Intensification of Low-Temperature Drying by Using Ultrasound

Abstract

28Low temperature air drying involves temperatures below or close to the freezing point 29 and aims to reduce the water content or to remove organic solvents keeping quality 30 attributes, thus it has a great potential in food, chemical, pharmaceutical and cosmetic 31 industries. Depending on the temperature and solvent involved, the removal is by 32 evaporation or sublimation, but in all cases, the drying process is slow due to the low 33 temperature used. An efficient ultrasound application at atmospheric pressure and 34 moderate temperatures could accelerate the drying process. Thus, the main aim of this 35 work was to test the feasibility of power ultrasound to intensify low temperature drying 36 processes. 37Drying kinetics of carrot, eggplant and apple cubes (side 10 mm) were carried out at 38 atmospheric pressure, 2 m/s, -14 ºC and relative humidity 7% with (acoustic power 39 19.5 kW/m 3 ) and without ultrasound application. At the same experimental conditions, 40 kinetics of ethanol removal from a solid matrix were also performed. Diffusion models 41 were used to describe drying curves and identify kinetic parameters in order to 42 evaluate and quantify the process intensification attained by ultrasound application. 43The effect of ultrasound application was similar for all products tested, being drying 44 time shortened between 65-70 %. In the case of ethanol removal, the time reduction by 45 ultrasound application was even higher achieving 120 %. Both, effective moisture 46 diffusivity and mass transfer coefficient increased from 96 to 170 % and from 407 to 47 428 % when ultrasound was applied, respectively. Therefore, ultrasound application 48 should be considered a potential and effective technology to intensify low temperature 49 drying processes, being capable to make more affordable and less energy and time-50 consuming these processes for all kind of industries.

Impact of applied ultrasonic power on the low temperature drying of apple

“…Drying experiments were carried out in a convective drier with air recirculation ( Figure 1), already described in the literature [20]. The system provides automatic air velocity and temperature control and an drying chamber excited by an ultrasonic transducer.…”

“…During the AFD (-6ºC) of peas, Bantle and Eikevik [19] reported an increase of 14.6% in the diffusion coefficient linked to US application. Meanwhile, Garcia-Perez et al [20] achieved an increase in effective diffusivity of around 400% during the drying of apples, carrot and eggplants at -14ºC. The ultrasonic performance variable found in the literature for LTD applications could be linked to the efficiency of the transducer used, and so to the different ultrasonic power applied.…”

Atmospheric freeze drying assisted by power ultrasound

“…This fact could be explained considering that in the AFD experiments the internal resistance to mass transfer was much higher than the external one, which was nearly negligible. In the case of USAFD experiments, US diminished the internal resistance to mass transfer, which leads to a more significant external resistance [8]. This fact explains also the low %VAR provided by the diffusion model in the USAFD experiments.…”

“…Atmospheric freeze drying experiments were conducted in a convective drier with air recirculation, temperature and air velocity control and an ultrasonically activated drying chamber, already described in literature [8]. Kinetics were carried out at constant air velocity (2 ms-1), temperature (-10ºC) and relative humidity of the air (10%) with (20.5 kWm-3, USAFD) and without ultrasonic application (AFD).…”

Description of an ultrasonic technology for food dehydration process intensification

“…Previous research like (Rodríguez et al, 2010;Riera et al, 2015) shows that these HPU technologies provide good performance in processes like defoaming, particle agglomeration or ultrasonic cleaning, among others. In the particular case of food dehydration at low temperature, it has been proved by (Garcia-Perez et al, 2012) that HPU provides a faster and more economic performance, that also improves the quality of the final product.…”