The production of biodiesel from food waste requires extraction
of lipids. Firstly, the food waste is mixed with water (typically
100 g of food waste in 1 l of water) to make slurry then it can be
mixed vigorously with non-polar organic solvents viz. n-hexane
and diethyl ether. This step is not compulsory but can be done.
Afterwards, the obtained mixture is then transferred into a
separating funnel. The organic layer is separated and evaporated
under reduced pressure to obtain the organic solvent-free lipid.
Yang et al. extracted oil from noodle waste. Typically, 100 g of the
instant noodle residue was boiled with one litre water. Subsequently,
the oil was extracted using 500 ml of n-hexane. Using this
method 5 ml of oil was isolated from 100 g of noodle waste
[42,43]. Alternatively, fungal hydrolysis of food waste using bienzymatic
catalytic system containing Aspergillus awamori and
Aspergillus oryzae can be carried out to separate the lipid, and food
hydrolysate rich in carbohydrate, amino acids and phosphate
[38,39]. After hydrolysis the obtained hydrolytic mixture is subjected
to centrifugation to separate the crude lipid from the food
hydrolysate. The obtained crude lipid is then heated to 100 °C to
obtain the water free lipid [18,19]. Additionally, lipid can be
extracted by using soxhlet extraction which is more efficient than
the conventional extraction methods. Along this line, supercritical
fluids are extensively utilized as solvents for lipid extraction from
natural herbs and as reaction mediums to perform chemical and
enzymatic reactions [40,41].
Particularly, supercritical carbon dioxide (scCO2) is used for the
extraction oils. This method can be tried for the extraction of lipid
from food wastes. Lipid extracted using scCO2 is not contaminated
with organic solvents unlike the conventional extraction processes.
Moreover, scCO2 extraction process can be optimized by
adjusting pressure and temperature [44]. Thus, lipid obtained is
clean and can be directly used for biodiesel production. Different
lipid extraction processes are summarized in Fig. 4.
Lipid obtained from food waste is first tested to determine the
acid value and moisture content. Base catalyzed transesterification
using KOH and NaOH as catalysts is reported for the biodiesel
preparation for low free fatty acid (FFA) containing feedstocks [8].
Additionally, acid catalyzed transesterification can be carried out
for biodiesel preparation. For high FFA containing feedstock twostep
reaction is carried out. In the first step, acid catalyzed esterification
and in some cases base catalyzed pretreatment is required
to lower the acid value of lipid feedstocks [8]. After this base