Application of hybrid drying of super critical CO2 and airborne ultrasonic in drying kinetic improvement of dunaliella salina

Dunaliella salina unicellular alga belonging to the Chlorophyceae family has been introduced as a extensive source of carotenoids (Zenouzi et al., 2013; Mendiola et al., 2008). The Carotenoids are strong antioxidants and bright pigments, and for this reason are used as human food and aquatics diets pigments (Denery et al., 2004). Investigations have shown that increasing the drying temperature will decrease the volatile and phenolic compounds of algae, and by decreasing the drying temperature, the amount of these compounds in the algae will be increased (Ling et al., 2015). According to the other research, the amount of beta-carotene after drying has been greatly decreased in conventional drying methods. (Ihns et al., 2011; Karabulut et al., 2007). The supercritical drying is an extraction process, that the supercritical fluid is the extraction solvent and water is the soluble substance (Brown 2010). The advantage of this drying method is that it is possible to prevent vapor-liquid contact in the homogeneous phase. For this reason, the tensile stress caused by the capillary that occurs during the air drying process does not exist in supercritical drying, and protects the structure of the material (Namatsu et al., 1999). In addition, carbon dioxide easily reached to critical temperature (31.1°C), so operating at low temperature (significantly lower than common dryers) prevents thermal damage (Brown 2010). Also Ultrasonic power at low frequency (20 to 100 kHz) is used for bacterial inactivation, improving process speed, heat and mass transfer, and water removal. In addition, the phenomenon of cavitation occurs in this composition and it causes an increase in the micro-mixing and communication between the solvent and microorganisms, and as a result, water exit and bacterial inactivation are improved (Morbiato et al., 2019). Non-contact ultrasonic, such as airborne processes, in which there is no contact surface or liquid to connect the transducer and the desired surface, can be used in various applications, including food drying. This system in combination with the other methods can overcome the limitations. For example, high temperature drying can damage the components of the material (color, texture, volatile compounds, etc.), while the combination with airborne can decrease the temperature and thus reduce the drying time, and the quality of the product and its nutritional ingredients should be maintained. The present work focused on investigation of the kinetics and drying rate of the Dunaliella salina algae by the supercritical carbon dioxide method in combination with airborne ultrasonic power, as well as the modeling of drying using mathematical and response surface methods.

The studied algae in this research was Dunaliella salina, which was obtained from the Artemia & Aquaculture Research Institute of Urmia University. In this research, 2.8 grams of algae paste with a thickness of 3 mm was placed in the drying vessel and methanol was added directly to the sample as a co-solvent. And the Suprex (MPS/225) in combination by airborne ultrasonic was used. The vessel that used in devise was made by stainless steel 316. Face center design consisting of 20 experimental runs with six replications at the central point was utilized to determine the effects of three independent variables in three levels: -1, 0, +1. The independent variables were Pressure at three levels (80, 110, 140 bar), temperature at three levels (40, 50, 60 °C) and airborne ultrasonic power at three levels (0, 20, 40 w) and the samples were weighed in dynamic mode in 30 minute intervals. The software of Design-Expert 13.0.5 (Stat-Ease, Minneapolis, MN, USA) was used for design experiment, data analysis and surface responsible diagram.

In 80 bar of pressure, the reduction of moisture content was constant for 150 min in temperature of 40 °C and without ultrasonic power and then was reduced. In other treatment the reduction of moisture content was continued for 120 min. In this pressure, after 180 min the moisture content in 40°C reached to 52% and in 60 °C and 40 w ultrasonic power reached to 31%. In 110 bar of pressure, the lowest moisture reduction was reached to 44% at 40 °C and 20 w. and the highest moisture reduction was 33% at 60°C and 20 w. The moisture reduction was constant for 120 min in temperature of 40°C and 20 w, while in temperature of 60°C and 20 w, the moisture content was constant for 90 min. in pressure of 140 bar, the maximum of moisture content was in 40 °C and without ultrasonic power and the minimum moisture content was 22% in 60 °C and 40 w. These results are due to the low viscosity, high diffusion rate and the near-absence of surface tension of the supercritical fluid. These properties allow the supercritical fluid to easily penetrate into the micro pores and surface and internal cells. Furthermore, new pores or channels could have been made by the supercritical fluid (Lee et al., 2011). The Midilli et al. model was the best ftting model. The highest values of R2, and the lowest values of SSE and RMSE obtained for the Midilli et al. model were 0.99925, 0.00023 and 0.0004, respectively. In RSM modeling, the results show that the changes of pressure, temperature and ultrasonic power had significant effect on moisture ratio in weighting time. While the interaction effect of independent variables had no significant effect on the factors. and the model has been investigated linearly. In the drying rate curve, at the beginning, drying was increased with a great slope, and then the curve decreases with a lower slope. The lowest drying rate at a pressure of 80 bar was occurred, in temperature of 40°C and the without ultrasonic power. And the highest drying rate was observed, in 60 °C and 40 w. the lowest drying rate in pressure of 110 bar was occurred in 40°C and 20w.

According to the results, the supercritical carbon dioxide method in combination with airborne ultrasonic power was suitable for dunaliella salina drying at low pressure and temperature. But in order to reduce the drying time and final moisture content, the pressure, temperature and ultrasonic power can be increased.