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R. Halim
12.2.8 Chemical Hydrolysis (Alkali and Acid)
Acid and alkali treatments have been shown to be effective means of hydrolysing
microalgal cell wall. Unlike mechanical treatments which generally use a form of
shear force in order to rupture the cell walls, chemical treatments work by attacking
certain linkages in the wall, hydrolysing these bonds and ultimately dissolving the
entire wall or the wall layer away (Dong et al. 2016; Laurens et al. 2015, 2017). Chemical treatments generally have low energy requirements and can often suppress emulsion formation as they hydrolyse long chains of surfactant molecules (e.g. polysaccharides, protein and phospholipids) into constituent units (Dong et al. 2016; Laurens
et al. 2015, 2017). The main drawback of chemical treatments will be their relatively
slow kinetics in comparison to mechanical treatments. Laurens et al. (2015) showed
that dilute acid pretreatment can assist lipid extraction from microalgal biomass by
degrading cell wall and enabling the release of oil droplets (lipid extraction from
acid-treated biomass can recover up to 97% of fatty acids). Dilute acid pretreatment
(1-10 N HCl at 70 °C) has also been applied to astaxanthin recovery from H. pluvialis biomass and was shown to be able to improve both the astaxanthin yield and the
antioxidant activity of the lipid extract. (Dong et al. 2014; Sarada et al. 2006).
12.3 Pigment Extraction
After cell rupture, released intracellular pigments are recovered from the debris
using an extraction solvent. For water-soluble pigments (such as phybiliproteins), the
extraction solvent can be water, the culture medium itself or an aqueous buffer. For
water-insoluble pigments (such chlorophylls and carotenoids), one of the following
extraction systems is used: organic solvent extraction, pressurised solvent extraction,
ionic liquid extraction or supercritical carbon dioxide extraction.
12.3.1 Organic Solvent Extraction
Pigment extraction is based on the principle of ‘like dissolves like’. Chlorophylls and
carotenoids have high partition coefficients in organic solvents and will migrate out of
the biomass (cell debris or intact cells) into the solvent during the extraction process
(Halim et al. 2012a; Rammuni et al. 2019). Various organic solvents have previously been used for the extraction of chlorophylls and carotenoids from microalgal
biomass (Table 12.1): acetone, chloroform/methanol, DMSO, dodecane, ethanol,
ethyl acetate, hexane, methanol, methylene chloride, vegetable oil and a mixture of
one or more of the above solvents. After the extraction step, a solid/liquid separation
step is used in order to separate solvent from the cell debris. The solvent phase is
then subjected to a distillation step in order to obtain crude lipid extract containing
R. Halim
12.2.8 Chemical Hydrolysis (Alkali and Acid)
Acid and alkali treatments have been shown to be effective means of hydrolysing
microalgal cell wall. Unlike mechanical treatments which generally use a form of
shear force in order to rupture the cell walls, chemical treatments work by attacking
certain linkages in the wall, hydrolysing these bonds and ultimately dissolving the
entire wall or the wall layer away (Dong et al. 2016; Laurens et al. 2015, 2017). Chemical treatments generally have low energy requirements and can often suppress emulsion formation as they hydrolyse long chains of surfactant molecules (e.g. polysaccharides, protein and phospholipids) into constituent units (Dong et al. 2016; Laurens
et al. 2015, 2017). The main drawback of chemical treatments will be their relatively
slow kinetics in comparison to mechanical treatments. Laurens et al. (2015) showed
that dilute acid pretreatment can assist lipid extraction from microalgal biomass by
degrading cell wall and enabling the release of oil droplets (lipid extraction from
acid-treated biomass can recover up to 97% of fatty acids). Dilute acid pretreatment
(1-10 N HCl at 70 °C) has also been applied to astaxanthin recovery from H. pluvialis biomass and was shown to be able to improve both the astaxanthin yield and the
antioxidant activity of the lipid extract. (Dong et al. 2014; Sarada et al. 2006).
12.3 Pigment Extraction
After cell rupture, released intracellular pigments are recovered from the debris
using an extraction solvent. For water-soluble pigments (such as phybiliproteins), the
extraction solvent can be water, the culture medium itself or an aqueous buffer. For
water-insoluble pigments (such chlorophylls and carotenoids), one of the following
extraction systems is used: organic solvent extraction, pressurised solvent extraction,
ionic liquid extraction or supercritical carbon dioxide extraction.
12.3.1 Organic Solvent Extraction
Pigment extraction is based on the principle of ‘like dissolves like’. Chlorophylls and
carotenoids have high partition coefficients in organic solvents and will migrate out of
the biomass (cell debris or intact cells) into the solvent during the extraction process
(Halim et al. 2012a; Rammuni et al. 2019). Various organic solvents have previously been used for the extraction of chlorophylls and carotenoids from microalgal
biomass (Table 12.1): acetone, chloroform/methanol, DMSO, dodecane, ethanol,
ethyl acetate, hexane, methanol, methylene chloride, vegetable oil and a mixture of
one or more of the above solvents. After the extraction step, a solid/liquid separation
step is used in order to separate solvent from the cell debris. The solvent phase is
then subjected to a distillation step in order to obtain crude lipid extract containing