Biosynthesis of succinic acid
Succinic acid is a dicarboxylic acid containing 4 carbons. Besides being crucial in (human)-life by its role in the citric acid cycle (CAC) it also is a key ingredient in several polymers, as part of polyurethane formulations and as an ingredient of food and cosmetics. As many organisms already produce succinic acid (or succinate) via their normal metabolic pathways, it was one of the earlier chemical building blocks which was predicted to be achievable via fermentation.
Multiple pathways of the CAC can lead to the formation of succinic acid, with a theoretical maximum yield of 1.72 moles of succinic acid per mole of glucose. Most of the times modified strains of the Escherichia coli bacteria are used for this process. Instead, current petrochemical routes are mostly based on the hydrogenation of maleic anhydride. Examples of the industries where succinic acid can be utilized are listed below.
Food and cosmetics
Naturally present in vegetables such as broccoli and rhubarb and with the number E363, succinic acid is approved for food usage, although used slightly less than citric acid and malic acid, it can be used as a flavour enhancer and to adjust the pH of a product. Succinic acid is linked to various cosmetic benefits, such as the stimulation of mitochondrial respiration, activity as an antioxidant and significant anti-microbial activity against the microorganism causing acne. Furthermore, it can be used as a pH modifier with excellent water solubility. Furthermore, succinate esters such as heptyl succinate or iso-octyl succinate are commonly used in cosmetics as emollients.
Succinic acid is used as a building block in various polyesters, such as polyethylene succinate and polybutylene succinate (PBS). Especially PBS is very promising as it reports similar mechanical properties as polyethylene and polypropene. By producing this polymer with bio-based succinic acid and bio-based 1,4-butanediol, it can be both biobased and biodegradable. Furthermore, PBS can be mixed with other biopolymers such as PLA and has good compatibility with natural fibres.
Polyester polyols based on succinic acids were compared to their analogues based on adipic acid. These polyols influence mechanical properties and are key structures for thermoplastic polyurethanes and elastomers. Succinic acid compares favourably against adipic acid in regards to softness, improved solvent resistance and good hydrolytic stability.
Starting material other bio-based chemicals
There are limitations to what type of products can be obtained by microbial fermentation. As such, a feasible method to obtaining these important chemical building blocks is to chemically or biologically convert bio-succinic acid into other molecules. Examples of molecules that can be produced from succinic acid are 1,4-diaminobutane, succinimide and N-methyl pyrrolidone. Although several synthetic steps might be required to produce these molecules, all carbons will have a bio-based origin.