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These NAD+molecules can be then utilised in glycolsis to split another glucose molecule
to maintain the flow of carbon through the process.
The process of aerobic respiration is the most energy efficient form of metabolism.
This efficiency, however, makes aerobic respiration rather useless for energy extraction
from biomass and its biochemical conversion to other compounds. All the energy from
respiration is quickly captured, transformed into ATP and used by the organism with-
out producing any stable intermediates that could be isolated and utilised as fuels or
platform chemicals by humans.
Fermentation– Anaerobic processes also known as fermentations include several
different types of metabolism that is capable of simultaneous production of ATP com-
bined with the release of high-energy products or intermediates that can be used as fuel
or platform chemicals: Fermentative production of ethanol, butanol, biohydrogen and
biomethane by specialised organisms are promising routes to produce these fuels from
renewable feedstocks. These four processes are described in detail in the later sections.
These processes share common characteristics such as major metabolic intermediates,
NADH regeneration capacity and to various extent production of additional ATP. They
may differ, however, in their capacity to use different feedstocks and of course yield
different products.
3.4.9 Harvesting Energy from Biochemical Processes
3.4.9.1 Ethanol Fermentation
Ethanol fermentation takes place under anaerobic conditions and is an example of
anaerobic metabolism that yields high-energy intermediate that could be easily isolated.
In the process of ethanol fermentation, pyruvate from the glycolysis is decarboxylated
to acetaldehyde with an enzyme pyruvate decarboxylase and subsequently reduced
to ethanol by alcohol dehydrogenase. The main biological function of this reaction is
anaerobic regeneration of NAD+from NADH, which is required for the process of
glycolysis and production of ATP through this route.
C 3 H 4 O 3 ↔C 2 H 4 O+CO 2
C 2 H 4 O+NADH+H+↔C 2 H 5 OH+NAD+
Ethanol is one of only few chemicals that are predominantly produced by microbial
fermentation. Although the process of ethanol fermentation can be carried out by
numerous organisms, yeast (Saccharomyces cerevisiae) is the industrial organism of
choice. The advantages of using yeast for ethanol production include high yield of
ethanol production; capacity of using short maltodextrins, sucrose and fructose in
addition to glucose; growth in slightly acidic conditions that inhibit bacterial contami-
nation and relatively high ethanol tolerance (10–15 %). Additionally, yeast is considered
as GRAS organism (Generally Regarded as Safe) that makes its biomass suitable as
animal feed and has been a subject of extensive genetic studies that facilitate further
strain improvement. Yeast can utilise numerous carbohydrate feedstocks to perform
fermentation process. Typically, sugar and starchy feedstocks are used; utilisation
of lignocellulose feedstocks has only recently entered commercial operations. Sugar
feedstocks are relatively easiest to use as yeast is capable of metabolising sucrose into
glucose and fructose and incorporating into glycolysis process. Starchy feedstocks
require an additional step called saccharification in which starch is hydrolysed into
