The consequences of extracting switchgrass, corn stover, and fescue
feedstocks with either 95%
ethanol or hot water prior to the chemical characterization of the
feedstock have been determined.
Glycans (glucan and xylan), Klason lignin, acid soluble lignin,
uronic acids, acetyl groups, protein,
and ash analyses were done on each feedstock. Extraction with 95%
ethanol or hot water
significantly reduced the measured Klason lignin values of all the
feedstocks. Ethanol extractions
reduced the apparent glucan content of two of the feedstocks, fescue
and switchgrass. Extractions
with hot water reduced the apparent glucan content of all the
feedstocks. The apparent content of
other components in the feedstock, such as ash and protein, was also
reduced as a result of doing
solvent extractions. The results indicate that more accurate
estimates of the true lignin and cellulose
content of a feedstock are obtained if extractives are removed from the
feedstock prior to the analysis.
However, in some cases the analysis of extracted feedstocks
resulted in lower values for the total
carbohydrate content of those feedstocks. This parameter is
critical when evaluating these feedstocks
for biomass-to-ethanol processes.
Keywords: Corn stover; switchgrass; fescue; cellulose; lignin;
extractives.
It is widely accepted that humans can taste mono- and disaccharides as sweet substances, but they cannot taste longer chain oligo- and polysaccharides. From the evolutionary standpoint, the ability to taste starch or its oligomeric hydrolysis products would be highly adaptive, given their nutritional value. Here, we report that humans can taste glucose oligomer preparations (average degree of polymerization 7 and 14) without any other sensorial cues. The same human subjects could not taste the corresponding glucose polymer preparation (average degree of polymerization 44). When the sweet taste receptor was blocked by lactisole, a known sweet inhibitor, subjects could not detect sweet substances (glucose, maltose, and sucralose), but they could still detect the glucose oligomers. This suggests that glucose oligomer detection is independent of the hT1R2/hT1R3 sweet taste receptor. Human subjects described the taste of glucose oligomers as "starchy," while they describe sugars as "sweet." The dose-response function of glucose oligomer was also found to be indistinguishable from that of glucose on a molar basis.
Acetic acid inhibition of yeast fermentation has a negative impact in several industrial processes. As an initial step in the construction of a Saccharomyces cerevisiae strain with increased tolerance for acetic acid, mutations conferring resistance were identified by screening a library of deletion mutants in a multiply auxotrophic genetic background. Of the 23 identified mutations, 11 were then introduced into a prototrophic laboratory strain for further evaluation. Because none of the 11 mutations was found to increase resistance in the prototrophic strain, potential interference by the auxotrophic mutations themselves was investigated. Mutants carrying single auxotrophic mutations were constructed and found to be more sensitive to growth inhibition by acetic acid than an otherwise isogenic prototrophic strain. At a concentration of 80 mM acetic acid at pH 4.8, the initial uptake of uracil, leucine, lysine, histidine, tryptophan, phosphate, and glucose was lower in the prototrophic strain than in a non-acetic acid-treated control. These findings are consistent with two mechanisms by which nutrient uptake may be inhibited. Intracellular adenosine triphosphate (ATP) levels were severely decreased upon acetic acid treatment, which likely slowed ATP-dependent proton symport, the major form of transport in yeast for nutrients other than glucose. In addition, the expression of genes encoding some nutrient transporters was repressed by acetic acid, including HXT1 and HXT3 that encode glucose transporters that operate by facilitated diffusion. These results illustrate how commonly used genetic markers in yeast deletion libraries complicate the effort to isolate strains with increased acetic acid resistance.
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