Sources for cellulose
Sources for cellulose
Cellulose is found in plants as microfibrils  (2-20 nm diameter
and 100 - 40 000 nm long). These form the structurally strong
framework in the cell walls. Cellulose (E460)
is mostly prepared from wood pulp. Cellulose is also produced in a highly hydrated form by some bacteria (for example, Acetobacter xylinum).
Cellulose is a linear polymer of β-(14)-D-glucopyranose
units in 4C1 conformation. The fully equatorial conformation of β-linked
glucopyranose residues stabilizes the chair structure, minimizing
its flexibility (for example, relative to the slightly more flexible α-linked glucopyranose residues in amylose).
Cellulose preparations may contain trace amounts (~0.3%) of arabinoxylans.a [Back to Top ]
Cellulose is an insoluble molecule consisting of between
2000 - 14000 residues with some preparations being somewhat
shorter. It forms crystals (cellulose Iα)
where intra-molecular (O3-HO5'
and O6H-O2') and intra-strand
(O6-HO3') hydrogen bonds holds
the network flat allowing the more hydrophobic ribbon faces
to stack. Weak C6-HO2' hydrogen bonds may also make some contribution to the crystal stability. Each residue is oriented 180° to the next with
the chain synthesized two residues at a time. Although individual
strand of cellulose are intrinsically no less hydrophilic,
or no more hydrophobic, than some other soluble polysaccharides
(such as amylose) this tendency to
form crystals utilizing extensive hydrophobic interactions  in addition to intra- and intermolecular
hydrogen bonding makes it completely insoluble in normal aqueous
solutions (although it is soluble in more exotic solvents
such as aqueous
N-methylmorpholine-N-oxide (NMNO, ~0.8 mol water/mol, then up to 30% by wt cellulose at 100 °C ), CdO/ethylene-diamine (cadoxen), LiCl/N,N'-dimethylacetamide
or near-supercritical water ). It is thought that water molecules
catalyze the formation of the natural cellulose crystals by
helping to align the chains through hydrogen-bonded bridging.
Part of a cellulose preparation is amorphous between these crystalline
sections. The overall structure is of aggregated particles with
extensive pores capable of holding relatively large amounts of water
The natural crystal is made up from metastable Cellulose I with all the cellulose strands parallel and no inter-sheet
hydrogen bonding. This cellulose I (that is, natural cellulose) contains two coexisting
phases cellulose Iα (triclinic) and cellulose Iβ (monoclinic) in varying proportions dependent on its origin; Iα being found more in algae and bacteria whilst Iβ is the major form in higher plants.
Cellulose Iα and cellulose Iβ have the same fibre repeat distance (1.043 nm for the
repeat dimer interior to the crystal, 1.029 nm on the
but differing displacements of the sheets relative to
one another. The neighboring sheets of cellulose Iα (consisting of identical chains with two alternating
glucose conformers -A-B-) are regularly displaced from each
other in the same direction whereas sheets of cellulose Iβ (consisting of two conformationally distinct alternating
sheets, (as shown right where the 2-OH and 6-OH groups
both change orientations so altering the hydrogen bonding
pattern) each made up of crystallographically identical
glucose conformers) are staggered .
It has been found that cellulose (Iβ)
significantly alters the water structuring at its surface
out to about 10 Å, which may affect its enzymatic
Cellulose Iα and cellulose Iβ are interconverted by bending during microfibril formation
and metastable cellulose Iα converts to cellulose Iβ on annealing.
Interactive structures are available (Jmol).
If it can be recrystallized (for example, from base or CS2)
cellulose I gives the thermodynamically more stable monoclinic Cellulose II structure with an antiparallel arrangement of the strands
and intermoleclar and both intra- and inter- sheet hydrogen-bonding.
This crystalline form of cellulose II, shown left, may also be prepared by deacetylation of cellulose acetate . Cellulose II contains two different types of anhydroglucose (A and B) with different backbone structures;
the chains consisting of -A-A-
or -B-B- repeat units , shown alternating from the top in the x-axis view. Both have intra-chain hydrogen bonding but only the -A-A- one has inter-chain hydrogen bonding. There are also hydrogen bonds between the A and B sheets such that an additional sheet structure consists of alternant A and B strands, with these sheets joined by hydrogen bonds, shown as diagonal sheets in the z-axis view. In the x-axis view the cellulose molecules go across the view (6 molecules shown) whereas in the z-axis view they disappear into the cartoon (18 molecules shown).
The interactive structure of cellulose II is available (Jmol).
Cellulose II hydrate may be prepared by immersion of mercerized cellulose II in anhydrous hydrazine followed by washing with water . it is similar to Cellulose II but has a larger unit cell containing the extra water molecules between the layers.
Cellulose III is formed from cellulose mercerized in ammonia and is similar
cellulose II but with the chains parallel, as in cellulose Iα and cellulose Iβ . For a
review of cellulose structure, see . Microcrystalline cellulose gives needle-shaped nanocrystals (microfibrillated cellulose ) of a relative uniform size with length of 90 ± 50 nm and width of 10 ± 4 nm . [Back to Top ]
Cellulose has many uses as an anticake agent, emulsifier, stabilizer,
dispersing agent, thickener, and gelling agent but these are generally
subsidiary to its most important use of holding on to water. Water
cannot penetrate crystalline cellulose but dry amorphous cellulose
absorbs water becoming soft and flexible. Some of this water is
non-freezing but most is simply trapped. Less water is bound by
direct hydrogen bonding if the cellulose has high crystallinity
but some fibrous cellulose products can hold on to considerable
water in pores and its typically straw-like cavities; water holding
ability correlating well with the amorphous (surface area effect)
and void fraction (that is, the porosity). As such water is
supercoolable, this effect may protect against ice damage.
Cellulose can give improved volume and texture particularly as a
fat replacer in sauces and dressings but its insolubility means
that all products will be cloudy.
Swelled bacterial cellulose (ex. Acetobacter xylinum),
in its never-dried state with much smaller fibrils (~1%) than
from plants, exhibits pseudoplastic viscosity like xanthan gels but this viscosity is not lost at high temperatures and
low shear rates as the cellulose can retain its structure.
Where individual cellulose strands are surrounded by water
they are flexible and do not present contiguous hydrophobic
surfaces. Bacterial cells may be removed by hot alkali and
the clean wet cellulose used as a substrate for immobilizing
or for covering wounds .
On drying the properties of bacterial cellulose irreversibly
lose their hydrated properties and tend to those of plant
About a third of the world's production of purified
cellulose is used as the base material for a number of water-soluble
derivatives with pre-designed and wide-ranging properties dependent
on groups involved and the degree of derivatization (for an extensive
review see ). Derivatizing
cellulose interferes with the orderly crystal-forming hydrogen bonding,
described above, so that even hydrophobic derivatives may increase
the apparent solubility in water. Methyl cellulose (E461)
 (made by methylating
about 30% of the hydroxyl groups) is thermogelling,
forming gels above a critical temperature due to hydrophobic interactions
between high-substituted regions and consequentially stabilized
intermolecular hydrogen bonding. Such gels break down on cooling
In a manner similar to that causing the solubility
minimum for non-polar gases; hydrophobic
saccharides becoming less soluble as the temperature increases
. This property is useful
in forming films as barriers to water loss and for holding on to
small gas bubbles.
Hydroxypropylmethylcellulose (HPMC, E464)
has similar properties and uses but with added water interaction and surface activity .
Both methylcellulose and HPMC may be used in gluten-free bakery
products as gluten substitutes. Hydroxypropyl cellulose (E463)
possesses good surface activity but does not gel as it forms open
helical coils. It is a water-soluble thickener, emulsifier and film-former
often used in tablet coating. Another important derivative of cellulose
Interactive structures are available (Jmol). [Back to Top ]
a Cellulose biosynthesis has been reviewed . [Back]