Page 485 - Biosystems Engineering

Super critical Fluid Extraction Applications 457

important factor physiologically, because oxygen limitation may
result in the suppression or even total inhibition of metabolite pro-
duction of lipids in aerobic fungi. PUFAs are formed through elonga-
tion and desaturation. Desaturation involves an aerobic reaction by
oxygenation. Therefore, oxygen availability is important to synthe-
size ARA, an n-6 PUFA with four double bonds, in M. alpina. Cellular
growth of fungi was restrained due to substrate limitation in the
region of the dense pellet core when the pellet radius exceeded a crit-
ical value. The lipid was produced on the edge of the pellet, where
mycelial density was high (Hamanaka et al. 2001). Higashiyama et al.
(1999a) studied the effects of dissolved oxygen (DO) on the morphol-
ogy of M. alpina in a 50-L fermentor. When DO concentration was
maintained at 20 to 50 ppm using the oxygen-enrichment method,
the morphology changed from filaments to pellets, and ARA yield
decreased drastically because of stress due to the limited mass trans-
fer through the pellet wall. Studies of fungal morphology and metab-
olite production in submerged mycelial processes over the past
50 years were reviewed by Papagianni (2004). The content included
growth mechanisms of filamentous fungi, dynamics of mycelial
aggregation, influence of process parameters on the morphology,
productivity and rheology, and some related models.
Optimum morphology for production usually varies among fungi
and products. There is still no obvious or unequivocal conclusion to
tell which is better for ARA production by M. alpina, the filamentous
or the pellet form. ARA is an intracellular primary metabolite that is
accumulated highly in active and mature cells between 3 to 7 days.
Small and compacted pellets consist of oil-rich cells. Furthermore,
small pellets reduce nutrients and oxygen limitation and autolysis of
cells in the core part. Small pellets also have better rheological prop-
erties than filamentous mycelia or large pellets, which may reduce
the problems associated with filamentous growth. Also, small pellet
formation has a relatively suitable fluid property for transportation in
downstream processing, such as biomass centrifuge recovery (prevent-
ing compacted cake forming), spraying, drying, and extraction.
Effect of Oil Supplement to the Culture Medium
Various studies have indicated that vegetable oil addition was benefi-
cial to enhance ARA yield (Shinmen et al. 1989; Singh and Ward 1997;
Jang et al. 2005) in M. alpina culture. The addition of corn, soybean,
peanut, and canola oils (1 percent) to the medium stimulated biomass
and lipid production. Cultures supplemented with corn and canola
oil exhibited the maximum ARA yield and was 50 percent higher
than that without any supplementation (Singh and Ward 1997). Jang
et al. (2005) also investigated the effect of added oil. Supplementation
with 1.0 percent of linseed oil gave the highest biomass, followed
by sunflower oil and soybean oil. Linseed oil gave a high ARA