Lactic acid from hexoses

Lactic add is a metabohc product of simple carbohydrates produced by many spedes of bacteria, yeasts, and mycehal fungi mainly through the fermentative metabolic pathway. The stoichiometry for homofermentative production of lactic acid from hexose can be expressed as ... 

Perhaps the acid-tolerant, thermophilic Bacillus coagulans is the only known biocatalyst that naturally produces lactic acid from xylose via the pentose phosphate pathway, not the phosphoketolase pathway (Patel et al., 2006). Three strains, 17C5, P4-102B, and 36D1, can ferment both hexoses and pentoses to pure L(+)-lactic acid at 50 °C and pFI 5.0, an optimal environment... 

Four major carboxylic acids, acetic, formic, glyceric, and lactic, are formed under mild alkaline oxidation conditions from xylose (Rahardja et al, 1994). At 40-60°C, lactic acid is formed from dilute aqueous solutions of xylose in the largest mass yield, about 50%, under continuous oxidation conditions. This is about 83% of the maximum theoretical yield of 60%. Under batch conditions, the yield of lactic acid from xylose is about 23%, while the yield from glucose is about 41%. These data indicate a commercial alkaline oxidation process for lactic acid from pentoses and hexoses is feasible, but it should be realized that the recovery of carboxylic acids having high water solubilities from dilute aqueous solutions can add signihcantly to production cost (Busche, 1984). 

Heterofermentative bacteria produce acetic acid from hexoses, but regulation mechanisms modify production. In anaerobic conditions, the NADH oxidase cannot regenerate NAD. Glucose preferentially leads to the formation of lactic acid and ethanol. When NADH can be reoxidized by another process, the amount of ethanol formed... 

Muscle extracts transform glycogen into lactic acid much more rapidly than they transform glucose into lactic acid, from which it is concluded that the polysaccharide is phosphorylated directly, and not first depolymerised to a hexose. 

The above observations suggested that hexoses arise in Nature by reaction of glycerose with dihydroxyacetone. A vast amount of practical information has been derived from investigation of plant- and muscle-extracts, two dissimilar systems that show many similarities in their biosynthetic manipulations. There is a close parallelism in the sequence of intermediates involved in the processes wherein D-glucose is converted to ethanol and carbon dioxide by yeasts, and to lactic acid by muscle during contraction. The importance of these schemes lies in their reversibility, which provides a means of biosynthesis from small molecules. 

Commercially, lactic acid is manufactured by controlled fermentation of the hexose sugars from molasses, corn, or milk. Lactates are made by synthetic methods from acetaldehyde and lactonitrile, a by-product acrylonitrile production. 

In principle, the same carbohydrates and their degradation products formed after hydrolysis of wood can be recovered from sulfite spent liquors. However, this requires complicated and expensive separation techniques. The industrial use of sulfite spent liquor components is mainly limited to fermentation processes. The most common product is ethyl alcohol which is produced from hexose sugars by yeast (Saccharomyces cerevisae) and separated from the mixture by distillation. Even the carbon dioxide formed in the process can be recovered. Other fermentation products, including acetone, n-butanol, and lactic acid, can be produced by certain microorganisms. Because some contaminants, for example, sulfur dioxide, inhibit the growth of the yeast, they must be removed from the liquor prior to the fermentation. 

Several heterofermentative LAB produce mannitol in large amounts, using fructose as an electron acceptor. Mannitol produced by heterofermentative bacteria is derived from the hexose phosphate pathway (Soetaert et al., 1999 Wisselink et al., 2002). The process makes use of the capability of the bacterium to utilize fructose as an alternative electron acceptor, thereby reducing it to mannitol with the enzyme mannitol dehydrogenase. In this process, the reducing equivalents are generated by conversion of one-third fructose to lactic acid and acetic acid. The enzyme reaction proceeds according to (theoretical) Equation 21.1 ... 

The first really definitive work on glutose was carried out by Benedict, Dakin and West. They showed that glutose in vitro resembles D-fructose in that it is converted by sodium hydroxide into hydroxy acids (principally optically inactive lactic acid) to about the same extent as D-glucose. In slightly alkaline solution phenylhydrazine reacts to form the phenylosazone of methylglyoxal, and zinc ammonium hydroxide converts glutose into methylimidazole in yield comparable to that obtained from fermentable hexoses. But in vivo glutose ... 

The role of substitution in determining the course of saccharinic acid formation has been critically examined recently by Kenner and coworkers. They conclude that an 0-glycosyl or 0-alkyl anion is more readily extruded from the sugar enediol anion of the Isbell mechanism than is a hydroxyl ion. In addition, substitution at certain positions in the sugar molecule may inhibit competing side-reactions. For example, a substituent at C4 of the hexose molecule inhibits cleavage (by reverse aldolization) into two three-carbon fragments and the resultant formation of lactic acid, a result that had been demonstrated earlier by the experiments of Evans and his associates. A combination of the two above effects, then, preferentially... 

As would be expected from the above considerations, substitution at C6 of a hexose has, at most, only a minor effect on the course of saccha-rinic acid formation. Thus, 6-0-(a-D-galactosyl)-D-glucose (melibiose) plus lime-water gives lactic acid and a mixture of the corresponding metasac-charinic, isosaccharinic, and saccharinic acids.The qualitative experiments with melibiose and with 6-0-methyl-D-glucose, based mainly on paper chromatography, suggest that the metasaccharinic acids may be the principal products from hexoses substituted at C6. 

Lactic acid is mainly prepared in large quantities (around 200 kT per year) by the bacterial fomentation of carbohydrates. These fermentation processes can be classified according to the type of bacteria used (i) the hetero-fermentative method, which produces less than 1.8 mol of lactic acid per mole of hexose, with otho- metabolites in significant quantities, such as acetic acid, ethanol, glycerol, mannitol and carbon dioxide (ii) the homo-fomentative method, which leads to greater yields of lactic acid and lower levels of by-products, and is mainly used in industrial processes [3]. The convo-sion yield from glucose to lactic acid is more than 90 per cent. 

Obligately heterofermentative Ferment both hexoses and pentoses by the 6-PG/PK (6-phosphogluconate/phosphoketolase pathway) and produce lactic acid, ethanol/acetic acid, and CO2 (this latter only from hexoses) in the ratio 1 1 1... 

Lactic acid occurs amongst the products of the action of alkali on hexoses. From 1 mole of D-glucose, treated at 25°C. with benzyltrimethylammonium hydroxide, Evans reported the production of 1.2 moles of racemic lactic acid (60 % of a theory of 2 moles per mole hexose). The substance apparently can be obtained by the action of alkali on any sugar (inclusive of trioses, pentoses, disaccharides, etc.) (99). Its preparation from sucrose under conditions of high temperature and pressure has been extensively studied from the viewpoint of potential industrial application. Lactic acid may be considered as the saccharinic acid (see below) related to glyceraldehyde and may arise from the rearrangement of a triose fragment. 

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