Technical note: A comparison of alkali treatment methods to improve neutral detergent fiber digestibility of corn stover.

The objective of this study was to compare neutral detergent fiber (NDF) digestibility of corn stover that had been treated by 2 alkali treatment methods. Two experiments were conducted to test a sodium hydroxide (NaOH) treatment method that uses an ethanol/water co-solvent (NaOH/ethanol-H2O, United States Patent No. 20140220228) and a calcium hydroxide (CaOH) treatment method, which uses water as a solvent (CaOH/H2O). An in situ trial was conducted to compare NDF digestion kinetics between NaOH/ethanol-H2O-treated stover, CaOH/H2O-treated stover, untreated corn stover, and soy hulls. The digestion rate of potentially digestible NDF (kd) of NaOH/ethanol-H2O-treated corn stover (5.36%/h) was higher than CaOH/H2O-treated stover (2.27%/h), or untreated corn stover (1.76%/h) and similar to the kd of soy hulls (4.93%/h). The indigestible NDF (iNDF) fraction of untreated corn stover (35.1% of NDF) was reduced by CaOH/H2O treatment (27.3% of NDF) and by NaOH/ethanol-H2O treatment (2.8% of NDF). The iNDF fraction in soy hulls (3.6% of NDF) was similar to iNDF of NaOH/ethanol-H2O-treated stover. An in vivo digestibility trial was also conducted to compare fiber digestibility of diets supplemented with untreated corn stover, NaOH/ethanol-H2O-treated corn stover, or soy hulls. Total-tract apparent dry matter (DM) and NDF digestibility were measured with 8 lactating Holstein cows in a replicated 4 × 4 Latin square with four 21-d periods. Apparent DM digestibility (DMD) was improved when supplemental soy hulls were added to the base diet (60.0% DMD) compared with the base diet with no supplemental fiber (57.7% DMD). Apparent DM digestibility was reduced when diets were supplemented with untreated stover (52.4%). Dry matter digestibility of NaOH/ethanol-H2O-treated stover was similar (54.8% DMD) to all other treatments. Digestibility of NDF was lowest when cows were fed the diet with supplemented untreated stover (35.5% of NDF), and improved when soy hulls (40.6% of NDF) or NaOH/ethanol-H2O-treated stover (43.8% of NDF) were added to the diets. The NaOH/ethanol-H2O treatment process improves the DM and NDF digestibility of corn stover to values similar to those of soy hulls.

Irreversible transformations of native celluloses, upon exposure to elevated temperatures.

Current research, basic and applied, assumes that observed recalcitrance of celluloses is an inherent characteristic associated with their state of aggregation in their native state; it is thought that processes of isolation remove other components of plant cell walls leaving the celluloses unchanged, even though elevated temperatures are routinely used during isolation. Since temperature elevation is known to influence the structures of all polymers, it is important to explore its influence on the character of isolated celluloses, almost always assumed to be still in their native state. Deuterium exchange is a measure of accessibility of reactive sites in celluloses. We report significant reduction in accessibility to deuterium and other probe molecules for celluloses isolated at ambient temperature and then exposed to elevated temperatures. Our results indicate that native celluloses, which are highly ordered biological structures, are irreversibly transformed and develop polymeric semi-crystalline character upon isolation at elevated temperatures.

The role of solid state 13C NMR spectroscopy in studies of the nature of native celluloses.

Published spectroscopic observations pertaining to the crystal structure of native celluloses are reviewed for the purpose of defining our current level of understanding about crystalline polymorphism in these materials. Emphasis is placed on observations from solid state 13C nuclear magnetic resonance (NMR), which first led to the postulate that most native, semicrystalline celluloses are composites of two crystalline allomorphs, labeled Ialpha and Ibeta. Historical background is presented, highlighting the structural controversies which mainly arose because different native celluloses were used, each one representing a different mixture of allomorphs. Input from Raman, infrared (IR) and electron diffraction data is included in the discussion of our current understanding of polymorphism in native celluloses. Also noted is the input from more recently studied celluloses (e.g., Halocynthia) as well as from newer processes that convert the Ialpha to the Ibeta form. On the basis of Raman and IR observations, it is argued that the Ialpha and Ibeta allomorphs differ in hydrogen bonding patterns only and that backbone conformations are nearly identical. Also, the point is made that the absence of correlation field splittings in the Raman spectra calls into question (although it does not disprove) whether the normal two-chain-per-unit-cell, monoclinic Ibeta allomorph really possesses two equivalent chains. Considerable discussion is devoted to the allomorphic composition of cellulose crystallites in higher plants. Published methods of NMR lineshape analysis for the higher plant celluloses are reviewed and critiqued, both from the point of view of lineshape theory and from the point of view of self-consistency of inferences that are based on lineshape analyses for different carbons (particularly C1 and C4). It is concluded that higher plant celluloses most likely possess a minor amount of the Ialpha allomorph where the Ialpha/Ibeta ratio is probably less than 0.25.

Production of cellulose II by Acetobacter xylinum in the presence of 2,6-dichlorobenzonitrile.

This report provides X-ray diffraction and Raman spectral evidence that, when 2,6-dichlorobenzonitrile is present in the culture medium, Acetobacter xylinum, which is a model system for investigation of the biosynthesis of native cellulose, produces cellulose II, as well as cellulose I. The significance of the observations with respect to the mechanism of biosynthesis of cellulose is discussed briefly.

Cellulose-Lignin Interactions (A Computational Study).

Within a broader program of study of the molecular structure of plant cell walls, molecular dynamics calculations were used to explore the character of the motion of lignin model compounds near a cellulose surface. Model cellulose microfibrils, which have a large number of hydroxyl groups on the surface, appear to have a net attractive interaction with the lignin models examined in this study. The lignin monomer coniferyl alcohol rapidly adsorbed onto the surface from a water layer after it was released 13 A from the surface. The major long-range force responsible for this adsorption is likely electrostatic. The attractive interaction is sufficient to restrict the motion of coniferyl alcohol when it is within 1 A of the surface and to orient the phenyl ring parallel to the surface. The [beta]-O-4-linked trimer also was observed to adsorb onto the surface with two of its phenyl rings parallel to the surface. These results suggest a mechanism by which the polysaccharide component of the plant cell wall could influence the structure of lignin. Furthermore, they provide a rationalization of the experimental observation that polysaccharides can change the course of dehydrogenation polymerization of cinnamyl alcohols.

Modification of crystallinity and crystalline structure of Acetobacter xylinum cellulose in the presence of water-soluble beta-1,4-linked polysaccharides: 13C-NMR evidence.

Cellulose produced by Acetobacter xylinum in medium containing 0.5% xyloglucan or glucomannan showed altered crystallinities and shifted I alpha/I beta ratios when analysed by solid-state 13C-NMR. By estimating the spectra of cellulose components in each composite, a decreased I alpha content was shown to be countered by increased I beta content in cellulose aggregated in the presence of xyloglucan, causing minimal loss of crystallinity. However, the I alpha decrease was linked primarily to increased disordered content in cellulose produced in medium containing glucomannan. These results are considered in the light of two models for the morphological disposition of the I alpha phase: (i) a series model, proposed on the basis of electron diffraction measurements for an algal cellulose, in which regions of I alpha and I beta alternate along the length of a microfibril, and (ii) a superlattice model, in which the I alpha and I beta domains co-exist throughout the cross-section of each microfibril and form as a result of hierarchical aggregation. The latter model offers clearer insight into the role of the polysaccharides in inhibiting the formation of I alpha crystalline regions. In this superlattice model, polysaccharides adsorbed on surfaces of the most elementary aggregates are displaced to varying degrees during subsequent aggregation, with the presence of these polysaccharides altering the extent of I alpha production at interfaces.

Hemicelluloses as structure regulators in the aggregation of native cellulose.

The influence of hemicelluloses on the aggregation of cellulose in higher plant cell walls was modelled by adding hemicelluloses to cultures of the cellulose producer Acetobacter xylinum. Characterization of the celluloses by X-ray diffractometry showed them to be more like those that occur in higher plants; the coaggregation of the hemicelluloses suggests their occlusion within and between the crystalline domains of the celluloses. The authors propose that hemicelluloses may be primary moderators of the tertiary structure of cell wall celluloses, allowing the development of a wide range of properties.

In-situ Raman microprobe studies of plant cell walls: Macromolecular organization and compositional variability in the secondary wall of Picea mariana (Mill.) B.S.P.

Native-state organization and distribution of cell-wall components in the secondary wall of woody tissue from P. mariana (Black Spruce) have been investigated using polarized Raman microspectroscopy. Evidence for orientation is detected through Raman intensity variations resulting from rotations of the exciting electric vector with respect to cell-wall geometry. Spectral features associated with cellulose and lignin were studied. The changes in cellulose bands indicate that the pyranose rings of the anhydroglucose repeat units are in planes perpendicular to the cross section, while methine C-H bonds are in planes parallel to the cross section. Changes in bands associated with lignin indicate that the aromatic rings of the phenyl-propane units are most often in the plane of the cell-wall surface. However, regions where lignin orientation departs from this pattern also occur. These results represent direct evidence of molecular organization with respect to cellular morphological features in woody tissue, and indicate that cell-wall components are more highly organized than had been recognized. Studies carried out in order to establish the usefulness and sensitivity of the Raman technique to differences of composition within the cell walls provide evidence of variations in the distribution of cellulose and lignin. Such compositional differences were more prominent between the walls of different cells than within a particular cell wall.

Raman microprobe evidence for lignin orientation in the cell walls of native woody tissue.

Raman microprobe spectra from the secondary wall of earlywood tissue from Picea mariana (black spruce) reveal evidence of the orientation of lignin relative to the plane of the cell wall. In most instances, the aromatic rings of the phenyl propane structural units are parallel to the plane of the cell-wall surface.

Native cellulose: a composite of two distinct crystalline forms.

Multiplicities in the resonances of chemically equivalent carbons, which appear in the solid-state carbon-13 nuclear magnetic resonance spectra of native celluloses, have been examined at high resolution. The patterns of variation are consistent with the existence of two distinct crystalline forms. One form is dominant in bacterial and algal celluloses, whereas the other is dominant in celluloses from higher plants.

Cellulose: its regeneration in the native lattice.

The regeneration of cellulose from solutions in 85 percent phosphoric acid into glycerol at 170 degrees C resulted in its recovery wholly in the native or cellulose I lattice. Identification of the structure of the regenerated material is based on comparisons of its x-ray diffractogram and its Raman spectrum with those of the native fiber, cellulose II, and cellulose IV.