Physicochemical properties and gluten structures of frozen steamed bread dough under freeze–thaw treatment affected by gamma-polyglutamic acid

Concerned to chemical structure, the viscous gamma-polyglutamic acid (γ-PGA) should have the similar characteristics with antifreeze hydrocolloids and polypeptides which have been broadly used in frozen foods. However, the cryoprotective functions of γ-PGA are little explored as compared to those of hydrocolloids and polypeptides. In this study, influences of γ-PGA addition on fermentative performance and rheological behavior of frozen steamed bread (SB) dough during frozen storage and freeze–thaw cycles were investigated, and the underlying mechanism governing these influences was clarified. Results showed the γ-PGA dispersed free water, remarkably reduced freezable water content, and restricted water migration, which facilitated the formation of smaller and more uniform ice crystals in frozen dough structure. In addition, γ-PGA interacted with gluten proteins through electrostatic attractions and hydrogen bonds, transforming the gluten protein conformation from loose coil structures to relatively compact β-structures. In these relatively compact structures, the free sulfhydryl (SH) groups, originally located far away in space, became closer and easier to be oxidized into disulfide bonds (SS) or be involved in SH–SS exchange reaction. This led to intensive protein aggregations and water–solid interplay, rendering the dough better resilience to cope with ice growth and recrystallization. Accordingly, the deterioration in frozen SB dough's fermentability and rheology was effectively alleviated, suggesting that γ-PGA has a great potential to serve as an effective cryoprotectant in frozen SB.