Sugar Beet, Energy Beet, and Industrial Beet - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 91

content would create less processing load on the conversion of complex cell wall
carbohydrates into readily fermentable energy resources.
The demonstrated ability of beets to accumulate high-value compounds such as
sucrose, betaine, and betalain suggests that other chemicals could be accumulated
in a similar industrial fashion. Which chemicals could be manufactured profitably
depends on economics; however, it is likely that current and additional coproducts
from beet sugar manufacture could be developed into specific industries on their
own, and thus breeding would need to address the needs of such industrial beets. A
US Dept. of Energy study [ 84 ] listed a dozen high-potential value-added chemicals
from biomass, for which beet coproducts might yield economic benefit in the
current bio-economy, namely, glutamic acid and sugar alcohols of xylose and
arabinose. Glutamic acid comprises roughly 8 % of amino-N compounds (exclud-
ing betaine) and glutamine 31 % in sugar beet roots, which is synthesized from
glutamate and ammonia enzymatically via glutamine synthetase, whose activity is
correlated with glutamine levels in high- and low-amino-N varieties of sugar beet
[ 85 ]. Inhibition of this enzyme in beet roots may result in larger pools of glutamic
acid, and it is suggested that total amino-N and specific ratios of amino acids in the
beet root are at least partially heritable but also markedly affected by environment,
including N fertilization [ 49 , 86 ]. Glutamic acid comprises 50 % of sugar beet

vinasse (liquid remaining after ethanol fermentation) [ 87 ], and it appears that some
glutamate-derived products can be made economically on par with their petro-
chemical equivalents [ 61 ].
Sugar beet cell walls are unusual when compared to nearly all other crops. Pulp
that remains after sucrose is extracted is mostly plant cell wall material. Sugar beet
has a highly atypical cell wall in that it has very low levels of xyloglucan and high
levels of pectin [ 88 , 89 ]. Sugar beet pectin is rich in neutral sugar side chains
(arabinan) and highly acetylated pectic homogalacturonan [ 90 , 91 ]. The neutral
sugars appear to directly link together pectin and cellulose [ 92 , 93 ]. These unusual
properties directly influence the properties of sugar beet pectin as a food
additive [ 94 ].
Sugar beet and other members of the Caryophyllales are unique in that ferulic
acid is esterified to pectic arabinosyl and galactosyl residues of the pectic side
chains [ 95 - 97 ]. Ferulic acid cross-linking is thought to influence the properties of
the cell wall such as extensibility, control of growth, intercellular adhesion, micro-
bial digestion, protein binding, and lignification [ 98 - 100 ]. Phenolic cross-linking
also occurs in grasses, although it occurs on arabinoxylans, in addition to lignin-
mediated reinforcement of secondary cell walls [ 101 , 102 ]. In contrast, sugar beet
storage roots contain mostly primary cell walls, so there is negligible lignin
(~1.5 %) [ 4 , 103 ]. The dry matter content of pulp is usually only 18-23 %, making
it a “wet” feedstock which, in combination to the low lignin content, makes it
unsuitable for combustion to produce heat and power [ 104 ]. Therefore, the chal-
lenges to decomposition of the cell walls are different to those of lignocellulosic
biomass crops such as corn stover or sugarcane bagasse. Deconstruction of the cell
wall involves the hydrolysis of polysaccharides in a process termed saccharifica-
tion. The products of saccharification can then be used for fermentation or as

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