Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 193

7.6.3 Propionate
Propionate is a common industrial chemical that is used in the food and feed industry as a
preservative. It is also used for the production of various plastics, pharmaceuticals, and
perfumes (Himmi et al ., 2000). Propionic acid produced from fermentation is primarily
produced by bacteria from the genus Propionibacterium . Propionic acid can be produced
using either glycerol or glucose as the carbon source. The precursor of propionic acid is
phosphenolpyruvate (PEP). Instead of PEP being dephosphorylated to produce pyruvate,
propionic acid bacteria dephosphorylate PEP while adding a carboxyl group from carbon
dioxide to form oxaloacetate. Oxaloacetate is then converted through four enzymatic
reactions to form propionate. Three of these reactions are actually part of the citric acid
cycle; however, the reactions are carried out in the reverse direction. The final reaction
combining water and succinate to form propionate and carbon dioxide is not part of the
citric acid cycle.
Propionic acid fermentations can achieve high yields and productivities. Himmi and
co-workers (2000) used Propionibacterium acidipropionici and achieved a propionate yield
of 0.79 mol/mol with a productivity of 0.42 g/l/h and a final concentration of 12 g/l when
using 20 g/l glycerol. They also found that with P. acidipropionici , using glycerol as the
carbon source resulted in more propionate and less acetate production than when they used
glucose. One of the keys for successful propionate fermentation is to minimize acetate
production, as acetate interferes with the distillation process used to purify propionate (da
Silva et al ., 2009 ).
7.6.4 Succinate
Succinate is an important precursor for plastics, pharmaceuticals, detergents, and other
consumer products (da Silva et al ., 2009). Several biodegradable plastics can be produced
with succinate, and the production of these plastics with renewable resources is desirable.
Also, one mole of carbon dioxide is fixed for every mole of succinate produced; thus,
fermentative production of succinate can be used to reduce carbon dioxide emissions (Song
and Lee, 2006). Succinate production can be accomplished using several metabolic
pathways, but the most prevalent in bacteria is PEP carboxylation. This is the same pathway
previously described for propionate production, with the exception that succinate is not
converted to propionate.
Most of the bacteria that have been isolated for succinate fermentation are rumen
bacteria, such as Anaerobiospirillum succiniciproducens , Actinobacillus succinogenes
and Mannheimia succiniciproducens (Guettler et al ., 1999 ; Lee et al ., 2000 ; Beauprez
et al ., 2010 ). Strains of E. coli have also been genetically modified to produce succinate
(Hong and Lee, 2000). Recent developments in fermentative succinate production have
involved the use of lignocellulosic substrates, such as corn stover (Zheng et al ., 2010 )
and various woods (Hodge et al ., 2009 ; Lee et al ., 2003 ), as feedstocks for succinate
production.
7.6.5 1,3 Propanediol
1,3 propanediol (PDO) is an important precursor for plastic manufacturing as well as an
effective solvent. Although PDO has been known as a product of glycerol fermentation for
many years (Freund, 1881), its commercial relevance has increased since Dupont began