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Advances in bio-oil extraction from nonedible oil seeds and algal biomass 189

conversion. Pyrolysis is one of the most efficient ways to produce bio-oil in which the
organic compounds are thermally decomposed at a moderate temperature in order to
obtain the liquid product known as bio-oil. Bio-oil is a dark brown viscous liquid with
complex chemical composition to form an emulsion. This review consolidates the
advances and challenges in extraction techniques from nonedible and algal sources.

7.2 Overview on downstream processing techniques

Bio-oil production from microalgal biomass is a consecutive process that consists of
the cultivation, harvest, oil extraction, and conversion of algal lipids into advanced
biofuels [19]. Apart from cultivation of algal biomass, the downstream processing
techniques contribute 60% of the total production cost of biofuels. The cost of micro-
algal biomass harvest is as high as 20% of the total production cost and it varies
depending on the harvest technology. Therefore, it is essential to reduce the cost of
downstream processing by combining certain downstream techniques [20].
Extraction is the first step in the synthesis of oil from the nonedible sources
(seeds, fruits, or other oil-bearing raw materials) using a variety of different
methods. In some cases, the oil is extracted directly from the fruit by cold pressing
with a simple mechanical press; this is done without any further processing. This
process is more complex for most of the oil seeds. The extract oil from seeds is
cleaned and dried to remove foreign material. Seeds with a high oil content are
usually mechanically pressed in expellers after preheating, followed by solvent
extraction to separate oil from seeds. The mixture of oil and solvent is separated
by distillation and the solvent is recycled into the extraction process (http://www.
neoda.org.uk/extracting-oil-from-seed).
There are different laboratory-scale processing techniques that play an eloquent
role during the production of biofuels, especially on using algae as source. The
scale-up potential differs for each downstream processing step. The pretreatment of
algal biomass after harvesting is an essential step prior to the extraction of lipid.
The extraction of lipid from cellular debris is carried out using an extraction solvent
that is selected based upon the solubility and nature of the strain [21–25].
The downstream processing techniques have gained wide attention due to the com-
plex structural organization and rigidity of the algal cell wall. The efficiency of lipid
extraction is mainly increased by a post dewatering method in which the concentrate is
pretreated, either in single or multiple steps. The concentrate is exposed to cell dis-
ruption, which enhances the release of intracellular lipids to the surrounding medium.
The other techniques adopted for pretreatment are drying and milling of algal biomass,
which eliminates the residual water and enhances the mass transfer rate. The disrupted
concentrate generally uses organic solvent or supercritical fluid as the solvent. The
resulting mixture after extraction contains residual water and solvent, which are fur-
ther subjected to solid-liquid separation for the removal of cell debris. The supercrit-
ical fluid extraction (SFE) does not require any extra step for the removal of extraction
solvent as it removes the solvent and residual water in single step with forced precip-
itation lipids. The isolated crude lipids known as crude lipids or total lipids contain