Thermo-responsive shellac/HPMC double-layered microcapsules for controlled release and stability enhancement of baker's yeast in frozen dough.

Frozen dough technology faces two major challenges: loss of yeast viability caused by freezing damage, and uncontrolled fermentation due to delayed freezing or temperature rise during thawing. In this study, a thermo-responsive double-layered microcapsule based on shellac and hydroxypropyl methylcellulose (HPMC) was developed to achieve cryoprotection and controlled release of yeast. Active dry yeast was encapsulated with HPMC (30 mPa·s) via a wet powder-layering method (loading capacity: 76.15%; survival after subsequent ethanol exposure: 93.25%). The outer shellac layer was plasticized with tributyl citrate (TBC), where 7.5% TBC was identified as the optimal concentration. Dynamic mechanical analysis (DMA) showed that the formulation with 7.5% TBC maintained relatively high rigidity at frozen storage temperatures. In contrast, it underwent significant softening within the proofing temperature range (35-40 °C). Dynamic vapor sorption (DVS) results showed that this group exhibited the lowest moisture absorption rate at 25 °C and enhanced moisture absorption response at 40 °C. Optical microscopy and spectrophotometric turbidity analysis verified that the rehydration of HPMC-yeast cores generated substantial volumetric expansion and internal swelling stress, triggering a temperature-dependent ductile shell deformation and rupture in the 7.5% TBC group, which was superior to the brittle fracture of the 0% TBC group. In frozen dough applications, the microcapsules effectively suppressed premature fermentation during cold holding, realized a controlled "delayed activation-concentrated fermentation" behavior, and preserved yeast survival at 86.47% after 6 freeze-thaw cycles (compared to 59.48% in the unencapsulated control). This encapsulation strategy provides a high-performance solution for stable, controllable frozen fermented products.