Harvesting the Heath Potentials of Oat Fibre : Xylanase bioprocessing of arabinoxylan

Popular science summary

Unhealthy eating habits are a growing threat to global health, as it increases the risk of obesity and diet related diseases such as cardiovascular diseases, diabetes, and cancer. One contributing factor to this is that we are eating too little healthy foods, rather than too much of foods that are considered unhealthy. We specifically need to eat more whole grains and plant foods rich in dietary fibre. In Europe, we are currently eating less than 20 g dietary fibre per day. Dietary fibres are plant substances, for example different types of carbohydrates, which are not broken down by our digestive system. The World Health Organization (WHO) recently updated their dietary recommendations (July 2023), stating that adults should eat at least 25 g of dietary fibre per day, to prevent obesity and reduce risk of diet related disease. This thesis explores ways of creating fibre rich food products from oats, and in that way help consumers increase their dietary fibre intake and adopt a healthier diet to reduce diet related diseases.

Oat is a suitable cereal grain to grow in the Nordic countries, due to its resilience to rain and colder climate. Although oats are a popular breakfast food and bread ingredient, most of the oats grown in Sweden is produced for animal feed. However, the fraction of oats produced for human consumption is steadily growing, as oat drinks have become a popular dairy milk alternative and there is an increased interest in using oats as an ingredient for innovative plant-based food products. For sustainability reasons, we should reduce our intake of animal-based foods, whose production causes larger greenhouse gas emissions and has a greater negative environmental impact compared to plant-based foods. The oat crop has great nutritional value, as it is rich in healthy fats and protein, but also dietary fibre such as β-glucan, which reduces the risk of heart disease, lower blood sugar and reduces levels of bad cholesterol. Another, lesser known, dietary fibre with promising health benefits found in oats is arabinoxylan.

The arabinoxylan (AX) fibre is a long-chained water-insoluble carbohydrate which bind to other fibre molecules in the outer layer of the oat grain, the bran layer. The water-insoluble bran fibres, such as AX, are often removed during certain types of oat processing, and is considered a low value by-product, which is sold as animal feed or fuel. To use AX in a more sustainable way and incorporate it into our diet, we can break this molecule down into smaller pieces which solubilizes it, to access all the potential health benefits. In this thesis, this was achieved by using enzymes as a mild and gentle processing method. Enzymes are protein-based biological catalysts, meaning they can be used to speed up the rate of a reaction without being consumed themselves, and are employed in several industrial applications. For instance, in food industry enzymes are commonly used in the process of making cheese, lactose free milk, and to improve the texture of bread. There are an endless number of enzymes with different specific functions; some specifically catalyse the reaction to break down AX and those are called xylanases. In one study, we evaluated the synergistic effects in combining several types of xylanases and other enzymes, in combination, in order to understand how enzymes with different activities can cooperate and solubilize as much AX from oat bran as possible. Cooperation between enzymes was shown to be dependent on the oat fibre source, for example oat bran or oat hull. Additionally, using chemical treatment of the oat fibres before enzymatic treatment could increase the activity of the enzymes, by breaking up the plant structures further and making the arabinoxylan more accessible. We also explored the possibility of adding xylanases in an oat drink manufacturing process, to break down insoluble fibre. We were able to create a liquid oat product rich in soluble arabinoxylan, fibres which without enzymatic solubilization would end up as a low value by-product. This is one step towards a future where you can pick up a high-fibre oat drink from your local store.

When the xylanase enzymes cut and break down AX, diverse shorter products with various lengths and branches are formed. These shorter products are collectively called arabinoxylan-oligosaccharides or (A)XOS, and their structures depend on where the enzyme cuts the fibre chain. Interestingly, some of these oligosaccharides have prebiotic properties. Prebiotics are molecules that are food for beneficial bacteria in our large intestine and these bacteria produce compounds which stimulate a healthy gut. Since only specific structures and lengths of (A)XOS can be utilized by these bacteria, it can be of interest to find xylanases that produce only the desired (A)XOS types in high amounts. In this thesis a new xylanase, HhXyn5A, was characterized for the intended use to create (A)XOS from oat AX. HhXyn5A showed high stability, temperature tolerance and promising activity, which are all desired qualities of enzymes intended for industrial use. By studying the relationship between HhXyn5A enzyme structure and its functional properties, we found that even though two xylanases can be very similar in structure, small differences or changes in enzyme architecture influence the catalytic activity and the resulting mixture of (A)XOS products.

This thesis investigated how enzymatic processing and oat fibres can be utilized in an innovative way to increase the value of oat AX, benefiting not only the planet but also human health. We demonstrated the potential of using specific xylanaser to solubilize oat AX and create prebiotic (A)XOS, both from insoluble oat fibre flours and in a process to make fibre-rich oat drinks. This is one step towards creating new healthier food options to improve our diet, where oat (A)XOS could be added to different kinds of food products, or eaten as supplements to stimulate a healthy gut. On the technical side, this research emphasises the importance of careful enzyme selection and design, based on the application and fibre structure, to maximize yield of desired products. In addition, this research showed that exploring new xylanases, with unique activities and characteristics, facilitates the discovery of suitable candidates for industrial oat processing.