China's modified atmosphere fresh-keeping packaging started in the 1990s, and the R&D and application of modified atmosphere fresh-keeping equipment are still at the initial stage. The key technology of advanced multifunctional air conditioning and fresh-keeping technology and equipment lies in the research and design of a suitable compound gas mixed distribution system.
1. The principle of composite modified atmosphere fresh-keeping packaging
Composite modified atmosphere fresh packaging also known as gas replacement packaging, internationally known as MAP packaging (ModifiedAtmospherePacking). The principle is to use composite gas (2-4 kinds of gas mixed according to the ratio of the packaged food characteristics) to replace the air in the packaging container, change the gas environment around the food in the packaging container, thereby inhibiting the growth and reproduction of bacteria (microorganisms). Relieve the metabolic rate of packaged foods and prolong the shelf life or shelf life of foods [1, 2].
Atmosphere-preserved gas is generally composed of carbon dioxide (CO2), nitrogen (N2), oxygen (O2), and a small amount of specialty gases. High-concentration CO2 gas can hinder the growth and reproduction of most microorganisms that cause food spoilage, prolong the stagnation period (or incubation period) of its reproductive growth, and delay its logarithmic growth phase; it is the main antibacterial component in fresh-keeping gas; O2 can inhibit large Most anaerobic bacteria grow and reproduce. High oxygen can maintain the color of fresh meat while hypoxia can maintain the fresh state of fruits and vegetables while inhibiting the respiration rate of fresh fruits and vegetables. N2 is an inert gas that does not have chemical interaction with food and can be used as a filling gas. Prevent the CO2 from escaping and make the package fall. Nitrogen-packed packaging can reduce the oxidation of fat, aroma, and color in foods [3]. Combining CO2, O2, N2, and specialty gases to form a composite preservative gas for the preservation of fresh fruits and vegetables is a trend in today's fresh-keeping packaging.
2. The basic structure of compound gas mixed distribution system
In order to send different proportions of carbon dioxide (CO2), nitrogen (N2), and oxygen (O2) into the packaging container, the CO2, N2, and O2 gases are generally sent to the mixing chamber (air chamber) in a certain proportion, mixed, and then injected. Into the packaging container, according to the needs of the characteristics of the packaged food, change the gas environment around the food in the packaging container, to achieve the purpose of preservation. Gas mixing and distribution can be performed in the following two ways.
(1) Controlling pneumatic composite gas distribution system
The control air pressure compound gas mixing and distribution system is mainly composed of air source, gas mixing room, vacuum inflator and pipeline, pressure reducing valve, pressure gauge and switch accessories, as shown in FIG. 2 . The gas source used in small batch production is compressed gas in cylinders. Carbon dioxide is used for food grade, nitrogen for dehydrated nitrogen. When the cylinder is full, the pressure can reach 15MPa, generally 10-12MPa.
After the various compressed gases are decompressed by the pressure reducing valve, they enter the gas mixing chamber to perform the optimal proportion mixing of various products, and then they are introduced into the air-packing machine for air-packing.
The gas distribution principle of the mixer, assuming that the depressurized gas is an ideal gas, there is a state equation:
PV=nRT
Where: P-gas pressure, Pa;
V-gas volume, m3;
N-gas molar number, mol;
R-gas state parameters, 8.31 J/mol·°K;
T—temperature, °K.
From the above equation, at a certain temperature, when the volume of gas V is constant, the number of moles of gas n is only related to its partial pressure P.
By controlling the partial pressures of the gases entering the constant volume gas mixing chamber, the gases can be mixed in the required proportions. For example, to mix with 70% CO2 + 30% N2, and assuming that the total pressure required for the mixture is P. The first volume of mixed room air is extracted first. When 0.7 P of CO2 and 0.3 P of N2 are fed in succession, the desired mixture ratio [4] can be obtained.
(2) Pressure composite gas distribution system
This gas mixing and distribution system requires that the pressure between each gas source is equal, and the ratio of the effective cross-sectional area of ​​the channels formed by the gas-filled solenoid valves of CO2, O2, and N2 to their pipelines is 1:1:1. The control end of the solenoid valve is respectively connected with the control device, and the control device includes a time control circuit corresponding to each solenoid valve. It is assumed that the mixing ratio of the three gases CO2, O2, and N2 is 7:2:1, and the control circuit of the gas conduction time is set to 7:2:1 by the control device so that when the gas mixing and distribution device works, three kinds The gas flows into the mixing chamber from the respective solenoid valves and pipes in a set amount, and is mixed in proportion in the mixing chamber to become a fresh gas suitable for packaged food. The gas is then filled into the packaging container through the stop valve, and the sealing is completed and packaged.
In the above-mentioned basic structure composition, the pressure of each air source is equal, the effective cross-sectional area of ​​each air filling channel is equal, and the flow of gas through the channel is equal in unit time. Therefore, as long as the opening time of each air-filled solenoid valve is adjusted, the purpose of adjusting the ratio of gas mixture can be achieved. During the working process, the control device controls the opening and closing of each pneumatic solenoid valve so that the opening time of each channel is the same, and the closing time is determined according to the gas mixture ratio.
This gas mixing distribution system has the following characteristics:
(1) The use of ordinary solenoid valve and pipe structure, by controlling the opening time of each inflatable solenoid valve to adjust the proportion of mixed gas, its accuracy can meet the requirements of the ratio of fresh food gas.
(2) The technical scheme of the system is rational and simple in structure.
(3) The gas mixing ratio of the system can be set and adjusted by the control device, and it can be easily controlled and operated automatically, and it has good function extensibility.
(4) The pressure requirements between various air sources are equal, and it is difficult for commercial cylinders to meet the requirements in operation. Therefore, there is a need for a gas source regulator, which is difficult to achieve continuous air-packing.
3. New composite gas distribution system
Through the research, design and testing of the gas mixing system of the composite modified atmosphere packaging machine, a new composite gas distribution system was developed. The key technology of this research is to eliminate the bulky gas mixing chamber in the system and use a pneumatic proportional valve to open and proportionally supply gas. Its working principle is: using a certain pressure of CO2 gas through a channel as a double pneumatic Proportional valve CO2 valve opening power, to promote the opening of the proportional valve, connected to CO2 gas; to open the CO2 gas as the opening power, open the O2 and N2 gas proportional valve through the other two channels, open the N2 gas proportional valve through the channel. In this way, the three gases enter the solenoid valve with equal pressure, and then the three gases are mixed into the packaging container for sealing and packaging.
The advantages of this system:
(1) The bulky gas mixing chamber is eliminated in the system, which further simplifies the structure and saves space.
(2) Pneumatic proportional valves are used. With reference to an air source pressure, various gas sources with the same pressure are obtained, and a constant ratio of mixed gas can be continuously supplied.
(3) The pressures of all kinds of air sources are the same, and the size of the pipes and solenoid valves that pass through are the same. By setting and adjusting the opening degree of the valve through the control device, the proportion of the mixed gas can be adjusted, and the accuracy thereof is further improved.
(4) When the pressure of any kind of air source fails to meet the requirements, the control unit can make the machine alarm and stop working. Replace the new gas source cylinder and continue working after connecting the system.
(5) It adopts computer automatic control and man-machine interface. It is convenient to adjust the gas combination with different proportions to meet the requirements of various packaged articles, and it can realize multiple uses of one machine.
(6) It can be designed and manufactured as a new independent compound gas mixing and distribution system. It can be used with any type of inflator packaging machine and has a wide range of application.
4. The key technology of the new composite gas distribution system
One of the key technologies of the new composite gas mixing and distribution system is the use of a pneumatic proportional valve in the system. The schematic diagram of its structure is shown in Figure 3. The set pressure gas is passed into the pneumatic proportional valve activation chamber and will be supplied. The gas is fed to the pneumatic proportional valve supply chamber. When the pressures of the two gases are equal, the supply valve will open to send gas. Therefore, a set pressure gas can be selected as the starting gas source in the system. When the other gas pressures that pass into the pneumatic proportional valve reach the pressure balance with the starting gas source, the gas pressure and other pressures are supplied.
The second key technology of the new composite gas mixed distribution system is the proportional control precision of the composite fresh gas. Due to the different performance characteristics of food and vegetables, their ratio of fresh gas is quite sensitive. With the new composite gas mixed distribution system, the ratio error of the feed gas mixture can be less than 2%.
The new composite gas distribution system adopts computer automatic control and man-machine interface in design, and it can be used in combination with a modified atmosphere fresh-keeping packaging machine. It can adjust the gas composition of different proportions to meet the requirements of the species to be packaged and extend the preservation period of the packaged products. Can really guarantee the original juice, flavor and appearance of food and vegetables. It is especially suitable for the production of agricultural and sideline products, agricultural and sideline product distribution centers, food processing companies, supermarkets and fresh agricultural and sideline products processing rooms and other places. The new composite gas distribution system and composite gas fresh-keeping packaging machine is a development direction of food and vegetable preservation technology in the future.
1. The principle of composite modified atmosphere fresh-keeping packaging
Composite modified atmosphere fresh packaging also known as gas replacement packaging, internationally known as MAP packaging (ModifiedAtmospherePacking). The principle is to use composite gas (2-4 kinds of gas mixed according to the ratio of the packaged food characteristics) to replace the air in the packaging container, change the gas environment around the food in the packaging container, thereby inhibiting the growth and reproduction of bacteria (microorganisms). Relieve the metabolic rate of packaged foods and prolong the shelf life or shelf life of foods [1, 2].
Atmosphere-preserved gas is generally composed of carbon dioxide (CO2), nitrogen (N2), oxygen (O2), and a small amount of specialty gases. High-concentration CO2 gas can hinder the growth and reproduction of most microorganisms that cause food spoilage, prolong the stagnation period (or incubation period) of its reproductive growth, and delay its logarithmic growth phase; it is the main antibacterial component in fresh-keeping gas; O2 can inhibit large Most anaerobic bacteria grow and reproduce. High oxygen can maintain the color of fresh meat while hypoxia can maintain the fresh state of fruits and vegetables while inhibiting the respiration rate of fresh fruits and vegetables. N2 is an inert gas that does not have chemical interaction with food and can be used as a filling gas. Prevent the CO2 from escaping and make the package fall. Nitrogen-packed packaging can reduce the oxidation of fat, aroma, and color in foods [3]. Combining CO2, O2, N2, and specialty gases to form a composite preservative gas for the preservation of fresh fruits and vegetables is a trend in today's fresh-keeping packaging.
2. The basic structure of compound gas mixed distribution system
In order to send different proportions of carbon dioxide (CO2), nitrogen (N2), and oxygen (O2) into the packaging container, the CO2, N2, and O2 gases are generally sent to the mixing chamber (air chamber) in a certain proportion, mixed, and then injected. Into the packaging container, according to the needs of the characteristics of the packaged food, change the gas environment around the food in the packaging container, to achieve the purpose of preservation. Gas mixing and distribution can be performed in the following two ways.
(1) Controlling pneumatic composite gas distribution system
The control air pressure compound gas mixing and distribution system is mainly composed of air source, gas mixing room, vacuum inflator and pipeline, pressure reducing valve, pressure gauge and switch accessories, as shown in FIG. 2 . The gas source used in small batch production is compressed gas in cylinders. Carbon dioxide is used for food grade, nitrogen for dehydrated nitrogen. When the cylinder is full, the pressure can reach 15MPa, generally 10-12MPa.
After the various compressed gases are decompressed by the pressure reducing valve, they enter the gas mixing chamber to perform the optimal proportion mixing of various products, and then they are introduced into the air-packing machine for air-packing.
The gas distribution principle of the mixer, assuming that the depressurized gas is an ideal gas, there is a state equation:
PV=nRT
Where: P-gas pressure, Pa;
V-gas volume, m3;
N-gas molar number, mol;
R-gas state parameters, 8.31 J/mol·°K;
T—temperature, °K.
From the above equation, at a certain temperature, when the volume of gas V is constant, the number of moles of gas n is only related to its partial pressure P.
By controlling the partial pressures of the gases entering the constant volume gas mixing chamber, the gases can be mixed in the required proportions. For example, to mix with 70% CO2 + 30% N2, and assuming that the total pressure required for the mixture is P. The first volume of mixed room air is extracted first. When 0.7 P of CO2 and 0.3 P of N2 are fed in succession, the desired mixture ratio [4] can be obtained.
(2) Pressure composite gas distribution system
This gas mixing and distribution system requires that the pressure between each gas source is equal, and the ratio of the effective cross-sectional area of ​​the channels formed by the gas-filled solenoid valves of CO2, O2, and N2 to their pipelines is 1:1:1. The control end of the solenoid valve is respectively connected with the control device, and the control device includes a time control circuit corresponding to each solenoid valve. It is assumed that the mixing ratio of the three gases CO2, O2, and N2 is 7:2:1, and the control circuit of the gas conduction time is set to 7:2:1 by the control device so that when the gas mixing and distribution device works, three kinds The gas flows into the mixing chamber from the respective solenoid valves and pipes in a set amount, and is mixed in proportion in the mixing chamber to become a fresh gas suitable for packaged food. The gas is then filled into the packaging container through the stop valve, and the sealing is completed and packaged.
In the above-mentioned basic structure composition, the pressure of each air source is equal, the effective cross-sectional area of ​​each air filling channel is equal, and the flow of gas through the channel is equal in unit time. Therefore, as long as the opening time of each air-filled solenoid valve is adjusted, the purpose of adjusting the ratio of gas mixture can be achieved. During the working process, the control device controls the opening and closing of each pneumatic solenoid valve so that the opening time of each channel is the same, and the closing time is determined according to the gas mixture ratio.
This gas mixing distribution system has the following characteristics:
(1) The use of ordinary solenoid valve and pipe structure, by controlling the opening time of each inflatable solenoid valve to adjust the proportion of mixed gas, its accuracy can meet the requirements of the ratio of fresh food gas.
(2) The technical scheme of the system is rational and simple in structure.
(3) The gas mixing ratio of the system can be set and adjusted by the control device, and it can be easily controlled and operated automatically, and it has good function extensibility.
(4) The pressure requirements between various air sources are equal, and it is difficult for commercial cylinders to meet the requirements in operation. Therefore, there is a need for a gas source regulator, which is difficult to achieve continuous air-packing.
3. New composite gas distribution system
Through the research, design and testing of the gas mixing system of the composite modified atmosphere packaging machine, a new composite gas distribution system was developed. The key technology of this research is to eliminate the bulky gas mixing chamber in the system and use a pneumatic proportional valve to open and proportionally supply gas. Its working principle is: using a certain pressure of CO2 gas through a channel as a double pneumatic Proportional valve CO2 valve opening power, to promote the opening of the proportional valve, connected to CO2 gas; to open the CO2 gas as the opening power, open the O2 and N2 gas proportional valve through the other two channels, open the N2 gas proportional valve through the channel. In this way, the three gases enter the solenoid valve with equal pressure, and then the three gases are mixed into the packaging container for sealing and packaging.
The advantages of this system:
(1) The bulky gas mixing chamber is eliminated in the system, which further simplifies the structure and saves space.
(2) Pneumatic proportional valves are used. With reference to an air source pressure, various gas sources with the same pressure are obtained, and a constant ratio of mixed gas can be continuously supplied.
(3) The pressures of all kinds of air sources are the same, and the size of the pipes and solenoid valves that pass through are the same. By setting and adjusting the opening degree of the valve through the control device, the proportion of the mixed gas can be adjusted, and the accuracy thereof is further improved.
(4) When the pressure of any kind of air source fails to meet the requirements, the control unit can make the machine alarm and stop working. Replace the new gas source cylinder and continue working after connecting the system.
(5) It adopts computer automatic control and man-machine interface. It is convenient to adjust the gas combination with different proportions to meet the requirements of various packaged articles, and it can realize multiple uses of one machine.
(6) It can be designed and manufactured as a new independent compound gas mixing and distribution system. It can be used with any type of inflator packaging machine and has a wide range of application.
4. The key technology of the new composite gas distribution system
One of the key technologies of the new composite gas mixing and distribution system is the use of a pneumatic proportional valve in the system. The schematic diagram of its structure is shown in Figure 3. The set pressure gas is passed into the pneumatic proportional valve activation chamber and will be supplied. The gas is fed to the pneumatic proportional valve supply chamber. When the pressures of the two gases are equal, the supply valve will open to send gas. Therefore, a set pressure gas can be selected as the starting gas source in the system. When the other gas pressures that pass into the pneumatic proportional valve reach the pressure balance with the starting gas source, the gas pressure and other pressures are supplied.
The second key technology of the new composite gas mixed distribution system is the proportional control precision of the composite fresh gas. Due to the different performance characteristics of food and vegetables, their ratio of fresh gas is quite sensitive. With the new composite gas mixed distribution system, the ratio error of the feed gas mixture can be less than 2%.
The new composite gas distribution system adopts computer automatic control and man-machine interface in design, and it can be used in combination with a modified atmosphere fresh-keeping packaging machine. It can adjust the gas composition of different proportions to meet the requirements of the species to be packaged and extend the preservation period of the packaged products. Can really guarantee the original juice, flavor and appearance of food and vegetables. It is especially suitable for the production of agricultural and sideline products, agricultural and sideline product distribution centers, food processing companies, supermarkets and fresh agricultural and sideline products processing rooms and other places. The new composite gas distribution system and composite gas fresh-keeping packaging machine is a development direction of food and vegetable preservation technology in the future.
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