Insights into the structural characteristics and in vitro starch digestibility on steamed rice bread as affected by the addition of okara

Starch structure and digestibility strongly depends on food composition during rice bread processing. This study investigates the effect of okara (0%, 7%, 14%, 21%) on the texture, in vitro starch digestibility, and multiscale structure (i.e. granule, crystallites, and short-range order) of steamed rice bread (SRB). Texture profile analysis results demonstrated that okara increased the elasticity and viscidity and reduced the hardness, cohesiveness, and chewiness of SRB. The addition of okara increased amylose content, slowly digestible starch (SDS), and resistant starch (RS) of SRB significantly, thereby reducing the predicted glycemic index (pGI) from 79.14 to 74.17–68.91. Meanwhile, SEM results showed that addition of 7% okara promoted the formation of the porous space starch–gel network structure in the SRB while 21% okara produced rougher surfaces and increased bread matrix density. XRD and FTIR revealed that suitable amounts of okara (<14%) assisted in starch arrangement while others (21%) reduced the ordered crystallization of starch. Results showed that the enhanced intermolecular hydrogen bonding between okara and water enhanced the viscosity and gas retention of SRB, thereby enabling the starch to retain more gas and form a loose starch gel structure spatial network. Furthermore, the addition of okara changed the multiscale structural and amylose contents when the SDS and RS contents are high and the pGI is low. As a result, the action sites of amylase can be shielded effectively to restrict the interaction of amylase with starch. Thus, this research suggests a potential approach in which rice complexed with okara can modify the texture and starch digestibility of SRB.