3.1 Physicochemical properties of the medium
Table 1 summaries the initial values of pH (-), surface tension (\(\frac{\text{mN}}{m}\)), electrical conductivity (\(\frac{\mu\text{S}}{\text{cm}}\)), and specific gravity (-) for four media (Table 1).
The statistical analysis of physicochemical parameters of different media is given in Tables 2. There was a significant effect of time and media on EC, pH, and S.G individually or in combination (P=0.05). No significant difference was observed between surface tensions values at different time levels (P>0.05).
The effect of sonication time on different properties is depicted in figure 3. According to Fig. 3-A the electrical conductivity follows an increasing trend in all solutions of MS, ½ MS, B5 and water during the ultrasonic application. A reason for the increasing trend of electrical conductivity by ultrasonic treatment application can be attributed to the concentration of the solution, and increase in the ions in the media that are an effective factor in the electrical conductivity. A direct effect of ion concentration on electrical conductivity has been reported by others (LeLAS, 2007; Assiry, 2011; Boldaji et al., (2015). The electrical conductivity of tomato juice also increased during concentration (Boldaji et al., (2015). Ultrasonic pre-treatment of button mushrooms, brussels sprouts and cauliflower caused migration of mineral salts and organic acids coming from samples. Another possible reason is related to cavitation which can alter the chemical structure of the soluble solution (LeLAS, 2007). Assiry (2011) applied ohmic heating method to evaporate seawater. The ohmic heating depends on electrical conductivity. The results showed that the electrical conductivity of seawater increased with the evaporation of water and increasing concentration in the sludge (Assiry, 2011).
Specific density values were raised for all media during the ultrasonic application (Fig. 3-B). This is expectable because the density represents the amount of material in the unit volume, the results showed that the greater loss of water leads to a greater residue dissolution and more ions thus, a higher density has resulted.
PH content values in MS, ½ MS, followed a decreasing trend during the ultrasonic application, while pH content in the B5 and water had an incremental trend (Fig. 3-C). The reason for the reversible behavior of the MS and ½ MS relative to water and B5 can be attributed to the chemical components, the response of different materials to ultrasonic treatment is different. During performing ultrasonic treatment, a number of secondary radicals may be created in Solution due to the chemical reactions of primary radicals and other soluble molecules. The report on the production of primary and secondary radicals from cavitation is also mentioned in other sources (Ashokkumar & Mason, 2007). Another possible reason is that the culture media are suitable for microorganism activity which might contaminate the MS and ½ MS.
Surface tension ranges from 47.8 to 58.1 mN/m. The MS and ½ MS followed an incremental trend during the ultrasonic process, while B5 and water were decreased gradually with treated time. (Fig. 3-D). The surface tension of liquids depends on the intermolecular force and also the temperature. The temperature was nearly constant during the experiment (\(23\pm 1\)), thus the effect of ultrasonic treatment was the main reason on the structural change of medium and the reduction of intermolecular force.
3.2 Mist generation system performance: Effect of different levels of liquid height on the \(\text{Mist}_{\text{us}}\) rate
The misting system as the most important part of the mist bioreactor has a significant effect on the growth performance and the production of hairy roots culture. The most effective factor which influences misting rate is the viscosity, density, surface tension and height of medium (Barreras et al., 2002; Knorr et Al., 2004; Avvaru et al., 2006). The ANOVA results of experimental factors are presented in table 3. The results showed that the ultrasonic misting rate (MistUS ) was influenced by medium and heights, either individually or in combination (P = 0.01(.
The effect of liquid height on mist generation (total and ultrasonic), surface evaporation rate, the power consumption of various media are shown in Fig. 4. The results showed that the ultrasonic mist generation rate increased with increasing liquid height, and after that reached to a maximum value and then decreased, this trend is similar for all media. When the liquid height is low on the piezo surface. The mist characteristics change. Instead of forming a relatively homogeneous mist, the diameter of particles are not uniform. In this case, larger diameter droplets form on the liquid surface. (Barreras et al., 2002). The droplets return to the liquid surface after detachment again and a few change of mass reduction was recorded by mass balance. Therefore, the only mass loss of vessels is due to the small fraction of misting. This result had been reasoned by the capillary wave hypothesis by researchers (Lang, 1962). Lang (1962) demonstrates that exciting vibration caused a wave. The peaks of wave tears and the collision and agglomeration of the droplets shortly after leaving the surface caused this uniformity (Lang, 1962). Due to bubble detection in bulk of liquid, Barreras et al., (2002) suggested that the other possible mechanism of misting (cavitation) should be considered for describing of atomization process (Barreras et al., 2002). At a high level of liquid, it seems that the wave damp and the surface stack formation fail to result again low misting rate generation. The maximum misting generation rate (total and ultrasonic) was obtained at the heights of 4, 3, 3, and 3 cm for water, B5, MS, and ½ MS respectively. The highest value of\(\text{Mist}_{\text{US}}\) rate was belonged to MS (\(2.78\times 10^{-2}\ \text{gr}/s\)), on the other hand, the lowest value was obtained for B5 (\(0.52\times 10^{-2}\text{gr}/s\)) at a height of 1 centimeter above piezoelectric. The power consumption analysis was depicted for water and MS as media with the highest misting rate (Fig. 4-A & 4-B). The power consumption increased with the increasing height of liquid above the piezo. The trends are not the same for MS and water due to different chemical compounds. The power consumption for a specified amplitude is proportional to the viscosity of the fluid (Figura & Teixeira, 2007).