Maltogenic α-amylase modification of pulse and maize starches to enhance the functional properties and resistant starch content

Abstract

The overarching objective of this thesis research was to modify granular lentil (LS), faba bean (FBS), pea (PS), waxy maize (WMS), normal maize (NMS) and high-amylose maize (HAMS) starches with maltogenic α-amylase (MGA) from Bacillus stearothermophilus and to characterize the functional properties and in vitro digestibility of modified starches. Pulse starches have higher amylose contents and longer amylopectin branch chains than most of the cereal starches. The first study aimed to modify LS, FBS and PS with MGA in a native granular form and to characterize and compare the structural features, functional properties, and in vitro digestibility of the modified pulse starches with that produced from NMS. Three pulse starches showed comparable characteristics before and after MGA treatment. Upon MGA hydrolysis, the granules of pulse starches were broken into small pieces. In contrast, numerous pores were observed in MGA-modified NMS granules. MGA treatment did not change the wide-angle X-ray diffraction (WAXD) patterns for all the starches, but slightly reduced the relative crystallinity of the three pulse starches. The chain lengths of amylose and long branch chain of amylopectin of all the starches were shortened by MGA hydrolysis. The shortening of amylopectin chain length reduced starch retrogradation rates. The degradation of molecular and granular structures led to the extremely low pasting viscosities of all MGA-modified starches. The 24-h MGA modification increased the resistant starch (RS) contents of cooked LS, FBS, PS and NMS by 5.9%, 6.5%, 4.2% and 4.7%, respectively, which could be attributed to the formation of retrograded amylose. The research demonstrated that MGA modification was an effective method to alter the functional properties and increase the RS contents of normal maize and pulse starches. In the second study, the effects of MGA modification on the starches from the same crop type but with different amylose contents were investigated. WMS, NMS and HAMS were selected for the modification with MGA. The structural features, functional properties, and in vitro digestibility of the modified maize starches were examined and compared with those of their respective controls. MGA performed limited enzymatic hydrolysis on HAMS because of its high amylose content and the B-type polymorphic structure. The MGA-modified WMS and NMS exhibited numerous pores in their granules. The WAXD patterns of the three maize starches were not changed by MGA treatment. MGA treatment significantly reduced the percentages of long branch chains and increased the percentages of short branch chains of WMS and NMS by hydrolyzing the α-1,4 linkages. The percentages retrogradation of WMS and NMS were substantially decreased due to the shortening of the branch chains. MGA hydrolysis degraded both granular and molecular structure of WMS and NMS, which resulted in extremely low pasting viscosities. Among the three modified maize starches, only the cooked NMS showed an increase of RS from 2.6% (control) to 7.3% due to the formation of retrograded amylose. The new findings from the research advanced our understanding of the hydrolysis patterns and resultant effects of MGA on starches with different amylose contents. The thesis research illustrated the new method of using MGA to modify starches in a native granular form and provided new insights into the impacts of MGA modification on the structures, functionality and in vitro digestibility of different pulse and maize starches. The most attractive features of the MGA-modified starches for food applications included substantially low retrogradation rate (except for HAMS) and increased RS contents (except for WMS). The new information as presented in this thesis will be meaningful for utilizing MGA treatment as a clean-label method to diversify the functional attributes and improve the nutritional value of starches from different botanical sources for wider industrial applications.