The static autoclave is the main equipment used in the canning industry from the early days. Usually it refers to a horizontal or vertical sealed high-pressure container that operates intermittently and uses no agitation, and can be used for sterilizing low-acid foods whose sterilization temperature needs to be higher than the boiling point of water.
2. Design of horizontal sterilization retorts
① If the steam supply is insufficient, it will be difficult to drive out all the air in the sterilization retort, and the heating temperature rise will be delayed and the expected requirements will not be met; the sterilization temperature will often fluctuate and be unstable. For this reason, the pressure in the steam pipeline during the sterilization operation must not be less than 6.19kg/cm2.
②The size of the steam inlet pipe of each sterilization retort should be sufficient to completely remove the air in the pot within a reasonable period of time. Steam can enter from the top or bottom of the sterilizer, but it must be in a position opposite to the air exhaust part.
③In order to ensure that the sterilization temperature can be accurately controlled within the fluctuation range of 0.5-1.0°C, directly controlled or air-operated automatic control valves and branch steam pipes are often installed on the steam inlet pipe for use during the heating period. The diameter of the former should be smaller than that of the latter to reduce the fluctuation range during automatic temperature control. However, the diameter of the branch pipe should be equal to the steam supply pipe to facilitate manual operation when the automatic control system fails.
④The steam distribution pipe is the continuation part of the steam entering the sterilization pot, that is, the steam pipe with holes so that the steam can be evenly distributed in the retort chamber. The length of the steam distribution pipe of the horizontal sterilizing retort should be the same as the pot length. Its holes are set at 90° on the top of the steam pipe, and the middle part of the steam distribution pipe is connected to the steam supply pipe from outside the retort.
⑤ It is strictly prohibited to have any baffles above the steam distribution pipe.
⑥If the pot length exceeds 6m, install a steam supply pipe 1/4 of the pot length from both ends of the sterilizer, and connect them to the same steam distribution pipe respectively; you can also install double rows of parallel steam distribution pipes.
As for the number of holes on the steam distribution pipe, it should be determined based on the requirement that the sum of the cross-sectional area of all holes on the pipe must be 1/2-2 times larger than the cross-sectional area of the steam supply pipe.
(2) Exclusion of air
Canned food must be sterilized in "pure" (air-free) steam to achieve good sterilization results. Because air is a poor conductor of heat, an insulation layer will be formed around the can, which will affect the sterilization effect of the can. In addition, the air present in the steam under high temperature conditions will also cause corrosion of the can, especially the crimp seal.
In fact, 70%-80% of the space in a horizontal sterilizing pot filled with cans is still filled with air. If the air in the pot is not removed before sterilization, there will be a cold area in the pot, and the heat distribution will be uneven and the canned food will be uneven. The sterilization effect will be inconsistent, leading to quality problems. For this purpose, the sterilization is equipped with exhaust holes, which are directly installed with valves or connected to pipes controlled by valves. The exhaust pipe has the following requirements:
1. It is necessary to ensure that the air in the pot is completely eliminated before sterilization officially starts.
2. It must be installed at a position opposite to the steam inlet part of the sterilizer.
3. Try not to use a ball valve, because steam and air flowing through this valve will encounter resistance and reduce the exhaust efficiency.
4. The exhaust holes of the horizontal sterilizing pot are located on the top of the pot, one at 50cm from both ends, and then one is added every 1.5m, usually parallel to the 25mm diameter pipe.
5. The exhaust pipe and all additional pipes that are connected to the outside world, the main air collecting pipe or the air collecting chamber should be as short as possible. Each exhaust pipe on a horizontal sterilizer is usually all connected to the main exhaust pipe, and only the valve is installed near its end. All exhaust pipes and manifolds can also be connected to a manifold that is no longer equipped with a control valve.
6. The diameter of the exhaust pipe must be larger than the steam supply pipe, and the cross-sectional area of the gas collecting pipe and gas collecting chamber should be equal to the total cross-sectional area of all exhaust pipes connected to it. When the length of the horizontal sterilization kettle is less than 4.6m, the diameter of the gas collecting pipe should be 32mm; when 4.6-6.2m, it is 64mm; when 6.2-9.2m, it is 76mm. This is because the longer the sterilization kettle, the more exhaust pipes connected to it. .
7. The exhaust holes should not be set up on the drainage pipe system. For example, when the overflow pipe must be used as an exhaust pipe at the same time, its pipe should have an interruption (connected to the outside world) to prevent the accumulation of water in the pipe from having a back pressure effect on it during exhaust.
8. The air removal method in the retort should depend on the model, size and shape of the retort, as well as the pressure and supply of steam, the stacking of cans in the retort, the required heating period, the resistance to air discharge and other factors .
9. Setting of the vent valve: ① The vent valve is a hole set up to eliminate the air entering with the steam and promote the continuous circulation of steam in the kettle and the measuring room. ② There is at least one relief valve on the top of the sterilization pot about 31cm from each end, and one is added every 2.5m.
(3) Water supply and discharge
1. Ball valves should be used as water pipe valves. The valve must be intact and in good condition to ensure that no cold water drips into the pot, resulting in insufficient sterilization.
2. The diameter of the drainage pipe is generally 35-50mm. When the pot length exceeds 4.5m, the pipe diameter should be increased to more than 63mm. Ball valves should be used as water pipe valves.
(4) Instrument design
Mercury thermometer: It is the most important instrument to correctly display the sterilization temperature. In order to prevent it from breaking, there is often a metal shield. The standard deviation of the Celsius engraved within the length of 18cm is below 60℃ (such as 65-125℃), and the graduation is 0.5-1.0℃, with 0.5℃ being the best.
The thermometer should be installed in a convenient position for reading, or inserted into the pot, or installed in the measuring chamber on the side of the pot. The wall of the pot connected to the measuring greenhouse should be equipped with at least holes with a diameter of 20 mm or more. There should be sufficient steam flowing around the thermometer to correctly display the sterilization temperature. The thermometer should be calibrated at least once a year. It should not be used if it differs by 0.5°C from the standard.
(5) Temperature recorder
It is best to have a temperature recorder in every retort, especially for low-acid foods. It provides a permanent record of sterilization temperature and time for each batch of cans. The general principle is as follows: the temperature sensing ball inserted in the sterilization pot is connected with the steel capillary tube and the rotating shaft of the recording pen. When the medium installed in the temperature sensing ball is heated and expands, it will push the rotating shaft connected to the recording pen, causing it to move accordingly. At this time, the temperature changes will be recorded on the recording paper that continuously rotates at a clockwise speed, so that it can be recorded from The temperature changes with time are observed on the temperature recording paper.
(6) Pressure gauge
In order to promptly grasp the changes in pressure within the sterilization kettle, the sterilization kettle must be equipped with a pressure gauge. It helps to ensure that the air in the steam is completely removed when the pressure and temperature gauges are consistent. The minimum diameter of the pressure gauge should be above 90mm. Below 2kg/cm2, the graduation should be 0.05-0.1 atmosphere or kg/cm2. The pressure gauge should be installed on the gooseneck, and the height should not exceed 10cm to protect the pressure gauge. The pressure gauge should be calibrated once a year or immediately if in doubt.
Installation of pressure gauge: ① The pressure gauge should be installed in a place that is easy to observe and repair. ②The installation location must avoid high temperature and vibration. ③ When measuring steam pressure, a condenser tube should be added to prevent direct contact with the pressure measuring element of high-pressure and high-temperature steam. The reading of the pressure gauge cannot represent the sterilization temperature in the kettle. It can only reflect the sterilization temperature in the kettle well. It will not be accurate if there is air in the kettle.
Safety valve: Any sterilization kettle must be equipped with a safety valve to prevent the steam from being released automatically in time when the pressure in the kettle is too high, and to reduce the pressure in the kettle to avoid accidents.
4. Operation of horizontal sterilization retort
(1) Sealing the can - putting it into the retort
1. After sealing, the canned food should not wait for sterilization for more than 0.5-1 hour to avoid "flat sour" rancidity caused by the proliferation of thermophilic bacteria. Mainly Bacillus stearothermophilus, its heat resistance is D121.1℃=4.0~50min, and its spoilage characteristics are: "Produces acid (lactic acid, formic acid, acetic acid) without producing gas or trace amounts of gas, does not expand the can, and the food has Sour.
2. The cage for cans should basically ensure that the surface of each can can be in contact with steam. For this reason, the upper and lower cans should not be stacked into a cylindrical shape. Do not use sacks, wooden boards, large towels or other items with poor thermal conductivity as pads between layers of cans. Because they affect the steam cycle, resulting in insufficient sterilization. Porous metal backing plates should be used.
3. “Initial temperature” refers to the average temperature of the contents of the can before steam sterilization begins. It should be shaken well before testing. If the initial temperature of the can does not reach the standard temperature, sterilization will be insufficient within the fixed sterilization program.
(2) Enter the chamber - exhaust
1. After the canned food enters the sterilizing autoclave and the lid of the autoclave is sealed, the period from entering the steam to heating the sterilizing autoclave to the sterilizing temperature is called the heating period.
2. In order to ensure that the cans can be sterilized in pure steam medium, the temperature should be raised slowly when starting to heat and the air in the retort should be drained. Therefore, when entering steam, it is required to fully open all valves that can remove air, and use steam to remove the air. When the color of the gas discharged from the exhaust valve is gray, there is no air.
3. Usually, the gauge pressure of the sterilization retort and the reading of the mercury thermometer are used to determine whether there is air remaining in the retort chamber. If the former is higher than the latter, it means there is still air remaining; if the two are corresponding, it means there is basically no air remaining in the cha. Of course it cannot absolutely reflect the real situation.
(3) High temperature and high pressure sterilization
1. The period of time from reaching the sterilization temperature until stopping the steam supply and external steam release is called the sterilization time. At this time, the pressure and temperature should be kept consistent, adjusted in time, and strictly controlled. The sterilization temperature is based on a mercury thermometer.
2. When the air in the sterilization kettle is completely discharged, close all valves except the relief valve, and at the same time continue to add steam to make the temperature in the kettle rise slowly and evenly.
3. When the sterilization retort chamber reaches the sterilization temperature, the temperatures of the mercury thermometer and temperature recorder should be checked. If the temperature of the recorder is slightly lower than the temperature of the mercury thermometer, the problem is not serious. The official sterilization time should only be calculated when the correct sterilization temperature is reached. At this time, the correct clock should be used instead of a watch or temperature recorder. Exhaust heating time, temperature and pressure should be recorded.
4. The sterilization time must be strictly and correctly implemented, especially when sterilizing at high temperatures for a short period of time. If there is a slight error in temperature and time, it will have a great impact on the sterilization effect.
5. During the sterilization process, in order to maintain the uniformity of heating in the kettle, the vent valve should be open to keep the steam from escaping continuously and promote the continuous circulation of steam in the kettle. At the same time, if there is any condensation water gathering at the bottom of the kettle, remove it in time to prevent the canned food from failing to achieve the expected sterilization effect.
6. High-temperature and high-pressure long-term sterilization cannot make food sterile, but can only achieve commercial sterilization, which means it does not contain pathogenic bacteria; does not contain toxins; does not contain any microorganisms that can multiply during the storage, transportation and sales of the product; Maintain stable quality and good commercial value without deterioration.
(4) Cooling of metal cans
1. When the sterilization of canned food is completed, it should be cooled quickly in time, because the canned food is still at a high temperature and continues to be heated. If the food in the can is not cooled immediately, the quality of the food will be seriously affected, such as the color will darken. , the flavor deteriorates, the texture becomes rotten, etc., and even the product value is lost.
2. If you stay in the high temperature stage for too long during the cooling process, it will promote the reproduction of thermophilic bacteria such as acidophilus, causing the canned food to deteriorate and become rotten. Continuing to heat will also accelerate the corrosion of the inner wall of the tank, especially for highly acidic foods. The faster the cooling rate, the better.
3. During the sterilization process, the temperature inside the tank is basically the same as that of the sterilizing kettle, and the steam pressure inside and outside the tank is also exactly the same. However, the expansion of the food in the tank and the expansion of the air remaining in the tank will cause the pressure inside the tank to be larger than the pressure outside the tank. It is larger, that is, there is positive pressure in the tank, and it varies with the size of the tank and the sterilization temperature.
If the exhaust valve is fully opened after sterilization, the pressure inside the sterilization kettle will drop sharply, but the temperature inside the tank will not drop accordingly. The pressure difference between the inside and outside of the tank will increase sharply, and the two lids of the can will be instantly subjected to a strong force. At this time, the can will not only be deformed but also damaged in appearance, resulting in loose curling and leakage, and can explosion accidents may also occur. For this reason, when canned food is cooled, it is advisable to slowly reduce the pressure of the sterilization kettle.
4. Back pressure cooling means that the can maintains a certain pressure in the retort during cooling until the pressure inside the can is close to the outside atmospheric pressure. At this time, the can continues to cool in the retort, or it can be cooled by spraying cold water of a certain temperature.
5. After stopping steam at the end of sterilization, the sterilization retort cannot be filled with cold water immediately, because the steam immediately forms a vacuum when it encounters cold water, expanding the pressure difference between the inside and outside of the tank, causing the can to be deformed or damaged. Therefore, the steam should be exhausted slowly. When the temperature in the sterilizing retort drops to about 100°C until the pressure in the retort shows 0, cold water should be poured in for cooling.
6. The exit temperature of the can after sterilization should be 40±2°C, because if the temperature is too low, the water in the tank cannot be completely evaporated, and the scratches on the tank will easily corrode after packaging; because the suitable temperature for mesophilic bacteria is 43 ~76.7℃, so the temperature of canned food should be below 43℃.
Measures to deal with abnormal situations of sterilization retort
1. Corrosion prevention measures for sterilizers
1. The horizontal sterilizing pot is a pressure vessel, but according to its operating process characteristics, it is a pressure vessel that can withstand alternating loads and frequent intermittent operations. Therefore, safety management should be strengthened and scientific and reasonable operating conditions and safety management measures should be formulated, which is a necessary condition to prevent corrosion.
2. When installing the horizontal sterilizing pot, the pot body can have a certain inclination angle (lower in front and higher in back) to ensure effective sewage discharge.
3. Strengthen management and promptly remove sewage or dirt in the pot. Especially when it is out of use, the pot should be kept dry and clean.
4. In order to reduce the oxygen content in the boiler, the boiler water supply should be installed with additional equipment, and the time for entering and exiting the feed truck should be as short as possible.
5. During normal operation, if hard objects such as iron hooks periodically enter the pot, try to avoid collision and friction with the pot drum.
6. The outer guide rail of the pot should be placed correctly to avoid hitting the pot body. At the same time, the outer guide rail of the pot should be as high as possible with the inner guide rail of the pot, and the gap should be as small as possible to ensure that the feed truck enters and exits the pot smoothly.
2. Measures to be taken during power outage
(1) Power outage during exhaust
Turn off the power supply and air pressure valves, and place them in the kettle to keep warm. After calling, we will work with the quality control to extend the exhaust time (the exhaust time depends on the temperature of the sterilizing kettle being 105°C, based on the mercury thermometer) for sterilization. After sterilization, the finished product is isolated and processed accordingly after the micro-inspection results are available.
(2) Power outage during sterilization
Turn off the power supply and air pressure valve, open the heating valve, use a manual steam valve to control the sterilization temperature, and make up for 12 minutes according to the actual sterilization time on the temperature record table. When making up the time, it is required to have more solid kettles and less empty kettles. After sterilization is completed, open Exhaust steam from the exhaust valve for 1.5 minutes, open the large cooling valve and drain valve to cool the milk with the water in the water storage bucket. After leaving the kettle, please ask quality control to determine whether isolation is needed.
(3) Power outage during cooling
Turn off the power supply and air pressure valve, open the large cooling valve, small cooling valve and drain valve, and rely on the water in the water storage bucket to continue cooling the milk. After sterilization is completed, ask the quality control personnel to check whether isolation is needed.
(4) Power outage when entering the cage
Turn off the power, unplug the oil pipe of the can sweeper, use a wrench to control the height of the elevator, put the milk on the conveyor belt and conveyor net into the basket and send it to the sterilization kettle for heat preservation.
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