Carbohydrate fermentation hexoses

Historically, techniques such as the formation of osazones and the demonstration of fermentation have contributed significantly to the separation and identification of carbohydrates. Observation of the characteristic crystalline structure and melting point of the osazone derivative, prepared by reaction of the monosaccharide with phenylhydrazine, was used in identification. This method is not completely specific, however, because the reaction involves both carbon atoms 1 and 2 with the result that the three hexoses, glucose, fructose and mannose (Figure 9.19), will yield identical osazones owing to their common enediol form. 

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. 

Xanthan Gum occurs as a cream colored powder. It is a high-molecular-weight polysaccharide gum produced by a pure-culture fermentation of a carbohydrate with Xanthomonas campestris, purified by recovery with isopropyl alcohol, dried, and milled. It contains D-glucose and D-mannose as the dominant hexose units, along with D-glucuronic acid and pyruvic acid, and it is prepared as the sodium, potassium, or calcium salt. It is readily soluble in hot or cold water, but it is insoluble in alcohol. Its solutions are neutral. 

Lactic acid is prepared by the fermentation of carbohydrates, such as glucose, sucrose, and lactose, with Bacillus acidi lacti or related microorganisms. On a commercial scale, whey, corn starch, potatoes, or molasses are used as a source of carbohydrate. Lactic acid may also be prepared synthetically by the reaction between acetaldehyde and carbon monoxide at 130-200°C under high pressure, or by the hydrolysis of hexoses with sodium hydroxide. 

There are many publications and comprehensive handbooks on the thin-layer chromatography (TLC) of carbohydrates (e.g., Refs. 1 and 2). The reason is their great importance in life science and the great diversity of cases monosaccharide, disaccharide, trisaccharide, oligosaccharide, polysaccharide, aldose, ke-tose, triose, tetrose, pentose, hexose, as well as reducing and nonreducing sugars. In addition, when extracted from natural products or produced by fermentation, carbohydrates are accompanied by many impurities. That is why separation methods are used predominantly for their analysis. 

LAB are non-respiring microorganisms, principally generating ATP by fermentation of carbohydrates coupled to substrate-level phosphorylation. The two major pathways for the metabolism of hexoses are homofermentative or glycolysis (Embden-Meyerhof pathway), in which lactic acid is virtually the only end-product, and heterofermentative (phosphoketolase pathway), in which other end-products such as acetic acid, C02, and ethanol are produced in addition to lactic acid (Axelsson et al., 1989 Kandler, 1983 Zourari et al., 1992). 

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 ... 

The action of enzymes is more or less specific. Zymase, for example, will not cause the alcoholic fermentation of all the hexoses. It appears that in most cases the stereochemical configuration of the molecule is an important factor in determining whether or not an enzyme will affect a particular carbohydrate or protein. 

ANAEROBIC CARBOHYDRATE METABOLISM Yeasts growing in media containing high concentrations of fermentable carbohydrate invariably metabolize it fermentatively to produce ethanol and CO2. If air is present, and when the sugar concentration has been lowered, the ethanol is respired using the metabolic routes described above. Under the anaerobic conditions of a brewery fermentation the hexoses derived from wort fermentable carbohydrates are catabolized by the EMP pathway (Fig. 17.2) to pyruvic acid. The pyruvate produced is decarboxylated by the enzyme pyruvate decarboxylase, with the formation of acetaldehyde and CO2. The enzyme requires the cofactor thiamine pyrophosphate (TPP) for activity and the reaction is shown in Fig. 17.10. The acetaldehyde formed acts (in the absence of the respiratory chain) as an electron acceptor and is used to oxidize NADH with the formation of ethanol ... 

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. 

A potentiometric method for the determination of D-glucose, D-mannose, and D-fructose in a mixture of hexoses and pentoses employs fermentation by Streptococcus mutans. The response is based on the selective fermentation of these three carbohydrates by the organism, followed by measurement of hydronium ion. The response time is 4 min and other hexoses and pentoses do not interfere. ... 

Pentoses aldoses containing five carbon atoms. P. are an important group of monosaccharides (see Carbohydrates). Naturally occurring P. include d- and l-arabinose, L-lyxose, D-xylose, D-ribose and 2-deoxy-D-ribose, and the ketopentoses (pentuloses) o-xylu-lose and D-ribulose. P. occur chiefly in the furanose form. They are not fermented by the usual yeasts. By distillation with dilute acids, P. are converted into furfural, a reaction which serves for the detection of P. and their differentation from hexoses. 

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