Fermentation Endproducts

FIGURE 7.1 Theoretical approach to assess the energy content of inulin-type fructans.


As extensively reviewed and discussed above (Chapter 4, Section 4.3) and (Chapter 5, Section 5.4), inulin-type fructans are resistant to absorption and hydrolysis in the stomach and the small intestine; they are completely fermented by the intestinal microflora (mostly in the large bowel) and are not excreted in feces. Only an insignificant proportion (0.12%/24 h) has been recovered in urine of human volunteers fed a low molecular weight oligofructose.14 Inulin-type fructans are thus not absorbed nor digested by the acid of the stomach or the mammalian digestive enzymes, but they are completely fermented by the intestinal bacteria. In the discussion above and in the proposed approaches to assess the caloric value of inulin-type fructans, the percentage of the ingested dose that is fermented by the intestinal microflora equals 100%. The key questions concern:

1. The stoichiometry of the metabolism by bifidobacteria

2. The stoichiometry of the fermentation by the intestinal microflora

3. The efficiency of microbial biomass production

4. The ATP yields of the metabolism of the fermentation end products by the host

7.3.1 Stoichiometry of Metabolism by Bifidobacteria

By reference to the stoichiometry of the bifidus pathway,10 the equation of the metabolism of the hexose (essentially fructose) units of inulin-type fructans by bifidobacteria is Equation (7.4):

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