Aromatic amino acids include tryptophan, phenylala­nine, tyrosine and histidine and all of them were isolated in the 1880s. Tyrosine was isolated by Liebig in 1846 and phenylalanine from lupins by Schulze in 1881. While histidine was reported by Kossel and Hedin in 1896, tryptophan was first isolated from casein by Frederick Hopkins in 1901. Even though the aromatic amino acids are produced by microbial fermentation, high produc­tion levels are not reached. Commercial production of these amino acids also includes extraction and enzymatic conversion. In 2006, Kyowa Hakko Kogyo claimed devel­opment of the world’s first fermentation-based method for the commercial production of L-tyrosine. Biodiesel industry-generated crude glycerol can be biorefined by phenylalanine-producing E. coli cells (Khamduang et al., 2009). The use of glycerol resulted in phenylalanine yields up to 0.58 g/g, which is twice as compared to pro­duction with sucrose. Tryptophan and histidine were produced from mixed sugars pentoses and hexoses by genetically modified E. coli (Savrasova et al., 2004).

Microbial amino acid production process for bio­refining application will be technically feasible, only if the nutrient requirements are met in invariably same quantity and quality. These raw material production costs must be lower than starch hydrolysates or refined sugars and the coproducts have to use the existing fermentation machinery and infrastructure for economic feasibility. Most of the microbial amino acid fermenta­tions occurs at a temperature range of 30—40 °C and at near neutral pH. The fermentation medium has to be neutralized and proper cooling systems for temperature maintenance, agitation and aeration has to be in place for the amino acid production to match with the expected values. Alternatives are development or utilization of strains that are pH and temperature tolerant and over­produce amino acids or choosing coproducts and organ­isms having the same substrate utilization spectrum and physical requirements. The use of microbial amino acid fermentation for biorefining resulted in improved ground water quality, lower ammonia and nitrate excre­tion from poultry and livestock. This is due to the substi­tution of optimal quantities of the limiting amino acids in place of soybean meal. The sugar solutions from the lignocellulose feedstock biorefinery will have a fair con­centration of inhibitors like furfural and hydroxymethyl furfurals and syringaldehyde, which is toxic to the micro­organisms. The inhibitor tolerance of amino acid pro­ducers will also be a deciding factor in the biorefining concept. Fortunately, the amino acid producer C. glutami — cum has shown tolerance toward inhibitors at growth — arrested conditions and high cell densities.