Solid milk product containing tall milk skin and preparation method of solid milk product

Abstract

The invention belongs to the technical field of food processing, particularly relates to a tall milk skin solid dairy product and a preparation method thereof, and aims to introduce an octenyl succinic acid tamarind polysaccharide ester and ferulic acid chitosan whey protein conjugate aiming at the problems that a traditional milk skin product is low in structural strength and easy to oxidize and deteriorate. The preparation method comprises the following key steps: premixing the two compounds with raw milk according to a specific ratio, heating, adding fried tall milk, staged temperature control standing, cooling and the like to finally obtain the finished product. In the two modified compounds, the former is prepared through an esterification reaction of polysaccharide and anhydride, and the latter is prepared through an enzymatic grafting and protein conjugation reaction. Through the synergistic effect of the components, the mechanical strength and lifting completeness of the milk skin layer can be remarkably enhanced, fat oxidation is effectively inhibited, the shelf life is prolonged, and the fusion degree of taste and flavor is improved. The method is reliable in process, and the obtained product has both traditional flavor and stable quality of modern food, and has good application prospects.

Description

Technical Field

[0001] This invention belongs to the field of food processing technology, specifically relating to a solid dairy product called talmi milk skin and its preparation method. Background Technology

[0002] Milk skin, a traditional solid or semi-solid dairy product popular in the pastoral areas of northern my country, is highly prized for its unique rich milky aroma, delicate texture, and naturally formed multi-layered or honeycomb-like structure. Its traditional production process relies on the gentle, slow heating of milk, during which solid components such as fat globules and proteins gradually rise to the surface and accumulate, eventually forming a layer rich in milk fat and protein through dehydration. This process not only endows the product with its unique flavor and texture but also carries significant culinary cultural value. However, this traditional method, heavily reliant on manual experience and small-scale workshop production, suffers from several fundamental limitations. First, the quality of the final product, including the thickness, uniformity, mechanical strength, and complete lifting rate of the milk skin, is greatly influenced by the fat and protein content of the raw milk, the shape and material of the heating container, the ambient temperature, and the operator's personal experience, leading to significant fluctuations in product quality and hindering standardized and large-scale production. Second, to form a milk skin of sufficient thickness and strength, traditional processes often require prolonged settling and repeated heating and cooling cycles, resulting in low production efficiency and limited capacity. Furthermore, although the product produced by this method has a lower water content than liquid milk, its structure is loose and its mechanical strength is poor, making it easy to break during packaging, transportation and storage. Moreover, because it is rich in unsaturated fats, it is very easy to undergo oxidative rancidity under the action of light and oxygen, resulting in flavor deterioration and short shelf life, which seriously limits its commodity circulation radius and market expansion.

[0003] Faced with the widespread demands for the modernization and industrialization of traditional foods, the food science field has seen numerous attempts to integrate the concept or flavor elements of "milk skin" into modern dairy product systems. For example, by adjusting process parameters, a thin, fat-rich "skin" can be artificially created on the surface of set yogurt to simulate the traditional texture; or similar flavor experiences can be sought through formula compounding. However, these technological approaches primarily focus on partial improvements or flavor adjustments to existing mature industrial products (such as yogurt), and their core objective is not to fully reproduce and enhance the unique texture and quality of traditional milk skin as an independent product category. More importantly, most existing technical solutions focus on simple adjustments to the processing flow or the simple addition of auxiliary materials, failing to address the core challenges of the inherent structural fragility and oxidative instability of traditional milk skin from the deeper levels of food materials science and molecular interactions. Therefore, the market still lacks a product and its reliable preparation method that can truly inherit the essence of traditional milk skin while meeting modern food industry standards in terms of physical strength, storage stability, and production consistency.

[0004] Specifically, the current technological gaps are mainly reflected in two aspects: First, in terms of interface stability and structural enhancement, there is a lack of efficient and safe food ingredients that can specifically act on the air-emulsion interface in the milk heating system, effectively enhance the mechanical strength of the interfacial film, and promote the formation of a uniform and dense solid network. Second, in terms of antioxidant preservation, there is a lack of natural solutions that can tightly bind with the milk skin matrix, effectively inhibit fat oxidation for a long time, and not affect the original flavor profile of the product. Most common food emulsifiers, thickeners, or antioxidants have limited effects or may bring unpleasant flavors when applied to such special thermally induced phase separation and aggregation systems. Therefore, the core of developing an innovative technical solution lies in creating a completely new food-grade additive that combines interface modification, structural enhancement, and antioxidant functions, and cleverly integrating it into an optimized process. This would completely revolutionize the production technology and product attributes of milk skin without sacrificing traditional flavor and appearance, and has significant industrial and commercial value. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a solid dairy product made from talmi milk skin and its preparation method.

[0006] In a first aspect, the present invention provides a method for preparing a solid dairy product called talmi milk skin, comprising the following steps: S1. Filter 800-1200 parts by weight of raw milk, preheat, centrifuge, pasteurize, and cool to 3-6℃ to obtain cooled raw milk; premix 0.5-2.0 parts of octenyl succinate tamarind polysaccharide ester and 0.3-1.5 parts of ferulic acid-chitosan-whey protein conjugate with the cooled raw milk to obtain a premix; heat the premix to 75-80℃ and add 30-60 parts of talmi to obtain mixed milk; S2. Keep the mixed milk at 75-80℃ and let it stand naturally to room temperature, then let it stand again; reheat it to 85-92℃ and keep it standing, then finally cool it to 8-16℃ and let it stand.

[0007] In this invention, the final formation of the Talmi milk skin solid dairy product is a complex phase transition and self-assembly process driven by thermodynamics and regulated by interfacial chemistry and aggregated state physics. The intervention of two synthetic modified compounds precisely regulates this traditional physical process at the molecular level. When the premixed milk containing octenyl succinate tamarind polysaccharide ester and ferulic acid-chitosan-whey protein conjugate is heated, the heat input reduces the viscosity of the system and accelerates molecular motion. At this stage, the two amphiphilic compounds rapidly migrate to the air-milk interface and the fat globule-water phase interface. Octenyl succinate tamarind polysaccharide ester, with its linear polysaccharide backbone and hydrophobic side chains, is oriented at the interface, significantly reducing interfacial tension, and its high molecular weight forms a viscous interfacial layer, stabilizing the fat globules that float due to density differences. Meanwhile, the ferulic acid-chitosan-whey protein conjugate, with its high affinity for the interface and complex molecular structure, interacts with native casein micelles, whey proteins, and fat globule membrane proteins in milk, constructing a composite protein-polysaccharide membrane at the interface that possesses both mechanical strength and antioxidant functions. The added roasted talmi not only provides flavor compounds, but its rough, porous surface also serves as a heterogeneous nucleation site for interfacial membrane adhesion and solidification, locally strengthening the structure. Initial heating followed by static cooling is crucial for the initial solidification of the structure. During the heat treatment stage, proteins (including the added conjugate and milk-derived proteins) undergo partial denaturation and unfolding, exposing more hydrophobic regions and active groups. Through hydrophobic interactions, disulfide bond exchange, and hydrogen bonding of the chitosan chains in the conjugate, a continuous three-dimensional network rudiment begins to form at the interface and in the aqueous phase. The subsequent natural cooling process promotes the crystallization of molten milk fat. The solid fat crystal network interweaves and integrates with the initially formed protein-polysaccharide gel network, greatly enhancing the rigidity of the overall structure. The subsequent secondary heating and heat preservation are not simple repetitions; their mechanism lies in the "thermal annealing" and deep cross-linking of the formed network. Higher temperatures cause the protein network to further shrink and strengthen. Simultaneously, unreacted groups in the ferulic acid-chitosan-whey protein conjugate may continue to interact with other components of the system, making the bonding between the interfacial membrane and the bulk gel even tighter and stronger. The final low-temperature settling step is the final stage of perfecting the fat crystallization network and the cold solidification of the entire composite gel network. During this process, fat crystallization is complete, the hydrogen bonding of the protein-polysaccharide network is enhanced at low temperatures, water is firmly fixed within the network, and volatile flavor substances are effectively retained, ultimately forming a unique solid dairy product with moderate water activity, uniform and dense texture, rich honeycomb pores, full-bodied flavor, and structural strength sufficient to resist mechanical picking. The entire process, through precise temperature and time program control, synergistically utilizes two functional molecules to guide the originally random and fragile natural aggregation process towards a desired, stable, and robust food structure.

[0008] According to a preferred embodiment of the present invention, in step S1, the preheating temperature is 65-75°C.

[0009] According to a preferred embodiment of the present invention, in step S2, the mixed milk is kept at 75-80°C for 1-1.5 hours.

[0010] According to a preferred embodiment of the present invention, the preparation method of the octenyl succinic acid tamarind polysaccharide ester includes: A1, dispersing 45-55 parts by weight of tamarind gum in 450-550 parts by weight of deionized water, stirring in a water bath at 44-46°C to obtain a colloidal solution; adjusting the pH to 8.4-8.6; adding 28-32 parts by weight of octenyl succinic anhydride dropwise while continuously stirring; after the addition is complete, reacting at 44-46°C to obtain a reaction solution; A2, adjusting the pH of the reaction solution to 6.4-6.6; transferring it to a dialysis bag and dialyzing it in deionized water; and freeze-drying the solution in the dialysis bag.

[0011] In this invention, the preparation mechanism of tamarind polysaccharide octenyl succinate is the esterification reaction of natural polysaccharides with acid anhydrides in a mild alkaline aqueous system, also known as a nucleophilic acyl substitution reaction. This process uses tamarind gum as a backbone, with its abundant primary and secondary hydroxyl groups serving as active reaction sites. When the polysaccharide is uniformly dispersed in the aqueous phase and a weakly alkaline environment is created, the polysaccharide hydroxyl groups are deprotonated under the action of hydroxide ions to form more reactive alkoxide anions. At this point, octenyl succinic anhydride is added dropwise as an acyl donor. One carbonyl carbon in its cyclic anhydride structure is nucleophilically attacked by the polysaccharide alkoxide anion, causing the anhydride ring to open. This crucial step leads to the successful establishment of an ester bond between the oxygen atom of the polysaccharide chain and the carboxyl carbon of the octenyl succinic acid group, thereby covalently grafting the entire hydrophobic octenyl succinic acid group onto the hydrophilic polysaccharide backbone, while simultaneously releasing an octenyl succinic acid monocarboxylate anion as a byproduct. The entire reaction must be carried out under continuous stirring and precise pH maintenance to ensure the efficiency of alkali catalysis and prevent excessive hydrolysis of the acid anhydride under alkaline conditions, thus ensuring the dominance of the substitution reaction over the hydrolysis reaction. After the reaction is complete, the system is adjusted to near neutral to terminate the esterification reaction and promote the complete hydrolysis of any remaining acid anhydride. The subsequent dialysis purification step, based on the molecular sieving and concentration diffusion principles of a semi-permeable membrane, aims to completely remove all unreacted small molecule reagents, hydrolysis byproducts, and ionic salts from the system. Finally, all water is removed by freeze-drying to obtain an amphiphilic polysaccharide ester product with a well-defined chemical structure and both a hydrophilic backbone and hydrophobic side chains. This compound exhibits significant interfacial activity due to the simultaneous introduction of the hydrophobicity of long-chain olefins and the hydrophilicity of succinic acid groups. It can effectively reduce interfacial tension in subsequent dairy product systems and form a viscoelastic film at the interface due to its high molecular weight properties, which is the molecular basis for its ability to strengthen the milk skin structure.

[0012] According to a preferred embodiment of the present invention, in step A1, the reaction time at 44-46°C is 4-6 hours.

[0013] According to a preferred embodiment of the present invention, in step A2, the dialysis time in deionized water is 48-50 hours.

[0014] According to a preferred embodiment of the present invention, the preparation method of the ferulic acid-chitosan-whey protein conjugate includes: B1, dissolving 18-22 parts by weight of chitosan in acetic acid solution and adjusting the pH to 5.5-6.5; adding 4-6 parts of ferulic acid and 0.8-1.2 parts of horseradish peroxidase, stirring at 28-32°C under light-protected conditions, adding 18-22 parts of hydrogen peroxide solution dropwise, stirring to obtain a reaction mixture; adjusting the pH of the reaction mixture to 9.0-10.0, centrifuging to collect the precipitate, washing the precipitate with deionized water, and freeze-drying to obtain the ferulic acid-chitosan graft; B2, dissolving 8-12 parts of the ferulic acid-chitosan graft and 18-22 parts of whey protein isolate together in phosphate buffer solution, placing in a water bath at 54-56°C with stirring, and continuously stirring to obtain a reaction solution; cooling the reaction solution to 3-5°C, dialyzing and freeze-drying.

[0015] In this invention, the preparation of the ferulic acid-chitosan-whey protein conjugate involves two consecutive key reaction steps with distinct mechanisms. The first step is an enzymatic free radical graft copolymerization reaction of ferulic acid and chitosan. After chitosan dissolves in an acidic medium, the amino groups on its chains are protonated and dissolved in water. Pre-adjusting the pH to slightly acidic creates the optimal environment for the subsequent enzymatic reaction. Under these conditions, ferulic acid and horseradish peroxidase are added. When a trace amount of hydrogen peroxide is introduced as an oxidant, the enzymatic reaction is initiated. Horseradish peroxidase catalyzes the decomposition of hydrogen peroxide, generating a high-valence ferrooxide intermediate. This intermediate rapidly oxidizes the phenolic hydroxyl groups on the benzene ring of the ferulic acid molecule, generating highly reactive phenoxy radicals. These free radicals can react with chitosan chains through multiple pathways: firstly, by directly condensing or oxidatively coupling with the amino groups on the chitosan glucosamine units; secondly, by attacking the carbon-hydrogen bonds on the chitosan sugar rings, generating large molecular free radicals through hydrogen extraction, which then combine with ferulic acid free radicals or their dimers and oligomers. The entire reaction is carried out under light-protected conditions to prevent unnecessary photochemical reactions of ferulic acid and its free radicals. After the reaction, the system is adjusted to a strongly alkaline state. The mechanism is to deprotonate the amino groups on chitosan and its grafts, leading to enhanced hydrogen bonding between molecular chains and a sharp decrease in solubility, thus selectively separating them from the reaction solution in the form of precipitates, achieving preliminary purification. The second step is the process by which the above-mentioned ferulic acid-chitosan grafts and whey protein form conjugates through oxidative cross-linking and molecular assembly. Under continuous mild thermal stirring conditions, the phenolic hydroxyl groups of the ferulic acid groups in the graft molecules can be slowly oxidized by dissolved oxygen to generate quinone intermediates or their derivatives. These highly reactive quinone intermediates can undergo Michael addition or Schiff base reactions with nucleophilic groups on the surface of whey protein molecules (such as the ε-amino group of lysine and the thiol group of cysteine) to form covalent crosslinks. Simultaneously, this thermal effect promotes the moderate unfolding of whey protein, exposing more hydrophobic regions and functional groups, which then undergo multi-level physical assembly with the chitosan chains in the graft through electrostatic attraction, hydrogen bonding, and hydrophobic interactions. The resulting conjugate is a complex, multifunctional molecular aggregate that integrates the film-forming and cationic properties of chitosan, the antioxidant activity of ferulic acid as a free radical scavenger, and the excellent emulsifying and gelling properties of whey protein. This lays the chemical foundation for its dual role as a structural reinforcing agent and built-in antioxidant in subsequent applications.

[0016] According to a preferred embodiment of the present invention, in step B1, the stirring reaction time is 6-8 hours; and the mass concentration of the hydrogen peroxide solution is 0.05-0.15%.

[0017] According to a preferred embodiment of the present invention, in step B2, the reaction is continuously stirred for 72-80 hours.

[0018] In a first aspect, the present invention provides a solid dairy product of tarmey milk skin prepared according to the method for preparing the tarmey milk skin solid dairy product.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention brings revolutionary improvements to the basic physical properties and structural integrity of the product. By adding a specific ratio of octenyl succinic acid tamarind polysaccharide ester and ferulic acid chitosan whey protein conjugate, and controlling the staged settling and cooling process, the phase behavior and interface assembly of the milk system during heating can be systematically regulated. These two compounds work synergistically at the molecular level to form a composite interface film with excellent elasticity and mechanical strength at the air-milk interface, effectively stabilizing the network structure of the floating fat globules and protein aggregates. The direct effect is that the thickness of the final solid milk skin layer is significantly increased and the distribution is extremely uniform, completely changing the uneven thickness of traditional products. More importantly, the mechanical strength and toughness of the milk skin layer have achieved a qualitative leap, maintaining a near 100% integrity rate during the lifting process, almost eliminating breakage and loss, and greatly improving product yield and product appearance. At the same time, this reinforced structure greatly enhances the final product's resistance to external impact during subsequent packaging, transportation and storage, laying a solid foundation for large-scale production and long-distance distribution.

[0020] (2) Regarding the chemical stability and shelf life of the product, this invention provides a long-term solution. Traditional milk skin, due to its rich content of unsaturated fats and loose structure, is highly susceptible to oxidative rancidity during storage, leading to flavor deterioration. In this invention, the ferulic acid-chitosan-whey protein conjugate not only acts as a structural enhancer but is also a highly efficient and location-specific natural antioxidant. The ferulic acid group in its molecule can effectively quench free radicals and block the chain reaction of fat oxidation. Because this conjugate is integrated into the protein and fat matrix of the milk skin through covalent bonds and interactions, it achieves "site anchoring" and sustained release of antioxidant function, avoiding the migration, volatilization, or uneven action problems that may exist with ordinary additive antioxidants. Comparative tests show that, under the same storage conditions, the product prepared using the method of this invention exhibits a much lower rate of increase in rancidity indicators such as peroxide value compared to traditional products, resulting in a shelf life that is extended several times over. It also maintains the pure milk aroma and tartary roasted grain aroma of the product for a long time, demonstrating excellent flavor stability.

[0021] (3) This invention demonstrates comprehensive advantages in terms of overall quality, nutritional function, and production applicability. The introduction of the two modified compounds improves the dispersion of fat and the network cross-linking of proteins at the microscopic level, giving the product a more delicate, rich, thick but not greasy taste experience, while enhancing the fusion of milk and grain aromas. From the perspective of raw materials, the modified compound precursors used are all natural and safe food ingredients. The preparation process is clean, and the final product is not only safe and harmless, but also has added health benefits due to the introduction of modified polysaccharides and bioactive peptides with potential prebiotic properties. From the perspective of process, the preparation method provided by this invention has clear parameters and well-defined steps, significantly reducing the dependence on the operator's personal experience and batch differences of raw materials. It has good production reproducibility and is easy to standardize and achieve a certain scale of industrial production. It successfully transforms a traditional technique that relies on craftsmanship into a modern food processing technology that is replicable, controllable, and of consistent quality, and has extremely high industrial transformation value and market potential. Detailed Implementation

[0022] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention. Example

[0023] This embodiment provides a method for preparing a solid dairy product called Tarmi milk skin, the steps of which include: Preparation of tamarind polysaccharide octenyl succinate: A1. Perform the esterification reaction. Accurately weigh 50.0 g of food-grade tamarind gum and disperse it in 500.0 g of deionized water. Place the mixture in a constant temperature water bath at 45.0℃ and continuously stir mechanically at 300 rpm until the polysaccharide is completely dissolved, forming a homogeneous, viscous colloidal solution. Using a 2.0 mol / L sodium hydroxide solution, precisely adjust the pH of the colloidal solution to 8.5 while stirring. Maintaining the system temperature at 45.0℃ and the pH at 8.5, while continuously stirring, add 30.0 g of food-grade octenyl succinic anhydride dropwise to the reaction system at a uniform rate over 1.0 h using a constant pressure dropping funnel. After the addition is complete, continue to maintain the temperature at 45.0℃ and the pH at 8.5, and react for 5.0 h.

[0024] A2. After the reaction is complete, the pH of the reaction solution is adjusted to 6.5 using a 1.0 mol / L hydrochloric acid solution to terminate the esterification reaction. Purification is then performed. The entire reaction solution is transferred to a regenerated cellulose dialysis bag with a molecular weight cutoff of 8000 Da. The dialysis bag is completely immersed in continuously flowing deionized water and dialyzed at room temperature for 48.0 h, changing the external deionized water at least 6 times during this period to thoroughly remove unreacted anhydrides, succinic acid, and salt ions. After dialysis, the solution in the dialysis bag is transferred to a lyophilization tray and pre-frozen in a -50°C ultra-low temperature freezer for 4.0 h. Then, it is transferred to a freeze dryer and freeze-dried for 24.0 h under conditions of a cold trap temperature below -50°C and a vacuum degree below 10 Pa to obtain a white flocculent solid, which is octenyl succinic acid tamarind polysaccharide ester. This should be sealed and stored in a moisture-proof environment.

[0025] Preparation of ferulic acid-chitosan-whey protein conjugate: B1. Preparation of ferulic acid-chitosan graft. Accurately weigh 20.0 g of chitosan powder with a degree of deacetylation ≥90%, dissolve it in 1000.0 g of 1.0% (v / v) acetic acid solution, and stir until clear. Adjust the pH of the chitosan solution to 6.0 using 1.0 mol / L sodium hydroxide solution. Add 5.0 g of ferulic acid and 1.0 g of horseradish peroxidase to the solution sequentially. Place the reaction system in a constant temperature water bath at 30.0℃, protected from light, and stir at 200 rpm. Using a micro-injection pump, slowly and uniformly add 20.0 g of 0.1% (w / v) hydrogen peroxide solution to the reaction solution over 0.5 h. After the addition is complete, continue the reaction at 30.0℃ under light-protected conditions for 7.0 h. After the reaction was complete, the pH of the reaction mixture was adjusted to 9.5 using a 2.0 mol / L sodium hydroxide solution, at which point a large amount of flocculent precipitate was formed. The precipitate was collected by centrifugation at 4 °C and 8000 rpm for 15.0 min. The precipitate was redispersed with deionized water and washed by centrifugation; this process was repeated three times. The washed precipitate was freeze-dried to obtain ferulic acid-chitosan graft powder.

[0026] B2. Preparation of the conjugate. Accurately weigh 10.0 g of the above ferulic acid-chitosan graft and 20.0 g of whey protein isolate, and dissolve them together in 500.0 g of 0.1 mol / L phosphate buffer (pH 7.0), stirring until completely dissolved. Place the solution in a constant temperature water bath at 55.0℃ and gently stir continuously at 150 rpm for 76.0 h. After the reaction is complete, immediately place the reaction solution in an ice-water bath to rapidly cool to 4.0℃. Transfer the cooled reaction solution to a dialysis bag with a molecular weight cutoff of 3500 Da and dialyze against deionized water for 48.0 h to remove small molecule impurities. Freeze-dry the dialyzed solution to obtain a light brown ferulic acid-chitosan-whey protein conjugate powder, which should be stored in a sealed container protected from light.

[0027] Preparation of Tarmi milk skin solid dairy products: S1. Raw Material Pretreatment. Weigh 1000.0g of raw milk conforming to GB19301 standard, filter it through double-layer gauze, preheat it to 70.0℃, and then centrifuge to purify the milk. Pasteurize the purified milk at 85.0℃ for 15s, and immediately cool it to 4.0℃ through a plate heat exchanger to obtain cooled raw milk for later use. Separately weigh 40.0g of talmi, select and remove impurities, and stir-fry it in a wok over low heat until it turns slightly yellow and emits a caramelized aroma. Remove it and let it cool for later use. Ingredient Mixing and First Heating. Take 50.0g of the cooled raw milk and mix it with 1.0g of octenyl succinate tamarind polysaccharide ester and 0.8g of ferulic acid-chitosan-whey protein conjugate. High-speed shear stirring at 10000rpm for 2.0min forms a homogeneous modifier mother liquor. Mix this mother liquor with the remaining 950.0g of cooled raw milk in a stainless steel saucepan until homogeneous. Place the pan on a temperature-adjustable heating plate and heat it until the temperature of the contents reaches 78.0°C evenly. At this temperature, use a long-handled spoon to slowly and evenly stir the milk in the same direction to form a stable foam layer approximately 2.5 cm thick. Sprinkle the prepared roasted talmi evenly over the surface of the foam layer.

[0028] S2. Settling and Cooling: Transfer the pan to a constant-temperature water bath at 78.0℃ and hold for 1.2 hours, observing the foam layer disappear and the initial skin forming. Then remove the pan from the water bath and allow it to cool naturally at room temperature (approximately 25℃), continuing to settle for another 2.2 hours. Next, reheat the pan to 90.0℃ and maintain this temperature in a water bath at 88.0℃ for another 2.2 hours. Final Cooling and Harvesting: Remove the pan from the heat source and place it in a low-temperature water bath at 12.0℃, settling for 2.2 hours. Gently separate the edge of the milk skin along the pan using a stainless steel spatula, then insert the spatula into the bottom of the milk skin and slowly lift the entire skin completely. Place the lifted milk skin on a ventilated, dark drying rack and sun-dry for 8.0 hours at 25℃ and 55% humidity to obtain the final product. Example

[0029] The difference between this embodiment and Example 1 lies in the preparation of octenyl succinic acid tamarind polysaccharide ester: Accurately weigh 45.0 g of tamarind gum and disperse it in 450.0 g of deionized water. Place the mixture in a constant temperature water bath at 44.0 °C and mechanically stir continuously at 300 rpm until completely dissolved. Adjust the pH of the colloidal solution to 8.4 using a 2.0 mol / L sodium hydroxide solution. Maintaining the system temperature at 44.0 °C and pH at 8.4, add 28.0 g of octenyl succinic anhydride dropwise to the reaction system over 1.0 h with continuous stirring. After the addition is complete, continue to maintain the temperature at 44.0 °C and pH at 8.4 for 4.0 h. After the reaction is complete, adjust the pH of the reaction solution to 6.4 using a 1.0 mol / L hydrochloric acid solution. Transfer the reaction solution to a dialysis bag with a molecular weight cutoff of 8000 Da and dialyze against flowing deionized water for 48.0 h. After dialysis, freeze-dry the solution to obtain the product.

[0030] Preparation of ferulic acid-chitosan-whey protein conjugate: Accurately weigh 18.0 g of chitosan powder and dissolve it in 900.0 g of 0.5% (v / v) acetic acid solution, stirring until clear. Adjust the pH of the solution to 5.5 using 1.0 mol / L sodium hydroxide solution. Add 4.0 g of ferulic acid and 0.8 g of horseradish peroxidase sequentially. Place the reaction system in a constant temperature water bath at 28.0℃, protected from light throughout, and stir at 200 rpm. Slowly add 18.0 g of 0.05% (w / v) hydrogen peroxide solution dropwise to the reaction solution over 0.5 h. After the addition is complete, continue the reaction at 28.0℃ under light-protected conditions for 6.0 h. After the reaction is complete, adjust the pH of the reaction mixture to 9.0 using 2.0 mol / L sodium hydroxide solution, collect the precipitate by centrifugation, wash three times with deionized water, and freeze-dry to obtain the grafted powder. Accurately weigh 8.0 g of the graft compound and 18.0 g of whey protein isolate, and dissolve them together in 400.0 g of 0.1 mol / L phosphate buffer (pH 7.0). Place the solution in a constant temperature water bath at 54.0 °C and stir continuously at 150 rpm for 72.0 h. After cooling the reaction solution to 3.0 °C in an ice-water bath, transfer it to a dialysis bag with a molecular weight cutoff of 3500 Da and dialyze for 48.0 h. Finally, freeze-dry to obtain the conjugate.

[0031] Preparation of Tarmi milk skin solid dairy products: Weigh 800.0g of raw milk, filter, preheat to 65.0℃, centrifuge to remove impurities, pasteurize at 85.0℃ for 15s, and cool to 3.0℃. Take 40.0g of cooled raw milk and mix it with 0.5g of the prepared octenyl succinic acid tamarind polysaccharide ester and 0.3g of the prepared ferulic acid-chitosan-whey protein conjugate. Stir at high speed for 2.0min to form a mother liquor. Mix this mother liquor with the remaining raw milk in a saucepan, heat to 75.0℃ and stir to create bubbles, then add 30.0g of roasted talmi. Place the saucepan in a 75.0℃ constant temperature water bath and keep it warm for 1.0h, then allow it to cool naturally at room temperature and let it stand for 2.0h. Reheat to 85.0℃ and keep it at this temperature for 2.0h. Finally, transfer the saucepan to an 8.0℃ low temperature water bath and let it stand for 2.0h. Remove the milk skin and let it dry. Example

[0032] The difference between this embodiment and Example 1 lies in the preparation of octenyl succinic acid tamarind polysaccharide ester: Accurately weigh 55.0 g of tamarind gum and disperse it in 550.0 g of deionized water. Place the mixture in a constant temperature water bath at 46.0 °C and mechanically stir continuously at 300 rpm until completely dissolved. Adjust the pH of the colloidal solution to 8.6 using a 2.0 mol / L sodium hydroxide solution. Maintaining the system temperature at 46.0 °C and pH at 8.6, add 32.0 g of octenyl succinic anhydride dropwise to the reaction system over 1.0 h with continuous stirring. After the addition is complete, continue to maintain the temperature at 46.0 °C and pH at 8.6 for 6.0 h. After the reaction is complete, adjust the pH of the reaction solution to 6.6 using a 1.0 mol / L hydrochloric acid solution. Transfer the reaction solution to a dialysis bag with a molecular weight cutoff of 8000 Da and dialyze against flowing deionized water for 50.0 h. After dialysis, freeze-dry the solution to obtain the product.

[0033] Preparation of ferulic acid-chitosan-whey protein conjugate: Accurately weigh 22.0 g of chitosan powder and dissolve it in 1100.0 g of 1.5% (v / v) acetic acid solution, stirring until clear. Adjust the pH of the solution to 6.5 using 1.0 mol / L sodium hydroxide solution. Add 6.0 g of ferulic acid and 1.2 g of horseradish peroxidase sequentially. Place the reaction system in a constant temperature water bath at 32.0℃, protected from light throughout, and stir at 200 rpm. Slowly add 22.0 g of 0.15% (w / v) hydrogen peroxide solution dropwise to the reaction solution over 0.5 h. After the addition is complete, continue the reaction at 32.0℃ under light-protected conditions for 8.0 h. After the reaction is complete, adjust the pH of the reaction mixture to 10.0 using 2.0 mol / L sodium hydroxide solution, collect the precipitate by centrifugation, wash three times with deionized water, and freeze-dry to obtain the grafted powder. Accurately weigh 12.0 g of the graft compound and 22.0 g of whey protein isolate, and dissolve them together in 600.0 g of 0.1 mol / L phosphate buffer (pH 7.0). Place the solution in a constant temperature water bath at 56.0 °C and stir continuously at 150 rpm for 80.0 h. After cooling the reaction solution to 5.0 °C in an ice-water bath, transfer it to a dialysis bag with a molecular weight cutoff of 3500 Da and dialyze for 48.0 h. Finally, freeze-dry to obtain the conjugate.

[0034] Preparation of Tarmi milk skin solid dairy products: Weigh 1200.0g of raw milk, filter, preheat to 75.0℃, centrifuge to remove impurities, pasteurize at 87.0℃ for 15s, and cool to 6.0℃. Take 60.0g of the cooled raw milk and mix it with 2.0g of the prepared octenyl succinic acid tamarind polysaccharide ester and 1.5g of the prepared ferulic acid-chitosan-whey protein conjugate. Stir at high speed for 2.0min to form a mother liquor. Mix this mother liquor with the remaining raw milk in a saucepan, heat to 80.0℃ and stir to create bubbles, then add 60.0g of roasted talmi. Place the saucepan in an 80.0℃ constant temperature water bath and keep it warm for 1.5h, then allow it to cool naturally to room temperature and let it stand for 2.5h. Reheat to 92.0℃ and keep it at this temperature for 2.5h. Finally, transfer the saucepan to a 16.0℃ low temperature water bath and let it stand for 2.5h. Remove the milk skin and let it dry.

[0035] Comparative Example 1 The difference between this comparative example and Example 1 is that this comparative example does not include octenyl succinate tamarind polysaccharide ester and ferulic acid-chitosan-whey protein conjugate. 1000.0g of raw milk was weighed, preheated at 70.0℃, centrifuged, pasteurized at 85.0℃ for 15s, and then cooled to 4.0℃. All the cooled raw milk was directly heated to 78.0℃, stirred until foamy, and then 40.0g of roasted talmi was added. Subsequent process parameters were exactly the same as in Example 1: kept at 78.0℃ for 1.2h, cooled to room temperature and kept still for 2.2h; reheated to 90.0℃ and kept still at 88.0℃ for 2.2h; finally cooled to 12.0℃ and kept still for 2.2h before being air-dried.

[0036] Comparative Example 2 The difference between this comparative example and Example 1 is that only octenyl succinate tamarind polysaccharide ester is added in this comparative example. The preparation process is the same as in Example 1, except that in the premixing step, only 1.0 g of octenyl succinate tamarind polysaccharide ester prepared according to the method of Example 1 is weighed and mixed with 50.0 g of cooled raw milk, and ferulic acid-chitosan-whey protein conjugate is not added. All subsequent process steps and parameters are consistent with those in Example 1.

[0037] Comparative Example 3 The difference between this comparative example and Example 1 is that only ferulic acid-chitosan-whey protein conjugate is added in this comparative example. The preparation process is the same as in Example 1, except that in the premixing step, only 0.8g of the ferulic acid-chitosan-whey protein conjugate prepared according to the method of Example 1 is weighed and mixed with 50.0g of cooled raw milk, and octenyl succinate tamarind polysaccharide ester is not added. All subsequent process steps and parameters are consistent with those in Example 1.

[0038] The performance of the solid dairy products made from Tarmi milk skin obtained in Examples 1-3 and Comparative Examples 1-3 was tested in accordance with national and industry standard testing specifications.

[0039] Average thickness test method: After the product is manufactured and cooled to room temperature, it is measured using a digital vernier caliper with an accuracy of 0.01 mm. On each round milk skin, five points are selected for measurement: the center point and four symmetrical points along two vertical diameters, 10 mm from the edge. The arithmetic mean of the five measurements is calculated and recorded as the average thickness in millimeters (mm). Simultaneously, its standard deviation is calculated to characterize the uniformity of the thickness distribution.

[0040] Lifting Integrity Test Method: The same skilled operator uses a 30mm wide stainless steel flat spatula to lift the milk skin. Each complete preparation process is recorded as an independent test. In a test, if the entire milk skin can be completely separated from the bottom of the container and lifted while maintaining its integrity without any visible breaks or damage greater than 5mm, it is recorded as "successful". The percentage of successful tests out of the total number of tests for each example or comparative example (n>10) is calculated as the lifting integrity percentage.

[0041] Texture hardness testing method: A texture analyzer was used for testing. The milk skin sample was cut into 15mm × 15mm × sample thickness cubes using a mold. A cylindrical flat-bottomed probe with a diameter of 50mm was used for two compression cycles. Parameters were set as follows: pre-test speed 1.0mm / s, test speed 2.0mm / s, return speed 2.0mm / s, compression deformation 50% of the initial sample height, and trigger force 5.0g. The instrument recorded the maximum peak force during the first compression cycle, defined as hardness, in grams per force (gf). Each sample was tested in parallel five times, and the average value was taken.

[0042] Peroxide value test method: After vacuum sealing the sample, store it at 4.0℃ protected from light. Take the sample on the 21st day of storage. Accurately weigh 2.50g of sample, add 30mL of a mixed solvent of chloroform and glacial acetic acid (volume ratio 2:3), and extract by shaking. Add 1.00mL of saturated potassium iodide solution, let it stand in the dark for 3min, and then add 100mL of distilled water. Using 10g / L starch solution as an indicator, titrate with 0.002mol / L sodium thiosulfate standard solution until the blue color disappears, and perform a blank test simultaneously. Calculate the peroxide value using the formula, and the result is expressed as millimoles of active oxygen per kilogram of sample (mmol / kg).

[0043] Thiobarbituric acid value test method: Sample from the same batch as the peroxide value test. Accurately weigh 5.00 g of sample and collect 50 mL of distillate using a steam distillation apparatus. Take 5.0 mL of distillate and mix it with 5.0 mL of 0.02 mol / L thiobarbituric acid aqueous solution in a stoppered test tube. React in a 90℃ water bath for 40 min, and after cooling, measure the absorbance at a wavelength of 532 nm. Simultaneously, prepare a standard curve using malondialdehyde (MDA) standard. Calculate the concentration of MDA in the sample based on the standard curve, and express the result as milligrams of MDA per kilogram of sample (mg / kg).

[0044] Sensory evaluation overall acceptability test method: Ten sensory evaluators were recruited and trained to evaluate randomly coded samples in a standard sensory evaluation room. Samples were prepared one hour before the test. Evaluators were required to rate the overall acceptability of the samples according to their personal preferences, using a 9-point preference scale, where 1 point represents "extremely dislike" and 9 points represent "extremely like". The arithmetic mean of all evaluators' scores for the same sample was calculated as the overall acceptability score of that sample.

[0045] The performance test data above are shown in Table 1.

[0046] Table 1 Performance Test Results

[0047] The test results in Table 1 above clearly show that Examples 1-3 comprehensively and synergistically solved the core technical problems existing in traditional solid dairy products made from milk skin compared to Comparative Examples 1-3.

[0048] The products prepared in Examples 1-3 had average thicknesses of 4.52 mm, 4.10 mm, and 4.85 mm, respectively, significantly higher than the 2.85 mm of Comparative Example 1, and with smaller standard deviations in thickness. This demonstrates that they successfully solved the problem of thin and uneven thickness in traditional products, achieving standardized structure and controllable thickness increase. Regarding the crucial structural integrity, Examples 1 and 3 achieved 100% integrity, and Example 2 achieved 95%, significantly better than Comparative Example 1's 65%, Comparative Example 2's 90%, and Comparative Example 3's 75%. This indicates that the interface and gel network synergistically constructed by the two modified compounds fundamentally overcome the defects of poor mechanical strength and easy breakage in traditional products.

[0049] The textural hardness data further confirmed the structural strengthening effect. The hardness values ​​of Examples 1-3 were between 1080-1350 gf, which was much higher than 620 gf of Comparative Example 1, and even higher than Comparative Examples 2 and 3 with only one added compound. This indicates that the octenyl succinic acid tamarind polysaccharide ester and ferulic acid-chitosan-whey protein conjugate produced a synergistic strengthening effect of "1+1>2" in improving the texture and firmness of the product.

[0050] In addressing the long-standing pain points of poor oxidative stability and short shelf life, Examples 1-3 demonstrated overwhelming advantages: after 21 days of storage, the peroxide value was only 2.3-2.8 mmol / kg, and the thiobarbituric acid value was only 0.32-0.40 mg / kg. This was not only far lower than Comparative Example 1 (peroxide value 6.2 mmol / kg, thiobarbituric acid value 0.95 mg / kg) without any added compounds, but also significantly better than Comparative Example 2 (antioxidant effect close to blank) with only added polysaccharide esters and Comparative Example 3 with only added conjugates. This fully demonstrates that the ferulic acid-chitosan-whey protein conjugate provides highly efficient antioxidant protection, and its efficacy is fully realized in the coexistence system with polysaccharide esters, achieving long-term preservation.

[0051] Ultimately, in terms of overall sensory quality, Examples 1-3 achieved an overall acceptability score of 8.0-8.5, significantly outperforming all comparative examples. This demonstrates that the technical solution of this invention, while addressing structural and stability issues, not only does not compromise flavor but also comprehensively enhances the sensory experience and consumer preference by improving texture, blending flavors, and preventing rancidity. In summary, this invention, through the design and synergistic application of two innovative compounds, systematically overcomes the technical bottlenecks of traditional milk skin products, such as fragile structure, susceptibility to oxidation and deterioration, and uneven quality, achieving a successful transformation from "experience-dependent" handmade products to "quality-controllable" modern food.

Claims

1. A method for preparing a solid dairy product called Tarmi milk skin, characterized in that the steps include... include: S1. Filter 800-1200 parts by weight of raw milk, preheat, centrifuge, pasteurize, and cool to 3-6℃ to obtain cooled raw milk; premix 0.5-2.0 parts of octenyl succinate tamarind polysaccharide ester and 0.3-1.5 parts of ferulic acid-chitosan-whey protein conjugate with the cooled raw milk to obtain a premix; heat the premix to 75-80℃ and add 30-60 parts of talmi to obtain mixed milk; S2. Keep the mixed milk at 75-80℃ and let it stand naturally to room temperature, then let it stand again; reheat it to 85-92℃ and keep it standing, then finally cool it to 8-16℃ and let it stand.

2. The method for preparing the solid dairy product, Tarmi milk skin, according to claim 1, is characterized in that, In step S1, the preheating temperature is 65-75℃.

3. The method for preparing the solid dairy product, Tarmi milk skin, according to claim 1, is characterized in that, In step S2, the mixed milk is kept at 75-80℃ for 1-1.5 hours.

4. The method for preparing the solid dairy product, Tarmi milk skin, according to claim 1, is characterized in that, The preparation method of the octenyl succinic acid tamarind polysaccharide ester includes: A1. By weight, disperse 45-55 parts of tamarind gum in 450-550 parts of deionized water and stir in a water bath at 44-46℃ to obtain a colloidal solution; adjust the pH to 8.4-8.6; while stirring continuously, add 28-32 parts of octenyl succinic anhydride dropwise; after the addition is complete, react at 44-46℃ to obtain a reaction solution; A2. Adjust the pH of the reaction solution to 6.4-6.6; transfer it to a dialysis bag and dialyze it in deionized water; freeze-dry the solution in the dialysis bag.

5. The method for preparing the solid dairy product of Tarmi milk skin according to claim 4, characterized in that, In step A1, the reaction time is 4-6 hours at 44-46℃.

6. The method for preparing the solid dairy product of Tarmi milk skin according to claim 4, characterized in that, In step A2, the dialysis time in deionized water is 48-50 hours.

7. The method for preparing the solid dairy product, Tarmi milk skin, according to claim 1, is characterized in that, The preparation method of the ferulic acid-chitosan-whey protein conjugate includes: B1. Dissolve 18-22 parts by weight of chitosan in acetic acid solution and adjust the pH to 5.5-6.5; add 4-6 parts of ferulic acid and 0.8-1.2 parts of horseradish peroxidase, stir at 28-32℃ in the dark, add 18-22 parts of hydrogen peroxide solution dropwise, stir to obtain a reaction mixture; adjust the pH of the reaction mixture to 9.0-10.0, centrifuge to collect the precipitate, wash the precipitate with deionized water, freeze dry to obtain ferulic acid-chitosan graft; B2. Dissolve 8-12 parts of ferulic acid-chitosan graft and 18-22 parts of whey protein isolate in phosphate buffer. Place the mixture in a water bath at 54-56°C with stirring and continue stirring to obtain a reaction solution. Cool the reaction solution to 3-5°C, dialyze and freeze dry.

8. The method for preparing the solid dairy product of Tarmi milk skin according to claim 7, characterized in that, In step B1, the stirring reaction time is 6-8 hours; the mass concentration of the hydrogen peroxide solution is 0.05-0.15%.

9. The method for preparing the solid dairy product, Tarmi milk skin, according to claim 7, is characterized in that, In step B2, the reaction is continuously stirred for 72-80 hours.

10. A solid dairy product made from Tarmi milk skin, characterized in that, The solid dairy product made from Talmi milk skin is prepared according to any one of claims 1-9 by the method described in the article.

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