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Fru, fructose; Gal, galactose; Xyl, xylose; Glc, glucose; Fuc, fucose; GlcNAc, N-acetylglucosamine; NeuAc, sialic acid.
Among the most studied prebiotics, lactulose, galactooligosaccharides, fructooligosaccharides, and inulin can be highlighted. Lactulose is a disaccharide produced synthetically through the chemical isomerization of lactose in alkaline conditions. Lactulose also appears naturally in milk, is not metabolized in the human digestive system, and is preferably used by lactobacilli and bifidobacteria [20]. Lactulose proved to be a prebiotic compound when 10 g was consumed daily by healthy adults, resulting in increases in the bifidobacterial counts, and decreases in clostridia counts [21]. Furthermore, the consumption of foods with lactulose (4 g), Ca (300 mg) and Mg (150 mg) by healthy adults (n = 24) resulted in increased absorption of magnesium and calcium [22]. The improvements in Ca absorption was also reported in postmenopausal women after consumption of lactulose for 9 days [23].
Figure 2.1 Health effects of orally administered prebiotic substrates. Adapted from Gibson et al. [10].
Galactooligosaccharides are naturally present in milk of several mammals and are produced industrially through the enzymatic synthesis of lactose. They are commonly used in infant formulas to replace the bifido-genic effects associated with the oligosaccharides of human milk [24]. The potential of galactooligosaccharides has been documented in studies with humans and they show improvement in constipation, reduction of harmful enzymatic activities, reduction of cancer incidence, stimulation of bone mineralization, and reduction of secondary bile acid production [20].
Fructooligosaccharides and inulin stand out as a prebiotic functional component in food applications. Thus, fructooligosaccharides are produced in two different ways, through the partial hydrolysis of fructose polymers of vegetable origin or the transfer of the fructose portion to sucrose [20]. Fructose polymers, such as inulin, occur naturally in several vegetables and fruits. Inulin occurs as a natural reserve of carbohydrates present in plants of the Asteraceae family and is industrially extracted mainly from the species Cichorium intybus (chicory roots) [25]. The inulin degree of polymerization (DP) is in the range of 10-60, consisting of fructose chains that end mainly with a glucose residue [26]. The oligofructose are produced by a partial enzymatic hydrolysis of inulin with endoinulinase, which, therefore, has a lower DP than inulin, ranging from 2 to 8 [27]. Fructooligosaccharides are produced using sucrose as substrate and the transfructosylation reaction by β-fructosylfuranosidase, which is produced by Aspergillus spp. FOS have DP of 3-5, a terminal glucose residue in each molecule, and, therefore, are non-reducing carbohydrates. They have similar stability to sucrose at neutral pH, and are resistant to high temperatures (< 150 oC) [28]. The bifidogenic effect, the reduction of colon pH, the reduction of pathogenic bacteria, the reduction of putrefactive compounds, and the improvement of constipation by fructooligosaccharides have been reported in clinical studies [20]. Furthermore, improvements on the metabolism of lipids and absorption of minerals have been reported in studies with animals. In clinical studies, the improvements in calcium absorption have been confirmed, but the effects on the metabolism of lipids were inconclusive [20].
Soy oligosaccharides are composed of stachyose, raffinoses, glucose, sucrose, and fructose, and have been isolated from soy extract [29]. The stachyose and raffinose contents in soy oligosaccharides are generally 24 and 8%, respectively, while the glucose, sucrose, and fructose content are 55%. Raffinose, in a pure form, is commercially available and produced using beet syrup [30]. Bifidobacterium strains usually grow in media containing soy oligosaccharides, stachyose or raffinose as unique carbon source, as they generally have α-galactosidase activity, which hydrolyses oligosaccharides [31]. Raffinose and soy oligosaccharides have been demonstrated bifidogenic properties in clinical studies with a daily effective dose of at least 0.5 oligosaccharides equivalent [20].
Xylooligosaccharides are xylose oligomers that show DP of 3-8 and are commercially obtained from xylan using partial enzymatic hydrolysis and endoxylanase [32]. Xylan is a type of hemicellulose and can be found on the cell walls of plant in association with pectin and cellulose. Xylooligosaccharides are obtained from plants with high concentrations of xylan, such as cotton seeds and bagasse [33]. Xylooligosaccharides can be fermented by Lactobacillus spp., Bifidobacterium spp., Peptostreptococcus and Bacteroides vulgatus. The bifidogenic effect has been demonstrated in humans where the minimum effective dose of xylooligosaccharides was 0.4 g per day. Stimulation of mineral absorption and relief from constipation have been reported in studies with rats and humans, respectively [20].
Chitin oligosaccharides are constituted by N-acetylglucosamine (GlcNAc) oligomers and can be produced using chitin from shrimp and crabs, which are subjected to a partial acid hydrolysis [34] or by using bacterial chitinase [35]. The consumption of chitin oligosaccharides is associated with improvements on the intestinal microbiota, and antimicrobial and immunomodulatory activities [36].
Maltooligosaccharides and isomaltooligosaccharides, palatinose oligomers, α-glycosyl saccharose, lactosaccharose, nigerooligosaccharides, gentiooligosaccharides, and chitosanoligosaccharides are other commercially available oligosaccharides. Although these compounds have not necessarily been used as prebiotics, they may have bifidogenic activity. Furthermore, prebiotic agents are possible fractions of oligosaccharides obtained from a partial hydrolysis of non-starch polysaccharides, such as acacia gum, guar gum, and wheat bran [20].
Recently, new sources are being explored in order to discover or isolate new prebiotic compounds [37]. For example, the consumption of
