The desorption isotherms and thermodynamic properties of cocoa beans were obtained during the drying process of this product. The isotherms were determined by dynamic method for various temperature (25, 35, 45 and 55°C) and relative humidity (RH) conditions (30, 40, 50, 60, 70 and 80%). Equilibrium moisture content data were correlated by the Guggenheim-Anderson-de Boer (GAB) model, which presented good fit to the data, according to statistical procedures. Equilibrium moisture content ranged from 5.90 to 16.67 d.b.; it increased with an increment in the RH and decreased with increased temperature at a constant RH. Enthalpy values for each model coefficient were encountered, ranging from )90.05 to 545.96 kJ kg )1 . The integral isosteric heat of desorption and differential entropy increased with decreased equilibrium moisture content, a tendency also found for Gibbs free energy.
It is important to understand how changes in the product formulation can modify its characteristics. Thus, the objective of this study was to investigate the effect of whey protein concentrate (WPC) on the texture of fat-free dairy desserts. The correlation between instrumental and sensory measurements was also investigated. Four formulations were prepared with different WPC concentrations (0, 1.5, 3.0, and 4.5 wt. (%)) and were evaluated using the texture profile analysis (TPA) and rheology. Thickness was evaluated by nine trained panelists. Formulations containing WPC showed higher firmness, elasticity, chewiness, and gumminess and clearly differed from the control as indicated by principal component analysis (PCA). Flow behavior was characterized as time-dependent and pseudoplastic. Formulation with 4.5% WPC at 10 °C showed the highest thixotropic behavior. Experimental data were fitted to Herschel-Bulkley model. The addition of WPC contributed to the texture of the fat-free dairy dessert. The yield stress, apparent viscosity, and perceived thickness in the dairy desserts increased with WPC concentration. The presence of WPC promotes the formation of a stronger gel structure as a result of protein-protein interactions. The correlation between instrumental parameters and thickness provided practical results for food industries.
Physical properties of fluid and semisolid foods, such as density and rheological behavior, must be carefully taken into account on designing unit operations for the processing of such kind of products. In this work, a rotational rheometer of concentric cylinders was used to evaluate the rheological behavior of red guava pulp (Psidium guajava L.), with different soluble solids content (5, 10, and 15°Brix), at four temperatures (10, 30, 50, and 70°C). Also density data were obtained using pycnometry. Models were fitted to the obtained experimental data, in order to mathematically represent the rheological parameters and the density as functions of temperature and soluble solids content. The rheological behavior of the red guava pulp was adequately described by the Ostwald-de-Waele model, with a pseudoplastic behavior. Models to describe the simultaneous effect of temperature and concentration on the density were also presented.
Infrared dehydration is more advantageous than the convective system under similar conditions, and studying this process is important to further develop equipment and procedures. Thus, the aim of this work was to study the dehydration process of tomato (Solanum lycopersicum L.) slices obtained by infrared drying at three different maturity stages throughout two different procedures: Firstly, the drying model was determined by applying the mass and energy balances under wet bulb temperature for the constant drying rate period and secondly, the mass effective diffusion coefficient was determined throughout the experimental data and the theory of diffusion of the liquid phase for the decreasing drying rate period. Tomato fruits cv. Santa Cruz were used. Three maturity stages were selected: green (stage 1), orange (stage 2), and red (stage 3). Mathematical models frequently used to represent drying of agricultural products were fitted to the experimental data of tomato drying. The effective diffusion coefficient was obtained by adjusting the liquid diffusion mathematical model to the experimental data of the descending period of dehydration. The two-term model was the best one to represent the tomato dehydration process. The critical moisture content for tomato dehydration was 2.97 kgw kgdm -1. There is an initial dehydration period in which the drying rate reaches its maximum (approximately 1.05 kgw kgdm -1 , about 3 min). Three different methods were used to obtain values of the effective diffusion coefficient, including the finite element method, which had the lowest values for the least square sum of deviation 1.00 × 10 -7 m 2 s -1 . The global coefficient of heat transfer was 12.45 W m -2 K -1 , and the global coefficient of mass transfer was 0.0105 m s -1 .
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