Florencia Cecilia Menegalli

Water sorption thermodynamic properties applied to persimmon skin and pulp

Abstract Moisture equilibrium data of persimmon skin and pulp were determined using the static gravimetric method. Adsorption and desorption isotherms were obtained in the range of 20–70°C, to water activities ( a w ) from 0.02 to 0.85. The application of the GAB model to the experimental results, using direct nonlinear regression analysis, provided a good 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 Δ H versus Δ S for skin and pulp provided the isokinetic temperatures, indicating an enthalpy controlled sorption process.

THERMODYNAMIC MODELS FOR WATER SORPTION BY GRAPE SKIN AND PULP

ABSTRACT The enthalpy-entropy compensation theory was applied to water sorption for grapes of Italy variety. The moisture sorption isotherms were analyzed using the static gravimetric method at 35, 40, 50, 60, 70 and 75° C. For isotherms construction, the skin and pulp of the grape were used separately and it was possible to observe significant differences. The GAB equation was fitted to the experimental data, using direct nonlinear regression analysis; the agreement between experimental and calculated values was satisfactory. The net isosteric heat or enthalpy of water sorption, determined from the equilibrium sorption data, showed a different behavior when compared with other works, as it was obtained for skin and pulp separately. Plots of Ah vs AS for skin and pulp provided the isokinetic temperatures TBS = 423.2 ± 27.6 K and TBP = 424.5 ± 25.3 K, respectively, indicating an enthalpy-controlled desorption process over the whole range of moisture content considered.

CHARACTERISTICS OF GELATIN BIOFILMS IN RELATION TO DRYING PROCESS CONDITIONS NEAR MELTING

ABSTRACT Biofilms are edible films formed from biopolymers. In the casting process technique, the biofilm is obtained by the drying of a colloidal solution in the final process stage The aim of this work was to analyse the drying process of gelatin based biofilms, in room frontier conditions. The biofilms were prepared initially by dissolving gelatin in water and adding sorbitol as plasticizer. The filmogenic solution was dried in an automatic drier under various drying conditions, for 1mm film: air temperature (T= 35, 40 and 45 °C), relative humidity (ϕ= 60 and 75 %) and air velocity (v= 3m/s). The drying kinetics was measured for 15, 30 and 45 % sorbitol concentrations. Sorption isotherms at 25, 35 and 45 °C were determined for the 15% sorbitol concentration films by the static gravimetric method. The glass transition and the melting temperature for various moisture contents were determined with a Differential Scanning Calorimeter TA 2010. The results showed an unusual behaviour for temperature dependen...

Effect of chemical pretreatment on the physical properties of dehydrated grapes

Abstract The effects of chemical pretreatment and air drying temperature on drying kinetics, shrinkage, density and rehydration ratio of grapes were determined at various moisture contents. It was observed that the chemical pretreatment employed - a solution of 2% CaCCb with 0 to 3% ethyl oleate - increased considerably the drying rate. It was established that the shrinkage increased with drying temperature between 40 to 80°C and decreased with increasing concentration of ethyl oleate in the chemical pretreatment solution.

Effects of Operating Conditions on the Quality of Mango Pulp Dried in a Spout Fluidized Bed

ABSTRACT The influence of process temperature, feed flow rate and spout, and annular gas velocity on the drying process of mango pulp in a spout fluidized bed dryer was evaluated using a full factorial design. The process was carried out with an intermittent paste flow rate since the high sugar concentration of the fruit led to bed collapse using continuous feeding. The drying performance was assessed by the efficiency of powder production, product moisture, feed, and process time. The final product quality was evaluated by color, vitamin C, and carotenoid content. High process temperatures provided a better drying performance and product quality.

Friction losses in valves and fittings for power-law fluids

Data on pressure drop were obtained in stainless steel, sanitary fittings and valves during laminar and turbulent flow of aqueous solutions of sucrose and xanthan gum, which were selected as model fluids. The rheological properties of these solutions were determined and the power-law model provided the best fit for experimental data. Friction losses were measured in fully and partially open butterfly and plug valves, bends and unions. Values of loss coefficients (kf) were calculated and correlated as a function of the generalized Reynolds number by the two-k method. The model adjustment was satisfactory and was better in the laminar flow range (0.976 < r2 < 0.999) than in the turbulent flow range (0.774 < r2 < 0.989). In order to test the adequacy of the results for predicting loss coefficients during flow of real fluids, experiments were conducted with coffee extract. Comparison between experimental and predicted loss coefficients showed very good agreement.

Evaluation of the drying process in spouted bed and spout fluidized bed of xanthan gum: focus on product quality

It has been shown that spouted bed (SB) dryers are an alternative way to obtain high quality dried powder products. Xanthan is a biopolymer, which gives highly elastic dispersions, while at the same time exhibiting great pseudoplasticity. This hydrocolloid is widely used as a stabilizer, thickener or as gelling agent when associated to other gums. In this work, the drying of xanthan gum in two-dimensional SB, and in a two-dimensional spouted bed with additional aeration (SFB) was evaluated. The product quality was studied as a function of the gum dispersability, and of the rheological properties under steady and dynamic shear. The dried gum dispersed in water shows a similar rheological behavior to the dispersion of commercial xanthan, i.e., significant break-up of the gum structure does not occur even if the biopolymer has undergone shear drying conditions. Turbidimetric measurements were done in order to quantify the dispersability of powder materials. A light microscopic analysis has shown that the material obtained via SFB acquires a shape of large agglomerates with spins, while the commercial powder exhibits an ellipsoid shape. The SB drying process yields a material with an intermediate shape. Such results were associated to the dispersability with the powdered gum.

Behavior of plant tissue in osmotic solutions

The effect of the concentration of sucrose solutions on the cellular structure of potato tissue in equilibrium at 27 °C was studied. Two different methods of investigation were used to determine the volume of the different phases composing the cellular tissue of the potato when in equilibrium with the solutions, one based on data of the concentration itself and the overall volume of 2 mm slices after 48 h at equilibrium, and the other on microscopic images of cells in thin slices of fresh tissue stained with neutral red after an hour in equilibrium to show protoplasts, vacuoles and plasmolysis spaces. The results of these methods were compared with those obtained by a predictive thermodynamic approach considering the semipermeability of cell membranes. Phase volume data obtained from microscopic analysis were more similar to what was predicted by the theoretical model than those obtained by means of composition measurement, where the long equilibrium time apparently led to the loss of semipermeability of the cell membranes, since total volumes calculated without consideration of the cell membranes were similar to those measured. This suggests that the length of time of osmotic dehydration brings about a change in cell structure and the consequent involvement of a different mechanism in mass transfer.

Influence of drying conditions on the rheological properties of prunes

ABSTRACT Rheological properties of rehydrated prunes were obtained applying compression–relaxation tests by using a Texture Analyzer TAXT2i. A mathematical development was adopted to determine the stress and area, along the deformation. Experimental data of stress versus time was fitted by using three different rheological models: generalized Maxwell, Normand & Peleg and Maxwell. Results showed that generalized Maxwell model can be used to describe the viscoelastic behavior of the samples. The rheological parameters obtained indicated that prunes exhibited elastic behavior more pronounced at low moisture content and drying air temperature. At high moisture content and temperature the sample became a more viscous and less rigid.

Effects of drying temperature and relative humidity on the mechanical properties of amaranth flour films plasticized with glycerol

Biofilms are made of biopolymers. In the casting technique, biofilms are obtained by the drying of a polymer suspension in the final stage of processing. The aim of the present paper was to analyze the effect of this drying process on the mechanical properties of films produced with amaranth flour. Variables considered include glycerol content (30, 35 and 40%, g/g dry flour) and air-drying conditions (air temperatures of 30, 40 and 50oC and relative humidities of 40, 55 and 70%). As amaranth flour films constitute a complex mixture of amylopectin and amylose as well as native protein and lipid, certain unexpected results were obtained. The toughest films were obtained at the lowest temperature and the lowest relative humidity (30oC, 40%).