The role of oat polysaccharide structure and processing in the quality of oat-fortified noodles

Abstract

Whole grain oat has a well-balanced nutritional composition, being a rich source of carbohydrates and quality protein with good amino acid balance, and a high percentage of unsaturated fatty acid and dietary fibre. Although oat is a minor crop and consumed in significantly lower qualities as compared to other major crops such as wheat, rice and maize, it has nutritional advantages in that it is usually consumed in the form of a whole-grain cereal rather than its processed products. Oat consumption in human food has been increasing because of the increasing evidence indicating the health benefits associated with dietary fibres present in the oat grain, especially β-glucan,such as reduced level of serum cholesterol, reduced risk of type II diabetes and some cancers. Therefore, oat is incorporated into various food products and noodles is one such product. Oat-fortified noodle quality depends on both eating quality (consumer sensory preferences) and nutritional expectations. Both of these characteristics of oat-fortified noodles strongly correlate with the structural and physicochemical properties of starch and β-glucan components in oat, which are affected during post-harvest processing from oat groats to noodles. The overall objectives of this thesis are to investigate the effects of processing on the structural and physicochemical properties of starch and β-glucan, understanding the contribution of starch and β-glucanon texture of noodles in order to establish structure – physicochemical properties – texture relationships, and assessing the nutritional functionality, particularly starch digestibility, of oat-fortified noodle with high oat content.The first chapter reviews current knowledge about noodles, starch and β-glucan in oatsand their effects on the quality of oat-fortified noodle. The current limits to knowledge from the literature review and overall objectives of the thesis are also presented in this chapter.Heat-moisture treatment is a crucial step in oat processing to inhibit activities of enzymes that cause undesirable effects on the end-product quality. It may also affect starch structure and functional properties of oats. Therefore, chapter 2 examined the effects of heat-moisture treatment processing on starch molecular structure, pasting and gelatinization properties of oat groats during processing. Heat-moisture treatment results in a more stable crystalline structure which degrades more easily with milling, producing more short amylopectin chains and enhancing the amount of long amylose chains as well as amylose content. Heat-moisture treatment increases onset (T0), peak (Tp) gelatinization temperatures and pasting RVA temperature but decreases gelatinization enthalpy (ΔH), breakdown viscosity, final viscosity, and setback viscosity. Whole meal oat flour is sometimes added to wheat flour noodles for nutritional value, but this can decrease eating quality. Chapter 3 presents the contributions of β-glucan and starch molecular fine structure to physicochemical properties of wholemeal oat flour and to the texture of oat-fortified white salted noodles. Hardness of oat-fortified noodles was controlled by the longer amylopectin chains (DP ≥ 26) and amount of longer amylose chains (DP≥1000). Higher levels of β-glucan, in the range from 3.1 to 5.2%, result in increased noodle hardness. Pasting viscosities of wholemeal oat flour positively correlate with the hardness of oat-fortified noodles. The swelling power of oat flour is not correlated with either pasting viscosities of oat flour or noodle hardness. Longer amylopectin chains and the amount of longer amylose chains both control the pasting viscosities of oat flour, which in turn affect noodle texture. This provides new means, based on starch and β-glucan molecular structure, to choose oats with optimal starch structure and β-glucan content for targeted oat-fortified noodle quality. Effects of sheeting and shearing during the noodle-making process and subsequent cooking on β-glucan solubility, molecular size and starch digestibility are reported in chapter 4. Samples with high levels of total β-glucan contains high amounts of soluble β-glucan. β-glucan levels reduce around 16% after cooking due to the loss of β-glucan into cooking water. Both the noodle-making process and cooking increase the solubility of β-glucan but interestingly they do not affect the molecular size of β-glucan. The digestion profiles show that the presence of β-glucan in wholemeal flour does not change starch digestion rates. In contrast, it significantly reduces starch digestion rate of oat-fortified noodle as compared to control (wheat) noodle. The interaction between β-glucan and protein from CLSM suggests that β-glucan contributes to the starch-protein matrix and changes the microstructure of noodles and thus alters digestibility of noodle.Chapter 5 summaries the major achievements in response to the objectives of this thesis. Recommendation for future work such as evaluating sensory characteristics of eating quality of noodle, measuring the postprandial blood glucose responses (PBGR) of oat-fortified noodle consumption, comparing hulled-oat genotypes with hull-less oat genotypes are also included in this chapter.