Upcycling Plant-Based Bevergae Residues (Okara) from Soy and Oat: Strategies for Shelf-Life Extension and Novel Food Product Development

In the plant-based beverage industry, a residue called okara, is generated in large quantities in the liquid-solid separation step. Despite the high nutritional value of okara from soy and oat beverages with respect to fiber and protein content, these residues are generally discarded or used as animal feed or compost. It would be more sustainable to utilize okara for human consumption, however, the main issue is the short shelf-life due to its high moisture content and water activity. The aim of this doctoral thesis was therefore to investigate strategies to extend the shelf-life of soy and oat okara and to find appropriate food applications to reduce food losses and thus improve the circular economy of plant-based beverages.
High-pressure processing was evaluated to improve the shelf-life of soy and oat okara at 200 MPa, 400 MPa, and 600 MPa. It was found that a treatment at 600 MPa could successfully extend the shelf-life for up to 2 weeks for soy okara, and almost 4 weeks for oat okara at storage in 4°C. High-pressure processing can therefore be a potential pasteurization technique for soy and oat okara to achieve a microbiologically safe ingredient. However, more research is required to optimize the process and to further investigate the microbiological species present in soy and oat okara to exclude any potential food safety risks. A beneficial nutritional outcome was also observed for the 600 MPa treatment, increasing the soluble fiber fraction of the total dietary fiber content in both soy and oat okara to 4.4% and 9.2%, respectively.
Dried oat okara was co-extruded together with corn grits to produce an expanded snack prototype. Five formulations were extruded (corn grits and dried oat okara ratios: 100:0, 90:10, 80:20, 70:30, 60:40) at three different feed moistures (14%, 16%, and 18%). Dried oat okara was successfully incorporated up to 30%. A threshold was reached at 40% dried oat okara, resulting in low specific mechanical energy input, melt temperature and pressure, leading to hard and dense extrudates with low expansion and porosity.
Dried oat okara and hempseed protein concentrate were co-extruded to produce a high-moisture meat analog. It was performed with a novel wet-feeding method of the hempseed protein concentrate, where the hemp slurry was fed through the water inlet. Three different feed moistures (49%, 52%, and 54%) and screw speeds (500 rpm, 700 rpm, and 900 rpm) were tested at a temperature profile of 40-70-110-130°C. The lowest feed moistures (49% and 52%) and the highest screw speeds (700 rpm and 900 rpm) were also tested for a higher temperature profile of 40-70-120-150°C. It was possible to achieve a high-moisture meat analog with thin-layered and fibrous structures, where dried oat okara could contribute up to 64% of the total protein. A higher feed moisture resulted in decreased hardness and chewiness, while a higher temperature profile had a reverse effect as it increased the degree of texturization. Change of screw speed did not have any consistent influence on the textural attributes.
Okara from soy and oat have great potential to become nutritious ingredients in a large variety of food products. In this doctoral thesis, applications for snacks and high-moisture meat analogs were explored, but okara could also be suitable for various bakery goods. There are, however, still issues regarding the microbial safety of okara that need to be further investigated. Upcycling of okara reduces food losses in the food industry by using all of the processed crops for food, leading to a better utilization of resources.