3.1. Model of sugar transfer in the solid-state fermentation of sweet
sorghum
In contrast to liquid fermentation, there is no continuous liquid
phase in SSF. SSF involves the growth of microorganisms on moist
substrates in the absence of free water, which makes the system
quite different from submerged fermentation (SmF) [13]. In particles
with water moisture above 70%, sugar transfer from the interior
of sweet sorghum plant cell to the surface is driven by sugar
concentration gradient. The schematic diagram of sugar transfer
in a sweet sorghum particle was shown in Fig. 1. From the microscopic
view, the sugar transfer from the interior to the surface of
substrates through the plant cell membrane is driven by the concentration
gradient. The sugar transfer from each particle takes
places in three steps. The first one is the sugar migration from
the interior to the exterior of the plant cell. The second is moving
into the interface between the solid substrate, the plant cell wall,
and the liquid phase on the surface of the plant cell. This interface
is defined as the liquid membrane. The third is dissolving into the
bulk liquid phase on the surface. However, it is so thin liquid film.
Although the system seems to be complex, the kinetic parameter is
described by a simple diffusion model because the driven force of
the entire system is sugar concentration gradient.
The yeast, Saccharomyces cerevisiae, grows in the thin liquid film
at the particle surface and reproduces asexually by budding. It does
not extend into the gaseous region or inside of substrate like other
fungi [14]. Fermentable sugars are extracted continuously from
sweet sorghum substrate and consumed simultaneously by yeast.
In SSF, sugar consumption by the propagation and metabolism
of yeast creates the concentration gradient, which drives the
continuously sugars diffusion to the surface of the particles. In this