Lilia Méndez-Lagunas

Drying of Garlic (Allium sativum) and Its Effect on Allicin Retention

Convective hot air-drying and freeze-drying were investigated as potential processes to preserve and concentrate allicin in garlic. Both temperature and air velocity had an important effect on hot air-drying kinetics. Sample size and temperature significantly affected the duration of freeze-drying, and thus the remaining moisture content of the garlic samples. Allicin content decreased with an increase of drying temperature in both convective hot air-drying or freeze-drying. Moderate air temperatures (40 and 50°C) allowed a better allicin retention than higher temperatures (60°C). However, retention of allicin was more important in garlic samples freeze-dried at a temperature of 20°C. The drying method did not show a significant impact on glass transition temperature values, indicating that garlic composition is a more important factor than internal structure. The predictions of the Gordon and Taylor model are in good agreement with the experimental data.

Convective drying kinetics of strawberry (Fragaria ananassa): Effects on antioxidant activity, anthocyanins and total phenolic content

The thermal drying effects on strawberries were investigated in terms of the kinetics of antioxidant activity (AA), anthocyanins (A) and total phenolic compound content (TPC), as well as the final colour. The evaluated drying temperatures were 50 and 60°C with an air rate of 1.5m/s. The 2,2-diphenyl-2-picryl-hydrazyl, pH differential and Folin-Ciocalteu methods were used to assess the antioxidant properties. The kinetics of TPC and AA showed an initial and final period of degradation attributed to inhibition of enzymes. A plateau between these two periods suggests that under certain conditions of temperature and water content, no degradation reactions occurred. Final losses of up to 74, 45 and 78% were found for AA, A and TPC, respectively. The total colour change (ΔE) was lesser degree at 60 than 50°C. Thermal degradation of the antioxidant compounds followed a first-order reaction kinetics and the degradation rate constants (k) were calculated.

Effects of Drying Air Temperature on the Structural Properties of Garlic Evaluated During Drying

Second order phase transitions may occur in foodstuffs during the convective drying process. These transitions involve physicochemical changes, which influence both structural properties and drying behavior. The aim of this study was to examine the effects of drying air temperature and the second order phase transition of garlic on the changes in particle density, apparent density, apparent porosity, effective diffusivity, and cracking produced during drying. Garlic slices were dehydrated at three air temperatures (40, 50, and 60°C). The moisture content (X), inside temperature (Ti), surface temperature (Ts), apparent (ρb) and particle (ρp) densities of garlic slices were measured during drying. Porosity (ϵ) was calculated based on the data collected for ρp and ρb. Glass transition temperatures (Tg) and micrographs were obtained for both raw and dehydrated garlic. A critical point in the intersection of Ti, Ts, and Tg was found; this point was identified as a second order phase transition. Diffusivity and slope changes in ρb, ρp, and ϵ with respect to moisture content were found to be related to this critical point. Experimental data for ρb, ρp, and ϵ was fitted to a nonlinear equation with three exponential terms with respect to moisture content, with an R2 > 0.85. Less dense products were found to be more porous, with more cracking, higher moisture diffusivity, and lower Tg at the end of the drying process.

True Density Prediction of Garlic Slices Dehydrated by Convection.

Physiochemical parameters with constant values are employed for the mass-heat transfer modeling of the air drying process. However, structural properties are not constant under drying conditions. Empirical, semi-theoretical, and theoretical models have been proposed to describe true density (ρp). These models only consider the ideal behavior and assume a linear relationship between ρp and moisture content (X); nevertheless, some materials exhibit a nonlinear behavior of ρp as a function of X with a tendency toward being concave-down. This comportment, which can be observed in garlic and carrots, has been difficult to model mathematically. This work proposes a semi-theoretical model for predicting ρp values, taking into account the concave-down comportment that occurs at the end of the drying process. The model includes the ρs dependency on external conditions (air drying temperature (Ta)), the inside temperature of the garlic slices (Ti ), and the moisture content (X) obtained from experimental data on the drying process. Calculations show that the dry solid density (ρs ) is not a linear function of Ta, X, and Ti . An empirical correlation for ρs is proposed as a function of Ti and X. The adjustment equation for Ti is proposed as a function of Ta and X. The proposed model for ρp was validated using experimental data on the sliced garlic and was compared with theoretical and empirical models that are available in the scientific literature. Deviation between the experimental and predicted data was determined. An explanation of the nonlinear behavior of ρs and ρp in the function of X, taking into account second-order phase changes, are then presented.

Texture analysis of dried papaya (Carica papaya L., cv. Maradol) pretreated with calcium and osmotic dehydration

Abstract The application of pretreatments before to convective drying improves the textural characteristics of minimally processed fruits. The aim of this work was to evaluate the texture of papaya (Carica papaya L., cv. Maradol) dried by convection pretreated with calcium hydroxide solutions and osmotic dehydration. Texture profile analysis was performed on fresh and dried cubes by convection at 70 °C not pretreated, as well as on those pretreated with calcium and osmotic solutions. During calcium immersion, the firmness increased, the TSS decreased, and the calcium gain was higher to low temperature. Water loss and solids gain were higher using high concentrations and temperatures during osmosis, resulting in lower values of moisture content and higher values of TSS. Calcium and osmotic pretreatments resulted in a product with intermediate moisture, stable color, firm, and minimal shrinkage. The modification of the papaya structure during all treatments caused a change in the textural characteristics. Statistical analysis of textural characteristics of dry cubes pretreated showed a significant difference (α = 0.05) compared to fresh ones and not pretreated. The pretreatments formed a firm (outer) and a soft (inside) structure that reduced the shrinkage and the deformation of dry cubes and decrease the drying time up to 37% compared to dry cubes no pretreated.

Solute Transfer in Osmotic Dehydration of Vegetable Foods: A Review

While various mechanisms have been proposed for the water transfer during osmotic dehydration (OD), little progress has been made to understand the mechanisms of solute transfer during osmotic dehydration. The transfer of solutes has been often described only by the diffusion mechanism; however, numerous evidences suggest the participation of a variety of mechanisms. This review deals with the main issues of solute transfer in the OD of vegetables. In this context, several studies suggest that during OD of fruits and vegetables, the migration of solutes is not influenced by diffusion. Thus, new theories that may explain the solute transport are analyzed, considering the influence of the plant microstructure and its interaction with the physicochemical properties of osmotic liquid media. In particular, the surface adhesion phenomenon is analyzed and discussed, as a possible mechanism present during the transfer of solutes in OD.

Diffusion of moisture in drying of sugar cane fibers and bundles.

ABSTRACT Sugar cane fibers and arrangements of fibers in cylindrical bundles were dried in a thermoanalyzer and their diffusive coefficients were calculated using the slope method. The effect of temperature, moisture content as well as structural changes were analyzed. Diffusion coefficients changed nanlineariy with moisture content and followed an Arrhenius-like functionality with temperature. The analysis of these effects suggested a liquid diffusion transport mechanism of moisture transfer inside sugar cane fibers and bundles.