Dietary fibre production from oat hulls
Abstract: The current threats of climate change connected to the unsustainable consumption of fossil resources paired with an increasing world population demands the exploration of alternative raw materials for the production of food, fuel and specialty chemicals. Agricultural waste streams present suitable sources due to their availability in large quantities at low costs without competing with current food production. Among them are oat hulls, the outer protective layer of the oat grain, which are currently mostly disposed via incineration. In this work, oat hulls were found to be an excellent source of dietary fibres as they are rich in lignocellulose (up to 83 % of its dry weight) and the hemicellulose arabinoxylan (35 %). Arabinoxylan has been identified as a promising dietary fibre with anti-inflammatory properties. The hull’s outstanding composition, however, is threatened by the presence of extreme weather events such as unusual heat and low precipitation during its growth phase leading to a loss of 25 % of its lignocellulose and 31 % of its hemicellulose content. Simultaneously, the separation of grain and hull is impeded leading to lower industrial grain yields and higher starch contamination of the hull fraction.Despite its excellent chemical composition for the production of dietary fibres, its optical properties are rather unfavourable in the eyes of consumers due to its dark coloration which challenges its seamless inclusion in a variety of products. In this work, a mild and effective bleaching method was developed using alkaline hydrogen peroxide. This method is suitable for the production of a white insoluble dietary fibre product with a CIE L' (lightness parameter) value of above 85, while retaining the hull’s lignocellulosic content. The method is robust yielding similar bleaching results despite large raw material batch variation caused by different weather conditions during the oat’s growth phase.For oat hulls to be used as source for soluble dietary fibres with potential prebiotic effects, their recalcitrance needs to be overcome. In this work, an ultrasonication assisted alkali pre-treatment method was developed that specifically targeted the solubilisation of the hemicellulose fraction. A 10 min ultrasonication stage in water followed by a 9 h sodium hydroxide (5 M) stage at 80 ºC resulted in the solubilisation of 75 % of the present hemicellulose.For soluble dietary fibres to have a prebiotic effect, shorter chain lengths are favourable. The production of (arabino-)xylooligosaccharides up to a degree of polymerisation of 6 was achieved in high yields (57 %) by combining a ferulic acid esterase (FAE) with GH11 and GH5 xylanases. The FAE provides more access of the hemicellulose backbone to xylanases and supports further solubilisation, while the two xylanases hydrolyse the backbone in different parts of the xylan chain.Overall, oat hulls were found to have a great potential as source for the production of both soluble and insoluble dietary fibres. In preliminary product evaluation and upscaling trials, the developed methods for the production of both fibre types were promising for application on industrial scale.
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