Ultracentrifugal milling and steam heating pretreatment improves structural characteristics, functional properties, and in vitro binding capacity of cellulase modified soy okara residues

[Display omitted] •Ultracentrifugal milling (M) reduces the size of okara from 740 to 147 µm.•The size of okara is critical for the accessibility of cellulase for hydrolysis.•Cellulase (E) converts 59% of IDF in milled (M) and steam heated (S) okara into SDF.•The M and S pretreatment allows E to improve the functionality of modified residues. Soy okara contains high levels of insoluble dietary fiber (IDF). The objective of this work is to investigate the composition, structure changes, and functionality of okara residues after the modification by ultracentrifugal milling (M), milling + steam heating (M + S), or milling + steam heating + enzymatic (M + S + E) treatment. The results showed that the combination of M + S could significantly convert okara IDF into soluble ones, and the highest conversion rate (59%) was achieved with the smallest size (147 µm). The structural characterization revealed that size reduction altered the functional groups and crystallinity of the modified okara residues with irregular and enlarged morphology. More importantly, the functionalities, including water and oil holding capacities, swelling capacity, as well as cholesterol and bile acid binding capacities were improved remarkably in okara residues pretreated by M + S prior to cellulase hydrolysis. The findings provide new insights on the effective biotransformation of okara into valuable food ingredients.