From insoluble to soluble: An overview of processing methods for dietary fibers in okara

Dietary fibers are commonly categorized as water-soluble (SDF) or water-insoluble (IDF) dietary fibers. SDF possesses more favorable techno-functional and textural features than IDF. Consequently, many different modification procedures have been suggested to increase the SDF content in various matrices, such as okara, an IDF-rich side stream from soy milk production. The present review summarizes the treatments reported until August 2024 to increase SDF in okara and systematically compares their efficiencies. Biological treatments resulted in the lowest increase in SDF followed by chemical treatments. Physical treatment techniques were very diverse, and some were found to be extremely efficient. They increased the SDF content five- to nine-fold, with size reduction being the most efficient treatment. Selected combinations of techniques boosted the SDF content even more. Most efficient were steam explosion (a subcategory of size reduction treatments) alone or in combination with enzymatic treatment, which increased SDF from 3.5 to 31.9% DM on average, and the alkaline treatment of extruded okara, which increased SDF from 2.8 to 34.1% DM. Although current treatments achieve high final SDF amounts, one-third of the measured fiber content remains insoluble. This limitation could be addressed by a sound understanding of the structure of the SDF-containing composites and of the dietary fibers themselves, which could enable the design of treatments optimized for higher SDF contents. Higher SDF contents can improve the techno-functional and nutritional properties, as well as the economic value of okara. • Different treatments for the increase of soluble okara fiber have been identified. • Physical and some combined treatments were the most efficient treatment categories. • The alkaline treatment of extruded okara yielded the highest final SDF content. • Steam explosion, alone or combined with enzymes, was similarly efficient.