XOS (xylooligosaccharides) are polymers of xylose units with a C5 molecular structure. This means that the xylose molecules are made up of 5 carbon atoms in a ring and an aldehyde group (functional group). The xylose molecules are linked together by β-1,4 bonds.
GOS (glucooligosaccharides) are polymers of glucose units with a C6 molecular structure. This means that the glucose molecules are composed of 6 carbon atoms in a ring and an aldehyde group (functional group). The glucose molecules are linked together by α-1,2-glycosidic bonds.
XOS and GOS are classified as complex polysaccharides of dietary fibre. Both are very soluble in water. XOS and GOS cannot be broken down by human digestive enzymes. Some microorganisms such as bifido- and lacto-bacteria can use these sugar molecules as a natural food source.