Because many current definitions are based on methods involving ethanol precipitation, oligosaccharides and fructans that are endogenous in foods, but soluble in ethanol, are not analyzed as dietary fiber. Yet endogenous human enzymes do not digest fructans which are found in plants such as chicory, onions, and Jerusalem artichoke; thus they are included in many definitions ( Table 3). Quantitation of fructans will be incomplete, even if the constituent monosaccharides of fructans are measured by a procedure that does not include ethanol precipitation, because the fructose component of fructans is labile in many acid hydrolysis procedures used during fiber analysis. Furthermore, fructose can be reduced to sorbitol and mannitol during preparation of derivatives for gas chromatographic analysis.
The oligosaccharides raffinose, stachyose, and verbacose that occur naturally in legumes and a variety of manufactured and enzymatically produced short-chain polysaccharides (e.g., fructooligosaccharides and partially hydrolyzed inulin and guar gum) also do not precipitate in ethanol. Several manufactured carbohydrates, such as methylcellulose, polydextrose, and oligosaccharides, are also resistant to human enzymatic hydrolysis. This would classify them as fiber under may definitions; however, they are not routinely analyzed as dietary fiber because they do not precipitate in ethanol.
No uniform approach has been developed to resolve the issue of fiber carbohydrates that do not precipitate in ethanol, even though many of these naturally occurring, hydrolyzed, or manufactured components are not analyzed as fiber but are considered to be fiber by many definitions. Recent analytical efforts have been directed toward the measurement of a specific carbohydrate or product, such as polydextrose or fructooligosaccharides. This individual approach has resulted in a proliferation of methods, some of which would overlap if applied to a product containing several manufactured or modified carbohydrates.
Typically, mono- and disaccharides have been found to be digestible by humans, and they do not precipitate in ethanol. Thus, no definition, except that used in China, includes these carbohydrates as dietary fiber ( Table 3). However, chemical and enzymatic modification of saccharides normally digested and absorbed in humans, such as glucose, or hydrolysis of fiber polysaccharides, such as a gum or inulin, result in mixtures that may contain monosaccharides and disaccharides that are not fully digested and absorbed. Theoretically, monosaccharides, such as arabinose, mannose, xylose, and galacturonic acid, that make up many fiber polysaccharides would be passively absorbed in the human small intestine, although unknown quantities would still reach the large intestine. Without specific disaccharidases, it is unlikely that disaccharides of these fiber-derived sugars or chemically modified disaccharides of glucose could be digested in the human small intestine. Because these mono- and disaccharides are nondigestible or poorly absorbed in the human small intestine, they could be classified as fiber.