Challenges in the Analysis of Sweet Sorghum Lignocellulosic Materials

Background

Sweet sorghum is a source of fermentable sugars in the form of both native water soluble sugars and polysaccharides which can be degraded. As with other forms of biomass, quantification of cellulose in sweet sorghum is critical to the evaluation of the material and processes for its utilization. Acid hydrolysis based methods, such as those detailed in the NREL LAPs and related ASTM methods, are used widely for herbaceous biomass analysis. While those methods are useful for sweet sorghum, they require some modification to account for of its high aqueous sugar content.

Stalk Fractionation Effects

Rationale

NREL LAP allows the optional sieving of biomass to remove small (-80 mesh), high-ash material. The LAP acknowledges the potential for skewing of results that may result from the removal of small particles. ASTM 1757? requires sieving, but recommends that the -80 mesh fraction be analyzed separately. In sweet sorghum, the pith, which is often sugar-rich and lignin-poor, tends to form fine particles during grinding. As a result, the entire sample, regardless of particle size, must be analyzed to accurately represent the composition of the stalk. However, accurate subsampling is also adversely affected by wide distributions in particle sizes and densities. Examining the separate contributions of the pith and rind to these factors may improve sampling accuracy.

Methods

The stalks of several different sorghum varieties were manually divided into pith and rind fractions after drying at 45°C and ground to pass a 2 mm screen. The material was Soxhlet? extracted and subjected to acid hydrolysis, with HPLC of the hydrolyzate. Bulk and absolute densities of the ground material were determined using a Micromeritics AccuPyc? 1330. Some material was also sputter coated and imaged with a Hitachi S3200 VPSEM.

Stalk Fractionation Results

As expected, both extractable and structural components of the sweet sorghum stalk vary between the stalk and the pith. The extent of this variability appears to be variety dependent. Both components are important to the accurate characterization of the sorghum stalk—though occupying much less volume, the rind corresponded to approximately 70%

Biomass Moisture Effects

Rationale

NREL LAP recommends that samples be dried to <10% moisture wet-basis; ASTM E1758 recommends <15% wet-basis. Sweet sorghum is high in moisture when fresh (~70% wet-basis) and hygroscopic when dried.

Methods

Soxhlet extracted sweet sorghum samples were allowed to reach moisture equilibrium at 105°C, 45°C, and 25°C; 50% and 75% w/w aqueous slurries were also generated using the 45°C dried biomass.

Moisture Results

Moisture content effects were not significant for acid insoluble residue or cellulose (p=.9891, p=.3945). A small increase in recovered hemicellulose was observed with increasing moisture content (p=.0134). (All values reported on a dry matter basis.)

Soxhlet Extraction

Rationale

NREL LAP describes water and/or ethanol extraction of biomass prior to lignocellulose analysis. ASTM 1690 describes Soxhlet extraction with ethanol only. Sweet sorghum often contains large quantities of sucrose? and/or glucose. It is critical that these be removed before acid hydrolysis as cellulose is ultimately quantified on an as-glucose basis. In addition other, acid-insoluble components (such as waxes) can interfere with the hydrolysis and inflate the lignin values.

Methods

Sweet sorghum samples were Soxhlet extracted with multiple two-solvent combinations

Label	1st Solvent	2nd Solvent
A	95% Ethanol	Water
B	95% Ethanol	50% Ethanol
C	50% Ethanol	50% Ethanol
D	50% Ethanol	Water
E	Water	95% Ethanol

Soxhlet Results

Generally, the most effective procedure, as determined by total mass of extraction, is to follow an ethanol extraction with a water extraction—A&D extracted significantly more solubles from the biomass than the other treatments (p>.0143), but were not significantly different from one another (p=.4413). As might be expected, 50% ethanol is slightly more effective if only a single extraction is performed (p>.0173), but the effects are not detectable after a second wash (p=.1205).

External links

PDF of the scientific poster