VEGETABLE FIBER FERMENTATION BY HUMAN FECAL BACTERIA - CELL-WALL POLYSACCHARIDE DISAPPEARANCE AND SHORT-CHAIN FATTY-ACID PRODUCTION DURING INVITRO FERMENTATION AND WATER-HOLDING CAPACITY OF UNFERMENTED RESIDUES

Dietary fiber from eight vegetables (broccoli, carrot, cauliflower, ce lery, cucumber, lettuce, onion and radish) was analyzed for chemical c omposition and potential in vitro fermentation by human fecal bacteria . Total dietary fiber concentration of substrates ranged from 34.9 (br occoli) to 5.8 (cucumber) g/kg edible matter. Substrate fiber fraction s were composed primarily of pectic substances and cellulose with smal ler concentrations of hemicelluloses and lignin. Total dietary fiber r esidues isolated from substrates were fermented in vitro for 24 h with fecal bacteria obtained from each of three human volunteers. Substrat e dry matter disappearance during fermentation was highest for carrot (63.7%) and lowest for cucumber (49.4%). Averaged across all substrate s, disappearances of arabinose, galactose, glucose, mannose, xylose an d uronic acids during fermentation were 96, 90, 54, 68, 51 and 97%, re spectively. Short-chain fatty acid (SCFA) production during substrate fermentation averaged 10.5 mmol SCFA/g dry matter fermented. Averaged across all substrates, production of the major SCFA, acetate, propiona te and butyrate, occurred in the molar ratio 76:14: 10. Potential wate r-holding capacity of substrates was not influenced by fiber source an d averaged 2.04 g H2O/g original substrate dry matter. Extent of subst rate fermentation, SCFA production and substrate potential water-holdi ng capacity were significantly different among inoculum donors, indica ting that considerable inter-individual variation exists in the potent ial in vivo fermentation of vegetable fiber.