Miguel E. Schmalko

Effect of growing and drying conditions on the phenolic composition of mate teas (Ilex paraguariensis).

Mate tea ( Ilex paraguariensis) has been used for centuries and is widely consumed in Brazil, Argentina, Paraguay, and Uruguay. The aim of the present study was to determine how growing and drying conditions affect the phenolic concentration and antioxidant capacity of 15 Mate teas from forest or plantation cultivations, dried either with hot air or wood smoke. The total polyphenol concentration determined with Folin-Ciocalteu ranged from 100.3 +/- 5.5 to 179.7 +/- 3.6 mg equiv chlorogenic acid/g dry leaves. The antioxidant capacity according to the oxygen radical absorbance capacity assay ranged from 1.5 +/- 0.3 to 4.1 +/- 0.1 mmol Trolox equiv/g dry leaves. Ten phenolic compounds were identified and correlated with antioxidant capacity ( R (2) = 0.80). Principle component analysis and multivariate linear regression were conducted to assess the effect of growing and drying conditions. Sun-exposed (plantation grown) Mate teas exhibited higher levels of all polyphenols as compared to shaded (forest grown) Mate teas ( P < 0.05). Lower rainfall, temperature, and drying conditions had varying effects on the phenolics. On average, plantation grown Mate teas had a greater concentration of phenolics than forest grown teas. Thus, plantation grown Mate teas represent better potential sources for their commercial extraction.

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.

DIFFUSION COEFFICIENTS OF WATER AND SUCROSE IN OSMOTIC DEHYDRATION OF PAPAYA

ABSTRACT Mass transfer during osmotic dehydration of papaya in sucrose solution has been studied. A mathematical model based on Ficks law for unsteady state mass transfer, considering shrinkage, has been used to calculate the moisture and sugars diffusion coefficients. Also, a diffusional model with simultaneous shrinkage was developed using a finite difference method to solve differential equations, assuming moisture content-dependent diffusivities for moisture and sugars. The validity of both proposed models was verified by the good fit between the experimental and calculated curves. The mean percent relative errors were 2.48 and 9.92 for moisture content, and 4.26 and 22.51 for total sugars content, for Model 1 and Model 2, respectively.

MOISTURE DIFFUSIVITY, SHRINKAGE, AND APPARENT DENSITY VARIATION DURING DRYING OF LEAVES AT HIGH TEMPERATURES

The variation of moisture diffusivity, apparent density, and shrinkage during drying of yerba maté leaves at high temperatures were studied. A simple mathematical model that describes the variation of the temperature and moisture content into the leaf during drying was developed. Temperatures of air varied between 100 and 130°C when the experiments were carried out in a convective cross-flow air dryer. Thickness and apparent density varied greatly with moisture content, but this variation does not depend on drying temperature. Moisture diffusivity varied from 2.3 × 10−11 to 2.5 × 10−10 m2/s, and activation energy values are in the range of other foodstuffs. A great dependence of moisture diffusivity on moisture content was also found.

Chlorophyll stability in yerba maté leaves in controlled atmospheres

The objective of this research was to investigate the stability of chlorophyll in yerba mate leaves in controlled atmospheres of CO2/air mixtures and different water activities at 25°C.Two levels of water activity were selected corresponding to saturated salt solutions of LiCl (aw=0.113) and MgCl2(aw=0.330) and three levels of CO2/air mixtures (0/100,20/80 and 40/60). The chlorophyll content was evaluated using a liquid chromatography HPLC technique. Experimental values varied between 2.16 and 0.61 mg/g of dry matter. For each sample, 5 determination were made during 58 days. Experimental values were fitted to an equation describing a first order reaction. In all cases, the agreement was good with P<3 10-3. The initial concentration of chlorophyll dropped, in average, to 30.5% after 58 days. However, after comparing the velocity constants, no differences were found between them.

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.

Modeling the drying of a deep bed of Ilex paraguariensis in an industrial belt conveyor dryer

The evolution of temperature and moisture content of leaves and twigs of yerba maté on different levels of a through-flow dryer was investigated by modeling heat and mass transfer using the finite-difference method. To validate the model, the temperature and moisture profiles were used to estimate chlorophyll losses. Great variations were obtained in moisture, temperature, and chlorophyll content at different levels of the bed. Leaf temperature quickly increased in the former nodes, and it then increased slowly until it reached air temperature. In the twigs, the temperature increase was slow and the air temperature was never reached.

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.

Influence of Drying Temperature on the Physical and Microbiological Parameters and the Quality of Dried Green Onion

The aim of this research was to determine the influence of drying temperature on the physical and microbiological parameters of the green onion and to evaluate the quality and shelf stability of the dried product. To evaluate the stability during storage, measurements of water activity, color parameters, and microbial survival were taken. Drying temperature had little influence on the adsorption isotherms and rehydration rates but had a strong influence on the color parameters and microbial survival. All stability parameters were improved at drying temperature of 70°C. Changes in color limit the shelf life of this product to about 30 days.

Extraction optimization of soluble compounds of yerba maté

The objective of this research was the extraction optimization of water soluble compounds of yerba mate. Measures variables were extract concentration and weight of leaves and twigs. Controlled variables were time and temperature of extraction and water/solid relation. A surface response method of three variables was used as experimental design, with 20 experiences in each case. The range of each variable, defined in the experimental design, was: extraction time, 13.2 to 46.8 minutes; temperature, 48.2 to 81.8°C and water solid relation, 4.64 to 11.36 g water/100 g of dry solid. Extract weight varied from 13.14 to 29.56 g in leaves and 8.98 to 16.32 g in twigs (each one per 100 g of dry solid). Extract concentration varied between 2.17 and 3.43 g/100 ml in leaves and between 1.32 and 2.31 g/100 ml in twigs. The results were fit to a linear equation in each case.