Lactic acid producing microbes

Table 11.6 Lactic acid producing microbes, their carbon sources, and raw materials Microorganism Carbon source and raw materials References...

Since the lactic acid industry started around 120 years ago, many industrial producers have started up their lactic acid operations then stopped their business due to technical problems and/or market competition. The history of the lactic acid industry and the retired producers has been well reviewed (Benninga 1990). Current key lactic acid producers are listed in Table 1. Musashino Japan is the only one still using the chemical synthesis process to produce racemic lactic acid and its esters. Others use fermentation to produce optically pure lactic acid, mainly l-LA but some Chinese companies ferment racemic mixture of l-LA and d-LA using nonspecific microbes. 

Figure 10 shows the inherit advantage of lactic acid produced by the universal metabohc pathway shared by most living organisms from humans, to plants, to microbes. Through the anaerobic glycolysis pathway, two lactic acid molecules (A/ =90) are produced from one glucose (M =180) with a 100% theoretical yield (i.e., no loss at all). For the anaerobic fermentation of ethanol, the theoretical yield is around 51% with two CO M =AA) molecules lost per glucose. After ethanol has been dehydrated to ethylene and polymerized into bio-PE, the theoretical yield from glucose is only around 31% compared with 80% for PLA (Fig. 10). For PHBV-like biopolymer synthesized in microbial cells, the building block is typically acetyl-CoA with around 51% theoretical yield after the loss of CO (decarboxylation) from...

With that problem solved, we can now take up the case of the exploding Swiss cheese. Characteristic of this type of cheese are the large holes produced by the carbon dioxide gas that forms during the aging process. Cheese makers add a variety of bacteria to their creations as a way of ripening them. To Swiss cheese they also add a strain of Propionibacter shermanii. This bacterium uses the lactic acid secreted by other microbes as... 

The mechanisms by which probiotics (and prebiotics) produce beneficial effects on the gut have not yet been fully elucidated. However, at least three mechanisms of action have been proposed (i) antibacterial agents produced by probiotic organisms may have an inhibitory effect on pathogenic microbes (ii) immune responses may be enhanced to suppress potential pathogens and (iii) competition in the gut epithelium may allow lactic acid bacteria and bifidobacteria to supplant pathogenic organisms. 

Another promising thermophilic Gram-positive microbe is Thermoanaero-bacterium saccharolyticum which ferments xylan and produces ethanol, acetate, lactic acid, CO2, and H2. The engineered strain TD1, in which the... 

L-ldh gene was inactivated, was reported to produce increased acetic acid (16.4-16.8 mM) and ethanol (38.6-40.8 mM) with neglectable amounts of lactic acid (Desai et al., 2004). Additional manipulations of this microbe are needed to achieve efficient simultaneous thermophilic saccarification and fermentation for cellulosic ethanol production. 

Biobased polymers from renewable materials have received increased attention recently. Lactate is a building block for bio-based polymers. In the United States, production of lactic acid is greater than 50,000 metric tons/yr and projected to increase exponentially to replace petroleum-based polymers. Domestic lactate is currently manufactured from corn starch using the filamentous fungus Rhizopus oryzae and selected species of lactic acid bacteria. The produced lactic acid can then be polymerized into polylactic acid (PLA) which has many applications (Hatti-Kaul et al., 2007). However, so far, no facility is built to use biomass derived sugars for lactic acid production. More research needs to be done to develop microbes using biomass derived sugars for lactate production. 

Odorant molecules are typically small organic molecules with a molecular weight of 26-300. An odor consists of millions of volatile molecules emitted into the fluid media of air or water. The vapor pressure of a compound is a measure that directly relates to its volatility odorants with high vapor pressures in ambient conditions volatilize readily. Inanimate odor sources passively emit odors animate odor sources may passively or actively emit them. The intensity and chemical nature of both passive and active emission are a function of the properties of the compounds present, the temperature, and the atmospheric pressure. Examples of inanimate passive emission would include an inanimate object such as a glass of wine emitting hundreds of compounds from the fluid surface into the headspace in the immediately vicinity of the fluid." Animate passive emission often occurs as a function of a metabolic process. For example, the odors typically associated with rotting meat are produced and emitted by microbes as a byproduct of metabolism." Mammals emit CO2 and lactic acid as a byproduct of metabolism CO2 and lactic acid may be used by insects, such as mosquitos, to locate mammalian hosts." " In active emission, odors are synthesized and emitted by an animate signaler. Examples... 

Lactic acid (2-hydroxypropionic acid) is perhaps the most widely occurring carboxylic acid in nature. The Swedish chemist Scheele first discovered it in 1780. It exists in two different forms (Fig. 1) the dextrorotatory form, called L(-i-)-lactic acid or (5)-lactic acid, and the levorotatory form, called D(-)-lactic acid or R)-lactic acid. The plus and minus signs indicate the direction of the rotation of plane-polarized light produced by a chemical. These 2 stereoisomers (scientifically known as enantiomers ) are produced by different enzymes [lactate dehydrogenases (LDH)] present in living organisms. In this chapter, they will be referred to as D-lactic acid (d-LA) and L-lactic acid (l-LA). Naturally formed lactic acid is usually in the l form, but d-LA may coexist with l-LA in some cases, especially if it is secreted by nonspecific microbes. 

This chapter focuses on the microbial fermentation process for lactic acid production. The first commercial operation was set up by Avery in the USA in 1881. Microbes contain enzyme(s) called LDH which can convert pyruvic acid to lactic acid. Depending on the particular microbe and the specificity of its LDH, the lactic acid fermentation process can produce rather pure d-LA or l-LA with high optical purity, or a mixture of them with low optical purity. Genetic engineering tools can be used to knockout the d-LDH gene(s) in the production strain to improve the optical purity of its l-LA fermentation process. 

A typical downstream process includes (1) removal of production microbes (biomass) and solids (e.g., gypsum) from the broth, (2) recovery of crude lactic acid, and (3) purification of lactic acid. The biomass and solid waste can be separated from the liquid streams by various means, such as filtration, centrifugation, and decantation. If calcium alkali is used to control the fermentation pH, it produces calcium lactate precipitates which must be dissolved by acids such as phosphoric or sulfuric acid to extract lactic acid back into solution. After sulfuric acid has been added, calcium sulfate (CaS0 -2H20, known as gypsum) is formed and must be removed from the liquid stream as a major solid waste. 

These materials can be degraded by moisture or microbes. Further, the materials and can be treated for composting in composters. Among these polymers, in particular, PLA has come into use in various applications as an ideal polymer that is a plant-derived raw material produced by polymerizing lactic acid obtained by fermenting materials such as various starches and sugars, and eventually... 

Compared with the chemical synthesis, microbial fermentation for lactic acid production is more ecofriendly, comparatively fast, has superior yields, and can produce one of the two stereoisomers of lactic acid as well as their racemic mixture. It is crucial to select the suitable microbes with high productivity, the low-cost raw materials, and most favorable fermentation conditions, for example, temperature, pH, aeration, agitation, and so on. For development of competitive processes, the search for low-cost raw materials... 

Chemicals. There is an on-going trend in the chemical industry to use bioprocess engineering instead of pure chemistry for production of a variety of chemicals such as amino acids, polymers, and organic acids (citric, acetic, and lactic). Some of these chemicals (citric and lactic acids) are used as food preservatives. Many chemicals are produced in traditional fermenters by the action of microbes. 

---