Fermentation-Based Building Blocks with Large Existing Markets

3 Fermentation-Based Building Blocks with Large Existing Markets 

The primary technology for citric acid recovery from fermentation broths is through precipitation with CaCOs, although other technologies include ion exchange, carbon adsorption, membrane filtration, chromatography, and liquid extraction [29]. Unlike other TCA cycle carboxylic acids, citric acid is excreted in the acid form rather than precipitated as citrate salt [30], which has the implication that the cell mass can first be filtered, followed by crystallization with iime with subsequent separation by filtration. The filter cake is next acidified with sulfuric acid, which precipitates the calcium as gypsum and the aqueous citric acid can next be decolourized and recrystallized by evaporation [29]. 

Acetic acid is a two-carbon monocarboxylic acid with many applications in food and as a building block for a wide range of industrial chemicals (Table 7.1). While much of the industrial acetic acid production is used directly for applications such as solvents, its primary use by volume is as a feedstock for the production of other chemicals such as vinyl acetate, which is used in plastics, adhesives and paints. Other products include cellulose acetate, acetic anhydride and acetate esters. The synthetic production route has been the dominant process since 1950 [31] using either hydrocarbon-derived ethylene or methanol as the feedstock. The 10% currently produced by microbial conversion is directed towards food applications. 

Acetic acid fermentation has been known since antiquity and both aerobic and anaerobic biological routes exist for acetic acid production [31] with Acetobacter species being the most significant aerobic strains and Clostridium for anaerobic strains. Ethanol and sucrose are the primary feedstocks although there is interest in utilizing sugars from lig-nocellulosic biomass as well [32]. Submerged bacterial cultivation is the most important 

Lactic acid is a three-carbon alpha-hydroxy acid that exists in two enantiomers, whose carboxyl and hydroxyl groups can undergo a number of reactions that make it a versatile building block. The global annual demand for lactic acid is estimated to be at least 130000 tonnes with approximately 85% of this demand directed towards food and food-related applications [8]. Out of the food applications, more than 50% is used for producing emulsifiers such as lactic acid esters of fatty acids and monoglycerides discussed previously [3]. However, the new applications, such as green solvents and polylactic acid (PLA) plastics, have driven a sharp increase in production in recent years [33]. 

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