Next Prebiotics Generation

Some oligosaccharides are emerging as alternative prebiotics in addition to those mentioned before (e.g., isomalto-oligodisaccharides (IMO), soybean-oligodisaccharides (SOS), xylo- oligodisaccharides (XOS), isomalto, lactosucrose (LS)). All these oligosac- cahrides are still under investigation because the data from human trials are low and are not yet considered officially fit for use in foods as prebiotics.

IMO are produced commercially from starch by a two-stage enzymatic process. The first consists of starch liquefied by a-amylase: The p-amylase hydrolyses starch to malto-oligosaccahrides and a-glucosidase catalyzes the transglycosylation, converting the a1-4 linkages to a1-6. The IMO produced contain mainly a1-6 linkages. IMO may be produced from dextrans as well, but products obtained in this way have not yet been commercialized. The IMO production from dextrans may represent a preferential application in food, because this process offers a good control of the molecular weight distribution of the components.

IMO are partially metabolized in the human small intestine. Studies carried out in rats have demonstrated that IMO are digested in the jejunum, with the higher DP fractions being less digestible. The undigested part reaches the colon, where it is metabolized. According to the last scientific literature, isomaltose, isomaltotriose, and panose are metabolized by Bifidobacterium longum, Bifidobacterium breve, Bifidobacterium adolescentis, and bacteroides, Enterococcus faecalis, and some species of Clostridia. In vitro studies, using pH-controlled fecal batch cultures, have shown that IMO are selectively fermented by bifidobacteria (Rycroft et al., 2001; Palframann et al., 2002). Further studies, and notably human trials, are needed to confirm the effective beneficial health effect of IMO.

Soybean oligosaccharides (SOS) are a-galactosyl sucrose derivates and they are extracted from soybean whey, a byproduct of tofu manufacture. SOS contain two main oligosaccharides: raffinose (Gal a 1-6 Glc a1-2p Fru) and stachyose (Gal a1-6 Gal a1-6 Glc a1-2p Fru). Raffinose and stachyose are resistant to digestion and reach the colon after feeding to humans. In vitro studies have shown SOS to have a bifidogenic effect, but other studies show that SOS administration may increase the bacteroides population (Rycroft et al., 2001). So there is a little information related to the selectivity of SOS: They can be fermented also by other bacteria, leading sometimes to abdominal discomfort due to the production of intestinal gas, particularly when large doses are used.

Xylo-oligodisaccharides (XOS) are low-molecular-weight, reducing oligosaccharides. They are usually used in soft drinks thanks to their acid stability. XOS may be produced as syrup or in powder form. For their production, xylan from corncobs or from oats or wheat arabinoxylans may be used. The end product consists of linear p1-4 linked XOS and oligosaccharides with branched arabinose residues. In vitro studies have been shown that XOS are metabolized by a range of lactobacilli and bifidobacteria as pure cultures, but not by other bacterial genera such as Bacteroides and Clostridia. Thanks to other in-vitro studies, it seems that the selectivity for health-positive bacteria is size- dependent; in particular, low-molecular-weight XOS are better metabolized by bifidobacteria. Further human studies are needed to evaluate better the prebiotic activity of XOS.

Isomalt is trademark product composed by a mixture of two components: 1-O-a-D- glucopyranosyl-D-mannitol and 1-O-p-D-glucitol. Isomalt is well tolerated in human volunteers at 30 g per day without adverse gastrointestinal effects. From the first results, it seems that the product induces a significant increase in bifidobacteria, whereas bac- teroides decrease (Gostner et al., 2006).

Lactosucrose (LS) is nonreducing trisaccharide (Gal p1-4 Glc a1-2p Fru), obtained by reacting a mixture of sucrose and lactose. It is available on the market as syrup or powder. Some human trials reported that a daily dose of 3 g provokes an increase of bifidobacteria and a decrease of bacteroides versus the initial levels of the same genera, in fecal samples. Several potential health benefits have been reported by administrating LS—for instance, enhancement of intestinal calcium absorption in healthy young women having lower than recommended calcium intakes or increased intestinal calcium absorption in growing rats (Kishino et al., 2006) and prevention of IgE-mediated allergic diseases in mice (Taniguchi et al., 2007).

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