Improving functional properties of normal maize starch granules by transglucosidase and branching enzyme

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

Abstract

This thesis describes the methods for permitting efficient modification of granular normal maize starch (NMS) by transglucosidase (TGA) and branching enzyme (BE), sequentially after the pre-treatments by maltogenic α-amylases (MA) and ethanol (ETH). We hypothesize that increasing the surface area of granules by MA and ETH pre-treatments can improve the catalysis efficiency of TGA and BE post-treatments. Without pre-treatments, TGA only treatment and BE only treatment had negligible effects on NMS. The first manuscript reported that MA pre-treatment produced porous granules with increased susceptibility to TGA. MA→TGA treatments decreased the molecular size of amylopectin molecules, increased the degree of branching, and increased the contents of amylopectin chains with the degree of polymerization (DP) <10, and decreased the contents of amylopectin chains with DP 10-28. MA pre-treatment and short-time (6 h) TGA post-treatment decreased the relative crystallinity, while long-time (20 h) TGA post-treatment recovered the relative crystallinity. NMS gels treated by MA→TGA showed a much lower increase of storage- and loss moduli during storage, showing the effect of MA→TGA on decreasing retrogradation. The second manuscript documented the effect of BE post-treatment on NMS, followed by the MA pre-treatment. In comparison with the effect of MA pre-treatment and TGA post-treatment on significantly decreasing the yield, BE post-treatments increased the yield slightly. BE post-treatments had no significant effect on amylopectin molecular structure and degree of branching, but remarkably increased the contents of long amylose chains and relative crystallinity. BE post-treatments also showed an effect on transforming A-type crystalline type to B-type. BE post-treatments improved the syneresis of granules and increased the content of slowly digested starch in the retrograded starch system. Rheology data showed the low and medium BE dosage produced the gel with higher storage- and loss moduli after storing 1 day, whereas high BE dosage led to the lower storage- and loss moduli. The third manuscript described the effect of ethanol pre-treatment (ETH) on permitting efficient modification of branching enzyme (BE) by significantly improved the swelling power of starch granules. ETH→BE treatments significantly decreased the yield and slightly decreased the degree of branching, and increased the relative crystallinity. ETH→MA treatments increased the branching degree, and the content of short amylopectin chains with DP<10. ETH→MA+BE treatments noticeably increased the contents of short and medium amylose chains and showed a lowered branching degree than ETH→MA treatments. Besides hydrolytic effect, ETH→MA treatments mainly exhibited the α -1,4 → α -1,6 transfer reaction, and ETH→BE treatments exhibited the α -1,6 → α -1,6 transfer reaction. In the combination of ETH→MA+BE treatments, BE is suggested to show the α -1,6 → α -1,6 and α -1,6 → α -1,4 transfer reactions.

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