12 Industrial Extraction of Microalgal Pigments
275
transfers during subsequent extraction step and ultimately increases the extraction
efficiencies/yields of the products (Dong et al. 2016). An ideal biomass pretreatment
needs to be able to avoid/minimise emulsion formation and be energetically efficient and scalable. Biomass can be subjected to more than one pretreatment prior to
extraction, though the energy and operational costs of performing multiple pretreatment steps should be taken into consideration. Cell disruption can be classified as
either mechanical, chemical or enzymatic treatment. Mechanical treatments include
high-pressure homogenisation, bead milling, ultrasonication, pulsed-electric field
treatment, microwaving and osmotic shock treatment.
12.2.1 High-Pressure Homogenisation (HPH)
HPH is a mechanical process that pumps cell culture under high pressure to achieve
disruption. During HPH operation, microalgal cellular suspension is pumped radially across a narrow valve seat before colliding with an impact ring and subsequently released into a low-pressure chamber. Cell disruption is attributed to several
phenomena: the impingement of the cells on the valve seat and impact ring, the pressure drop that the cells experience as they pass from the valve to the chamber and
the cavitation caused by the rapid release of gas bubbles from within the cells under
sudden pressure drop (Dong et al. 2016; Halim et al. 2013; Martin 2016). HPH is
currently used in the biotechnological industry for the recovery of high-value protein.
HPH has frequently been shown to be an effective means to rupture microalgal cells
for lipid and pigment recoveries (Halim et al. 2012b, 2013, 2019; Jubeau et al. 2013;
Yap et al. 2015). In Halim et al. (2012), HPH was found to be more effective in
rupturing Chlorococcum cells than ultrasonication, bead milling and sulphuric acid
treatment. Martin (2016) recently demonstrated that HPH could be an energetically
scalable approach for microalgal cell disruption provided that a species with a weak
cell wall and high lipid content is used. Dong et al. (2016), however, noted that the
efficiency of HPH varies remarkably across different species and growth conditions
due to differences in cell wall rigidity. They also noted that the use of HPH could
lead to the formation of severe emulsion which hinders solvent recovery and reduces
extraction yield.
12.2.2 Bead Milling
Bead milling achieves cell disruption by violently grinding the biomass against solid
bead surfaces (Dong et al. 2016; Doucha and Lívanský 2008). The technology has
been routinely applied on microalgal biomass for both lipid and pigment extraction
(Doucha and Lívanský 2008; Halim et al. 2012b). The use of bead milling, however,
can often lead to a dramatic temperature rise which denatures pigments as well as
the formation of micelles which hinders solvent recovery and reduces extract purity.
275
transfers during subsequent extraction step and ultimately increases the extraction
efficiencies/yields of the products (Dong et al. 2016). An ideal biomass pretreatment
needs to be able to avoid/minimise emulsion formation and be energetically efficient and scalable. Biomass can be subjected to more than one pretreatment prior to
extraction, though the energy and operational costs of performing multiple pretreatment steps should be taken into consideration. Cell disruption can be classified as
either mechanical, chemical or enzymatic treatment. Mechanical treatments include
high-pressure homogenisation, bead milling, ultrasonication, pulsed-electric field
treatment, microwaving and osmotic shock treatment.
12.2.1 High-Pressure Homogenisation (HPH)
HPH is a mechanical process that pumps cell culture under high pressure to achieve
disruption. During HPH operation, microalgal cellular suspension is pumped radially across a narrow valve seat before colliding with an impact ring and subsequently released into a low-pressure chamber. Cell disruption is attributed to several
phenomena: the impingement of the cells on the valve seat and impact ring, the pressure drop that the cells experience as they pass from the valve to the chamber and
the cavitation caused by the rapid release of gas bubbles from within the cells under
sudden pressure drop (Dong et al. 2016; Halim et al. 2013; Martin 2016). HPH is
currently used in the biotechnological industry for the recovery of high-value protein.
HPH has frequently been shown to be an effective means to rupture microalgal cells
for lipid and pigment recoveries (Halim et al. 2012b, 2013, 2019; Jubeau et al. 2013;
Yap et al. 2015). In Halim et al. (2012), HPH was found to be more effective in
rupturing Chlorococcum cells than ultrasonication, bead milling and sulphuric acid
treatment. Martin (2016) recently demonstrated that HPH could be an energetically
scalable approach for microalgal cell disruption provided that a species with a weak
cell wall and high lipid content is used. Dong et al. (2016), however, noted that the
efficiency of HPH varies remarkably across different species and growth conditions
due to differences in cell wall rigidity. They also noted that the use of HPH could
lead to the formation of severe emulsion which hinders solvent recovery and reduces
extraction yield.
12.2.2 Bead Milling
Bead milling achieves cell disruption by violently grinding the biomass against solid
bead surfaces (Dong et al. 2016; Doucha and Lívanský 2008). The technology has
been routinely applied on microalgal biomass for both lipid and pigment extraction
(Doucha and Lívanský 2008; Halim et al. 2012b). The use of bead milling, however,
can often lead to a dramatic temperature rise which denatures pigments as well as
the formation of micelles which hinders solvent recovery and reduces extract purity.