A method for prolonging the storage period of pre-baked toast by pre-cooling and segmenting liquid nitrogen flash freezing

Abstract

The present application belongs to the technical field of liquid nitrogen quick freezing, and particularly relates to a method for pre-cooling and segmenting liquid nitrogen quick freezing of pre-baked toast. The toast is pre-cooled in a refrigerator freezing chamber, and then frozen by using a segmenting liquid nitrogen quick freezing mode. Temperature is measured by using a temperature sensor during freezing, and temperature and time are adjusted to find the best segmenting freezing parameters. The toast is frozen by using the method, which on the one hand delays the aging of the toast by using pre-cooling and segmenting liquid nitrogen quick freezing technology, and prolongs the storage period; on the other hand, temperature is measured by using a temperature sensor, temperature and time are adjusted, and the best segmenting freezing parameters are found, which greatly improves the energy saving effect and reduces the cost.

Description

Technical Field

[0001] This invention belongs to the field of liquid nitrogen quick-freezing technology, specifically relating to a method for pre-cooling segmented liquid nitrogen quick-freezing of pre-baked toast. Background Technology

[0002] Plain toast is a can-shaped bread made from high-gluten flour, sugar, salt, yeast, milk, eggs, butter, and other ingredients, kneaded, proofed, and baked in a can with or without a lid. Due to the influence of temperature, humidity, and the production process, toast transported to stores via ordinary cold chain usually has a short shelf life. Currently, ordinary quick-freezing is generally used to extend the shelf life of bread. However, due to the long freezing time, when the temperature passes through the ice crystal formation zone, large and irregularly sized ice crystals easily form inside the toast. This easily damages the gluten network structure inside the toast, leading to accelerated quality deterioration, increased crumbling during storage, hardening of the toast's texture, and ineffective preservation of flavor.

[0003] Liquid nitrogen is a colorless, odorless, and low-viscosity liquid. When used for refrigeration, the temperature difference can reach 200°C. Liquid nitrogen rapidly absorbs heat from objects to achieve a cooling effect. Furthermore, when liquid nitrogen freezes, it produces an inert gas that does not oxidize when in contact with food, making it an ideal refrigerant.

[0004] Existing technologies already disclose methods for freezing food using liquid nitrogen. For example, Patent Document 1 (application number 201410443531.8) discloses a method for quick-freezing rice and flour products using liquid nitrogen. This method improves the quality of frozen food by first controlling the moisture content of the rice and flour products and then freezing them using a liquid nitrogen tunnel freezer. Patent Document 2 (application number 202011099826.X) discloses a liquid nitrogen preservation process. This process achieves quick-freezing and preservation by spraying liquid nitrogen into an ultra-low temperature cooking pan and completely immersing the food inside. While these documents all use liquid nitrogen to freeze food and achieve a certain freezing effect, the large amount of liquid nitrogen used results in high costs, making it unsuitable for practical production. Furthermore, Patent Document 1 involves drying the rice and flour products before liquid nitrogen freezing and adding sorbitol solution to control the moisture content. This process is cumbersome and can affect the texture of bread and other wheat-based foods.

[0005] Therefore, how to control the freezing cost of toast while ensuring its authentic taste and extending its shelf life is a problem that urgently needs to be solved. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for pre-cooling segmented liquid nitrogen quick-freezing of pre-baked toast. The toast is first placed in a refrigerator compartment for pre-cooling, and then frozen using a segmented nitrogen-filling mode. Temperature sensors are used to measure the temperature, and the temperature and time are adjusted. This combined refrigerator pre-cooling and segmented quick-freezing process aims to maintain the good taste of the toast, reduce the cost of freezing the toast, and extend its shelf life.

[0007] The present invention achieves the above objectives through the following technical solutions:

[0008] A method for extending the shelf life of pre-baked toast by pre-cooling segmented liquid nitrogen quick-freezing involves the following steps: first, pre-cooling the toast in the refrigerator compartment, and then freezing it using a segmented nitrogen-filling mode to obtain frozen toast for storage.

[0009] The method for extending the shelf life of pre-baked toast using pre-cooled segmented liquid nitrogen quick-freezing, as described above, specifically includes the following steps:

[0010] (1) Pack the baked toast in a bag and place it in the refrigerator to cool before use;

[0011] (2) Place the pre-cooled toast into a liquid nitrogen quick-freezing machine for quick-freezing, and select one toast to insert a temperature sensor as a reference to monitor the temperature change inside the toast.

[0012] (3) Set the temperature, holding time, nitrogen addition time and interval time of the three stages of quick freezing of the liquid nitrogen quick freezer respectively, and carry out quick freezing;

[0013] (4) Store the frozen toast in the refrigerator.

[0014] Furthermore, in step (1), the temperature of the cold storage compartment is set to 4-6℃, and the pre-cooling time is 30min.

[0015] Furthermore, in step (1), the packaging bag is a PE packaging bag.

[0016] Furthermore, in step (2), the temperature sensor is inserted into the center of the toast.

[0017] Furthermore, in step (3), the parameters for the three-stage quick-freezing of the liquid nitrogen quick-freezing machine are as follows: the first stage quick-freezing temperature is -70℃ to -50℃, the holding time is 5 to 8 min, the nitrogen addition time is 2 to 4 s, and the interval time is 2 s; the second stage quick-freezing temperature is -90℃ to -80℃, the holding time is 4 to 6 min, the nitrogen addition time is 2 to 4 s, and the interval time is 2 s; the third stage quick-freezing temperature is -70℃ to -65℃, the holding time is 3 to 6 min, the nitrogen addition time is 2 to 4 s, and the interval time is 2 s.

[0018] Furthermore, in step (4), the storage temperature is -18℃ and the storage period is 5 to 7 days.

[0019] This invention first uses a refrigerator to pre-cool the toast, ensuring the moisture inside the toast and further guaranteeing its texture after subsequent freezing. Then, liquid nitrogen is used to freeze the toast. The low temperature of liquid nitrogen causes the surrounding air to drop rapidly, creating a hot-cold alternation with the food to achieve freezing. Its freezing speed is very fast, quickly passing through the maximum ice crystal formation zone (-5 to -1℃), producing small and uniform ice crystals. This ensures that the gluten network structure inside the toast is not damaged, thus guaranteeing its texture. Moreover, it consumes very little energy, only 0.25% to 0.5%, achieving cost savings.

[0020] The beneficial effects of this invention are as follows:

[0021] (1) Before freezing with liquid nitrogen, the present invention pre-cools the bread in the refrigerator compartment and then uses liquid nitrogen segmented quick-freezing mode to process the bread. The bread after pre-cooling in the refrigerator and segmented quick-freezing with liquid nitrogen avoids the evaporation of its internal moisture, locks in as much moisture as possible, and ensures that the bread still has a good taste after freezing.

[0022] (2) The present invention adopts a segmented nitrogen-filling mode for freezing. The first stage of liquid nitrogen quick freezing and air alternating between hot and cold can rapidly reduce the temperature inside the liquid nitrogen quick freezer to -70℃ to -50℃. At this time, the temperature of the center of the toast can be reduced to below -4℃, which shortens the time for the second stage of freezing to pass through the maximum ice crystal formation zone. This is conducive to the formation of small and uniform ice crystals, reduces the formation of large ice crystals, protects the integrity of the gluten network structure, and ensures the good taste of the toast.

[0023] (3) The third stage of freezing can lower the temperature of the center of the toast to -18-20℃, which puts almost all microorganisms into a dormant state and reduces their metabolic activity to a very low level, thereby extending the storage period of the toast.

[0024] (4) Nitrogen is a disposable product. This invention uses a temperature sensor to measure the temperature, adjust the temperature and time, and explore suitable liquid nitrogen quick-freezing machine settings. It adopts a three-stage quick-freezing mode, which consumes less nitrogen and achieves energy-saving effect. Compared with toast frozen in a single-stage liquid nitrogen freezing mode, it can save up to 49.8% of the cost per 250g of toast, greatly improving the energy-saving effect and reducing the cost of use. Detailed Implementation

[0025] To enable those skilled in the art to better understand the present invention, the present invention will now be further described in conjunction with specific embodiments.

[0026] Example 1

[0027] (1) Pack the baked toast in a PE bag and place it in a refrigerator at 5°C for 30 minutes to cool.

[0028] (2) Place the pre-cooled toast into a liquid nitrogen quick-freezing machine and insert a temperature sensor in the center to monitor the temperature change inside the toast;

[0029] (3) Set the parameters of the liquid nitrogen quick-freezing machine: the first stage quick-freezing temperature is -65℃, the holding time is 7min, the nitrogen addition time is 3s, and the interval time is 2s; the second stage quick-freezing temperature is -80℃, the holding time is 6min, the nitrogen addition time is 3s, and the interval time is 2s; the third stage quick-freezing temperature is -65℃, the holding time is 6min, the nitrogen addition time is 3s, and the interval time is 2s.

[0030] (4) Store the quick-frozen toast in a -18℃ freezer.

[0031] Example 2

[0032] The difference from Example 1 is that the first heat preservation time is 6 minutes.

[0033] Example 3

[0034] The difference from Example 1 is that the heat preservation time for the third stage is 3 minutes.

[0035] Example 4

[0036] The difference from Example 1 is that the quick-freezing temperature in the third stage is -70°C.

[0037] Example 5

[0038] The difference from Example 1 is that the second quick-freezing temperature is -90°C.

[0039] Comparative Example 1

[0040] The difference from Example 1 is that the toast is cooled in one step with liquid nitrogen until the bread core temperature reaches -20°C.

[0041] Comparative Example 2

[0042] The difference from Example 1 is that the toast is not pre-cooled in a refrigerator, but is quick-frozen to -18°C using a regular quick-freezing machine before being placed in the freezer.

[0043] Comparative Example 3

[0044] The difference from Example 1 is that the toast is neither pre-cooled nor subjected to liquid nitrogen staged quick-freezing; it is directly frozen in a regular refrigerator after baking.

[0045] Comparative Example 4

[0046] The difference from Example 1 is that the toast is not pre-cooled in a refrigerator, and after being baked and cooled to room temperature, it is directly subjected to three-stage liquid nitrogen freezing.

[0047] Comparative Example 5

[0048] The difference from Example 1 is that in step (3), the first quick-freezing temperature is -20°C and the holding time is 3 minutes.

[0049] Comparative Example 6

[0050] The difference from Example 1 is that in step (3), the second quick-freezing temperature is -110℃ and the holding time is 8min.

[0051] Comparative Example 7

[0052] The difference from Example 1 is that in step (3), the quick-freezing temperature of the third stage is -90℃. The holding time is 8 minutes.

[0053] Comparative Example 8

[0054] The difference from Example 1 is that in step (3), the first segment adopts the parameter settings in Comparative Example 4, the second segment adopts the parameter settings in Comparative Example 5, and the third segment adopts the parameter settings in Comparative Example 5.

[0055] Test case

[0056] Experiment 1: Determination of Toast Hardness

[0057] Hardness is a direct factor reflecting starch retrogradation and texture. During starch retrogradation, moisture migrates and is lost, starch molecules rearrange, and hardness increases accordingly. The toasts obtained in Examples 1-5 and Comparative Examples 1-8 were stored for 7, 30, and 60 days, and their hardness was measured under the conditions of thawing temperature of 20℃-25℃ and thawing time of 4 hours. The results are shown in Table 1.

[0058] Table 1. Hardness of toast at different storage periods (taking 250g covered toast as an example)

[0059] name Hardness after 7 days / gf Hardness after 30 days / gf Hardness after 60 days / gf Example 1 78.3 225.5 310.2 Example 2 85.1 230.4 335.3 Example 3 80.7 229.5 298.5 Example 4 78.1 220.4 279.7 Example 5 78.5 228.7 282.8 Comparative Example 1 85.3 243.4 291.5 Comparative Example 2 88.2 240.2 378.9 Comparative Example 3 110.6 279.3 420.7 Comparative Example 4 83.7 255.2 331.2 Comparative Example 5 87.8 240.4 297.7 Comparative Example 6 85.0 237.8 288.2 Comparative Example 7 83.5 241.3 297.1 Comparative Example 8 82.7 261.0 397.8

[0060] Table 1 shows that the hardness values ​​of the examples after different storage periods were all lower than those of the comparative example. This indicates that the pre-cooling in the refrigerator compartment followed by segmented quick-freezing with liquid nitrogen is beneficial to the integrity of the internal structure of the toast. After long-term low-temperature storage, the damage to the gluten network caused by ice crystal growth is less than that caused by single-stage cooling. However, the increase in hardness varied with the increase in storage period. Comparing the examples with Comparative Example 2, Example 4 showed a smaller increase in hardness, indicating that Example 4 has a better effect on delaying starch aging than the other examples and can effectively extend the storage period.

[0061] Experimental Example 2

[0062] The end temperatures of the second and third stages of liquid nitrogen freezing, the time taken to pass through the maximum ice crystal formation zone, and the nitrogen consumption in the methods of Examples 1-5 and Comparative Examples 1-8 were measured, and the results are shown in Table 2.

[0063] Table 2. Results of toast measurement (taking 250g toast with lid as an example)

[0064]

[0065]

[0066] As shown in Table 2, although the nitrogen consumption in Example 4 is not the lowest, Example 4 is in a better state by comprehensively comparing the hardness value, the end temperature of the second and third stages, and the time required to pass through the maximum ice crystal formation zone with Example 1. This indicates that the parameter settings in Example 4 can delay starch aging to the greatest extent and reduce nitrogen costs. Furthermore, the toast center temperature is within the range of -20℃ to -18℃, which meets the requirements for extending the storage period and saving energy.

[0067] Experiment 3 calculates the energy consumption cost of different freezing methods in Examples 1-5 and Comparative Examples 1-8, and the results are shown in Table 3.

[0068] Table 3 Energy Costs

[0069]

[0070]

[0071] According to the energy consumption cost data in Table 3, Comparative Example 6 has the highest cost, which is also the highest after 30 and 60 days of storage. Compared with Example 4, although Comparative Example 2 has a lower energy consumption cost, it takes longer to pass through the maximum ice crystal formation zone, resulting in a higher hardness value and faster aging rate after storage. While Comparative Example 1 also has a relatively low cost, it consumes a large amount of nitrogen, which increases the cost and does not meet the purpose of energy saving and cost reduction.

[0072] Experiment Example 4 Sensory Evaluation

[0073] Ten evaluators with relevant assessment experience were selected to conduct sensory evaluations of the bread according to the scoring criteria in Table 4. The evaluation indicators included the bread's appearance, surface color, baking uniformity, crust texture, grain and air pockets, internal color, taste, mouthfeel, and texture. The maximum score was 100 points, and the average score was taken as the final sensory evaluation result. The test results of the sensory scores are shown in Table 5.

[0074] Table 4 Sensory Evaluation Scoring Criteria

[0075]

[0076]

[0077] Table 5. Sensory rating test results

[0078]

[0079]

[0080] As shown in Table 5, the sensory evaluation score in Example 4 was relatively high. In this case, the three quick-freezing parameters were beneficial to delay the aging of the toast, extend the storage period, and reduce the cost of using liquid nitrogen.

[0081] In summary, Example 4 achieves the effect of ensuring a good taste for the toast while also saving costs and extending shelf life. Therefore, taking 250g of toast as an example, the following liquid nitrogen quick-freezing parameters are selected: first stage quick-freezing temperature -65℃, holding time 6min, nitrogen addition time 3s, and interval time 2s; second stage quick-freezing temperature -80℃, holding time 6min, nitrogen addition time 3s, and interval time 2s; third stage quick-freezing temperature -70℃, holding time 6min, nitrogen addition time 3s, and interval time 2s are the preferred parameters.

[0082] Experiments have determined the specific size of the toast and its corresponding liquid nitrogen treatment parameters, as shown in Table 6. The parameters shown in Table 6 can be used for industrial production.

[0083] Table 6. Toast Size and Temperature Parameters

[0084] Toast size First stage of quick freezing Second stage of quick freezing Stage 3 quick-freezing 250g (10×10×10cm) -65℃ / 6min -80℃ / 6min -70℃ / 6min 300g (17.5×10×10cm) -65℃ / 6.5min -80℃ / 6min -70℃ / 6.5min 450g (19.5×11×11cm) -68℃ / 6min -80℃ / 6min -73℃ / 6.5min 600g (31×9×10.5cm) -70℃ / 7min -83℃ / 6min -75℃ / 6.5min 750g (31.5×10×11.5cm) -73℃ / 7min -78℃ / 6.5min -75℃ / 7min 900g (33×10.5×12cm) -80℃ / 7min -78℃ / 7min -77℃ / 7min 1000g (33×12×12cm) -83℃ / 7min -79℃ / 7min -79℃ / 7min 1200g (37×12×12cm) -85℃ / 7min -79℃ / 7min -80℃ / 7min

Claims

1. A method for extending the shelf life of prebaked toast by precooling segmented liquid nitrogen flash freezing, characterized in that, The steps are as follows: (1) The toasted toast is packaged with a PE packaging bag and placed in a 5°C refrigerator cold storage room for precooling for 30 min, ready for use; (2) The pre-cooled toast is placed in a liquid nitrogen quick freezer, and one of the toasts is inserted into a temperature sensor as a reference to monitor the internal temperature change of the toast; (3) The temperature, holding time, nitrogen adding time and intermittent time of the three-stage quick freezing of the liquid nitrogen quick freezer are set respectively, and quick freezing is carried out; The parameter settings of the three-stage quick freezing of the liquid nitrogen quick freezer are as follows: ① The first-stage quick freezing temperature is-65°C, the holding time is 7 min, the nitrogen adding time is 3 s, and the intermittent time is 2 s; ② The second-stage quick freezing temperature is-80°C, the holding time is 6 min, the nitrogen adding time is 3 s, and the intermittent time is 2 s; ③ The third-stage quick freezing temperature is-70°C, the holding time is 6 min, the nitrogen adding time is 3 s, and the intermittent time is 2 s; (4) The quick-frozen toast is placed in a-18°C refrigerator for storage.

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