Abstract The effects of pellet morphology, diameter, density, and interior structure on Llactic
acid fermentation by Rhizopus oryzae were characterized for different inoculum
sizes and concentrations of peptone and CaCO3. Inoculum size was the most important
factor determining pellet formation and diameter. The diameter decreased with increasing
inoculum size, and larger pellets were observed for lower inoculum sizes. Peptone
concentration had the greatest effect on pellet density, which increased with increasing
peptone concentration. L-lactic acid production depended heavily on pellet density but not
on pellet diameter. Low-density pellets formed easily under conditions of low peptone
concentration and often had a relatively hollow structure, with a thin condensed layer
surrounding the pellet and an extraordinarily loose biomass or hollow center. As expected,
this structure greatly decreased production. The production of L-lactic acid increased until
the density reached a certain level (50–60 kg/m3), in which the compact part was
distributed homogeneously in the thick outer layer of the pellet and loose in the central
layer. Homogeneously structured, denser pellets had limited mass transfer, causing a lower
overall turnover rate. However, the interior structure remained nearly constant throughout
all fermentation phases for pellets with the same density. CaCO3 concentration only had a
slight influence on pellet diameter and density, probably because it increases spore
germination and filamentous hypha extension. This work also provides a new analysis
method to quantify the interior structure of pellets, thus giving insight into pellet structure
and its relationship with productivity.
