CHAPTER 3-B: HEMICELLULOSE Flashcards
hemicellulose also known as __________
polyose
defined as cell wall polysaccharides that have the capacity to bind strongly to cellulose micro fibrils by hydrogen bonds
hemicellulose
degree of polymerization of glucose units in hemicelluloses is in the range of ___________, which is much lower than cellulose
100–200 units
usual DP values in wood are reported to be between ___________
300 and 1700
in nature, cellulose has a DP of approximately ___ glucopyranose units in wood cellulose and of ______ units in plant cellulose
10,000 ; 15,000
What subunits does hardwood hemicellulose consist of?
Pentose, specifically xylose
_____ is a monosaccharide with five carbon atoms.The chemical formula of many pentoses is _______.
pentose; C ₅H ₁₀O ₅.
What subunits does softwood hemicellulose consist of?
Hexose subunits like glucose
__________ any of the class of simple sugars whose molecules contain six carbon atoms, such as glucose and fructose. They generally have the chemical formula ______.
Hexose; C6H12O6
What percentage of hemicellulose do non-wood fibers contain?
Up to 40%
What percentage of hemicellulose do wood fibers contain?
25% to 35%
What are the classifications of hemicellulose?
a. β-(1→4)-linked backbones
b. Includes xyloglucans, xylans, mannans, glucomannans
c. β-(1→3,1→4)-glucans in some groups
Xyloglucan has been found in every land plant species that has been analyzed, including mosses EXCEPT in ______
Charophytes
Xyloglucan the most abundant hemicellulose in primary walls of spermatophytes except for ______
grasses
What is the main component of xylans?
Xylose residues
What percentage of dry weight in plants can xylans constitute?
More than 30%
mainly constituted by _____ as the monomeric unit, and traces of_________ are also present
D-xylose; L-arabinose
Xylansof several wood species, particularly of hardwoods, are ______
acetylated
What are glucomannans primarily composed of?
β-(1→4)-linked polysaccharides containing mannose
What role do glucomannans play in plants?
Energy reserves and structural roles
What components make up the plant cell wall?
a. Cellulose
b. Hemicellulose
c. Pectin
d. Glycoproteins
What is the purpose of hemicelluloses in the cell wall?
Cross-link glycans and interlock cellulose fibers and form a mesh like structure to deposit other polysaccharides.
What are the methods for extracting hemicellulose from lignocellulosic biomass?
- Hydrothermal extraction
- Steam explosion
- Acidic pretreatments
- Alkaline pretreatments
- Solvent extraction
- Ultrasonic-assisted processes
- Microwave-assisted processes
Why is pretreatment of lignocellulosic biomass necessary?
To improve bioconversion for sugar production
What requirements must an effective pretreatment for lignocellulosic biomass meet?
- Improve sugar production
- Avoid degradation of cellulose and hemicelluloses
- Minimize inhibitor production
- Be cost-effective
What is hydrothermal extraction also known as?
Autohydrolysis
What is a key advantage of hydrothermal extraction?
It uses only water as a reaction medium
can be considered to be a green technology, because of its ability to prevent corrosion problems and not need chemical recycling
Hydrothermal Extraction (Autohydrolysis)
What is the typical temperature range for hydrothermal treatments?
160-240 °C
What happens to hemicelluloses during hydrothermal treatment?
They are solubilized and converted to monomers
What pH should be maintained during hydrothermal treatment?
Between 4 and 7
What is the purpose of steam explosion in biomass pretreatment?
To destroy cell wall structure and separate components
a common process for separating and extracting hemicellulose from lignocellulosic biomass
Alkaline extraction
What is the most common alkali used in alkaline extraction?
NaOH solution
What does alkaline extraction promote in hemicellulose?
Swelling of cellulose and hydrolysis of hemicelluloses
What is the main outcome of acid extraction of hemicellulose?
Separation of main components of biomass
What is a significant drawback of acid hydrolysis?
Low yield and molecular weight of extracted hemicellulose
What are the by-products formed during acid extraction?
- Furfural
- 5-hydroxymethylfurfural
- Acetic acid
