Thermally responsive yeast microcapsules, methods of making and uses

Abstract

The application discloses a kind of thermally responsive yeast microcapsules, preparation method and application, with yeast as core, hydrophilic inner layer and hydrophobic outer layer are successively coated to form microcapsule;The hydrophilic inner layer is hydroxypropyl methyl cellulose (HPMC) layer, and the viscosity grade of hydroxypropyl methyl cellulose is 5-100 mPa·s;The hydrophobic outer layer is shellac layer, and tri-butyl citrate (TBC) is added as plasticizer, and the addition amount of the tri-butyl citrate is 0%-10% (w / w) based on the solid mass of shellac, and the end point value excluding 0 is not included.By adjusting the viscosity grade of HPMC and the addition amount of TBC, the microcapsule keeps structural stability and reduces water migration at frozen storage temperature, the shell softens at proofing temperature zone and absorbs the inner layer to trigger release, so as to realize yeast activity maintenance and on-demand fermentation in frozen fermentation dough system, and can be used in frozen fermentation dough or its products, reduce yeast inactivation caused by freezing and thawing and inhibit early fermentation in low temperature stage.

Description

Technical Field

[0001] This invention relates to the fields of food biotechnology and agricultural product processing engineering, specifically to a method for preparing a thermoresponsive yeast microcapsule and its application in a frozen dough system. Background Technology

[0002] Frozen dough technology, which involves freezing dough at low temperatures and storing and transporting it in a cold chain, extends shelf life and improves production efficiency and quality consistency, thus becoming an important part of modern baking and cold chain food systems. However, freezing, refrigeration, and freeze-thaw cycles can cause ice crystal formation and recrystallization, moisture migration, etc., leading to damage to the gluten network in the dough, a decrease in the dough's gas-holding capacity, and ultimately, a decrease in specific volume, coarse texture, and other quality degradation. For fermented frozen dough, cell damage caused by ice crystal damage, osmotic pressure fluctuations, and oxidative stress during freezing and storage reduces yeast survival rate and gas production capacity, further exacerbating insufficient fermentation dynamics and product volume loss. In addition, if there is a delay in freezing the dough before freezing or premature fermentation occurs during the thawing and warming stage, the yeast will consume fermentable substrates and produce metabolites such as CO2 in non-target stages, resulting in insufficient fermentation capacity or inconsistent response during the proofing stage, further reducing product quality and process stability.

[0003] Existing improvement approaches mainly include: adding antifreeze agents or modifiers, optimizing freezing and storage processes, and improving freeze resistance through strain selection. However, the protective effect of adding modifiers has an upper limit, and excessive addition may affect flavor and formula cleanliness; process optimization has limited adaptability to supply chain fluctuations and repeated freeze-thaw cycles; and strain modification is also constrained by regulations and consumer acceptance.

[0004] Microencapsulation technology, as a method to construct physical barriers for sensitive active ingredients or probiotics and effectively isolate them from external environmental stresses, has seen highly systematic development in the fields of pharmaceuticals and functional food additives. In the field of frozen dough, strategies such as alginate hydrogel immobilization of yeast can improve freeze resistance and the sensory qualities of the finished product, but they largely rely on hydration systems, requiring pre-rehydration of the yeast and failing to fundamentally achieve freeze protection. Meanwhile, conventional preparation routes such as spray drying involve severe thermal stress and shear force collisions during processing, often resulting in the death of a large number of active microorganisms during the encapsulation stage. Therefore, there is an urgent need to develop a novel yeast microcapsule preparation route that is environmentally mild, highly biocompatible, and possesses an environmental temperature-driven rupture mechanism. Summary of the Invention

[0005] This invention provides a thermoresponsive yeast microcapsule, its preparation method, and its application, aiming to solve at least the following technical problems:

[0006] Firstly, it reduces yeast inactivation and maintains subsequent fermentation capacity during frozen storage or repeated freeze-thaw cycles. Secondly, it inhibits or delays premature yeast fermentation during the low-temperature thawing or warming stage, thereby improving the concentration and controllability of the fermentation response during the waking stage. Thirdly, while ensuring food applicability and mild preparation, a reproducible method and parameter window for preparing bilayer microcapsules are provided.

[0007] To solve the above problems, the technical solution adopted by the present invention is as follows: A heat-responsive yeast microcapsule, which consists of yeast as the core, and sequentially encapsulates a hydrophilic inner layer and a hydrophobic outer layer to form a microcapsule; The hydrophilic inner layer is a hydroxypropyl methylcellulose layer, and the viscosity grade of hydroxypropyl methylcellulose is 5-100 mPa·s; The hydrophobic outer layer is a shellac layer, and tributyl citrate is added as a plasticizer. The amount of tributyl citrate added is 0% to 10% w / w based on the solid mass of shellac, excluding the endpoint value of 0.

[0008] Optionally, the viscosity grade of the hydroxypropyl methylcellulose is 30 mPa·s; The amount of tributyl citrate added is 5% to 7.5% w / w.

[0009] Optionally, the particle size of the microcapsules ranges from 250 to 850 μm.

[0010] The preparation method of any of the heat-responsive yeast microcapsules of the present invention includes: (1) Preparation of HPMC-yeast kernel: wet dry yeast and nonionic surfactant were premixed and moistened, and then mixed with HPMC dry powder of different viscosities. Atomized water was sprayed in a rotating rolling state to moisten and activate the yeast, so that HPMC formed a film in situ to coat the yeast. Then, the yeast was dried at low temperature to constant weight to obtain HPMC-yeast kernel with a particle size in the range of 250-850 μm. (2) Preparation of thermally responsive yeast microcapsules: Yeast microcapsules were prepared by solvent evaporation method. The HPMC-yeast kernel obtained in step (1) was mixed with shellac-TBC ethanol solution for casting and embedding, followed by drying and separation. Particles with a particle size in the range of 250-850 μm were collected.

[0011] Optionally, in step (1), the mass-to-volume ratio of dry yeast, nonionic surfactant, and HPMC powder is 3.0 g:(0.5-1.0) mL:(2.0-2.5) g; Optionally, in step (1), the nonionic surfactant is Tween 80; the rotation speed is controlled at 50-80 rpm, and the drying temperature is 25-35℃.

[0012] Optionally, in step (2), the shellac concentration in the shellac-TBC ethanol solution is 4% to 10% w / v.

[0013] Optionally, in step (2), the shellac concentration is 6% to 8% w / v.

[0014] Optionally, in step (2), a "double encapsulation, concentration control" process is adopted; The volume ratio of the added shellac-TBC ethanol solution to the HPMC-yeast kernel was controlled between 1.5 mL:1.0 g and 1.2 mL:1.0 g. The HPMC-yeast kernel was first cast and embedded using 60% to 70% of the total volume of the shellac-TBC ethanol solution, and after drying, a primary shellac layer was formed. The dried microcapsules were then encapsulated a second time using the remaining shellac-TBC ethanol solution, and after drying and separation, thermoresponsive yeast microcapsules were obtained.

[0015] The application of any of the heat-responsive yeast microcapsules described in this invention in the fermentation of frozen dough, refrigerated fermented dough, and pre-baked foods.

[0016] The beneficial effects of this invention are: (1) The preparation process is mild and retains the activity: The heat-responsive yeast microcapsules provided by the present invention are prepared by wet powder stratification method to prepare HPMC-yeast kernel and by solvent evaporation method to prepare shellac-TBC shell. The preparation process is mild and can maintain the original activity of highly active dry yeast to the greatest extent.

[0017] (2) Dual protection mechanism: When the temperature is below the glass transition temperature of shellac, the shellac on the outer layer of the microcapsule is always maintained in a highly dense, low-permeability glass state, preventing external moisture from seeping into the core and preventing the moisture in the dough from activating the yeast prematurely. HPMC not only acts as an ethanol barrier layer to maintain the original activity of the yeast during the preparation process, but also plays a water-absorbing buffering role during freezing, reducing the physical damage of ice crystals to the yeast.

[0018] (3) Intelligent thermal response release: When the temperature is higher than the glass transition temperature of shellac, the shellac barrier layer rapidly undergoes mechanical relaxation and significant softening. Water molecules that migrate freely in the dough begin to penetrate the softened shellac membrane and are rapidly absorbed by the hydrophilic HPMC in the inner layer, causing it to expand violently inside the core and generate expansion pressure. This coupled triggering mechanism, consisting of "external temperature softening" and "internal moisture swelling", ultimately causes irreversible structural rupture of the microcapsules, successfully achieving intelligent controlled release of yeast in the dough during the proofing stage. Attached Figure Description

[0019] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings: Figure 1 Scanning electron microscope images of yeast kernels prepared with HPMC of different viscosities; Figure 2 Mechanical and thermal properties of shellac films with different TBC concentrations; Figure 3 Conventional fermentation curves of thermoresponsive yeast microcapsule dough under different freeze-thaw cycles and freezing times; Figure 4 Cold fermentation and conventional fermentation curves of thermoresponsive yeast microcapsule dough under different freeze-thaw cycles and freezing times. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The present invention relates to thermoresponsive yeast microcapsules, their preparation method, and applications. The microcapsules have a double-layer structure, comprising: a hydrophilic inner layer (primarily composed of hydroxypropyl methylcellulose (HPMC), providing both protection and moisture-induced swelling) encapsulating the yeast, and an outer hydrophobic barrier shell (primarily composed of shellac, containing tributyl citrate (TBC) to regulate thermomechanical transition). By adjusting the viscosity grade of HPMC and the amount of TBC added, the microcapsules maintain structural stability and reduce moisture migration at frozen storage temperatures. During proofing, the outer shell softens and synergistically triggers release with the swelling of the inner layer, thereby maintaining yeast activity and enabling on-demand fermentation in frozen fermented dough systems. The preferred preparation method includes: preparing the HPMC-yeast core using a wet powder stratification method and preparing the shellac-TBC outer shell using a solvent evaporation method. These microcapsules can be used in frozen fermented dough or its products (such as frozen dough, frozen proofed dough, and quick-frozen baked goods) to reduce yeast inactivation caused by freeze-thaw cycles and inhibit premature fermentation at low temperatures.

[0022] The microcapsules of this invention possess both low-temperature dormancy protection and high-temperature thermal response with site-specific rupture characteristics. They have a multi-layered core-shell structure, with active dry yeast at the core, surrounded by an inner protective layer constructed of hydroxypropyl methylcellulose (HPMC) and an outer functional barrier layer constructed of natural decolorized and dewaxed shellac. Furthermore, this invention introduces the plasticizer tributyl citrate (TBC) into the shellac to adjust its glass transition temperature, enabling the microcapsules to exhibit thermally responsive controlled-release characteristics at dough proofing temperatures.

[0023] The method for preparing the heat-responsive yeast microcapsules of the present invention includes the following steps: (1) Preparation of HPMC-yeast kernel: HPMC-yeast kernel was prepared by wet powder deposition method. Highly active dry yeast was premixed and moistened with nonionic surfactant, and then mixed with HPMC dry powder of different viscosities. Atomized water was sprayed in a rotating rolling state for wetting and activation, so that HPMC formed a film in situ to coat the yeast. Then, it was dried at low temperature with hot air to constant weight. (2) Remove HPMC powder not coated on the dry yeast particles using a 60-mesh (250 μm) sieve. Then remove excessively large aggregates using a 20-mesh (850 μm) sieve to obtain HPMC-yeast kernels with a particle size in the range of 250 to 850 μm.

[0024] (3) Preparation of thermally responsive yeast microcapsules: Yeast microcapsules were prepared by solvent evaporation method. The HPMC-yeast kernel obtained in step (2) was mixed with shellac-TBC ethanol solution of a specific mass concentration for casting and embedding. The ratio of the volume of solution added to the mass of the composite kernel was controlled. The shell thickness and loading were controlled by adjusting the concentration of shellac solution. Then, the mixture was dried and separated.

[0025] (4) The dried product is subjected to standard sieving according to step (2) to collect particles with a particle size in the range of 250 to 850 μm, thus obtaining the heat-responsive yeast microcapsules.

[0026] In an embodiment of the present invention, in step (1), the viscosity grade of HPMC is one or more of 5 to 100 mPa·s, with a preferred viscosity grade of 30 mPa·s.

[0027] In the embodiments of the present invention, in step (1), the mass-volume ratio of highly active dry yeast, nonionic surfactant and HPMC powder is 3.0 g: (0.5-1.0) mL: (2.0-2.5) g; the nonionic surfactant is preferably Tween 80; the spray water addition amount corresponding to HPMC of different viscosity grades is determined according to the optimization of nucleation and adhesion efficiency and yield; the rotation speed of the rotary granulator is controlled at 50-80 rpm and the drying temperature is 25-35℃.

[0028] In an embodiment of the present invention, in step (3), the amount of TBC added is 0% to 10% (w / w) based on the shellac solids mass, and preferably the amount of TBC added is 5% w / w to 7.5% w / w based on the shellac solids mass.

[0029] In an embodiment of the present invention, in step (3), the concentration of shellac is 4% to 10% w / v, preferably 6% w / v to 8% w / v.

[0030] In an embodiment of the present invention, step (3) employs a "double-layer encapsulation, concentration control" process; the volume ratio of the added shellac-TBC ethanol solution to the HPMC-yeast kernel is controlled between 1.5 mL:1.0 g and 1.2 mL:1.0 g; 60% to 70% of the total volume of the shellac-TBC ethanol solution is used for the first layer of shellac-yeast kernel encapsulation, and after drying, a primary shellac layer is formed. The remaining shellac-TBC ethanol solution is used for the second layer of shellac-TBC ethanol encapsulation on the dried microcapsules to improve the continuity of the shell and seal the pores. After drying and separation again, thermoresponsive yeast microcapsules are obtained.

[0031] Furthermore, this invention discloses the use of the above-described heat-responsive yeast microcapsules in frozen fermented dough.

[0032] Material Commodity Information: Highly active dry yeast ( Saccharomyces cerevisiae (Low-sugar type) was purchased from Angel Yeast Co., Ltd. Food-grade decolorized and dewaxed shellac was purchased from Aigao Wenbao (Beijing) Technology Co., Ltd. Hydroxypropyl methylcellulose (HPMC), tributyl citrate (TBC), and Tween 80 of different viscosity grades (5 mPa·s, 15 mPa·s, 30 mPa·s, 50 mPa·s, 100 mPa·s) were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0033] Part One: This section mainly studies the effect of HPMC viscosity on the physicochemical properties of HPMC-yeast kernel. 1.1 Preparation of HPMC-yeast kernel Five different viscosity grades of hydroxypropyl methylcellulose (HPMC, with viscosities of 5, 15, 30, 50, and 100 mPa·s, respectively) were screened through a 100-mesh (150 μm) standard sieve and dried at 60°C to constant weight for later use. 2.0 g of highly active dry yeast ( Saccharomyces cerevisiae The yeast cells (low-sugar type) were premixed with 0.35 g of Tween 80 to enhance interfacial affinity, followed by the addition of 1.5 g of HPMC powder of different viscosities, which were then mixed thoroughly. The mixture was placed in a small rotary granulator (60 rpm) and continuously tumbled to allow the HPMC powder to form an initial layer on the surface of the yeast particles. Subsequently, a fine water mist was intermittently sprayed to wet and activate the HPMC powder, promoting the formation of a continuous and stable protective film on the yeast surface. To ensure optimal film-forming quality of the microcapsule core, the water spray volume was pre-optimized based on the adhesion efficiency and yield of HPMC at various viscosities. The optimal water spray volumes for the 5 mPa·s, 15 mPa·s, 30 mPa·s, 50 mPa·s, and 100 mPa·s groups were set to 1.8 g, 1.7 g, 1.6 g, 1.3 g, and 1.2 g, respectively. The film-forming core particles were immediately dried at 30°C until constant weight and stored in a desiccator for further encapsulation.

[0034] 1.2 Load Capacity Measurement Accurately weighed HPMC-yeast kernel sample (M core Disperse the yeast in 50 mL of deionized water and shake for 30 min to ensure complete dissolution of HPMC and Tween 80. Then centrifuge the suspension at 8000 g for 10 min, discard the supernatant, wash the yeast precipitate twice with water, collect the precipitate, and dry it at 45°C until constant weight (M). yeast The kernel load is calculated using the following formula. Unencapsulated dry yeast was used as a control for data correction.

[0035] Load capacity (%) = (M yeast ×M0) / (M core ×M1)×100; Among them, M core M represents the initial mass of the core sample. yeastM0 represents the mass of yeast precipitate in the kernel sample; M1 represents the initial mass of the control group; and M2 represents the mass of yeast precipitate in the control group.

[0036] 1.3 Determination of Adhesion Efficiency and Yield The adhesion efficiency of HPMC was determined using the mass balance method. The initial mass of highly active dry yeast (M) was accurately weighed. y HPMC quality (M) h Tween 80 mass (M) t After encapsulation, HPMC powder (M) not coated on the dry yeast particles was removed using a 60-mesh (250 μm) sieve. r Then, use a 20-mesh (850 μm) sieve to remove excessively large aggregates. At this point, the HPMC-yeast kernel (M) f The final product is ( ). Adhesion efficiency (%) and yield (%) are calculated using the following formulas: Adhesion efficiency (%) = (M h –M r ) / M h ×100; Yield (%) = M f / (M) y +M h +M t ) × 100; 1.4 Particle size distribution The particle size distribution of yeast microcapsules was determined using a laser particle size analyzer. A dry dispersion mode was selected. An appropriate amount of microcapsule sample was added to the sample inlet, and the measurement was initiated. The software automatically acquired and analyzed the scattered light signal to calculate the particle size distribution.

[0037] Table 1 shows the loading, adhesion efficiency, and yield of kernels prepared with HPMC of different viscosities. Different HPMC viscosities significantly affected the loading, adhesion efficiency, and yield of yeast kernels. The 30 mPa·s HPMC group had the highest loading (76.15%), moderate adhesion efficiency (68.8%), and a yield of 80.36%. Low-viscosity (5, 15 mPa·s) HPMC had high adhesion efficiency, but slightly lower loading and yield. High-viscosity (50, 100 mPa·s) HPMC showed decreased loading and significantly reduced adhesion efficiency. Medium-viscosity (30 mPa·s) HPMC formed a sufficiently thick protective film during tumbling and wetting without affecting the uniform coating of particles, thus achieving a balance between loading and yield.

[0038] The particle size distribution of the cores prepared by HPMC with different viscosities is shown in Table 2. The low viscosity group (5 mPa·s, 15 mPa·s) showed a single-peak characteristic, indicating good particle size uniformity. The high viscosity group (30 mPa·s, 15 mPa·s) showed a double-peak characteristic, with a wider particle distribution. Due to the slow gelation speed and high viscosity of high viscosity HPMC, some powder failed to adhere effectively to the yeast surface, forming small HPMC aggregates or powder fragments.

[0039] Table 1. Loading capacity, attachment efficiency, and yield of yeast kernels prepared with HPMC of different viscosities.

[0040] Table 2. Particle size distribution of yeast kernels prepared by HPMC with different viscosities

[0041] 1.5 Microscopic morphological observation The surface morphology of HPMC-yeast kernels with different viscosities was observed using field emission scanning electron microscopy. A thin layer of HPMC-yeast kernel sample was fixed on a copper sample holder using double-sided carbon ribbon and then plated with a thin layer of gold. The instrument accelerating voltage was 10.00 kV.

[0042] Figure 1 Scanning electron microscopy (SEM) images of yeast kernels prepared with HPMCs of different viscosities. The control group consists of clearly visible naked yeast cells. In the 5 and 15 mPa·s groups, the particle surface is uneven, with debris or irregular adhesions, and the coating layer is thin and discontinuous. The outline of yeast cells is still visible at high magnification (×1000 and ×3000). In the 30, 50, and 100 mPa·s groups, the surface is smooth and the film layer is uniform, forming a "hill-like" aggregated structure. As the viscosity continues to increase, the particle surface becomes rougher and tends to clump. Therefore, the 30 mPa·s group has the greatest advantage in constructing a high-quality, continuous protective barrier.

[0043] 1.6 Ethanol Tolerance Assessment Accurately weigh 0.1 g of yeast kernels prepared with HPMC of different viscosities, add 0.15 mL of pure ethanol, and then dry at room temperature (25°C) to simulate the chemical shock to the yeast in the kernel during the subsequent shellac-ethanol encapsulation process. The HPMC-yeast kernels were then resuspended in 5 mL of sterile physiological saline. The viable count of unencapsulated active dry yeast was used as a control. The yeast suspension was serially diluted 10-fold with 0.85% sterile physiological saline. 100 μL of each appropriate dilution was plated onto YPD solid medium (chloramphenicol final concentration 50 mg / L). After the bacterial suspension dried, the plates were inverted and incubated at 30°C for 36 h. The colony count of effective plates within the range of 30-300 was recorded. The yeast survival rate (%) was calculated using the following formula: Live yeast count (CFU / g) = Average colony count on plate × Dilution factor; Yeast survival rate (%) = (number of viable cells after ethanol treatment / number of viable cells before ethanol treatment) × 100; Table 3. Yeast survival rate of yeast kernels prepared with HPMC of different viscosities after ethanol treatment.

[0044] Table 3 shows the yeast survival rates of yeast cores prepared with HPMC of different viscosities after ethanol treatment. The yeast survival rates in all HPMC-encapsulated groups were higher than those in the unencapsulated control group, and the survival rate increased significantly with increasing HPMC viscosity. Although the yeast survival rates in the 5 mPa·s and 15 mPa·s groups were slightly higher than the control, the protective effect was limited. At high viscosities (30 mPa·s, 50 mPa·s, and 100 mPa·s), the yeast survival rates were significantly improved, and there was no significant difference among the three groups. This was because the polymer chains formed a highly entangled network, effectively blocking the penetration of small-molecule ethanol into the core.

[0045] Part Two: This section mainly studies the effect of TBC plasticizer concentration on the glass transition behavior of shellac films.

[0046] 2.1 Preparation of Shellac Film Prepare 15% (w / v) shellac-ethanol solutions containing 0%, 2.5%, 5%, 7.5%, and 10% TBC (by weight of shellac). Pour the solutions into silicone molds and cast them into films at 25°C. Dry at room temperature for 48 hours to ensure complete solvent evaporation. After demolding, films with different plasticizer concentrations are obtained.

[0047] 2.2 Dynamic Mechanical Analysis The thermomechanical properties of shellac films were evaluated using a dynamic mechanical analyzer. The films, measuring 5 mm × 25 mm × 0.15 mm, were heated from -20°C to 50°C at a rate of 5°C / min. Temperature scanning was performed in film stretching mode at a frequency of 1 Hz and an amplitude of 10 μm to determine the storage modulus. E '), loss modulus ( E '') and tangent (tan δ (Changes with temperature)

[0048] Mechanical and thermal properties of shellac films with different TBC concentrations, such as Figure 2 As shown. Shellac films with 0% TBC concentration, i.e., unmodified shellac films, are typically brittle and frequently break during the initial stage of low-temperature scanning (around -20°C), resulting in no valid data. Therefore, plasticization modification is a necessary means to improve the thermomechanical stability and processability of shellac. Initial storage modulus of the 2.5% TBC group.E The lower values ​​and smaller fluctuations indicate insufficient rigidity under frozen or low-temperature storage conditions. As temperature increases, the rigidity of each group... E The significant decrease in both values ​​indicates the onset of the glass transition process. E "The peak temperature reaches 37.24℃, which means that in the 35-40℃ evaporation temperature range, the outer shell is still mostly in a glassy state or in the early stages of transformation, with the energy storage modulus remaining above 100 MPa, indicating excessive rigidity. The tan of the 2.5% TBC group..." δ The lowest value and the latest rise also indicate the longest glassy state retention time and the latest occurrence of chain segment relaxation. The 5% and 7.5% TBC groups showed higher values ​​in the range of -20°C to approximately 15°C. E (≥750 MPa), with the 7.5% TBC group exhibiting the highest initial modulus in the low-temperature region, indicating its ability to maintain strong glassy support under freezing or low-temperature storage conditions. The 5% and 7.5% TBC groups... E "The peak temperatures were 22.71℃ and 19.79℃, respectively. The energy storage moduli of both groups decreased to below 94 MPa and below 56 MPa in the 35-40℃ aging temperature range, respectively. tan..." δ The increase occurred earlier than in the 2.5% TBC group and the 10% TBC group, indicating a significant change in thermal response. (10% TBC group) E The levels were significantly lower than in the 5% TBC and 7.5% TBC groups. E The peak becomes significantly lower and wider. E "The peak temperature rebounded to 26.42℃, and the energy storage modulus in the 35-40℃ aging temperature range increased, indicating that excessive plasticization weakened the overall rigidity of the membrane."

[0049] Part Three: This section mainly studies the application of different yeast microcapsules in frozen dough.

[0050] As we know from the first two parts, the optimal HPMC viscosity is 30 mPa·s and the optimal TBC concentration is 7.5%.

[0051] 3.1 Preparation of yeast microcapsules HPMC cores with a viscosity of 30 mPa·s were prepared and encapsulated in a two-step process using a shellac-TBC ethanol solution. A two-step dip-coating method was employed to ensure structural integrity and effective pore sealing. 6% (w / v) shellac-ethanol solutions containing 0% and 7.5% TBC were prepared. 1.0 g of HPMC-yeast core was uniformly mixed with 1.0 mL of the 6% (w / v) shellac-ethanol solution. The mixture was gently stirred at room temperature to ensure complete coverage of the particle surface, followed by drying at room temperature for 1 h to form a primary shellac layer. To eliminate potential surface defects and macropores, a sealing step was then performed: 0.5 mL of the same shellac-ethanol solution was added. After thorough drying at room temperature for 24 h, the microcapsules were sieved through standard sieves (20 mesh and 60 mesh) to remove loose HPMC powder and excessively large aggregates. The resulting microcapsules, with a particle size range of 250–850 μm, were stored in a desiccator for later use.

[0052] 3.2 Preparation and Storage of Frozen Dough The dough consisted of 100 g all-purpose flour, 55 g deionized water, and 1 g active dry yeast powder or an equivalent number of yeast microcapsules. All dry ingredients were premixed, then water was added, and the mixture was kneaded at low speed for 6 minutes using a dough mixer to form a dough. 8.0 g of dough was accurately weighed and placed in a 15 mL graduated glass tube. The dough was gently pressed down with a glass rod to fill the bottom of the tube and smooth the surface, and the initial volume (V0) was recorded. All dough samples were grouped according to testing requirements. The samples underwent 3 and 6 freeze-thaw cycles (FT) (one freeze-thaw cycle consisted of freezing at -18°C for 23 h followed by thawing at 4°C for 1.5 h) and 3 and 6 weeks of frozen storage (FS) (at -18°C). Fresh dough served as a control.

[0053] 3.3 Fermentation Characteristics Assessment The sample was placed in a fermentation chamber at 40°C and 85% RH, and the dough volume V was recorded every 10 minutes. t Volume increase rate (%) = (V t Plot the fermentation curve using (V0) / V0×100. Place the sample in a 4°C refrigerator and record the dough volume V every 4 hours. t The volume increase rate was calculated until 12 hours. The partially fermented dough was then placed in a fermentation chamber at 40℃ and RH 85%, and the dough volume V was recorded every 10 minutes. t Fermentation curves were plotted. The positive control was dough containing an equal amount of dry yeast (Con-1), and the negative control was dough without yeast (Con-2).

[0054] The conventional fermentation curves and post-refrigeration fermentation curves of thermoresponsive yeast microcapsule dough under different freeze-thaw cycles and freezing times were respectively... Figure 3 and Figure 4As shown, repeated freeze-thaw cycles (FT) and long-term frozen storage (FS) treatments significantly weakened the fermentation potential of the dough. With the increase in the number of freeze-thaw cycles from 0 to 3 and 6, or the extension of the frozen storage time to 3 or 6 weeks, the volume growth curves of all samples shifted significantly to the right, indicating delayed initiation time, reduced volume growth rate, and decreased final fermentation volume. Although the initial fermentation rate of the 7.5% TBC group was slightly lower than that of Con-1, it maintained relatively stable fermentation behavior after repeated freeze-thaw cycles and long-term frozen storage. In the test of "fermentation at 40°C after 12 h of refrigeration at 4°C", the microcapsule system successfully achieved spatial and temporal control of the fermentation process. The encapsulated group significantly inhibited dough volume growth and reduced premature yeast fermentation during the low-temperature stage, while the Con-1 control group, lacking a diffusion barrier, showed significant premature fermentation during the refrigeration stage after thawing. When the temperature was increased to 40°C, the dough in the 7.5% TBC microcapsule group quickly recovered its fermentation activity, indicating that the microcapsules can effectively delay low-temperature metabolism and achieve a more concentrated fermentation response.

[0055] 3.4 Yeast survival rate determination Take 1 g of dough sample, add 50 mL of 0.85% physiological saline and homogenize for 2 min. Determine the total number of yeast colonies and calculate the survival rate according to the method described in 1.6.

[0056] Table 4. Yeast survival rate in microencapsulated doughs subjected to different freezing treatments.

[0057] As shown in Table 4, the 7.5% TBC microcapsule group exhibited significant cryoprotection under both freeze-thaw and frozen storage conditions, with a yeast survival rate as high as 86.47% after 6 freeze-thaw cycles, significantly better than the unencapsulated group Con-1 (59.48%). Although the Con-1 group consistently showed the fastest fermentation rate, this advantage mainly stemmed from the rapid water absorption and immediate metabolism of yeast after complete exposure to the dough system, rather than superior environmental stability. The yeast in the Con-1 group rapidly absorbed water and initiated metabolism after thawing or during refrigeration, thus accelerating dough expansion but also making it more susceptible to freeze-drying damage and premature fermentation. In contrast, the 7.5% TBC microcapsule dough sacrificed some initial fermentation speed for higher freeze-drying stability, lower yeast mortality, and more controllable fermentation behavior. From an industrial application perspective, this "delayed activation-concentrated fermentation" model is more beneficial for the processing stability and quality consistency of frozen dough products.

[0058] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0059] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0060] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A thermoresponsive yeast microcapsule, characterized in that, Microcapsules are formed by sequentially coating yeast with a hydrophilic inner layer and a hydrophobic outer layer. The hydrophilic inner layer is a hydroxypropyl methylcellulose layer, and the viscosity grade of hydroxypropyl methylcellulose is 5-100 mPa·s; The hydrophobic outer layer is a shellac layer, and tributyl citrate is added as a plasticizer. The amount of tributyl citrate added is 0% to 10% w / w based on the solid mass of shellac, excluding the endpoint value of 0.

2. The thermoresponsive yeast microcapsule according to claim 1, characterized in that, The viscosity grade of the hydroxypropyl methylcellulose is 30 mPa·s; The amount of tributyl citrate added is 5% to 7.5% w / w.

3. The thermoresponsive yeast microcapsule according to claim 1 or 2, characterized in that, The microcapsules have a particle size range of 250–850 μm.

4. The method for preparing the thermoresponsive yeast microcapsules according to any one of claims 1-3, characterized in that, include: (1) Preparation of HPMC-yeast kernel: wet dry yeast and nonionic surfactant were premixed and moistened, and then mixed with HPMC dry powder of different viscosities. Atomized water was sprayed in a rotating rolling state to moisten and activate the yeast, so that HPMC formed a film in situ to coat the yeast. Then, the yeast was dried at low temperature to constant weight to obtain HPMC-yeast kernel with a particle size in the range of 250-850 μm. (2) Preparation of thermally responsive yeast microcapsules: Yeast microcapsules were prepared by solvent evaporation method. The HPMC-yeast kernel obtained in step (1) was mixed with shellac-TBC ethanol solution for casting and embedding, followed by drying and separation. Particles with a particle size in the range of 250-850 μm were collected.

5. The method for preparing thermoresponsive yeast microcapsules according to claim 4, characterized in that, In step (1), the mass-volume ratio of dry yeast, nonionic surfactant and HPMC powder is 3.0 g: (0.5-1.0) mL: (2.0-2.5) g.

6. The method for preparing thermoresponsive yeast microcapsules according to claim 4 or 5, characterized in that, In step (1), the nonionic surfactant is Tween 80; the rotation speed is controlled at 50-80 rpm, and the drying temperature is 25-35℃.

7. The method for preparing thermoresponsive yeast microcapsules according to claim 4 or 5, characterized in that, In step (2), the shellac concentration in the shellac-TBC ethanol solution is 4% to 10% w / v.

8. The method for preparing thermoresponsive yeast microcapsules according to claim 4 or 5, characterized in that, In step (2), the shellac concentration is 6% to 8% (w / v).

9. The method for preparing thermoresponsive yeast microcapsules according to claim 4 or 5, characterized in that, In step (2), a "double encapsulation, concentration control" process is adopted; The volume ratio of the added shellac-TBC ethanol solution to the HPMC-yeast kernel was controlled between 1.5 mL:1.0 g and 1.2 mL:1.0 g. The HPMC-yeast kernel was first cast and embedded using 60% to 70% of the total volume of the shellac-TBC ethanol solution, and after drying, a primary shellac layer was formed. The dried microcapsules were then encapsulated a second time using the remaining shellac-TBC ethanol solution, and after drying and separation, thermoresponsive yeast microcapsules were obtained.

10. The application of the thermoresponsive yeast microcapsules according to any one of claims 1-3 in the fermentation of frozen dough, refrigerated fermented dough and pre-baked food.

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