32.4 Microbiology and Physico-Chemical Process in AD

495

Table 32.1

Action of microbial enzymes on feedstock polysaccharides.

Substrate

Enzyme produced

Product

Cellulose

endo-l,4-Glucanases

exo-l,4-Glucanases cellobiase

ß-Glucosidase

Glucose

Starch

α-Amylases

ß-Amylases

Amyloglucosidases

Debranching enzyme

Maltase

Glucose

Pectin

Pectinase

Galacturonic acids

Xylans

α-endo-Xylanase

α-Xylosidase

Xylose

Fats

Lipase

Fatty acids, glycerol

Proteins

Protease

Amino acids

bacteria cultured. These limitations can be overcome by co-culturing the aceto-

genic bacteria with H2-consuming bacteria, such as methanogens and sulfate-

reducing bacteria. In addition, there is breakdown of lactate and ethanol to acetate

and H2 by Clostridium formicoaceticum during acetogenesis [3].

32.4.3

Methanogenesis and the Essential Microbial Consortia

During methanogenesis, acetate, hydrogen, and carbon dioxide are utilized by

methanogenic bacteria to generate methane. Methanogens can be classified as

hydrogenotrophic, acetotrophic, and methylotrophic methanogens. Hydrogenotro-

phic methanogenesis (Methanothermobacter thermautotrophicus, Methanosarcina

thermophila, and Methanoculleus sp.) involves utilization of H2 and CO2 for the

production of methane as indicated in the following chemical reaction.

CO2 + 4H2 →CH4 + 2H2O

Hydrogenotrophic methanogenesis are leading in energy crop digestion. Some of

the bacteria belonging to this class are capable of consuming formate for the produc-

tion of methane.

Acetotrophic methanogenesis is a primary pathway associated with methane gen-

eration from acetate by a few strains such as Methanosaeta spp. and Methanosarcina

spp. as shown in the following chemical reaction.

CH3COOH →CO2 + CH4

Methylotrophic methanogenesis which involves the production of methane from

the substrate methanol, a less common pathway

CH3OH + H2 →CH4 + H2O