Satyanarayan R. S. Dev

Advancements in Drying Techniques for Food, Fiber, and Fuel

Removal of moisture from biological materials, popularly called drying, has numerous benefits, including ease of handling due to reduction in bulk, resulting in reduced handling costs. Moreover, drying prevents microbial growth and spoilage. Though different drying techniques share a common objective, conceptually they are different and require modification/adaptation based on the biomaterial that is dried. There have been significant scientific advancements in the past century in the field of drying of foods, fibers, and fuel. This article will provide an extensive review of various drying pretreatments and different hybrid drying techniques, including supercritical and fluidized bed concepts, microwave drying, superheated steam drying, and heat pump drying to meet tomorrows food and energy needs.

Drying of Pineapple by Microwave-Vacuum with Osmotic Pretreatment

Drying of pineapple by microwave-vacuum (MW-V) with osmotic dehydration (OD) as a pretreatment was studied. Central composite design was used for the independent variables sugar concentration (40, 50, and 60 °Brix), immersion time (0, 3, and 6 hours) and product temperature in the microwave cavity (50, 60, and 70°C). The dependent variables were moisture content, shrinkage, rehydration coefficient, visual quality, color, and texture. The osmotic dehydration pretreatment was also evaluated with respect to weight reduction (WR), water loss (WL), and solid gain (SG). Maximizing moisture loss was the main objective of OD pretreatment for various combinations of sugar concentration and immersion time. OD preserved the color of the samples while drying. In the complete process composed by OD and MW-V, only the influence of temperature of the product on the drying rate was statistically significant (P < 0.05), whereas OD reduced shrinkage and improved rehydration capacity. The best visual quality was obtained when the independent variables were diminished. OD also significantly (P < 0.05) affected the texture of product producing softer, less surface-hardened product compared to untreated ones.

A Comparative Study on the Effect of Chemical, Microwave, and Pulsed Electric Pretreatments on Convective Drying and Quality of Raisins

In order to discourage the use of chemicals in raisin processing, the effect of microwave and pulsed electric field (PEF) pretreatments on the drying rate and other quality parameters like color, total soluble solids, bulk density, appearance, and market quality were compared with that of chemically pretreated raisins dried at 65°C. The untreated and pretreated samples had a statistically significant difference in drying rate (P < 0.05). The drying rate of chemically pretreated raisins was the highest when compared to others. The results showed that the PEF and microwave-treated samples had a significantly high Total Soluble Solids (TSS), along with good appearance and market quality.

Effects of Microwave-Assisted Hot Air Drying and Conventional Hot Air Drying on the Drying Kinetics, Color, Rehydration, and Volatiles of Moringa oleifera

In this study, the fresh Moringa oleifera pods (Drumsticks) were dehydrated by microwave-assisted hot air drying (MAHD) and conventional hot air drying methods. The samples were dried at three different temperatures, viz. 50, 60, and 70°C, with and without the application of microwaves. Microwave power density of 1 W/g was used for the MAHD. The final moisture content was targeted as 13% wb. The drying curves and drying rate curves were plotted and compared. The kinetics of drying obtained experimentally were correlated with the Page equation. The constants K and N of the Page equation were determined to predict the drying kinetics for varying conditions. The quality attributes, namely, color, rehydration ratio, and volatile compounds, were analyzed and compared with that of the fresh Moringa pods. The volatile compounds were analyzed using z-Nose (an electronic nose; Electronic Sensor Technology, Newbury Park, CA) and bioactive molecules were analyzed using gas chromatography–mass spectrometry. The results showed that the MAHD method had significantly (P < 0.05) reduced the loss of volatiles during drying. Also, MAHD preserved most of the bioactive molecules when compared to the conventional hot air drying method. The samples dried at 50°C using MAHD were the best in terms of all of the quality attributes tested in this study. Also, the results established that the z-Nose can be used as a quick and inexpensive means to assess the effect of different process parameters on the aromatic quality of the product and quantitatively classify quality based on aroma.

A Comparative Study on the Effects of Microwave and High Electric Field Pretreatments on Drying Kinetics and Quality of Mushrooms

This research evaluated the effects of microwave and high-voltage pretreatments on convective freeze drying of mushrooms (Agaricus bisporus) independently. The effect of the microwave (5 W/g for 5 min) and high electric field (HEF; 430 kV/m for 15 min) as pretreatments on enhancement of the drying rates during subsequent drying and the value addition due to the above pretreatments on the quality of the final dehydrated products were investigated. An exponential mathematical model was developed by fitting the drying kinetics to the Page equation to predict the effects of the pretreatments on the drying kinetics of the mushrooms. The parameters considered for the evaluation of product quality project included color, texture, shrinkage properties, and rehydration ratio of the dried mushrooms. The drying rate of HEF-pretreated mushrooms was found to be unaffected overall when compared to the control and the HEF pretreatment resulted in better quality product and less overall shrinkage. Freeze drying of the pretreated mushrooms was found to result in slower drying rate but better overall quality and rehydration ratio.

FDTD Modeling and Simulation of Microwave Heating of in - Shell Eggs

Considering microwaves as a viable alternative for the pasteurization of In-shell eggs, preliminary trials performed had conflrmed that microwave at 2450MHz can be successfully used to raise the temperature of in-shell eggs to the required pasteurization temperatures in a few minutes. Based on these trials a flnite difierence time domain (FDTD) model was developed using C language and MATLAB to simulate the E fleld and power distribution in lossy dielectric media like that of the egg components (egg white and yolk) taking into consideration the complex shape, dielectric properties and heterogeneous composition of the in-shell egg. This can be used to assist in the design and development of an industrial microwave in- shell eggs pasteurization unit.

Effect of Radiofrequency Heating on the Dielectric and Physical Properties of Eggs

Eggs are one of the most nutritious foods available in nature. This rich nutritive environment attracts microbes to invade, feed and multiply. Salmonella enteritidis is one such microbe that is highly pathogenic and is the causative agent for the disease salmonellosis. To ensure safety of eggs, processing them without afiecting their unique physical properties is essential. In this study, the impact of radiofrequency (RF) heating on the dielectric properties (dielectric constant and dielectric loss factor) of the egg at varying temperatures (5 - C{56 - C) and frequency (10MHz{3GHz) is evaluated. This study on the dielectric parameters is essential to devise a better heating paradigm wherein there is minimal detrimental efiect to the egg components. Based on the dielectric study, the heating process parameters were determined. The efiect of such heat treatment on the physical properties viz. Viscosity, foam density, foam stability and turbidity of the egg white were also studied. This study was conducted to provide su-cient literature and experimental background for employing RF in pasteurization of in-shell eggs. This study showed that if careful process parameter optimization and meticulous equipment design is done, RF heating can be successfully employed to pasteurize in-shell eggs.

Selected Post-Heating Properties of Microwave or Hot Water Heated Egg White for In-Shell Pasteurization

Heat treatments adversely affect the functional properties of the egg white. The selected properties of egg white after in-shell pasteurization by microwave heating were compared to that of egg white after in-shell pasteurization by immersion in hot water. The effects of the heat treatments on egg white enthalpy of denaturation, viscosity, turbidity, foam density, and stability, and also the dielectric properties, were investigated and compared to that of untreated egg white. Results demonstrated that microwave pasteurized in-shell eggs had nearly 20% higher enthalpy of denaturation, 4 times more viscosity, 50% more stable foam with 30% less foam density and 78% clearer, on an average, when compared to the water bath pasteurized ones.

Optimization of Radiofrequency Heating of in-Shell Eggs through Finite Element Modeling and Experimental Trials

Considering Radio Frequency (RF) heating as a viable alternative for the in-shell heating of eggs, Finite Element Modeling and simulation of RF heating of in-shell eggs at 27.12MHz were carried out to assess the feasibility and heating uniformity of the process. According to the recommendations of USDA-FSIS for the pasteurization of eggs, egg white must be heated up to 57.5 - C, and the egg yolk has to be heated up to 61.1 - C for 2min. The objective of the simulation was to determine the location of hot and cold spots generated due to non-uniform heating. A parallel plate setup for Radio Frequency heating was simulated for difierent electric fleld strength levels and orientations of the egg (long axis parallel and long axis perpendicular to the plates). The simulation results were experimentally verifled and the simulation procedure was validated using a laboratory parallel plate RF setup. A coaxial cavity design was simulated with a similar approach. Results indicated that both the parallel and coaxial cavity designs were suitable for in-shell pasteurization of eggs provided that the eggs were rotated to maintain the uniformity in heating. After the simulation of RF heating process, the process optimization was carried out to determine the most efiective procedure for the process. The varying parameters obtained by using difierent modeling techniques for radiofrequency heating of in-shell eggs, were optimized using MATLAB. Laboratory scale experimental trials were conducted to test the validity and efiectiveness of the optimized parameters. The optimal parameters set forth were found to be more e-cient in terms of heating time and uniformity.

FINITE ELEMENT MODELING OF SELECTIVE HEATING IN MICROWAVE PYROLYSIS OF LIGNOCELLULOSIC BIOMASS

Microwave pyrolysis overcomes the disadvantages of conventional pyrolysis methods by efficiently improving the quality of final pyrolysis products. Biochar, one of the end products of this process is considered an efficient vector for sequestering carbon to offset atmospheric carbon dioxide. The dielectric properties of the doping agents (i.e., char and graphite) were assessed over the range of 25◦– 400◦C and used to develop a finite element model (FEM). This model served to couple electromagnetic heating, combustion, and heat and mass transfer phenomena and evaluated the advantages of selective heating of woody biomass during microwave pyrolysis. The dielectric properties of the doping agents were a function of temperature and decreased up to 100◦C and thereafter remained constant. Regression analysis indicated that char would be a better doping substance than graphite. The simulation study found that doping helped to provide a more efficient heat transfer within the biomass compared to non-doped samples. Char doping yielded better heat transfer compared to graphite doping, as it resulted in optimal temperatures for maximization of biochar production. The model was then validated through experimental trials in a custom-built microwave pyrolysis unit which confirmed that char doping would be better suited for maximization of biochar. Received 25 August 2013, Accepted 16 October 2013, Scheduled 22 October 2013 * Corresponding author: Baishali Dutta (baishali.dutta85@gmail.com). 2 Dutta, Dev, and Raghavan