Fructans are defined as any compound where one or more fructosyl- fructose linkage constitutes a majority of the linkages.1 This refers to polymeric material as well as oligomers as small as the disaccharide, inulobiose. Material included in this definition may or may not contain D-glucosyl substituents. The terms oligomer and polymer are used by fructan researchers to distinguish between materials which can be specifically characterised and those which cannot.1 Fructans are widely distributed in the plant kingdom. They are present in monocotyledons, dicotyledons and in green algae. Fructans differ in molecular structure and in molecular weight. They may be classified into three main types: the inulin group, the levan group and the branched group. The inulin group consists of material that has mostly or exclusively the (2→1) fructosyl-fructose linkage. Levan is material which contains mostly or exclusively the (2→6) fructosyl-fructose linkage. The branched group has both (2→1) and (2→6) fructosyl-fructose linkages in significant amounts (e.g. graminan from Gramineae).
Several procedures have been described for the measurement of fructan in plant material and food products, but it is generally accepted that they are best measured after hydrolysis to D-fructose and D-glucose. This introduces the challenge of independently removing, or measuring, sucrose, D-fructose and D-glucose. Pontis (1966)2 hydrolysed sucrose with crystalline yeast invertase and destroyed the resulting D-glucose and D-fructose as well as existing monosaccharides by boiling with sodium hydroxide. However, yeast invertase also hydrolyses lower degree of polymerisation (DP) fructo-oligosaccharides (FOS). At a concentration of 10 mg/mL, 1-kestose is hydrolysed at approx. 20% the rate of sucrose and 1,1-kestotetraose is hydrolysed at approx. 10% the rate of sucrose.