environment and handling, the enzymatic hydrolysis has become attractive, especially if the hydrolysate is considered as fermentation feedstock. Hence, xylanases
and mannanases are used to convert the polymeric xylan and mannan to fermentable
sugars in hemicellulose biorefineries. In addition to bioethanol production [17, 186,
187], hemicellulose hydrolysates have been used as fermentation feedstocks to
produce a variety of chemicals such as xylitol [188], 2,3-butanediol [16, 189],
3-hydroxypropionic acid [190], succinic acid [191], lactic acid [192], amino acids
[193], 1-hexadecanol [15], biosurfactants [194], etc. Some of these chemicals
produced from hydrolysates are platform chemicals, chemicals that subsequently
can be converted to a range of other high-value chemicals. This widens the potential
product portfolio from hemicelluloses. Moreover, with advances in metabolic engineering and microbiology, it is possible to get more organisms that can industrially
valorize hemicellulose hydrolysates to different products, and this will significantly
contribute to further expand the product portfolio from hemicelluloses.
In addition to sorted hemicelluloses, hydrolysis of hemicelluloses in unsorted
biomass can increase productivity by improving cellulose accessibility to cellulases.
This synergy can enhance the total conversion of biomass to fermentable sugars. An
increase in the amount of fermentable sugars would ultimately increase the amount
of fermentation products from the biomass.
The use of alkaline active hemicellulases in hemicellulose biorefineries has
advantages over that of neutral active counterparts for the same reasons described
in the above, better solubility of hemicelluloses at high pH and direct hydrolysis.
Often hemicelluloses are extracted from lignocellulosic biomass by alkaline extraction methods and hence the use of alkaline active enzymes allow direct conversion of
alkali extracted hemicellulose to its monomeric sugars, which avoids the time
consuming and corrosive (addition of acid) pH adjustment. The other benefit is the
high solubility of hemicelluloses at high pH which facilitates an efficient hydrolysis
of the hemicellulose. There is another advantage of direct hydrolysis of alkaliextracted hemicelluloses in biorefineries, and that is its alkalinity. Often, microbial
fermentation is a companied by pH drop, and to maintain the pH of the culture within
the tolerance limit of the microorganism, it is a standard protocol that bases are
pumped to the fermentation tank. The base consumption is high, especially when the
target product is organic acid such as lactic acid or succinic acid. Thus, the use of
alkaline hydrolysate can buffer the pH drop, which can substantially cut the base
consumption. This may require starting the cultivation around neutral condition
(as most of the industrial fermentation is accomplished around neutral to mildly
acidic conditions) with enough substrate to induce acid production and then feed the
major substrate, the alkaline hydrolysate.
6.6 Animal Feed
Cereal grains such as barley and wheat, which contain a lot of arabinoxylans, are
among the major ingredients in poultry diets. For instance, in barley arabinoxylans
276
G. Mamo
and mannanases are used to convert the polymeric xylan and mannan to fermentable
sugars in hemicellulose biorefineries. In addition to bioethanol production [17, 186,
187], hemicellulose hydrolysates have been used as fermentation feedstocks to
produce a variety of chemicals such as xylitol [188], 2,3-butanediol [16, 189],
3-hydroxypropionic acid [190], succinic acid [191], lactic acid [192], amino acids
[193], 1-hexadecanol [15], biosurfactants [194], etc. Some of these chemicals
produced from hydrolysates are platform chemicals, chemicals that subsequently
can be converted to a range of other high-value chemicals. This widens the potential
product portfolio from hemicelluloses. Moreover, with advances in metabolic engineering and microbiology, it is possible to get more organisms that can industrially
valorize hemicellulose hydrolysates to different products, and this will significantly
contribute to further expand the product portfolio from hemicelluloses.
In addition to sorted hemicelluloses, hydrolysis of hemicelluloses in unsorted
biomass can increase productivity by improving cellulose accessibility to cellulases.
This synergy can enhance the total conversion of biomass to fermentable sugars. An
increase in the amount of fermentable sugars would ultimately increase the amount
of fermentation products from the biomass.
The use of alkaline active hemicellulases in hemicellulose biorefineries has
advantages over that of neutral active counterparts for the same reasons described
in the above, better solubility of hemicelluloses at high pH and direct hydrolysis.
Often hemicelluloses are extracted from lignocellulosic biomass by alkaline extraction methods and hence the use of alkaline active enzymes allow direct conversion of
alkali extracted hemicellulose to its monomeric sugars, which avoids the time
consuming and corrosive (addition of acid) pH adjustment. The other benefit is the
high solubility of hemicelluloses at high pH which facilitates an efficient hydrolysis
of the hemicellulose. There is another advantage of direct hydrolysis of alkaliextracted hemicelluloses in biorefineries, and that is its alkalinity. Often, microbial
fermentation is a companied by pH drop, and to maintain the pH of the culture within
the tolerance limit of the microorganism, it is a standard protocol that bases are
pumped to the fermentation tank. The base consumption is high, especially when the
target product is organic acid such as lactic acid or succinic acid. Thus, the use of
alkaline hydrolysate can buffer the pH drop, which can substantially cut the base
consumption. This may require starting the cultivation around neutral condition
(as most of the industrial fermentation is accomplished around neutral to mildly
acidic conditions) with enough substrate to induce acid production and then feed the
major substrate, the alkaline hydrolysate.
6.6 Animal Feed
Cereal grains such as barley and wheat, which contain a lot of arabinoxylans, are
among the major ingredients in poultry diets. For instance, in barley arabinoxylans
276
G. Mamo