration of other natural fruit sugars (glucose and fructose) were studied during osmotic dehydration of pineapple slices (0.6 mm thick) in sucrose solution (60 % w/w) at three temperatures (30, 40 and 50oC). As temperature increased from 30 to 50oC, the apparent moisture and sucrose diffusivities (Dw and Ds) increased 3.8 and 2.8 times, respectively; therefore, the dehydration efficiency index (Dw/Ds) increased with temperature. The loss of glucose and fructose increased with temperature too. It was found that the solute content was a linear function of the moisture content and this relation was independent of the temperature during the first 600 minutes of dehydration.

AN APPLICATION OF SIMULTANEOUS HEAT AND MASS TRANSFER IN A CYLINDER USING THE FINITE-DIFFERENCE METHOD

ABSTRACT A study of simultaneous heat and mass transfer during drying an infinite cylinder shape material (twigs of ilex paraguayenais saint hilaire) was carried out. The finite-difference method was used to solve the drying model and a simultaneous heat and mass balance in each node was made. Models with different assumptions were tested and the external mass transfer coefficient was used as a parameter to fit the model to experimental data. The thickness of the node and the time step were selected considering the system stability. Drying temperature, twig diameter and air velocity were selected as study variables. The models results were in good agreement with experimental measurements giving mass coefficient values between 1.97 10−4and 9.55 10−4 Kg/m2 s.

Dimethoate degradation in plants and during processing of yerba maté leaves

The objective of this research was to study degradation kinetics of dimethoate in plants of Ilex paraguariensis Saint Hilaire (or yerba mate) and during its processing. To determine dimethoate concentration, a capillary gas chromatography technique with a mass selective detector was used. Half-life times in plants ranked between 9.8 and 11.8 days. During processing, with a blanching and two drying steps, dimethoate concentration decayed to a 22.7% of its initial value (in dry basis); while during seasoning step (at 45°C), half-life time was 17.3 days. With these values, preharvest safety interval was determined.

Effect of Pulsed Vacuum Treatment on Mass Transfer and Mechanical Properties during Osmotic Dehydration of Pineapple Slices

Abstract The influence of operating pressure during osmotic dehydration on mass transfer and mechanical properties in pineapple fruits was analyzed. Dehydration trials were performed at atmospheric pressure (OD) and by applying a vacuum pulse (VPOD), in sucrose solution at 60°Brix and 40°C for 300 min. Seven operation conditions were implemented with a vacuum pulse of 100 mbar or 250 mbar for 0, 5, 15 or 25 min at the beginning of the process. The decrease of pressure favored the solute uptake, but the water loss has not been significantly affected. No significant effect of vacuum time was observed. However, solute uptake in trials with vacuum pulse of 100 mbar was significantly higher than in OD process. In general, mechanical properties and shrinkage were not affected by operation conditions. Osmotic dehydration process (both OD and VPOD) originates a more resistant tissue structure than the one in fresh pineapple fruit.

Effect of Processing Conditions on Calcium Content, Firmness, and Color of Papaya in Syrup

Calcium impregnation is used as a pretreatment in the processing of papaya in syrup. The effect of process temperature (30 and 45°C), calcium source (calcium gluconate and calcium lactate), calcium concentration (0.5 and 1.5% w/w), and pH (4.2 and 6) were studied. The mineral source affected significantly the calcium uptake and the fruit firmness, and therefore, the product quality maximum content of calcium in the fruit was 240 and 72 mg/100 g fresh fruit in 8 h of treatment with calcium lactate and calcium gluconate, respectively. Greater firmness was observed in samples impregnated with calcium lactate. Impregnation treatments did not affect the surface color of fruit. Finally, the effect of cooking in sucrose syrup on product quality attributes (calcium retention, firmness, and color) was analyzed. Cooking in syrup had a positive effect on tissue firmness, despite the decrease of calcium content. During cooking in syrup, calcium content of treated fruit decreased between 9% and 37%. However, the calcium content of fruit in syrup was up to 6 times higher than in fresh fruit. Moreover, the cooking stage had a strong influence on color parameters, leading to a processed product darker than fresh fruit.