Recovery and Extraction of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 55

method has been limited to a small scale because of its low yields and high
energy requirement. 25

3.2.5 Other PHA Recovery Methods
Recently, new methods such as supercritical fluids, spontaneous liberation
of PHA, dissolved air flotation, and air classification, are being investigated
and probably promise to have much more success. Improvement of these
new extraction and purification methods should cause an optimal recovery
of PHA, with a high purity and recovery level at a low production cost.

d n 2 r 4 n g | 7

3.2.5.1 Supercritical Fluids
Supercritical fluids have unique physicochemical properties such as high
densities and low viscosities that make them suitable as extraction solvents.
For that purpose, CO 2 is most widely used because of its low toxicity and
reactivity, moderate critical temperature and pressure (31 1C and 73 atm),
availability, low cost and non-flammability. However, this method still
seems to be costly compared with other techniques. 31

3.2.5.2 Dissolved-air Flotation
This method continues with some of the enzyme pretreatments such as
lysozymes and novozymes. The function of this process is based on the fact
that cell debris and PHA polymers have an iso-electric point at a specific pH
thereby selective aggregation and flotation can occur. Particle properties,
such as particle size, hydrophobicity and surface charge, have a significant
effect on the performance of the flotation process. However, this system does
not use chemicals but it needs some subsequent flotation steps which affect
the economics of the overall process. 32

.

3.2.5.3 Air Classification
Air classification begins with an ultrasonic sonicator to treat the cells and
create a suspension of polymer granules. The suspension is freeze dried and
pulverized subsequently using a fluid energy mill. Then the grinded sample
is air classified to make a 38% fine fraction and a 62% coarse fraction. The
fine fraction is exposed to chloroform extraction followed by methanol
precipitation to form PHA particles. This method leads to high purity but low
recovery yield. 33

3.2.5.4 Spontaneous Liberation
Bacterial strains such as E. coli which are capable of spontaneously liber-
ating PHA from cells are favourably used in this system. Currently, spon-
taneous liberation is drawing much attention since no chemicals are