Cell growth is then shifted to
PHB synthesis by limiting nutrients others than the carbon
source, which is continually fed as a high-concentration
sugar syrup. After 45–50 h, the fed-batch fermentation
process is stopped, with a final dry cell mass of
125–150 kg/m3, containing nearly 65–70% PHB. The
fermented medium is thermally inactivated in a heat exchanger,
diluted with water, and flocculated. Separation
and concentration procedures yield a cell sludge containing
25–30% solids which is then submitted to a multistage
extraction process with medium-chain-length alcohols
in continuous-stirred tank reactors. The extract is
purified for cell debris removal and then cooled down to
recover a PHB gel. Solvent from the gel is removed by
mechanical and thermal concentration. The resulting
PHB paste is mixed with water and distilled to remove
the remaining solvent. PHB granules are then collected
by a sieve, vacuum dried, compounded and extruded as
pellets.