Introduction and Literature Review
1.3 Intensification strategies for enhanced lipid (biodiesel) production
1.3.2 Application of sonication during fermentation
Further, Salama et al. (2013) reported that the maximum growth was achieved with a concentration of 25 mM NaCl for C. Mexicana and S. obliquus exhibiting maximum lipid content of 37% and 34%, respectively. They also studied the fatty acid composition reporting the oleic acids (41%) and linoleic acids (41%) to be the major fractions. Though the reports on the effect of salt concentrations on the composition of fatty acids in microalgal lipids are limited and variable, it has also been shown that high salt concentrations in microalgae such as C. Mexicana and S. obliquus can improve the fatty acid composition. Thus, different salt (NaCl) levels can be used to alter the composition of fatty acids, depending on the type of lipid in microalgae. Pandit et al. (2017) reported that high lipid content of 49% and 43% was obtained with the growth of C. vulgaris and A.
obliquus, respectively, in a culture medium containing different salt concentration ranging from 0.06 to 0.4 M NaCl. Moreover, considerable lipid accumulation of 33.40 ± 2.29% was observed in Acutodesmus dimorphus with 200 mM NaCl; that increased to 43% on extending the salinity stress to 3 days (Chokshi et al., 2017).
The lipid accumulation in microalgae also gets affected by the type of salt used to give salinity stress. Srivastava and Goud (2017) carried out the cultivation of Chlorella sorokiniana and Desmodesmus with different types of salts such as NaCl, KCl, CaCl2, etc.
They reported that the maximum production of lipids was obtained with CaCl2. It was assumed that calcium played a vital role in cell signaling under conditions of salt stress that may have increased the production of lipid compounds.
fermentation can also be enhanced with application of ultrasound (Jeon et al., 2013).
Sonication induces modifications in the physicochemical properties of the algal cell that facilitates an improved cellular transport with better accessibility to the carbohydrate substrates in the fermentation media, thus, enhancing their utilization and conversion during fermentation. Sonication at low intensity has been reported to promote cell growth or cell density in the medium and improve the porosity of the cell membrane that results in higher protein production (Chaunyan et al., 2004). For example, the cell membrane porosity was observed to increase in Pseudomonas aeruginosa by sonication that resulted in an increased uptake of 16–doxylstearic acid through the membrane (Rapoport et al., 1997).
Also, diffusion rates can be enhanced due to the increased cell permeability ultimately boosting the overall growth rate and cell productivity (Pitt and Ross, 2003). Literature review reveals very little activity in the area of sonication-enhanced lipid production by microalgae.
Most of the previous literature reports application of sonication in downstream processing, i.e. biomass pretreatment and extraction of lipids from microalgae. Han et al.
(2016) have demonstrated that application of sonication during the end of log phase was most effective for enhancing fermentative lipid production. Sonication under optimum conditions could enhance extent as well as rate of lipid accumulation by 57.5%.
1.3.2.1 Basic principles of ultrasound and cavitation
Ultrasound: Ultrasound are basically the longitudinal acoustic waves that are beyond the upper limit of human hearing range which is above 20 kHz. The frequency of ultrasound waves ranges from 20 kHz – 20 MHz. Since ultrasound is a longitudinal wave, it can pass in the form of alternate compression and rarefaction cycles through a compressible medium like air or water. A periodic variation is generated in bulk pressure and density of the
medium due to the propagation of ultrasound waves in the medium. Such a propagation causes an oscillatory motion of fluid elements in the medium (Shah et al. 1999). The ultrasound wave is characterized by physical properties of frequency, velocity and pressure amplitude. The properties of the sound wave in gaseous medium are strongly influenced by the static pressure in the medium. Since the liquid properties are comparatively insensitive to moderate variations of static pressure, the ultrasound waves in liquid medium are practically uninfluenced by the static pressure.
Cavitation: Cavitation refers to the growth, nucleation, oscillation or collapse of gas bubbles due to variation in the bulk pressure of the medium. Such a pressure may arise due to propagation of an acoustic wave, variation in the flow geometry, or energy dissipation in the system. The efficiency of any process whether it is biological, physical or chemical, is usually dependent on the method through which energy is introduced into the system.
Cavitation is one of the methods that is efficient in introducing the energy into the system for intensification of large number of biological processes. Ultrasound uniquely provides energy available on very small time and spatial scales that are generally unavailable from any other kind of source.
Physical effects of cavitation and sonication on system: Both cavitation and ultrasound exhibit different physical effects on a particular reaction system. However, the primary purpose of the final outcome is the induction of intense micro–mixing and micro–
convection in the reaction system. The physical effects generated due to cavitation and ultrasound are briefly described below (Young, 1989; Shah et al., 1999):
Micro–streaming: It refers to an oscillatory motion of fluid elements having small amplitude around a mean position that is generated due to propagation of ultrasound wave.
The micro–streaming velocity is approximately 0.08 m/s for an ultrasound wave having a pressure amplitude of 120 kPa in water (C = 1500 m/s; ρ = 1000 kg/m3).
Acoustic streaming: The wave momentum is absorbed by the medium during propagation of ultrasound wave due to finite viscosity. Such phenomenon leads to generation of unidirectional currents of fluid having low velocity and is referred to as acoustic streaming (Nyborg, 1958).
Micro–turbulence: The oscillatory motion of fluid generated due to oscillations of the bubble is known as micro–turbulence. This process usually comprises of two phases.
During the expansion phase of the radial motion of cavitation bubble, the liquid is pushed away from the bubble interface. While in the collapse phase the liquid is attracted towards the bubble as filling the generated vacuum in the liquid due to the size reduction of bubble.
The mean velocity of micro–turbulence is dependent on amplitude of the bubble oscillation.
Acoustic (or shock) waves: In the compression phase of radial motion, a contraction of the cavitation bubble takes place, generating a void space in the liquid. The fluid elements are then spherically converged with high velocity in the void space created during compression, thus transferring kinetic energy to the bubble. Thus, work is done on the bubble. In a cavitation bubble that contains a non–condensable gas (air), the adiabatic compression leads to a rapid rise of the pressure within the bubble. At the minimum radius i.e. maximum compression, the bubble wall experience a sudden halt. At this point, the fluid elements reflect from the interface that were converged towards the bubble. Such a reflection generates a highly pressurized shock wave that travels through the entire medium. Finally, the pressure within the bubble due to non–condensable gas results in rebounce of the bubble.
Micro–jets: If the motion of liquid in proximity of the cavitation bubble is uniform and symmetric, a spherical geometry can be maintained by radial motion due to ultrasound
waves such that pressure gradient is not there. If the location of the bubble is in close proximity to a phase boundary– solid–liquid, liquid–liquid or gas–liquid, the motion of liquid is altered in its vicinity, thereby leading to emergence of pressure gradient. Such a non–uniform pressure results in the loss of bubble’s spherical geometry. In the asymmetric radial motion, the bubble portion which is exposed to a higher pressure ruptures faster in comparison to other bubbles, thereby resulting in the formation of a high speed liquid jet directed towards the boundary. The velocity of such micro–jets were predicted to be in the range of 120 to 150 ms–1 which can cause acute damage such as cell disruption, particle size reduction, polymer degradation or particle size reduction.