Tetra- and penta-fructooligosaccharide (FOS) isomers assessment in onion bulb tissues: effect of temperature and storage time

Abstract

Tetra- and penta-fructooligosaccharide (FOS) isomers and their hydrolysis parameters––% hydrolysis, hydrolysis rate constant ( k obs) and half-lifes ( t 1/2) in onion bulb tissues kept for 6 months at 10, 15, and 20 °C, were investigated. The hydrolysis of the tetra- and pentaFOS isomers ranged between 67% and 86% and isomers of b type 4b and 5b were hydrolyzed preferentially. The k obs showed large variation from 51 × 10 −3 week −1 (lowest value) to 82 × 10 −3 week −1 (highest value), while the half-life t 1/2 of the isomers ranged between 8.5 and 16.6 weeks. DP 4 content showed close correlation at 15 and 20 °C but different at 10 °C. At 10 °C, DP 4 isomers decreased sharply during the first 2 months but remained stable during the last 4 months. At 15 and 20 °C, DP 4 increased slightly within the first month, and then decreased during the second month. During the last 4 months, the 4a isomer remained stable, while 4b and 4c increased slightly. PentaFOS isomers varied in a similar fashion to tetraFOS isomers. At 10 °C, DP 5 isomers decreased sharply during the first 2 months and remained stable during the last 4 months except for a slight increase observed during month 3 for 5b and 5c + 5d. At 15 and 20 °C, they showed a similar pattern to those of the tetra-isomers. It was concluded that pentaFOS isomers are preferentially hydrolyzed with respect to tetraFOS isomers, because they have a relatively high content of fructosyl ends chain. Results also allow us to distinguish that the DP xb (type b) isomers are preferentially hydrolyzed.