In vitro fermentation by human faecal bacteria of total and purified dietary fibres from brown seaweeds

Abstract

The in vitro degradation of dietary fibre from three brown seaweeds (Himanthalia elongata, Laminaria digitata and Undaria pinnatiJda) was studied, using human faecal flora. Two sets of fibre were tested:(1) total algal fibres extracted from the whole algae, mainly composed of alginates, and (2) purified fibres (sulphated fucans, Na-alginates and laminarans) representative of those contained in the whole brown algae. Mannuronate, one algal component, was also investigated. Substrate disappearance and shortchain fatty acid (SCFA) production were monitored after 6, 12 and 24 h fermentation. Gas production was followed hourly during the first 9 h and then at 12 and 24 h. Sugarbeet fibre was used as a fermentation reference substrate. According to the fermentative indices used, most of each of the total algal fibres disappeared after 24 h (range 60-76 %) hut, unlike the reference substrate, they were not completely metabolized to SCFA (range 47-62 %). Among the purified algal fibres, disappearance of laminarans was approximately 90% and metabolism to SCFA was approximately 85% in close agreement with the fermentation pattern of reference fibres. Sulphated fucans were not degraded. Naalginates exhibited a fermentation pattern quite similar to those of the whole algal fibres with a more pronounced discrepancy between disappearance and production of SCFA: disappearance was approximately 83 % but metabolism was only approximately 57 YO. Mannuronate was slowly fermented hut its metabolism corresponded to its disappearance from the fermentative medium. Thus, the characteristic fermentation pattern of the total fibres from the three brown algae investigated was attributed to the peculiar fermentation of alginates, and mannuronate was shown not to be directly involved.In vitro fermentation: Faecal bacteria: Dietary fibre: Brown algae Most algal polysaccharides are resistant to hydrolysis by human endogenous digestive enzymes and thus are considered as dietary fibre (Trowel1 et af. 1976; Southgate, 1977; Salyers et al. 1977~-c). The recent approval of ten seaweeds for human consumption in France (Fleurence, 1991) led to a revival of interest in these sources of fibre, and information concerning methods of quantification, amounts and physico-chemical properties of algal dietary fibres has been published recently (Kishi et af. These reports demonstrate that seaweeds are rich in dietary fibre, with a particularly high amount of the soluble forms in brown algae. In these seaweeds, soluble fibre consists of laminarans (Pl-3, Pl-6 glucans associated with mannitol residues; * For reprints.

Alginate as a Source of Dietary Fiber

“…However, in separate studies on algal oligosaccharides, alginate oligosaccharides were not found to alter the total aerobe/anaerobe balance in faecal inoculum (Michel et al, 1999). In other faecal inoculum studies, the fermentability of faecal alginate is suggested as being much lower than the values quoted by Michel et al, (1996), as only 50% of alginate had disappeared after 24 h (Bobin-Dubigeon et al, 1997). Alginates appear to have a slightly lower fermentability in vivo in rats, as 64% of alginate is recovered in faeces upon initial feeding with alginate.…”

“…Although over 80% of alginate was degraded in 24 h incubation with faecal inoculum, levels of SCFA release were significantly lower than this (Michel et al, 1996). This could suggest that alginates are increasing the numbers of aerobes in the inoculum, therefore reducing the levels of measured SCFA released (due to the aerobic metabolism of alginate/SCFA), or that SCFA are not being produced as a byproduct of alginate metabolism.…”

“…Upon incubation in human faecal microflora, alginate degradation does not show signs of SCFA and gas production until after 6 h (Michel et al, 1996). Although over 80% of alginate was degraded in 24 h incubation with faecal inoculum, levels of SCFA release were significantly lower than this (Michel et al, 1996).…”

“…Previous studies have reported the bacterial degradation of, and short-chain fatty acid production from, alginates (Suzuki et al, 1993a;Michel et al, 1996;Kuda et al, 1998). However, relatively few models have considered the effects of alginate inclusion on the bacterial microflora, and the potential effects of this on the host, or how alginate structure may effect its degradation.…”

“…Two previous studies have cited the production of various SCFA from sodium alginate by human faecal microflora innoculates (Michel et al, 1996;Kuda et al, 1998). These are shown below in Table 3.…”