Agriculture Reference
In-Depth Information
1.
I
NTRODUCTION
:
C
HEMICAL
C
OMPOSITION
OF
S
UGAR
B
EET
S
HREDS
Sugar beet shreds are by-product of the sugar beet industry representing the fibrous
portion of the roots of sugar beet left after the sucrose is removed. Sugar beet roots contain
about 15% sucrose and about 5% cell wall polysaccharides on a wet weight basis. In order to
reduce water content sugar beet shreds can be mechanically pressed and dried. As a result
wet, pressed and dried beet shreds differ only in dry matter (DM) content which is 6 - 12%,
18 - 30% and 87 - 92%, respectively [1].
Composition of sugar beet shreds given in Table 1 shows relatively low content of fat and
high percentages of both crude fiber and nitrogen free extractives which indicate high
carbohydrate content.
Table 1. Chemical composition of sugar beet shreds (based on DM)
Čobić et al. [2]
Kelly [3]
Dinand et al. [4]
9.9 - 10.7
Crude protein (%)
10.1 - 10.3
7
Crude fat (%)
0.7 - 1.0
nd
*
2
Crude fiber (%)
20.7 - 24.2
19.6 - 20.4
24
Nitrogen free extractives (%)
58.1 - 61.8
62.7 - 63.1
59
Ash (%)
4.4 - 6.9
6.0 - 6.5
8
*nd - not determined.
It is known that plant cell walls are composed predominantly of the structural
polysaccharides - cellulose, hemicelluloses and pectins. Typical primary cell walls of
dicotyledonous plants to which sugar beet belongs are composed of almost equal amounts of
three types of polysaccharides: a) pectin, rich in galacturonic acid and containing the main
neutral sugars galactose, arabinose, and rhamnose, b) hemicelluloses, typically xyloglucans
with minor amounts of (gluco)mannans, and c) cellulose. However, sugar beets which are
mainly composed of parenchymal tissue with elactic hydrophilic cell walls differ in
composition from cell walls of typical dicotyledonous plants - they are characterized by very
high pectin content which is methylated and acetylated. In addition, sugar beet cell walls have
very low amounts of xylose, mannose, non-cellulosic glucose, and fucose - the sugars usually
indicating hemicelluloses. Extraction of sugar beet cell walls with KOH produced
hemicellulosic extract which was predominantly xylan [5]. However, methods for
polysaccharides extraction usually are not selective enough; also the same sugars might exist
in different polysaccharides which might lead to confusing results. It have to be pointed out
that hemicellulose content in sugar beet cell walls is mainly calculated on the base of the
presence of extremely high content of arabinans which actually originate from side chains of
the sugar beet pectins and are a small part of hemicellulose [6]. Sugar beet cell walls also
contain phenolic acids (<10 mg/g), mainly ferulic acid. Although quantitatively minor, these
phenolic acids are thought to be of major importance in the structure of the beet cell wall.
Although there are minor differences in global carbohydrate composition between cell
wall material isolated from raw beets and sugar beet shreds [6] processing of sugar beet might
induce changes in structure and extractability of its polysaccharides [5]. Results of several
