Structures, properties, and digestibility of resistant starch

This study aimed to understand effects of different treatments and modifications on the structures, properties, and digestibility of selected starches. Different lipids, including corn oil (CO), soy lecithin (SL), palmitic acid (PA), stearic acid (SA), oleic acid (OA) and linoleic acid (LA), were selected for the study, and their impacts on the properties and digestibility of normal corn (NCS), tapioca (TPS), waxy corn (WCS) and high-amylose corn (HA7) starch were investigated to elucidate mechanisms of starch-lipid interactions. After cooking with the lipids (10%, w/w, dsb), NCS, TPS and HA7 showed significant decreases in the percentage enzymatic hydrolysis, and their DSC thermograms displayed an amylose-lipid-complex dissociation peak except the one cooked with the CO. 13C-NMR spectra of amylodextrin with the presence of CO showed downfield changes in the chemical shifts of carbons 1 and 4 of the anhydroglucose unit, indicating helical-complex formation. In general, free fatty acids (FFAs) reduced, but SL increased the peak viscosity of starch. FFAs and SL decreased, but CO increased the gel strength of NCS. All the lipids displayed little impacts on the digestibility and properties of WCS because it lacked amylose. Resistant starch Type 5 (RS5) was prepared by complexing debranched HA7 with SA. Because of amylose-helical-complex formation with SA, the RS5 showed restricted swelling of starch granules at 95 °C. The RS5 displayed a larger RS-content (67.8%) than the HA7 (33.5%) and NCS (0.8%) analyzed using the AOAC Method 991.43. When the cooked RS5, HA7 and NCS were used to prepare diets for rats with 55% (w/w) starch content, RS contents of the diets were 33.7%, 15.8% and 2.6%, respectively. After feeding to the rats in Week 1, ~16% of the starch in the RS5-diet was found in the feces, substantially greater than that of the HA7-diet (~6%) and NCS-diet (0.1%). The percentage of starch not being utilized in the RS5-diet decreased to ~5% in Week 9, which could be partially attributed to the fermentation of RS5 by gut microflora. Large proportions (68%-99%) of the SA in RS5-diet remained unabsorbed and were discharged in the rat feces. Effects of octenyl succinic anhydride (OSA) modification of the NCS and HA7 on their digestibility were examined. After the modification with 3% and 10% OSA, RS contents of the cooked OS-NCS increased from 0.8% of the control starch to 6.8% and 13.2% (Englyst Method), respectively, whereas that of the cooked OS-HA7 decreased from 24.1% to 23.7% and 20.9%, respectively. When the cooked NCS, HA7 and OS (10%)-HA7 were used to prepare diets for rats at 55% (w/w) starch, RS contents of the diets were 1.1%, 13.2% and 14.6%, respectively. After feeding to the rats, 20%-31% of the starch in the OS (10%)-HA7-diet was not utilized in vivo and was found in rat feces, which was substantially larger than that of the HA7-diet (≤ 5%) and NCS-diet (≤ 0.2%). Characteristics of starch and ethanol production of five sorghum lines (6B73, 6C21, 6C69, 7R34, and X789) were investigated and compared with that of B73 corn. Sorghum starches displayed higher gelatinization-temperatures (66.6-67.4 °C), greater gelatinization enthalpy-changes (13.0-14.0 J/g) and percentages retrogradation (60.7-69.1%), but slower enzymatic-hydrolysis rates (83.8-87.8% at 48 h) than the B73 corn starch (61.7 °C, 10.1 J/g, 51.5%, and 88.5%, respectively). These differences could result from that the sorghum amylopectins consisted of fewer short branch-chains (DP 6-12) (12.8-14.0%) than the corn amylopectin (15.0%). After 96 h fermentation, most ground sorghums exhibited lower ethanol-yields (30.5-31.8%) than the ground B73 corn (31.8%). Structure, properties, and digestibility of starch isolated from bamboo seeds (Bambusa textilis species) were studied and compared with that of indica and japonica-rice starch. The bamboo seeds also had compound starch granules, with morphology and sizes comparable to the rice starches. The amylopectin of bamboo-seed starch showed similar branch-chain-length distribution to that of the indica-rice starch, and both of them had longer branch-chains (DP = 19.1 and 19.7, respectively) than the japonica-rice amylopectin (DP = 17.1). Consequently, the bamboo-seed and indica-rice starch granules exhibited higher gelatinization-temperatures (To = 68.9 °C and 71.9 °C, respectively), larger enthalpy-changes (ΔH = 14.2 J/g and 15.3 J/g) and percentages of retrogradation (57.1% and 55.4%), but slower enzymatic-hydrolysis rates (50.6% and 46.3% at 24 h) than the japonica-rice starch counterpart (56.4 °C, 14.0 J/g, 11.4%, and 66.2%, respectively)