7.3.2 Stoichiometry of Fermentation by Intestinal Microflora

In the complex intestinal microflora, bifidobacteria is far from being the only microorganism. Some of the metabolites they produce (especially lactate and pyruvate) are thus further oxidized, as shown in their principal microbial, metabolic path-


2 CH3COCOO- + 2 NADH + 2 H+ □ 2 CH3CHOHCOO- + 2 NAD+ (75) 2 CH3COCOO- + 4 NAD(P)H + 4 H+ + 2 ATP □ 2 CH3CH2COO- + 4 NAD(P)+ + 2 ADP + 2 P + 2 H2O (76)

2 CH3COCOO- + ADP + Pi + H2O CH3 CH2 CH2COO- + ATP + 2CO2 (7.7)

The overall equation, Equation (7.8), for the fermentation of fructose units by the mixed intestinal microflora is thus:

Based on Equation (7.7), it can be calculated that from the C atoms of a fructose unit in inulin-type fructans:

• 78% are recovered in the SCFAs, essentially acetate (64%), propionate (21%), and butyrate (15%)

• 15% are recovered in lactate

• 5% are recovered in ethanol, formate, and CO2

These figures compare reasonably well with those obtained using the generalized colonic fermentation equation derived by different authors, the major difference being the relatively high proportion of acetate (Table 7.2).1215-21

7.3.3 Efficiency of Microbial Biomass Production

However, the above analysis does not account for the fact that the fermentation of hexose also provides bacteria with maintenance energy as well as ATP and substrates (essentially acetyl-S-CoA and to a lesser extent succinic acid)for biosynthetic path-

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