Autoclave sterilizer retort

Autoclave sterilizer retort

Autoclave is a major sterilization equipment for food products. Here in this article we will have an overview of autoclaves, including what is autoclave, why autoclave is important in food produciton, how does autoclave work, main types of autoclaves, and some tips for using of autoclaves.

The autoclave for food packaging is fully customized, customers are allowed to personalize their own autoclave by different capacity, sterilization method, pressure vessel structure, and control type.


What is autoclave

In food production, an autoclave is a machine that uses steam or hot water under pressure to kill harmful bacteria, viruses and spores on food packages or inside food products that are placed inside a pressure vessel. The food packages are heated to a required sterilization temperature for a given amount of time. The temperature and time for sterilizaiton is preset and automatically controlled by computer. The moisture in the steam efficiently transfers heat to the food packages to destroy the protein structure of the bacteria and spores. This sterilization process offers the food products longer shelf life up to 8-16 months.

autoclave for food packaging

Main types of autoclaves

Based on the status of the products when they are placed inside the autoclaves for sterilization, the autoclave can be classified into 2 main types:

  1. Static type: Products are placed onto stainless steel plates, then slide into the autoclaves and keep still during the sterilization process
  2. Rotary and swing type: Food products and plates keep rotating inside the autoclave. Rotary type autoclaves can effectively enhance heating transfer speed, shorten the sterilization time, improve the tast of foods.
autoclave retort sterilizer
Inside of “Fixed type” autoclave
Autoclave sterilizer retort
Inside of  “Rotary type” autoclave

There are main 3 types of sterilization methods for autoclaves, we can use a drawing to show:

autoclave sterilization retort

Static- Steam autoclave

Basic points:Food products keep still during sterilization process, sterilization method is steam.

Steam autoclave sterilizer retort is the most common type of autoclave. It is generally heated by steam with a pressure of 0.2~0.3MPa. Steam autoclave has a simple structure and is easy to use. However, it is directly heated by steam, due to high temperature, high pressure, long time, and large thermal shock , the thermal damage to the food is relatively large, and some foods may cause thermal denaturation, and there will be a distillation odor and a burnt smell. When the steam autoclave is used for sterilization of glass bottled food should, the operation should be careful to prevent the temperature difference between the inside and outside of the glass bottle from being too large.

steam autoclave

The main points in designing and using of steam autoclaves are:

① When entering steam, the steam should fully and effectively remove the air in the tank in a short time (there should be a large enough exhaust port opposite the steam inlet);

② During pressure-maintaining sterilization, the water vapor should be fully circulated to ensure that there is no cold spot in the tank (there should be several steam vents opposite to the steam inlet);

③ During cooling, the compressed air should be able to satisfy the pressure balance of the food in the package during cooling.

In addition, the reliability and accuracy of the readings of thermometers and pressure gauges should also be guaranteed. Therefore, in order to ensure the safety of food, FDA has formulated many regulations for canned food, including many specific regulations on the equipment, operating procedures and management of thermal sterilization and low-acid food steam sterilization, including thermometers, The accuracy and setting position of the pressure gauge, the size, position and structure of the steam inlet, exhaust port and compressed air port.


Static type – Water immersion autoclave

The water immersion autoclave uses hot water as heating medium to heat and sterilize food packages.

Water immersion autoclave advantages:

①The thermal shock to food packages at the beginning of sterilization and cooling is small, minimize the risk of package broken

②Energy saving. After the sterilization cycle is completed, the hot water can be recycled for next time

③The temperature in the retort can be uniform without cold spots by changing the structure of the autoclave or changing the flow direction of the water.

The immersion type autoclave can be divided into normal pressure and high pressure autoclaves.

The normal pressure sterilizaiton machine is suitable for canned fruits and jams with high acidity (highly acidic food is sterilized because of its high acidity). The pasteurization machine blanching machine is one of a major normal pressure sterilization machine.

High-pressure type autoclave refers to the use of water in the pot. As the medium, the water is heated by steam, and the compressed air is passed into the sterilization pot to pressurize, so that the water temperature in the sterilization pot reaches between 118 and 121 ℃. The immersion type autoclave is a typical high-pressure sterilization machine, and its main structure and working principle are as follows:

autoclave retort sterilizer

The immersion type autoclave has a set of water circulation system to ensure uniform temperature in the retort. The heating steam enters the steam distribution pipe at the bottom of the pot to heat the hot water in the pot. A high-flow hot water circulating pump is sent from the bottom of the pot to the top or side water distribution pipe through the suction pipe, and the water in the pot should be circulated once within 5-7 minutes. After the sterilization is completed, the hot water is recovered and cooled. At this time, there are two methods:

①Replacement method, the cold water pump injects cold water into the sterilization pot, and the cold water pushes the hot water in the sterilization pot back to the hot water storage tank (water storage pot). When the temperature drops, the temperature of the cold water will rise, and the temperature difference with the food will not be too high, so the heat will be lost, but it is necessary for glass bottles;

②Direct recovery method, the superheated water in the sterilization pot is directly pumped into the water storage pot by the circulating pump. After the recovery, the cold water pump will inject cold water into the sterilization pot. During the recycling process, the pressure of the retort should be maintained at the sterilization pressure, and a certain pressure should also be maintained during cooling to prevent the package from bursting.

Double layer pot water-immersion autoclave

1. Save energy. The hot water used in the sterilization process can be recycled, which can effectively save energy and reduce production costs.
2. Shorten the sterilization time. The double-tank hot water cycle is used for sterilization, and the water in the hot water tank is heated to the temperature required for sterilization in advance, thereby shortening the sterilization time and improving the work efficiency.

autoclave for food sterilizaiton

Double layer tank water-immersion autoclave

3. The sterilization pot adopts hot water circulation and immersion sterilization methods. During the sterilization process, the circulating water in the sterilization tank is constantly switched up and down, which ensures the heat distribution at any point in the sterilization tank from heating, heat preservation to cooling. Uniformity, effectively eliminating the dead angle phenomenon in the sterilization process, making the shelf life of the product more stable and long.
4. The temperature in the sterilization tank is always stable in all stages of the sterilization process, which ensures the qualified rate of the F value.
5. Fully automatic control system. The entire sterilization process is controlled by computer PLC, which can be completed at one time without manual operation. Using a graphic tablet, using the screen for touch control, all periods are controlled by analog sensors and automatic valves, which can be switched from a single sterilization process to a multi-stage sterilization process. Pressure and temperature are displayed digitally, and various pressure and temperature modes suitable for any product can be pre-programmed into the computer.
6. The function of measuring F value. Each autoclave can be equipped with an F-value measuring instrument. The measuring instrument has the function of measuring the value. The F-value software calculates the F-value every 3 seconds. All sterilization data, including sterilization conditions, F value, time-temperature curve, time-pressure curve, etc., can be saved or printed after being processed by the data processing software to facilitate production management.
7. The simulated temperature control system can set up a multi-stage heating mechanism. According to the requirements of different foods for sterilization conditions, different heating and cooling programs can be set at any time. The multi-stage heating sterilization method can be used to limit the heat that the food can withstand. To the smallest, the color, aroma and taste can be perfectly preserved, so that each food can be conditioned and sterilized in the best state. The sterilization temperature is accurate to ±0.3℃, which can avoid the phenomenon of over-cooking or incomplete sterilization of products.
8. 100 sterilization formulas can be stored in the PLC for selection and use.


Double tanks parallel connected-water immersion autoclave

1. Improve sterilization efficiency:This type of autoclave retort sterilizer uses double-tank hot water circulation for sterilization. After the food in one tank is sterilized, high-temperature water is directly injected into the other tank. The 2 tanks work alternatively as how water tank and sterilization tank to enhance sterilization efficiency.

double tank autoclave
2. Save energy: The hot water used in the sterilization process can be recycled between the 2 tanks, which saves energy and heating time. During sterilization, the two tanks are alternately used as sterilization tanks, and the output is increased at the same time.
3. Fully automatic control system; the entire sterilization process is controlled by computer PLC, which can be completed at one time without manual operation. The screen is used for touch control, and all periods are controlled by analog sensors and automatic valves, which can switch from a single sterilization process to a multi-stage sterilization process. Digital display of pressure and temperature range, various pressure modes and temperature modes suitable for any product can be pre-programmed and input into the computer. 100 sterilization formulas can be stored in the computer for selection and use.
4. The simulated temperature control system can set a multi-stage heating mechanism; according to the requirements of different foods for sterilization conditions, different heating and cooling procedures can be set at any time, and the multi-stage heating sterilization method can be used to make the food withstand the heat Restricted to the minimum, in order to preserve its color, aroma and taste as perfectly as possible, so that each food can be conditioned and sterilized in the best condition. The sterilization temperature is accurate to ±0.3°C, which can avoid the phenomenon of product cooking pot or incomplete sterilization.

water immersion autoclave
5. The function of measuring F value; each sterilizer can be equipped with an F value measuring instrument, and the measuring instrument has the function of measuring value. The F value software calculates the F value every 3 seconds. All sterilization data, including sterilization conditions, F value, time-temperature curve, time-pressure curve, etc., can be saved or printed after being processed by the data processing software to facilitate production management.


Triple tanks-water immersion autoclave

Triple tank autoclave ownes 1 top hot water storage tank and 2 sterilization tank that works alternatively for best performance.

automatic autoclave for food sterilization

Static type – Water spray autoclave

According to the different heating methods, the water spray autoclave can be divided into direct heating and indirect heating. Direct heating is to directly use steam to pass into the bottom of the autoclave for heating, and indirect heating is heat transfer by heat exchanger. The main features and working principles of the water spray autoclave are introduced below with indirect heating of the heat exchanger: the structure is simple and the maintenance is easy. The only transmission parts of the entire equipment are a circulating water pump and several pneumatic valves.

water spray autoclave

The water spray autoclave uses a certain amount of water (about 400 ~ 500L) in the tank as a heat transfer medium, and a large-flow circulating pump sends the water into the heat exchanger, and the steam or cooling water passes through the heat exchanger. Heating or cooling, because the steam and cooling water can be fully recycled, saving energy and cost, and avoiding the secondary pollution problem of the product during the cooling process, thus effectively ensuring the sterilization effect of the product.

The heat transfer is stable and the temperature distribution is uniform. Due to the use of a large amount of circulating water, the water flow through the specially designed spray structure forms a bunch of fast water flow from top to bottom, forming a uniform flowing water film on the surface of the product , to achieve the effect of fast heat transfer and uniform temperature distribution at each point.

The product has a wide range of adaptability. Because the water spray sterilizer uses water as a heat transfer medium, it is heated or cooled through a heat exchanger, and in the heating stage, the sterilizer and cans are gradually heated together, which overcomes the inherent heat of other direct methods. Impact, widely used in cans, plastic bottles, glass bottles, cooking bags, etc.

In the heating stage, only a small amount of cold water in the pot is heated; in the cooling stage, there is only a small amount of hot water in the pot, so the heating speed and cooling speed of the sterilization pot are fast, thus shortening the time of the whole sterilization process.

The working principle of direct steam heating is similar to that, that is, the heat exchanger is eliminated. During heating, steam is directly used to heat hot water from the bottom of the pot, and the hot water is sprayed on the outer surface of the product from top to bottom through a circulating pump; when cooling, it is directly on the top of the pot. For spraying cold water, of course, the relationship between the flow rate and time should depend on different foods.

autoclave retort sterilizer
There is also a side spray type, because when hot water is sprayed from top to bottom, for soft cans packaged with composite films, the water flow of the following layers may be in a line except for the first layer. The shape is not uniform, so the side spray type appears. As the name suggests, the side spray type is to set up a large number of nozzles on both sides of the sterilizer to spray wave-like hot water to the sterilized object to form a very uniform sterilization temperature.


Rotary type autoclave

The main feature of the rotary type autoclave is that during the sterilization process (heating, sterilization and cooling) the food packages and the sterilization plates are rotated or swayed around the axis of the sterilizer. Since the food packages are stacked vertically in the sterilization basket, when the sterilization basket rotates, the liquid material in the tank is in a state of constant stirring due to the change of position, which greatly improves the heat transfer rate from the outside to the center of the tank, which is conducive to shortening the sterilization time and improving the The taste of canned food, especially for viscous food and heat-sensitive nutritional food. Rotary autoclaves are also divided into water-immersion type and water-spray type according to their different structures.

rotary type autoclave food sterilization machine retort

For rotary type autoclaves, the sterilization method is same as the Fix-type autockaves, including water-immersion type, steam type and water-spray type.  The difference is that the rotary-type autoclaves have the rotating mechanism.


How to choose autoclaves for food packages sterilization

1. The basic rules in choosing autoclaves is mainly based on the temperature control accuracy and the uniformity of heat distribution. If the product requires a very strict temperature, the fully automatic PLC constrolled autoclaves should be the best choice. You can also consider semi-automatic controlled autoclaves which offers almost same opration process as the fully-auto autoclaves, while the price is much less. Manual type autoclave is difficult to use because the temperature and pressure control are all done manually.

2. If the product is packaged with gas (MAP modified atmosphere packaging) or it’s important to keep the packages appearances, you should choose a fully-auto or semi-automatic autoclave.

food sterilization machine autoclave sterilizer retort

3. If the product is a glass bottle or tin can packages, because the heating and cooling speed need to be be controlled precisely, try not to choose a double-layer autoclave.

4. If you consider energy saving, you can choose a double-layer autoclave, which is characterized in that the upper tank is a hot water tank, the lower tank is a processing tank, and the hot water in the upper tank is reused, which can save a lot of steam and is suitable for a daily output of more than 10 tons food production enterprises.

5. If the output is small or there is no boiler, you can consider using an electric-steam heating autoclave. The principle is that the steam is generated by the electric heating in the upper tank and the lower tank is for sterilization.

6. If the product has a high viscosity and the product needs to be rotated during the sterilization process, you should choose a rotary type autoclave.

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