Taste of common prebiotic oligosaccharides: impact of molecular structure

Abstract

Prebiotic oligosaccharides are naturally occurring nondigestible carbohydrates with demonstrated health benefits. They are also a chemically diverse class of nutrients, offering an opportunity to investigate the impact of molecular structure on oligosaccharide taste perception. Accordingly, a relevant question is whether these compounds are detected by the human gustatory system, and if so, whether they elicit sweet or "starchy" taste. Here, in 3 psychophysical experiments, we investigated the taste perception of 3 commercially popular prebiotics [fructooligosaccharides (FOS), galactooligosaccharides (GOS), xylooligosaccharides (XOS)] in highly pure form. Each of these classes of prebiotics differs in the type of glycosyl residue, and position and type of bond between those residues. In experiments I and II, participants were asked to discriminate a total of 9 stimuli [FOS, GOS, XOS; degree of polymerization (DP) of 2, 3, 4] prepared at 75 mM in the presence and absence of lactisole, a sweet receptor antagonist. We found that all 9 compounds were detectable (P < 0.05). We also found that GOS and XOS DP 4 were discriminable even with lactisole, suggesting that their detection was not via the canonical sweet receptor. Accordingly, in experiment III, the taste of GOS and XOS DP 4 were directly compared with that of MOS (maltooligosaccharides) DP 4-6, which has been reported to elicit "starchy" taste. We found that GOS and MOS were perceived similarly although narrowly discriminable, while XOS was easily discriminable from both GOS and MOS. The current findings suggest that the molecular structure of oligosaccharides impacts their taste perception in humans.

Keywords: carbohydrates; conformation; degree of polymerization; glycosidic bond; prebiotics; starch taste.

Fig. 1.

Chemical diversity in oligosaccharides. Oligosaccharides can differ in (A) base residue; for example, glucose, galactose, fructose, and xylose are common. (B) The linkages between residues can be in α or β conformation, and are formed between (C) the anomeric carbon (carbon 1 in the case of glucose) on one residue and one of several carbons on the adjacent residue (numbered here). (D) The terminal residue of the chain may have a free anomeric carbon (i.e. not in a glycosidic linkage). When an anomeric carbon (*) is free, the residue can convert to an acyclic form; in such cases the oligosaccharide is said to have a reducing end (denoted above as diagonal lines across the appropriate terminal residue). (E) The degree of polymerization of residues varies between 3 and 20.

Fig. 2.

List and visualization of selected prebiotic stimuli (DP 3, 4) and corresponding sugars (DP 2).

Fig. 3.

Detectability (dʹ) for fructooligosaccharides (FOS), galactooligosaccharides (GOS), and xylooligosaccharides (XOS), determined using triangle tests. Stimuli were tested either without (A-C) or with (D-F) lactisole, a sweet taste antagonist. Gray horizontal line represents the level at which samples are considered significantly detectable (P < 0.05).

Fig. 4.

(A) Data are directly compared discriminability (dʹ) for galactooligosaccharides (GOS) DP 4, xylooligosaccharides (XOS) DP 4, and maltooligosaccharides (MOS) DP 4-6. Discriminability was determined using triangle tests. Dotted line represents the level at which targets are considered significantly discriminable from nontargets (P < 0.05). (B) Distance, represented by the magnitude of dʹ, between GOS, MOS, and XOS, demonstrating that GOS and MOS are perceptually “closer” than XOS is to either.

Fig. 5.

Structural conformations of MOS (A), cyclodextrin (B), and PDOS (C). (D) Demonstration of the structural heterogeneity in the GOS DP 4 sample (adapted from van Leeuwen et al. 2014). Green circles represent galactosyl units; light blue circles represent glucosyl units.