How do food jar factories put negative pressure into the jar to begin with?
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Hot air is less dense than cold air, so if you heat the jar first then you’ll close the jar with hot air in it, and when it cools the hot air shrinks, thus creating a negative pressure differential.
This is part of the reason why some jars may become much more difficult to open after storing in the fridge, as you filled the jar with room temp air when you opened it last, then when the air shrank in the jar after cooling in the fridge, it formed a sort of seal in the jar.
Hot air is less dense than cold air, so if you heat the jar first then you’ll close the jar with hot air in it, and when it cools the hot air shrinks, thus creating a negative pressure differential.
This is part of the reason why some jars may become much more difficult to open after storing in the fridge, as you filled the jar with room temp air when you opened it last, then when the air shrank in the jar after cooling in the fridge, it formed a sort of seal in the jar.
There are a few ways to achieve this; the main way is what everyone else has stated: heat the product before filling, and when it is sealed and cooled, the product will contract, vapors will condense, leaving more space for less gasses in the headspace, creating negative pressure. Hot filled products typically use this method.
For some seals, a blast of steam to the headspace of the product before sealing will create the negative pressure. It acts the exact same way as above to create negative pressure, but without the need to heat the product first. This could be done to seal a jar before the retort process. The retort process is essentially a large scale pressure cooker that can bring water to temperatures well above its typical boiling point (250F compared to 212F, respectively). It is important to note that the containers must be sealed before going into the retort.
The negative pressure helps in a few ways. First, it acts as a fail safe after processing. If the lid does not pop, then the seal is likely compromised and you should not eat that food. For canned products like this, the worry is that the bacteria clostridium botulinum could have some remaining cells alive in the food and can cause botulism. Second, during the thermal process in a retort, the sealed containers will see massive increases in pressure inside the container. Without creating an initial negative pressure, the rise in pressure could cause the seals to fail.
Whether using the heated product or by steam, the cooling after sealing is what creates that negative pressure.
Hot air is less dense than cold air, so if you heat the jar first then you’ll close the jar with hot air in it, and when it cools the hot air shrinks, thus creating a negative pressure differential.
This is part of the reason why some jars may become much more difficult to open after storing in the fridge, as you filled the jar with room temp air when you opened it last, then when the air shrank in the jar after cooling in the fridge, it formed a sort of seal in the jar.
There are a few ways to achieve this; the main way is what everyone else has stated: heat the product before filling, and when it is sealed and cooled, the product will contract, vapors will condense, leaving more space for less gasses in the headspace, creating negative pressure. Hot filled products typically use this method.
For some seals, a blast of steam to the headspace of the product before sealing will create the negative pressure. It acts the exact same way as above to create negative pressure, but without the need to heat the product first. This could be done to seal a jar before the retort process. The retort process is essentially a large scale pressure cooker that can bring water to temperatures well above its typical boiling point (250F compared to 212F, respectively). It is important to note that the containers must be sealed before going into the retort.
The negative pressure helps in a few ways. First, it acts as a fail safe after processing. If the lid does not pop, then the seal is likely compromised and you should not eat that food. For canned products like this, the worry is that the bacteria clostridium botulinum could have some remaining cells alive in the food and can cause botulism. Second, during the thermal process in a retort, the sealed containers will see massive increases in pressure inside the container. Without creating an initial negative pressure, the rise in pressure could cause the seals to fail.
Whether using the heated product or by steam, the cooling after sealing is what creates that negative pressure.
You mean for jarring / canning of food? You heat the jar / can to a temperature that sterilizes the contents and leave a small amount of air at the top. Place the seal / lid on, and as the jar / can cools the volume of the trapped air decreases (because it cools off), creating a vacuum and securing the seal / top to the container.
When I do this at home, I fill the jar with the hot contents (jelly or preserves) up to a line on the jar, I place a lid on, and lightly screw on a retainer ring. Then, I submerge the jar in boiling water and allow it to sit for a certain amount of time (depending on what I’m canning). The gasses at the top expand and leak out a bit. When the time’s up, I pull the jar out with tongs and set it on a towel on the counter to cool. If, after 45 minus, the lid has not deformed (pulled in by the vacuum), then I know the seal is bad and it can’t go on the shelf — I put it in the fridge and use it in the next few days. Those with a good seal will keep a few years on the shelf.
You mean for jarring / canning of food? You heat the jar / can to a temperature that sterilizes the contents and leave a small amount of air at the top. Place the seal / lid on, and as the jar / can cools the volume of the trapped air decreases (because it cools off), creating a vacuum and securing the seal / top to the container.
When I do this at home, I fill the jar with the hot contents (jelly or preserves) up to a line on the jar, I place a lid on, and lightly screw on a retainer ring. Then, I submerge the jar in boiling water and allow it to sit for a certain amount of time (depending on what I’m canning). The gasses at the top expand and leak out a bit. When the time’s up, I pull the jar out with tongs and set it on a towel on the counter to cool. If, after 45 minus, the lid has not deformed (pulled in by the vacuum), then I know the seal is bad and it can’t go on the shelf — I put it in the fridge and use it in the next few days. Those with a good seal will keep a few years on the shelf.
There are a few ways to achieve this; the main way is what everyone else has stated: heat the product before filling, and when it is sealed and cooled, the product will contract, vapors will condense, leaving more space for less gasses in the headspace, creating negative pressure. Hot filled products typically use this method.
For some seals, a blast of steam to the headspace of the product before sealing will create the negative pressure. It acts the exact same way as above to create negative pressure, but without the need to heat the product first. This could be done to seal a jar before the retort process. The retort process is essentially a large scale pressure cooker that can bring water to temperatures well above its typical boiling point (250F compared to 212F, respectively). It is important to note that the containers must be sealed before going into the retort.
The negative pressure helps in a few ways. First, it acts as a fail safe after processing. If the lid does not pop, then the seal is likely compromised and you should not eat that food. For canned products like this, the worry is that the bacteria clostridium botulinum could have some remaining cells alive in the food and can cause botulism. Second, during the thermal process in a retort, the sealed containers will see massive increases in pressure inside the container. Without creating an initial negative pressure, the rise in pressure could cause the seals to fail.
Whether using the heated product or by steam, the cooling after sealing is what creates that negative pressure.
You mean for jarring / canning of food? You heat the jar / can to a temperature that sterilizes the contents and leave a small amount of air at the top. Place the seal / lid on, and as the jar / can cools the volume of the trapped air decreases (because it cools off), creating a vacuum and securing the seal / top to the container.
When I do this at home, I fill the jar with the hot contents (jelly or preserves) up to a line on the jar, I place a lid on, and lightly screw on a retainer ring. Then, I submerge the jar in boiling water and allow it to sit for a certain amount of time (depending on what I’m canning). The gasses at the top expand and leak out a bit. When the time’s up, I pull the jar out with tongs and set it on a towel on the counter to cool. If, after 45 minus, the lid has not deformed (pulled in by the vacuum), then I know the seal is bad and it can’t go on the shelf — I put it in the fridge and use it in the next few days. Those with a good seal will keep a few years on the shelf.
They just close the jars while it’s hot, cooling down creates the vacuum. You don’t need a factory for that, same thing happens when you make jam or other preserved foods at home.
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