Biomedical Engineering Reference
In-Depth Information
the choice of fermentation often depends on the strain characteristics, material
characteristics, product characteristics, equipment status, technical feasibility, and
cost accounting. Currently, production of SCP feed using lignocellulose as raw
material employs solid-state fermentation of mixed bacteria according to the
characteristics of the materials, which means simultaneous saccharification and
fermentation in the same container by the fermentation of yeast and microorganisms
that can produce cellulase. Strains are generally divided into two categories: one is
various genera of mold, such as
Aspergillus
spp. and
Trichoderma
spp. Individual
bacteria and actinomycetes, which can decompose the lignocellulosic materials,
also show potential value for application. The other category is various genera
of yeast, which mainly feeding yeast,
Candida utilis
,
Candida tropicalis,
and
Candida parapsilosis
. In the process, the sugars from the enzymatic hydrolysis
of cellulose can be used for fermentation immediately without accumulation and
eliminate feedback inhibition to the saccharification response and finally improve
saccharification and fermentation efficiency. Increasing attention has been paid to
the process because of its advantages of simple operation, low investment, and
energy consumption [
2
,
12
].
5.1.1.4
Hemicellulose and SCP
Hemicelluloses are main chemical components of raw materials from plant fiber.
They can be not only homogeneous glycan and heterologous glycan but also
various glycan consisting of different monosaccharides with different linkages
among the monosaccharides [
13
]. According to the different proportions and
different substituents, hemicelluloses are composed of the different sugar units,
which include
D
-xylose,
L
-arabinose, glucose, galactose,
D
-mannose, glucuronic
acid, 4-O-methyl-glucuronic acid, galacturonic acid, traces of rhamnose, fucose, and
O-methylation neutral sugars. These structural units prefer to form heterogeneous
glycans by 2-4 linkage of various structural units rather than forming homogeneous
glycans by identical structural units when they constitute the hemicelluloses [
14
].
The content of hemicelluloses is a fourth to a third of the total amount of plant
resources; for example, the content of hemicelluloses is about 28 % in cornstalks,
34.9 % in barley grass, and 35.8 % in rice straw [
13
]. Therefore, hemicellulose is a
rich and inexhaustible source from renewable plant resources.
Hemicellulose is an important component of cellulosic materials. Steam explo-
sion technology, which not only can improve enzymatic hydrolysis of cellulose but
also recover the hemicelluloses, can be used to extract the water-soluble hydrolysis
materials of hemicelluloses from the steam-exploded materials and then produce
SCP, alcohol, and other products that are urgent for human beings [
15
]. Making
full use of the hemicellulose is one of the keys to fully utilize biomass [
16
].
Currently, fermentation production of xylitol [
17
], 2,3-butanediol [
18
], SCP [
15
],
and ethanol [
19
] uses pure xylose as a substrate at home and abroad. Submerged
liquid fermentation for SCP production emits large quantities of organic wastewater
