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Estimation of the fermentability of dietary fibre in vitro: a European interlaboratory study

Five European laboratories tested a simple in vitro batch system for dietary fibre fermentation studies. The inoculum was composed of fresh human faeces mixed with a carbonate-phosphate buffer complex supplemented with trace elements and urea. Five dietary fibre sources (cellulose, sugarbeet fibre, soyabean fibre, maize bran and pectin) were used by each laboratory on three occasions to determine pH, residual non-starch polysaccharides (NSP) and short-chain fatty acid production during fermentation. Cellulose and maize bran degradabilities were very low (7·2 (SE 10·8) and 6·2 (SE 9·1) % respectively after 24 h), whereas pectin and soyabean fibre were highly degraded (97·4 (SE 4) and 91·1 (SE 3·4)% respectively after 24 h). Sugarbeet fibre exhibited an intermediate level of degradability (59·5 (SE 14·9)%). Short-chain fatty acid production was closely related to NSP degradation (r 0·99). Although each variable was ranked similarly by all laboratories, some differences occurred with respect to absolute values. However, the adaptation of donors to the experimental substrates was not an influential factor. Interlaboratory differences could be reduced either by adding less substrate during incubations or using less-diluted inocula. In vitro fermentations with inocula made from human faeces and from rat caecal contents gave similar results. There was a close correspondence between the data obtained in the present experiment and those previously published in in vivo studies in the rat using the same fibres. The in vitro batch system tested during the present study provides a rapid means of obtaining quantitative estimates of the fermentation and the estimation of the energy content of new sources of dietary fibre.