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R. Halim
biofuel lipid extraction as it facilitates facile solvent recovery. For biofuel application,
since large-scale biomass dehydration is energetically prohibitive, the lipid recovery
process needs to be able to effectively extract lipids from wet microalgal biomass
(e.g. slurry or paste). Recent studies have asserted hexane extraction from previously
ruptured wet microalgal concentrate as a biofuel production pathway that is able to
achieve large-scale energy scalability (Dong et al. 2016; Martin 2016). Hexane is a
water-immiscible solvent that readily forms a biphase with wet microalgal concentrate and thus does not require energy-intensive distillation for its separation (and
recycling) from the post-extracted concentrate.
A biorefinery production strategy takes advantage of microalgal biomass compositional complexity and exploits both its nutritional and calorific fuel values by fractionating the biomass into an array of food and fuel products. The strategy reduces the
cost of producing a single product through a shared processing scheme, maximises
total possible revenue from the biomass and thus has a higher chance of achieving
commercial viability than a linear strategy that focuses on the sole production of
a single food or a single fuel product. In this context, microalgal pigment extraction will likely form part of a larger food-and-fuel biorefinery system that includes
lipid recovery steps for biofuel production as well as steps to co-generate other
product streams (such as the recovery of biomass sugar for ethanolic fermentation
and the isolation of processed cell debris for animal or aquaculture feed). The biomass
processing pathway deployed for pigment recovery will therefore have to comply
with the requirements of both food and fuel production, i.e. using solvent that is safe
for human consumption while at the same time using processing steps and solvents
that facilitate a positive net energy balance. Even though we are unable to provide
specific recommendations on the perfect solvent at this juncture in time, we postulate that the selected solvent will likely be one that is able to achieve a compromise
between the two requirements. Hexane, for example, is a volatile solvent that requires
minimal amount of energy for its evaporation and recovery (boiling point at 69 °C and
enthalpy of vapourisation at 365 kJ/kg), while at the same time can still be considered
safe for human consumption as long as it is present below its mandatory residual
limit in the final food product. The European Commission has imposed the maximum
residual limit for hexane in food products to be between 1 and 30 mg/kg depending
on the type of food product and the extraction process used (Council 2009); we note
that there is currently no safety guideline for solvent content in algal-derived food
products.
To avoid energetically prohibitive large-scale biomass dehydration, the lipid or
pigment recovery steps in a microalgal biorefinery system will have to be performed
on wet microalgal biomass (slurry or paste). As shown in Tables 12.2–12.6, the
majority of studies previously investigating microalgal pigment recovery, however,
have used freezed/spray- or thermal dried biomass as their raw materials. Future
studies must stop this practice and direct their efforts into developing scalable pigment
recovery pathways from wet microalgal concentrate instead.
Equally importantly, more research efforts should also be directed into the development of a scalable lipid/pigment purification system. The solvent used for the lipid
or pigment extraction step is unlikely to be selective only to the targeted pigment or
R. Halim
biofuel lipid extraction as it facilitates facile solvent recovery. For biofuel application,
since large-scale biomass dehydration is energetically prohibitive, the lipid recovery
process needs to be able to effectively extract lipids from wet microalgal biomass
(e.g. slurry or paste). Recent studies have asserted hexane extraction from previously
ruptured wet microalgal concentrate as a biofuel production pathway that is able to
achieve large-scale energy scalability (Dong et al. 2016; Martin 2016). Hexane is a
water-immiscible solvent that readily forms a biphase with wet microalgal concentrate and thus does not require energy-intensive distillation for its separation (and
recycling) from the post-extracted concentrate.
A biorefinery production strategy takes advantage of microalgal biomass compositional complexity and exploits both its nutritional and calorific fuel values by fractionating the biomass into an array of food and fuel products. The strategy reduces the
cost of producing a single product through a shared processing scheme, maximises
total possible revenue from the biomass and thus has a higher chance of achieving
commercial viability than a linear strategy that focuses on the sole production of
a single food or a single fuel product. In this context, microalgal pigment extraction will likely form part of a larger food-and-fuel biorefinery system that includes
lipid recovery steps for biofuel production as well as steps to co-generate other
product streams (such as the recovery of biomass sugar for ethanolic fermentation
and the isolation of processed cell debris for animal or aquaculture feed). The biomass
processing pathway deployed for pigment recovery will therefore have to comply
with the requirements of both food and fuel production, i.e. using solvent that is safe
for human consumption while at the same time using processing steps and solvents
that facilitate a positive net energy balance. Even though we are unable to provide
specific recommendations on the perfect solvent at this juncture in time, we postulate that the selected solvent will likely be one that is able to achieve a compromise
between the two requirements. Hexane, for example, is a volatile solvent that requires
minimal amount of energy for its evaporation and recovery (boiling point at 69 °C and
enthalpy of vapourisation at 365 kJ/kg), while at the same time can still be considered
safe for human consumption as long as it is present below its mandatory residual
limit in the final food product. The European Commission has imposed the maximum
residual limit for hexane in food products to be between 1 and 30 mg/kg depending
on the type of food product and the extraction process used (Council 2009); we note
that there is currently no safety guideline for solvent content in algal-derived food
products.
To avoid energetically prohibitive large-scale biomass dehydration, the lipid or
pigment recovery steps in a microalgal biorefinery system will have to be performed
on wet microalgal biomass (slurry or paste). As shown in Tables 12.2–12.6, the
majority of studies previously investigating microalgal pigment recovery, however,
have used freezed/spray- or thermal dried biomass as their raw materials. Future
studies must stop this practice and direct their efforts into developing scalable pigment
recovery pathways from wet microalgal concentrate instead.
Equally importantly, more research efforts should also be directed into the development of a scalable lipid/pigment purification system. The solvent used for the lipid
or pigment extraction step is unlikely to be selective only to the targeted pigment or