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).