Phenolic compounds are useful bioactive molecules with important medicinal properties. Wheat dried distillers grain (DDG), a coproduct of the ethanol production process, is rich in potentially health-promoting phenolic compounds. As the concentration of natural medicinal compounds (e.g. phenolic compounds) in plant materials (e.g. DDG) is low, an efficient solid-solvent extraction process would improve their solubility and effective diffusivity. In the extraction of phenolic compounds from DDG, the DDG cell wall is an important barrier for mass transfer from inside to outside the cell. High-power ultrasound pretreatment of plant material (e.g. DDG) can break down the cell wall and increase the extraction rate and yield of natural medicinal compounds (e.g. phenolic compounds). Radio frequency (RF) heating can provide uniform internal heating of all particles and solvent in the packed-bed extraction unit which results in improved, uniform solubility of the solute in the solvent and diffusivity of the desired compound without overheating specific areas.
The kinetics and mechanism of the extraction were studied and the rate constant of extraction, the saturated concentration, the activation energies, and the temperature independent factors of solid-liquid extraction of phenolic compounds from DDG under different extraction conditions were determined, assuming second-order extraction kinetics. The effect of particle moisture content, ethanol fraction of solvent and extraction temperature on the kinetics, rate and yield of extraction were analyzed. The results of these calculations were compared and discussed with a view to optimizing the extraction process. The maximum extraction rate and yield were obtained with 70% ethanol concentration, 70°C extraction temperature and 59% particle moisture content (w.b.). The effective diffusivity of phenolic compounds in DDG particles was also determined for each extraction condition.
The effect of high-power ultrasound pretreatment on destruction of DDG cell walls and the extraction yield and rate was investigated. Direct sonication by an ultrasound probe horn at 24 kHz was applied and factors such as ultrasound power, treatment time and consumed energy were investigated. The effect of ultrasound on destruction of DDG cell walls was studied by characterizing the physical properties (specific surface area, pore volume and pore size) of the untreated and treated samples at different levels of ultrasound power and treatment time using the method of nitrogen (N2) adsorption at 77 K. The increased surface area, pore volume, pore size, and extraction yield and rate after ultrasonic treatment showed the positive effect of ultrasound pretreatment on breaking down cell walls and pore developement. Among tested ultrasound conditions, 100% ultrasound power for 30 seconds was determined to be the best pretreatment with appropriate consumed energy compared to other tested conditions. Under this extraction condition a 14.29% increase in extraction yield was observed compared to the control, and the BET (Brunauer, Emmett, and Teller) surface and extraction rate constant increased from 13.90 to 18.85 m2/g and 0.057 to 3.933 Lg-1min-1, respectively.
The dielectric properties of the packed bed of wheat DDG particles with ethanol/water solution were measured for more than eight different frequencies using a precision LCR (inductance, capacitance and resistance) meter and a liquid test fixture. The power penetration depth of the packed bed was measured for all applied experimental conditions at 13.56 and 27.12 MHz. The effect of the ethanol fraction of the solvent, moisture content of the DDG particle and temperature on the dielectric constant, loss factor and power penetration depth were investigated. Both the dielectric constant and the loss factor of the packed decreased with frequency for all levels of ethanol fractions and temperatures. The dielectric constant and loss factor of the bed increased with temperature for all levels of particle moisture content and ethanol fraction; however, for the particle moisture content of 0.0373 d.b. with 100% and 70% ethanol, and also for the particle moisture content of 1.58 d.b. with 100% ethanol, the effect of temperature on the dielectric constant was insignificant. The dielectric constant and loss factor of the packed bed were significantly decreased with ethanol volumetric fraction of solvent for all levels of temperature and particle moisture content. The dielectric constant and loss factor increased with moisture content for 40%, 70% and 100% ethanol; however, for 0% ethanol, the effect of moisture content was not significant. Power penetration depth decreased with temperature, and particle moisture content increased with ethanol fraction. Multiple regression equations for the dielectric constant and dielectric loss factor of the packed bed were developed for frequencies of 13.56 and 27.12 MHz. The dielectric properties of the packed beds with solvent in this study assure the possibility of applying RF-assisted extraction for extraction of phenolic compounds from DDG
