Nelson O. Moraga

Stevia rebaudiana Leaves: Effect of Drying Process Temperature on Bioactive Components, Antioxidant Capacity and Natural Sweeteners

Stevia leaves are usually used in dried state and undergo the inevitable effect of drying process that changes the quality characteristics of the final product. The aim of this study was to assess temperature effect on Stevia leaves through analysis of relevant bioactive components, antioxidant capacity and content of natural sweeteners and minerals. The drying process was performed in a convective dryer at constant temperatures ranging from 30 to 80xa0°C. Vitamin C was determined in the leaves and as expected showed a decrease during drying proportional to temperature. Phenolics and flavonoids were also determined and were found to increase during drying below 50xa0°C. Antioxidant activity was determined by DPPH and ORAC assays, and the latter showed the highest value at 40xa0°C, with a better correlation with the phenolics and flavonoids content. The content of eight natural sweeteners found in Stevia leaves was also determined and an increase in the content of seven of the sweeteners, excluding steviol bioside, was found at drying temperature up to 50xa0°C. At temperatures between 60 and 80xa0°C the increase in sweeteners content was not significant. Stevia leaves proved to be an excellent source of antioxidants and natural sweeteners.

Parallel Finite Volume Method Simulation of Three-Dimensional Fluid Flow and Convective Heat Transfer for Viscoplastic Non-Newtonian Fluids

Three-dimensional fluid mechanics and heat transfer for viscoplastic flows are described by finite volume method, FVM. The open multi-processing approach has been implemented to parallelize the numerical code. Results for the elapsed times, speed-ups and efficiencies are presented. The code was used to describe the natural convection (Ra = 104; 106) and the lid-driven cavity (Re = 100; 1000) processes with Bingham, Casson and Herschel–Bulkley fluids (Bn = 0.01; 1.0). Results describing isotherms, velocity distributions and streamtraces, as a function of Ra, Re, Pr and Bn numbers are shown. The grid size analysis shows that different sizes are required to obtain precise results for Nusselt number and friction factor.

VOF/FVM prediction and experimental validation for shear-thinning fluid column collapse

Dam break problems for non-Newtonian fluids can be found in sudden collapse of mine tailings, snow avalanches, debris and lava flows, and casting solidification. A numerical simulation and experimental validation of collapse of a shear-thinning fluid column with a high viscosity is presented. The 2D fluid mechanics, described in terms of the non-linear coupled continuity and momentum equations, was solved by the finite volume method (FVM) with the Pressure Implicit with Splitting of Operators (PISO) coupling algorithm and the volume of fluid method (VOF). The shear-thinning fluid column that collapses was described by the rheological model of Carreau-Yasuda. The numerical results obtained for the instantaneous position of the free surface were validated with 7% accuracy in comparison with experimental measurements, determining that when a small container is used the walls affect the transport of fluid, causing a creeping flow.

Modelling of Rheological Behaviour of Pulps and Purées from Fresh and Frozen-Thawed Murta (Ugni molinae Turcz) Berries

Abstract The rheological behaviour of purées and pulps from fresh and frozen-thawed murta (Ugni molinae Turcz) berries was studied at 30, 45, 60 and 75 °C. The Mitschka method was shown to be applicable to the murta berries, with its products showing non-Newtonian pseudoplastic behaviour. Rheological models as Ostwald-de Waele, Herschel-Bulkley, Bingham, Casson and Mizrahi-Berk fitted the experimental data with high values for correlation coefficients. Effect of temperature was evaluated using an Arrhenius-type equation. Compared to the murta pulp, the fresh or frozen-thawed murta purées had higher apparent viscosity at any shear rate and would undergo a more rapid change in viscosity with temperature. Activation energy for fresh and frozen-thawed purées were 22.335 kJ mol-1 (R2 = 0.9225), and 16.478 kJ mol-1 (R2 = 0.9654) respectively, while for fresh and frozen-thawed pulp Ea values of 8.967 kJ mol-1 (R2 = 0.9618) and 6.943 kJ mol-1 (R2 = 0.9773) were determined respectively.

Quality Assessment and Mathematical Modeling of Hot-Air Convective Drying of Persimmon (Diospyros kaki L.) Fruit

Abstract The influence of drying on the color, porosity, shrinkage and moisture of persimmon fruit during convective drying was determined by computer vision. The experiments were performed with persimmon fruit that were cut into slab 20 × 20u2006mm, which were arranged into a bigger slab, 60 × 60u2006mm. Drying process was carried out at 60u2006°C. Noticeable changes in quality parameters (color, porosity and shrinkage) could be observed during the drying process, where the central region of the sample evidenced less changes. Persimmon’s physical properties were experimentally obtained as the temperature function and heat and mass convective coefficients were adjusted as a time function. A numerical simulation using the Finite Volume Method allowed to describe the evolution of temperature and moisture content distributions during drying. The numerical and experimental results of temperature and moisture during persimmon drying were found to be in a good agreement.

Numerical Simulation of Experimental Freezing Process of Ground Meat Cylinders

The efficient prediction of freezing processes is important for the frozen food industry with the purpose of preserving food quality. A comparison is made between the finite difference and finite volume methods to predict the unsteady 2D temperature distribution for the freezing of three different ground meat cylinders, with variable convective boundary conditions. Experiments were carried out to measure the local heat transfer coefficients as a function of freezing time and location. Time-dependent air temperature conditions caused by thermostat action were included in the analysis. The meat thermophysical properties as density, specific heat, and thermal conductivity are assumed to vary non-linearly with temperature. The experimentally measured time-temperature data were compared with the simulation results obtained by both numerical methods. According to statistical test results (RE %), the finite volume method gave the best fit quality. The finite volume method is more accurate for simulating the freezing process, as well as estimating the freezing time of this product

Influence of backfill soil shear strength parameters on retaining walls stability

DOI: 10.7764/RDLC.16.2.175 One of the problem of geotechnical engineering is the analysis of the retaining walls stability, where are several uncertainty sources. Some of these uncertainties relate to the inherent variability of fill soil, retained by the retaining wall, strength parameters. This feature can be quantified through a retaining wall stability probabilistic analysis using the Monte Carlo Simulation method (SMC), which enables to determine the wall reliability from collected testing shear strength information of existing soils in the cities of Coquimbo and La Serena, Coquimbo Region, Chile. An application example, the study of a cantilever retaining wall stability filled with silty sand soil (SM) with three different degrees of compaction (DC%: 80, 90 and 95%), with the objective to analyze the wall stability through statistical analysis of safety factors.