Properties of starch from Australian waxy wheat

Abstract

The number of genetically different wheat varieties has increased in recent years to produce starches with different properties due to the limitation of native starch for uses in the food industry. The goal of this study was study the differences between the normal and waxy wheat starch properties, which may be useful in specific end-uses. In this thesis, QAL2000, a commercial soft wheat variety, and seven soft waxy wheat genotypes, Qa-wx-1, Qa-wx-39, Qa-wx-53, Qa-wx-59, Qa-wx-69, Qa-wx-70, Qa-wx-83, which were derived from QAL2000 by mutagenesis were used to isolate starch and study its properties. Differential scanning calorimetry (DSC), for thermal properties, X-ray diffraction (XRD) for intensity of crystallinity and high performance anion exchange chromatography (HPAEC) for amylopectin chain length distribution have been employed to investigate the starch granular structure at the nanometre scale. Starch chemical and physical structure was characterized in terms of amylose content and starch granule size distribution. Starch functionality was studied by a series of tests including swelling power, solubility, pasting properties by Rapid Viscosity Analyser (RVA) and in vitro enzymatic digestibility methods. The swelling power of the waxy granules was higher, whereas the solubility value and starch granule size distribution were lower than the QAL2000 granules. The difference in the amylose content of the two types of starches was shown to have an effect on the amylopectin chain length distribution. The absence of amylose was shown to the influence the relative crystallinity (RC), thermodynamic and functional properties of the starch granules. The study showed that, in comparison to the non-waxy parent, the waxy wheat starches displayed characteristic pasting properties that presented increased peak, and lower breakdown and final viscosities. After adding lipid to both waxy and non-waxy wheat starches pasting properties featured increased peak, breakdown and final viscosities. The interaction of granular starch with iodine solution and vapour was used to reveal information about the physical state of the polymers in the waxy and normal wheat starch granules. The interaction of granular starch to iodine solution and vapour established different levels of molecular organization between the waxy and normal starches. In vitro enzymatic digestibility of native waxy and normal wheat starches showed clear differences related to the amylose content. Wheat starches with normal amylose content had slower digestion and were more resistant to enzymes than the low amylose starches. The in vitro enzymatic digestibility of all of the cooked starches was the same, whereas in vitro of the digestibility cooked and cooled waxy and normal wheat starches was influenced by storage times. Conditioning under different relative humidities strongly influenced starch properties. Variability of moisture content in the starch granule (low and high) and storage time had large effects on starch granule structure and functional properties including swelling power, solubility and pasting properties as well as the reaction to iodine vapour. The results of this study can be applied to improve the quality of starch storage for use in food applications. The effect of alkaline treatment, which is used for starch modification and in methods for starch isolation, on starch granular structure and other physicochemical properties was studied. Alkali treated starch granules were not greatly changed morphologically but there were substantial effects on starch structure, functional properties and the in vitro enzymatic digestibility. In conclusion, this study showed that the absence of amylose significantly affected physicochemical, functional and in vitro digestion properties of waxy mutants in comparison to the non-waxy parent. The effects of moisture content and alkali treatment were also clearly different on the waxy starches compared to the QAL2000 parent. The variability in these properties due to the genetic manipulation by plant breeding the variability in these properties can have positive effects on starch properties, which may be useful for starch industrial applications.