Swelling and Gelatinization of Oat Starches

Abstract

A comparison was made between the structure and physicochemical properties of starches extracted from five normal and one naked cultivar of oat. There was little difference in the molecular size or polydispersity of the native amylose and amylopectin determined by gel permeation chromatography (GPC), or in the unit chain distribution of isoamylase-debranched amylopectin, where three peaks were resolved at modal degree of polymerization of 15, 23, and 46. Apparent, total, and A-amylose (difference between apparent and total amylose due to lipid complexing) contents ranged from 19.7 to 22.0%, 27.5 to 29.8%, and 7.1 to 8.1, respectively, with lipid content, as fatty acid methyl esters, ranging from 0.66 to 0.75% on a dry basis and comprising the following fatty acids : 46.6% C16 ; 2.1% C18 ; 15.0% C18 :1 ; 35.3% C18 :2, and 0.7% C18 :3. The naked oat starch had the lowest lipid content and Δ-amylose. Lintner type solubilization in 2M HCl (six days of incubation at 35°C) ranged from 56.6 to 60.0% of the original dry α-glucan. The 80°C swelling factor corresponding to postgelatinization swelling (determined by a blue dextran dye exclusion method) ranged from 8.6 to 10, with the naked oat being at the top of the range. At this temperature, the amount of α-glucan leached from the granules ranged from 4.1 to 6.6% (dry basis), where the highest value corresponded to the naked oat. Amylose (determined colorimetrically) accounted for 42.1-47.6% of the α-glucan in the leachate. No intermediate material could be detected by GPC. Damaged starch levels were very low in the starches analyzed (1.9-2.2% on a dry basis). The high proportion of branched α-glucan in the leachate was confirmed by GPC and was attributed to the fragile nature of the gelatinized oat starch granules. Gelatinization parameters by differential scanning calorimetry were 44.7-47.3, 56.2-59.5, and 68.7-73.7°C for T 0 , T p and T c respectively, with ΔH ranging from 8.1 to 9.5 J/g. The mean diameter of native granules, determined by Coulter Counter, ranged from 4.96 to 5.63 μm, with the naked oat starch being at the low end.