Carbohydrates are homo- or heteropolymers of hexoses, pentoses, or sugar derivatives, which occur in soluble form or as particles, forming grains or fibers of various sizes. In some plants, starch forms grains up to 1 mm in diameter, which is 1000 times the diameter of bacteria. Starch metabolism by bacteria requires hydrolytic cleavage by amylases to form soluble monomers or dimers, since only soluble substrates can be taken up and metabolized.
The anaerobic degradation of biopolymers in general and of cellulose in particular can be divided into hydrolytic, fermentative, acetogenic, and methanogenic phases (Fig. 1.3). Hydrolysis and fermentation of the hydrolysis products can be catalyzed by the same trophic group of microorganisms. The distinction of the two phases is of more theoretical than practical relevance. Concerning reaction rates in a methane fermenter that is fed with a particulate substrate, the rate-limiting step is hydrolysis rather than the subsequent fermentation of the monomers, if acetogenesis and me-thanogenesis proceed faster. The hydrolysis rates of polymers can be very different. Hemicellulose and pectin are hydrolyzed ten times faster than lignin-encrusted cellulose (Buchholz et al., 1986, 1988). In the acidification reactor of a two-stage anaerobic process, hydrolysis of polymers to monomers is normally slower than fermentation of monomers to fatty acids and other fermentation products. For this reason, no sugar monomers can be detected during steady-state operation. In the methane reactor, p oxidation of fatty acids, especially of propionate or n-butyrate, is the rate-limiting step (Buchholz et al., 1986). Fatty acid degradation is the slowest reaction overall in a two-stage methane reactor fed with carbohydrate-containing wastewater from sugar production. Thus, the methane reactor has to be larger than the acidification reactor to permit longer hydraulic retention times.
The rate of cellulose degradation depends strongly on the state of the cellulose in the wastewater. If cellulose is lignin-encrusted, lignin prevents access of cellulases to the cellulose fibers. If cellulose is mainly in a crystalline form, cellulases can easily attach to it, and then hydrolysis can be a relatively fast process. At increasing loading in an anaerobic reactor fed with crystalline cellulose, acetogenesis became the rate-limiting process, leading to propionate and butyrate formation (Winter and Cooney, 1980). In decaying plant material, cellulose is very often lignin-encrusted. Due to the highly restricted access to these complexes by cellulases, hydrolysis of cellulose is the rate-limiting step in its degradation to methane and CO2.
Whether microorganisms are capable of degrading lignin under anaerobic conditions is still under discussion. In a natural environment without time limitation, lignin was reported to be degraded anaerobically (Colberg and Young, 1985; Colberg, 1988). However, since these results were based on long-term experiments performed in situ and anaerobiosis was not controlled, it remains doubtful, whether the small amount of lignin that disappeared was really degraded under strictly anaerobic conditions. The occurrence of coal and fossil oil suggests that lignin compounds are highly resistant to microbial attack.
During anaerobic degradation of starch, hydrolysis by amylases proceeds with high velocity if good contact between starch grains and amylases is maintained. Whereas in an anaerobic reactor at low loading, starch degradation can proceed in the absence of acetogenic bacteria, as indicated in Figure 1.4 (route a), at high loading, volatile fatty acids are formed and acetogens are essential for total degradation (Fig. 1.4, route b). Figure 1.4 illustrates how the rate-limiting acetogenic reactions may be avoided by adjusting the conditions so that hydrolysis and fermentation occur no faster than methanogenesis. The rationale behind this is that many fermen tative bacteria produce only acetate, formate, CO2, and hydrogen when H 2-scaveng-ing methanogens or sulfate reducers are able to maintain a sufficiently low H2 partial pressure, but a wide spectrum of fermentation products, typical for the metabolism of the respective bacterial species in pure culture, is produced at higher H2 partial pressure (Winter, 1983, 1984). Methanogenesis in continuous syntrophic methanogenic cultures can be disturbed by spike concentrations of sugars or - at low concentrations of sugars - by the presence of inhibitory substances like NH3, H2S, antibiotics (Hammes et al., 1979; Hilpert et al., 1981), or xenobiotics. In consequence, the H2 partial pressure increases and volatile fatty acids are generated (Winter, 1984; Winter et al., 1989; Wildenauer and Winter, 1985; Zellner and Winter, 1987b). Once propionate or n-butyrate are produced, anaerobic degradation requires acetogens for p oxidation (Fig. 1.4, route b).
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