Hydrolysis kinetic parameters of DP 6, 7, 8, and 9-12 fructooligosaccharides (FOS) of onion bulb tissues. effect of temperature and storage time

The objective of this study was to report on the variation of DP 6 isomers (6b, 6c + 6d1 + 6d2), 7a, 8, and 9-12 fructooligosaccharides (FOS) and their hydrolysis parameters percent hydrolysis, consumption rate, hydrolysis rate constant (k(obsd)), and half-life time (t(1/2))] in onion bulb tissues stored for 6 months at 10, 15, or 20 degrees C. The hydrolysis of DP 6 isomers, 7a, and 8 ranged from 74 to 85%, whereas that of DP 9-12 averaged 86%. The consumption rate of 6b, 6c + 6d1 + 6d2, 7a, 8, and -12 averaged 25, 58, 38, 26, and 48 microg/g of fresh weight per week, respectively. The k(obsd) showed large variation from 53 x 10(-3) week(-1) (lowest value) to 92 x 10(-3) week(-1) (highest value), whereas the half-life t(1/2) of the different FOS ranged between 7.5 and 13.1 weeks. DP 6b isomers increased during the first month, and then the content decreased sharply during the second month at 20 degrees C but remained stable during the last 4 months, whereas at 10 and 15 degrees C, 6b decreased progressively from the first to the six month. In contrast, DP 6c + 6d1 + 6d2 decreased abruptly within the first 3 months at 10, 15, and 20 degrees C; however, during the last 3 months they remained stable, ranging between 0.32 and 0.39. Variation of DP 7a, 8, and 9-12 FOS was close to that of DP 6 FOS isomers. DP 7a increased slightly during the first month, and afterward 7a started decreasing progressively during the last 5 months. DP 8 FOS showed a similar pattern independent of temperature regime: they increased slightly within the first month, but from the second month, DP 8 began decreasing progressively and continued decreasing to the sixth month. DP 9-12 FOS also varied similarly to DP 6c + 6d1 + 6d2. They decreased sharply within the first 2 months, and during the last 4 months, they continued to decrease slowly. Surprisingly, these variations occurred independently of temperature regimes and were affected only by storage duration. It was concluded that highly polymerized FOS are preferably hydrolyzed rather than low DP FOS isomers because they have a relatively high content of fructosyl end chains.