Composite phase change cold storage agent, preparation method and application thereof

Abstract

The application provides a composite phase change cold storage agent and a preparation method and application thereof, and comprises the following raw materials in terms of mass fraction: 2wt%-20wt% of a main energy storage agent, 2wt%-8wt% of a temperature adjusting agent, and the balance of water; the main energy storage agent is potassium nitrate, and the temperature adjusting agent is one or more of maltitol or D-sorbitol. The composite phase change cold storage agent has high phase change latent heat and low cost, the onset temperature is-6 to-4 DEG C, the phase change latent heat reaches 290 J / g or more, and the preservation requirement of low-temperature meat products in cold chain circulation can be met.

Description

Technical Field

[0001] This invention relates to the field of phase change cold storage technology, specifically to a composite phase change cold storage agent, its preparation method, and its application. Background Technology

[0002] The quality assurance of chilled meat products relies on the support of a cold chain distribution system. However, the application of traditional active cooling systems in the cold chain presents two problems: on the one hand, the lagging development of low-carbon technologies leads to high energy consumption; on the other hand, frequent temperature fluctuations caused by cold chain breaks significantly affect the quality of chilled meat products.

[0003] Phase change energy storage technology, as an important innovative path for low-carbon emission energy systems, demonstrates unique advantages in the field of energy regulation due to its high energy storage density and dynamic temperature regulation capabilities. This technology achieves efficient energy storage by utilizing the absorption and release of latent heat during the phase transition of phase change energy storage materials, significantly improving the flexibility of energy allocation in time and space, and providing key technical support for building low-carbon and precisely temperature-controlled cold chain logistics. Furthermore, in recent years, researchers have applied phase change energy storage technology to passive cooling systems, further realizing low-carbon, economical, and temperature-constant cold chain logistics.

[0004] Single-component phase change cold storage materials suffer from high costs, insufficient matching between phase change temperature and heat release characteristics, and poor long-term stability. By combining and optimizing materials, composite phase change cold storage agents can effectively compensate for the performance deficiencies of single materials, solving the aforementioned problems while also possessing broader practical application value. Currently, there are many composite phase change cold storage agents for vaccines and cold chain logistics of fruits and vegetables; however, those with a latent heat of phase change exceeding 290 J / g are relatively few, and some exhibit supercooling or high costs, making it difficult to meet the preservation requirements of low-temperature meat products in cold chain distribution. Summary of the Invention

[0005] The technical problem to be solved by this invention is how to improve the latent heat of phase change of the refrigerant.

[0006] The present invention solves the above-mentioned technical problems through the following technical means:

[0007] The first aspect of the present invention provides a composite phase change cold storage agent, comprising the following raw materials by mass fraction: 2wt%~20wt% main energy storage agent, 2wt%~8wt% temperature regulator, and the balance being water; the main energy storage agent is potassium nitrate, and the temperature regulator is one or more of maltitol or D-sorbitol.

[0008] Beneficial effects: The composite phase change cold storage agent of the present invention has a high latent heat of phase change and low cost. Its onset temperature is -6~-4℃, and its latent heat of phase change reaches more than 290 J / g. It can also meet the preservation requirements of low-temperature meat products in cold chain circulation.

[0009] Preferably, the raw materials include the following by mass fraction: potassium nitrate 2wt%~20wt%, D-sorbitol 2wt%~8wt%, and the balance being water.

[0010] Preferably, the raw materials include the following by mass fraction: 4 wt% potassium nitrate, 2 wt% D-sorbitol and the balance being water.

[0011] Preferably, it also includes a thickener, which is carboxymethyl cellulose, and the amount added is 0.5wt%~2.0wt%.

[0012] Beneficial effects: This invention increases the viscosity of the composite phase change cold storage agent by adding a thickener, thereby reducing its fluidity and the possibility of phase separation, thus enabling the composite phase change cold storage agent to be repeatedly used in freeze-thaw cycles.

[0013] Preferably, the raw materials include the following by mass fraction: 4 wt% potassium nitrate, 2 wt% D-sorbitol, 1 wt% carboxymethyl cellulose, and the balance being water.

[0014] Preferably, it also includes a nucleating agent, which is borax, and the amount added is 0.6wt%~3.0wt%.

[0015] Beneficial effects: This invention adds a nucleating agent after adding a thickener to the composite phase change cold storage agent, which enables the composite phase change cold storage agent to achieve the best phase change performance. Adding a nucleating agent can suppress the supercooling phenomenon of the composite phase change cold storage agent.

[0016] Preferably, the raw materials include the following by mass fraction: 4 wt% potassium nitrate, 2 wt% D-sorbitol, 1 wt% carboxymethyl cellulose, 3 wt% borax, and the balance being water.

[0017] Beneficial effects: The composite phase change cold storage agent of the present invention has significant advantages in thermal properties; and after multiple freeze-thaw cycles, it still has good thermal properties, and no supercooling or phase separation phenomenon was found, which proves that it has excellent cycle stability.

[0018] The second aspect of the present invention provides a method for preparing the above-mentioned composite phase change cold storage agent, wherein the main energy storage agent and the temperature regulator are dissolved in water and mixed evenly to obtain the composite phase change cold storage agent.

[0019] The third aspect of this invention provides the application of the above-mentioned composite phase change cold storage agent in cold chain logistics.

[0020] The fourth aspect of this invention provides the application of the above-mentioned composite phase change cold storage agent in the cold chain distribution of low-temperature meat products.

[0021] Beneficial effects: The composite phase change cold storage agent of the present invention is suitable for refrigerated temperature zones and has certain application value and potential in cold chain circulation. Attached Figure Description

[0022] Figure 1 This is a DSC heat flux curve of potassium nitrate solution; Figure 2 This is a graph showing the relationship between the concentrations of potassium nitrate, sodium sulfate, polyethylene glycol, maltitol, and D-sorbitol and the latent heat of phase transition. Figure 3 This is a graph showing the relationship between the concentrations of potassium nitrate, sodium sulfate, polyethylene glycol, maltitol, and D-sorbitol and the phase transition temperature. Figure 4 These are DSC heat flow curves of the composite phase change cold storage agents in Examples 1-11; A1-A3 in the figures represent Examples 1-3; B1-B4 represent Examples 4-7; and C1-C4 represent Examples 8-11. Figure 5 These are DSC heat flow curves of the composite phase change refrigerants of Examples 12-15; D1-D4 in the figures represent Examples 12-15. Figure 6 This is a step cooling curve diagram of KDH, KDHC and the composite phase change refrigerant of Examples 16-20; E1-E5 in the figure represent Examples 16-20; Figure 7 These are DSC heat flow curves of the composite phase change refrigerants of Examples 17-20; E2-E5 in the figures represent Examples 17-20. Figure 8 These are the DSC heat flux plots for KDHCP, Comparative Example 1, and Comparative Example 2. Figure 9 These are the cooling curve and melting curve of KDHCP; Figure 10 This is a temperature-time variation curve inside the insulation box under different numbers of cold storage plates in Example 21; Figure 11 This is a process design diagram simulating the cold chain distribution of beef balls in Example 22; Figure 12 This is a graph showing the change in TVC of beef balls during the simulated cold chain distribution in Example 22; Figure 13 This is a graph showing the change in pH value of beef balls during simulated cold chain distribution in Example 22; Figure 14 This is a graph showing the change in TVB-N value of beef balls during simulated cold chain distribution in Example 22; Figure 15 This is a graph showing the changes in TBARS value (A) and total thiol content (B) of beef balls during the simulated cold chain circulation in Example 22; Figure 16 This is a graph showing the changes in the cooking loss rate (A) and drip loss rate (B) of beef balls during the simulated cold chain circulation in Example 22. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0024] Unless otherwise specified, all test materials and reagents used in the following examples are commercially available.

[0025] Unless otherwise specified in the embodiments, the techniques or conditions described in the literature in this field or in accordance with the product manual may be followed.

[0026] according to Figure 1-3 It can be seen that the latent heat of phase change of potassium nitrate solution exhibits relatively small changes overall under varying concentrations, ranging from 324.15 to 365.70 J / g. Specifically, within the concentration range of 2wt% to 8wt%, its latent heat of phase change changes slightly with the solution concentration, reaching a maximum of 365.70 J / g at a concentration of 4wt%. When the concentration exceeds 8wt%, the latent heat of phase change begins to decrease, finally reaching a minimum of 324.15 J / g at 20wt%. Therefore, potassium nitrate solution meets the requirement of high latent heat (>290 J / g) for phase change materials. Under varying concentrations, the onset temperature of potassium nitrate solution remains relatively stable, essentially maintaining at -2.5 ℃, indicating that it is a good composite monomer material and can serve as the main energy storage agent for composite phase change refrigerants.

[0027] Examples 1-11 This embodiment provides a composite phase change cold storage agent. The raw materials for the composite phase change cold storage agent are shown in Table 1. The raw materials in Table 1 are mixed evenly to obtain the composite phase change cold storage agent.

[0028] In Table 1, A1-A3 are Examples 1-3; B1-B4 are Examples 4-7; and C1-C4 are Examples 8-11.

[0029] Table 1

[0030] This embodiment is based on Figure 4 Further analysis of the DSC heat flow curves yielded the Onset temperature and latent heat of phase change of the composite phase change refrigerants in Examples 1-11, as shown in Tables 2-4 below.

[0031] Thermophysical property data of Examples 1-3

[0032] Table 2 Thermophysical property data of Examples 4-7

[0033] Table 3 Thermophysical property data of Examples 8-11

[0034] Table 4 according to Figure 4 As shown in Table 2-4, with a constant potassium nitrate concentration, the onset temperature and latent heat of phase change of the composite phase change refrigerant both decrease with increasing mass fractions of the temperature regulators polyethylene glycol, maltitol, and D-sorbitol. The table also shows that C1 has the highest latent heat of phase change. The preferred formulation is a composite of 4 wt% potassium nitrate and 2 wt% D-sorbitol, with the balance being water, named KDH.

[0035] Examples 12-15 This embodiment provides a composite phase change cold storage agent. The raw materials for the composite phase change cold storage agent are shown in Table 6. The raw materials in Table 6 are mixed evenly to obtain the composite phase change cold storage agent. In Table 1, D1-D4 represent Examples 12-15.

[0036] Table 6

[0037] The use of carboxymethyl cellulose (CMC) can effectively improve the performance of low-hydrate inorganic salts. CMC has significant hydrophilic properties, rapidly dissolving in both cold and hot water to form a highly transparent, viscous colloidal solution. Based on this property, CMC is widely used as a highly efficient thickener in oil extraction, food processing, and building materials, and also plays an important role in thermal energy storage systems (especially cold energy storage technology). Its thickening mechanism mainly stems from the formation of hydrogen bond networks between the hydrophobic groups of the molecular chain and water molecules, causing a significant expansion of the polymer fluid volume, thereby compressing the free movement space of the dispersed phase and ultimately achieving an effective increase in system viscosity. In this embodiment, by adding CMC to the composite phase change cold storage agent, the viscosity of the composite phase change cold storage agent is increased, reducing fluidity and the possibility of phase separation, thereby improving the repeated freeze-thaw cycle utilization rate of the composite phase change cold storage agent.

[0038] This embodiment is based on Figure 5 Further analysis of the DSC heat flow curves yielded the Onset temperature and latent heat of phase change of the composite phase change refrigerants of Examples 12-15, as detailed in Table 7 below.

[0039] Thermophysical property data of Examples 12-15

[0040] Table 7 according to Figure 5 As shown in Table 7, with the increase of carboxymethyl cellulose addition, the latent heat of phase change of the composite phase change cold storage agent showed a trend of first decreasing and then increasing in the range of 316.00 J / g to 335.60 J / g. However, the overall decrease in onset temperature was relatively small (maximum difference -0.59 ℃), and the latent heat of phase change of all composite phase change cold storage agents was higher than 290 J / g, with their onset temperature remaining stable in the range of -6 to -4 ℃. Further analysis of the flowability results revealed that when the carboxymethyl cellulose addition was 1 wt%, the composite phase change cold storage agent could still maintain a high latent heat of phase change while improving flowability. Therefore, the optimal formulation of the composite phase change cold storage agent was finally determined to be: 4 wt% potassium nitrate, 2 wt% D-sorbitol, 1 wt% carboxymethyl cellulose, with the balance being water, named KDHC.

[0041] Examples 16-20 This embodiment provides a composite phase change cold storage agent. The raw materials for the composite phase change cold storage agent are shown in Table 8. The raw materials in Table 8 are mixed evenly to obtain the composite phase change cold storage agent. E1-E5 in Table 8 are from Examples 16-20.

[0042] Table 8

[0043] according to Figure 6It can be seen that the supercooling of KDHC is 4.61 ℃, which is greater than that of KDH (2.96 ℃), indicating that the addition of thickener CMC increases the supercooling of the cold storage agent. From E1, it can be seen that when 0.6 wt% borax is added to KDHC, the supercooling of the composite phase change cold storage agent decreases from 2.96 ℃ to 1.39 ℃. Furthermore, when the borax addition is ≥1.2 wt% (E2-E5), the supercooling of the composite phase change cold storage agent approaches 0 ℃, proving that using borax as a nucleating agent can completely suppress the supercooling phenomenon of the composite phase change cold storage agent. However, the optimal addition amount of borax still needs to be determined through comprehensive analysis, combining the onset temperature and latent heat of phase change from E2 to E5.

[0044] according to Figure 7 Further analysis of the DSC heat flow curves yielded the Onset temperature and latent heat of phase change of the composite phase change refrigerants of Examples 17-20, as detailed in Table 9 below.

[0045] Thermophysical property data of Examples 17-20

[0046] Table 9 according to Figure 7 As shown in Table 9, with the increase of borax addition, the onset temperature and latent heat of phase change of the composite phase change refrigerant exhibit a fluctuating phenomenon of first decreasing, then increasing, and then decreasing again, with the highest latent heat of phase change being E2. Therefore, the optimal formulation of the composite phase change refrigerant is: 4wt% potassium nitrate, 2wt% D-sorbitol, 1wt% CMC, 1.2wt% borax, and the remainder being water. It is named KDHCP, with an onset temperature of -5.28 ℃ and a latent heat of phase change of 326.50 J / g.

[0047] Comparative Example 1 This comparative example uses commercially available Earl of Loca refrigerant (350ML), with a temperature specification of -5℃ and a service temperature of -5~0℃.

[0048] Comparative Example 2 This comparative example uses commercially available Jiruosen refrigerant (350ML), with a phase change temperature of -5℃ and a supply temperature of -10~0℃.

[0049] according to Figure 8 Further analysis of the DSC heat flux plots yielded the Onset temperature and latent heat of phase change for KDHCP, Comparative Example 1, and Comparative Example 2, as shown in Table 10 below.

[0050] KDHCP, thermophysical property data of Comparative Example 1 and Comparative Example 2

[0051] Table 10 according to Figure 8 It can be seen that KDHCP exhibits only one absorption peak throughout the entire phase transition process, while commercially available comparative examples 1 and 2 both show two absorption peaks, demonstrating the instability of the phase transition system. Figure 8 As can be seen, the KDHCP phase transition temperature range is relatively narrow, making it suitable for cold chain logistics that require precise temperature control.

[0052] As shown in Table 10, the latent heat of phase change of KDHCP is 64.90% and 88.56% higher than that of Comparative Example 1 and Comparative Example 2, respectively, indicating that it has superior thermal properties.

[0053] according to Figure 9 As can be seen from the comparison of melting curves at different cycle numbers, the melting curves of subsequent cycles did not show significant shifts compared to the first cycle, and they closely matched the initial curves, indicating that KDHCP maintains stable thermodynamic properties even after multiple phase change cycles. Similarly, the cooling curves for each cycle number showed no significant separation, and no supercooling was observed. Furthermore, no phase separation was observed in KDHCP after repeated thawing and freezing. These experimental results collectively demonstrate that KDHCP exhibits excellent cycle stability.

[0054] Example 21 This embodiment provides an application of a composite phase change refrigerant, KDHCP. KDHCP is injected into blank refrigerant plates (size: 21 cm × 2.5 cm × 18 cm), with the injection volume in each plate approximately 85% of its capacity. The refrigerant plates are placed in a -20℃ freezer for pre-cooling. An insulated box (size: 27.5 cm × 27.5 cm × 22 cm) is used as the experimental carrier, and 1, 2, 3, and 4 refrigerant plates are placed inside and numbered ①, ②, ③, and ④, respectively. A multi-channel temperature recorder (thermocouple accuracy ±0.2 ℃) is used to monitor temperature changes. The probe is fixed at the geometric center of the insulated box, and the entire device is placed in a constant temperature environment of 25℃. Temperature data is recorded at 20-second sampling intervals. A control group (without refrigerant plates) is also included in the experiment, with temperature changes recorded synchronously. Each experiment is repeated twice, and the final data is the average of the two measurements.

[0055] according to Figure 10It can be seen that the temperature-time curve inside the insulated box clearly demonstrates the effect of the number of cold storage plates on the control of the internal temperature. Firstly, compared to the control group (without cold storage plates), which reached near-equilibrium with the ambient temperature in 155 minutes, the use of cold storage plates effectively slows down the temperature rise inside the insulated box. Secondly, the temperature control time is positively correlated with the number of cold storage plates; as the number of cold storage plates increases, the insulation effect of the insulated box improves. Specifically, 1, 2, 3, and 4 cold storage plates can control the internal temperature of the insulated box at or below 4℃ for 15, 151, 615, and 1250 minutes, respectively. In practical applications, the number of cold storage plates can be flexibly selected according to requirements.

[0056] Example 22 This embodiment provides an application of a composite phase change cold storage agent in the cold chain distribution of low-temperature meat products. The composite phase change cold storage agent is KDHCP, and the low-temperature meat product is beef balls.

[0057] The beef balls were randomly assigned to insulated boxes and processed according to the following conditions: (1) CL group: The insulated box contains only beef balls and is placed in a constant temperature environment of 4 ℃.

[0058] (2) DL group: The insulated box contains only beef balls and is placed in an environment with fluctuating temperature.

[0059] (3) DK group: The insulated box contains beef balls and a cold storage plate (filled with 85% composite phase change cold storage agent KDHCP) arranged in a tight layout on all four sides, and is placed in the same temperature fluctuation environment as the DL group.

[0060] S1 Sensory Evaluation Referencing SB / T 10610-2011 with slight modifications, ten graduate students majoring in food science were selected and, after sensory training, conducted sensory evaluations of the beef ball samples. The evaluation criteria are shown in Table 11, and the sensory evaluation results are shown in Table 12.

[0061] Table 11 Sensory Evaluation Criteria for Beef Balls

[0062] Table 12 Sensory Evaluation Results

[0063] Color, odor, texture, and overall acceptability are important sensory indicators for evaluating the freshness of meat products, and are primary factors for producers and consumers in judging meat quality. Table 12 shows that the sensory scores of the CL, DK, and DL groups all showed a significant downward trend (P<0.05). The DL group had significantly lower sensory scores for odor, texture, and overall acceptability than the CL and DK groups. This indicates that temperature fluctuations during distribution have a negligible impact on color, but a significant impact on texture, odor, and overall acceptability. For the DK group with the added composite phase change cold storage agent KDHCP, its sensory characteristics were close to those of the isothermal control group (CL group) and superior to the temperature fluctuation group (DL group), indicating that the cold storage agent KDHCP, by maintaining a stable low-temperature environment, can effectively delay the quality deterioration of beef balls in terms of color, odor, and texture, and has a significant preservation effect.

[0064] S2 Total Colony Count Analysis Total bacterial count (TVC) determination: The determination was performed according to GB / T 4789.2-2016 "Microbiological Examination of Food Hygiene: Determination of Total Bacterial Count". 10 g of sample was placed in a sterile homogenizing bag with 90 mL of sterile physiological saline (0.85% NaCl), and then homogenized in a sterile homogenizer for 120 s. A series of decimal dilutions (1:10) were then performed with sterile physiological saline. Finally, the appropriate dilution suspension was inoculated onto PCA medium and incubated at 37 ℃ for 48 h. The microbial count for each sample was expressed as the logarithm of colony-forming units per gram of sample, i.e., lg(CFU / g). Measurements were performed independently three times at each sampling point.

[0065] Meat is a highly perishable food, and its high moisture and protein content provides an ideal environment for bacterial growth. Therefore, the total bacterial count is considered one of the most important indicators for assessing the microbiological safety of meat. The trends in the total bacterial count of beef balls in different groups during cold chain distribution are shown below. Figure 12As shown, the total bacterial count in groups CL, DK, and DL all increased with the extension of the circulation time, and the growth rate of the total bacterial count in the three groups differed significantly (P<0.05). Overall, during the 102-hour cold chain circulation process, the microbial proliferation in group CL was the slowest, with the total bacterial count increasing from an initial 2.94 lg (CFU / g) to 4.44 lg (CFU / g). Due to the influence of temperature fluctuations, group DL experienced the most rapid increase in the total bacterial count, reaching 5.19 lg (CFU / g) at the end of the circulation, significantly higher than the other groups (P<0.05). The final total bacterial count in group DK was 4.64 lg (CFU / g), falling between that of groups CL and DL. It is noteworthy that before 42 hours of distribution, the total bacterial count in the DK group was not significantly different from that in the CL group (P>0.05), indicating that the composite phase change refrigerant KDHCP maintained a relatively stable low-temperature environment during this period. However, in the later stages of distribution, the bacterial growth trend in the DK group began to be significantly higher than that in the CL group (P<0.05), which is consistent with the phenomenon that the temperature inside the chamber accelerated after the composite phase change refrigerant KDHCP completely melted at 1250 min. However, compared with the DL group, the use of refrigerant KDHCP in the DK group could still significantly (P<0.05) reduce the impact of temperature fluctuations on the accelerated spoilage of beef balls.

[0066] S3 pH value analysis pH determination: Weigh 10g of sample and chop it, mix it with 90 mL of sterile physiological saline (0.85% NaCl), and then filter the mixture through filter paper. Finally, measure the pH value using a pH meter. Each sample was measured in triplicate.

[0067] pH value is an important indicator of changes in meat quality. Changes in its pH value not only affect the color stability, flavor characteristics, and water-holding capacity of meat, but may also regulate the growth and metabolic activities of microorganisms by altering the internal environment. According to... Figure 13 The initial pH of the beef ball samples was 6.73. With prolonged flow time, the pH of the beef balls under the three flow conditions showed different upward trends. At the end of the flow, the pH values ​​of the three groups increased by 4.01% (CL group), 4.46% (DK group), and 6.39% (DL group), respectively. The pH increase in the DL group was significantly higher than that in the other two groups due to temperature fluctuations (P<0.05). Furthermore, there was essentially no significant difference between the DK group and the CL group (P>0.05), but a significant difference was found between the DK group and the DL group (P<0.05). This indicates that the application of the composite phase change refrigerant KDHCP can effectively reduce the impact of temperature fluctuations on the pH value of the beef balls.

[0068] S4 Volatile Basic Nitrogen Analysis Determination of volatile basic nitrogen (TVB-N): The content of total volatile basic nitrogen (TVB-N) was determined using the Kjeldahl method. 10 g of chopped sample was weighed, added to 100 mL of purified water, shaken for 15 min, and filtered. 10 mL of the supernatant was mixed with 10 mL of magnesium oxide suspension (0.01 g / mL). Finally, the consumption of hydrochloric acid (0.01 mol / L) was accurately determined using an automated Kjeldahl nitrogen analyzer, and the TVB-N value was calculated. Each sample was tested in triplicate, and the results are expressed in mg / 100 g.

[0069] TVB-N, a major compound produced by spoilage bacteria during protein decomposition, is often used as a biomarker to assess the degree of spoilage in various meat products. The changes in TVB-N in different groups of beef balls during distribution are shown below. Figure 14 As shown in the figure. The results indicate that TVB-N in all groups continued to increase within 102 hours of circulation. Initially, the CL group showed the most gradual increase in TVB-N, rising from an initial value of 5.24 mg / 100g to 7.17 mg / 100g after 102 hours of circulation. However, the DL group, affected by temperature fluctuations, experienced a significantly faster increase in TVB-N, with its value becoming significantly higher than the other two groups after 15 hours of circulation (P<0.05), reaching 9.68 mg / 100g at the end of circulation. In the DK group, which added the composite phase change refrigerant KDHCP, the trend of TVB-N change was between that of the CL and DL groups, ultimately reaching 7.17 mg / 100g. It is noteworthy that the TVB-N values ​​of the DK group were not significantly different from those of the CL group during the first 42 hours of distribution (P>0.05), indicating that the composite phase change refrigerant KDHCP effectively maintained the stability of the low-temperature environment during this period. However, after 42 hours of distribution, the TVB-N growth trend of the DK group began to be significantly higher than that of the CL group (P<0.05), which is consistent with the TVC change trend. This may be related to the accelerated temperature rise inside the chamber after the composite phase change refrigerant KDHCP completely melted at 1250 min. Nevertheless, compared with the DL group, the refrigerant KDHCP still significantly inhibited the promoting effect of temperature changes on the spoilage of beef balls (P<0.05).

[0070] S5 Oxidation Index Analysis Determination of thiobarbituric acid (TBARS): Thiobarbituric acid (TBARS) was determined spectrophotometrically. 5 g of sample was thoroughly homogenized with 50 mL of a trichloroacetic acid mixture (7.5% TCA, 0.1% EDTA-2Na) and filtered. Then, 5 mL of the filtrate was mixed with 5 mL of thiobarbituric acid solution (0.02 mol / L) and incubated at 90 °C for 30 min. Finally, the absorbance was measured at 532 nm and quantitatively compared with a series of malondialdehyde (MDA) standard solutions. Results are expressed as MDA content (mg / kg).

[0071] Determination of total thiol content: The total thiol content was determined by the micro-method as described in the instructions of the BC1725 Total Thiol Content Detection Kit. The absorbance at a wavelength of 412 nm was measured using an ELISA reader. The results are expressed as total thiol content (µmol / g).

[0072] The deterioration of the nutritional value and sensory characteristics of meat is primarily due to the degradation of its main components, such as proteins and lipids. Lipid oxidation leads to rancidity, giving meat products an "oxidized taste" and darkening or yellowing their color, affecting consumers' perception of freshness. Protein oxidation, on the other hand, damages the physical properties of meat, making it tough or sticky and producing unpleasant odors. By detecting indicators of lipid oxidation (such as thiobarbituric acid value) and protein oxidation (such as total sulfhydryl content), the degree of oxidation in meat products can be effectively monitored, ensuring product quality.

[0073] Figure 15 (A) shows the trend of TBARS values ​​for each group of beef meatballs during 102 h of cold chain circulation. First, the TBARS value of the CL group slowly increased from 0.40 mg / kg at the beginning of circulation to 0.66 mg / kg at the end, indicating that a relatively stable low-temperature environment can effectively reduce the degree of fat oxidation in meat products. Second, the TBARS value of the DL group was significantly higher than that of the CL and DK groups at 15 h of circulation (P<0.05) and then increased rapidly, reaching a maximum of 0.98 mg / kg at the end of circulation, which was obviously affected by temperature fluctuations. In addition, in the DK group with the addition of the composite phase change cold storage agent KDHCP, its TBARS value was basically between that of the CL and DL groups, and was 0.75 mg / kg at the end of circulation, which was 23.47% lower than that of the DL group, indicating that the composite phase change cold storage agent KDHCP effectively reduced the impact of temperature fluctuations on lipid oxidation of beef meatballs.

[0074] Figure 15(B) shows the trend of total thiol content changes in beef meatballs of each group during 102 h of cold chain circulation. Similarly, there were different trends among the groups. Among them, the total thiol content of group CL decreased slowly from the initial value of 2.53 µmol / g to 1.48 µmol / g at the end of circulation (a decrease of 41.50%); the total thiol content of group DL showed the most significant decreasing trend, decreasing to 0.82 µmol / g at the end of circulation (a decrease of 67.59%); while the total thiol content of beef meatballs in group DK was 1.37 µmol / g at the end of circulation (a decrease of 45.85%), which was 21.74% less than that of group DL. This indicates that the composite phase change cold storage agent KDHCP effectively inhibited the protein oxidation reaction induced by temperature fluctuations by stabilizing the cold chain environment.

[0075] S6 water-holding capacity analysis Determination of cooking loss rate: Before cooking, weigh and record the mass m0 of the beef ball sample. Place it in a cooking bag, gently press to remove air, and seal. Fix the thermometer probe to the center of the meatball. Then, completely immerse the packaged sample in a 75℃ water bath and heat until the center temperature of the meatball reaches 70℃. Immediately remove it and cool it at room temperature for 30 min. After removing the meatball, pat it dry with kitchen paper, weigh it, and record the mass as m1. The formula for calculating the cooking loss rate (%) is: .

[0076] Determination of drip loss rate: Before the test, weigh and record the mass m0 of the beef ball sample. Suspend it on a toothpick and place it horizontally in a disposable paper cup. Cover with plastic wrap, seal, and place in a 4 ℃ refrigerator for 24 h. After removal, blot the surface moisture with kitchen paper, weigh, and record the mass m1. The formula for calculating the drip loss rate (%) is: .

[0077] Water-holding capacity is a key indicator characterizing the ability of meat products to retain moisture during processing and storage. A decrease in water-holding capacity leads to increased water loss, directly affecting the product's weight, appearance, and sensory characteristics. Changes in water-holding capacity are assessed using cooking loss rate and drip loss rate. Figure 16The trends show that the cooking loss rate and drip loss rate of beef balls under the three circulation conditions all increased with the extension of circulation time. The CL group showed the slowest increase rate, the DL group the fastest, and the DK group was in between. Specifically, at the end of circulation, the cooking loss rate of the CL group increased from the initial 2.72% to 4.84%, and the drip loss rate increased from 0.14% to 0.76%; the cooking loss rate and drip loss rate of the DL group increased to 8.50% and 1.52%, respectively; while the DK group increased to 4.88% and 0.81%, respectively, with no significant difference from the CL group (P>0.05). However, compared to the DL group, the DK group reduced the cooking loss rate and drip loss rate by 42.59% and 46.71%, respectively, indicating that the use of the composite phase change refrigerant KDHCP alleviated the decrease in water holding capacity caused by temperature fluctuations to some extent.

[0078] The composite phase change cold storage agent KDHCP of this invention is integrated into a cold storage and insulation box and applied in cold chain logistics. Its temperature control effect and application value in practical use were investigated. It was found that the composite phase change cold storage agent exhibits excellent heat preservation performance in the early stages of cold chain logistics due to continuous phase change and the release of cold energy. Overall, it can reduce the impact of temperature fluctuations on the sensory properties of beef balls (texture, odor, overall acceptability), microbial growth, pH, fat oxidation, protein oxidation, and moisture loss. In conclusion, the composite phase change cold storage agent KDHCP has certain application value and potential in cold chain logistics.

[0079] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite phase change refrigerant, characterized in that, The raw materials include the following by mass fraction: 2wt%~20wt% main energy storage agent, 2wt%~8wt% temperature regulator, and the balance being water; the main energy storage agent is potassium nitrate, and the temperature regulator is one or more of maltitol or D-sorbitol.

2. The composite phase change cold storage agent according to claim 1, characterized in that, The raw materials include the following by mass fraction: potassium nitrate 2wt%~20wt%, D-sorbitol 2wt%~8wt%, and the balance being water.

3. The composite phase change cold storage agent according to claim 1, characterized in that, The ingredients, by mass fraction, are: 4 wt% potassium nitrate, 2 wt% D-sorbitol, and the remainder is water.

4. The composite phase change refrigerant according to claim 3, characterized in that, It also includes a thickener, which is carboxymethyl cellulose, with an addition amount of 0.5wt% to 2.0wt%.

5. The composite phase change cold storage agent according to claim 4, characterized in that, The ingredients, by mass fraction, are: 4 wt% potassium nitrate, 2 wt% D-sorbitol, 1 wt% carboxymethyl cellulose, and the balance being water.

6. The composite phase change cold storage agent according to claim 5, characterized in that, It also includes a nucleating agent, which is borax, with an addition amount of 0.6wt%~3.0wt%.

7. The composite phase change cold storage agent according to claim 6, characterized in that, The ingredients, by mass fraction, are: 4 wt% potassium nitrate, 2 wt% D-sorbitol, 1 wt% carboxymethyl cellulose, 1.2 wt% borax, and the balance being water.

8. The method for preparing the composite phase change cold storage agent according to any one of claims 1-7, characterized in that, The main energy storage agent and the temperature regulator are dissolved in water and mixed evenly to obtain a composite phase change cold storage agent.

9. The application of the composite phase change cold storage agent as described in any one of claims 1-7 in cold chain logistics.

10. The application of the composite phase change cold storage agent as described in any one of claims 1-7 in the cold chain distribution of low-temperature meat products.

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