What happens when you heat up soup in a vacuum?
January 29, 2019 3:53 PM Subscribe
I can do this! if it's not dangerous.
Some guy on youtube popped a bowl of popcorn on a stovetop, but first he sucked all the air out with a vacuum pump. Since the steam had no where to be vented out I was surprised nothing bad happened. I mean, my vitamix blender comes with a warning that some of the blender containers are not vented and therefore shouldn't be used for hot items... but in a vacuum there's no vent either. Nothing happened to the popcorn guy except he got much fluffier and bigger popcorn.
I'd like to try it, but I don't have a professional vacuum... I just have a non-professional electric vacuum pump (think foodsaver vac) and a lid that I can put on a pot that would work. Can I safely do this or do you need a pro pump like that guy on youtube had?
And can I heat up other things like soup this way? I'm curious.
Some guy on youtube popped a bowl of popcorn on a stovetop, but first he sucked all the air out with a vacuum pump. Since the steam had no where to be vented out I was surprised nothing bad happened. I mean, my vitamix blender comes with a warning that some of the blender containers are not vented and therefore shouldn't be used for hot items... but in a vacuum there's no vent either. Nothing happened to the popcorn guy except he got much fluffier and bigger popcorn.
I'd like to try it, but I don't have a professional vacuum... I just have a non-professional electric vacuum pump (think foodsaver vac) and a lid that I can put on a pot that would work. Can I safely do this or do you need a pro pump like that guy on youtube had?
And can I heat up other things like soup this way? I'm curious.
Well, if you pull a strong enough vacuum your soup will start to boil at room temperature. It won't be hot, but it will boil. For less strong vacuums, it will boil at temperatures that are lower than normal but higher than room temp, proportional to the strength of your vacuum.
When I say "strength of your vacuum" what I really mean is simply how much air pressure still remains after your pump has done all it can do.
If you heat your soup while the pump is running, the water vapor from the soup will get sucked out through the pump. Probably this is fine, but depending on how much soup stuff it takes with it, it might be bad for your pump. And if you get it really hot, you might be pulling vapor that's hotter than what the pump was designed to deal with. It's probably fine, but no guarantees.
If the pump is off, vapor pressure from the hot soup will quickly fill the vessel and take it back to standard pressure. If you leave it a way to get out, it will escape just like when a pot of boiling soup boils over on the stove. If your system is sealed against positive pressure (like, if your lid is screwed onto the pot and your pump's valves are designed such that they don't open when the internal pressure pushes on them) then the pressure will increase until something breaks, perhaps catastrophically. That seems unlikely, though.
posted by Anticipation Of A New Lover's Arrival, The at 4:03 PM on January 29, 2019 [2 favorites]
When I say "strength of your vacuum" what I really mean is simply how much air pressure still remains after your pump has done all it can do.
If you heat your soup while the pump is running, the water vapor from the soup will get sucked out through the pump. Probably this is fine, but depending on how much soup stuff it takes with it, it might be bad for your pump. And if you get it really hot, you might be pulling vapor that's hotter than what the pump was designed to deal with. It's probably fine, but no guarantees.
If the pump is off, vapor pressure from the hot soup will quickly fill the vessel and take it back to standard pressure. If you leave it a way to get out, it will escape just like when a pot of boiling soup boils over on the stove. If your system is sealed against positive pressure (like, if your lid is screwed onto the pot and your pump's valves are designed such that they don't open when the internal pressure pushes on them) then the pressure will increase until something breaks, perhaps catastrophically. That seems unlikely, though.
posted by Anticipation Of A New Lover's Arrival, The at 4:03 PM on January 29, 2019 [2 favorites]
Oh, and as for the blender - the reason it's dangerous to use it on hot liquids is that you can build up a nice head of pressure with the lid on the jar (as steam is generated), which would rapidly decompress when you pop the lid off (or the pressure gets high enough that it does it on its own) and potentially cause the hot liquid to sort of volcano out and scald you.
posted by backseatpilot at 4:04 PM on January 29, 2019 [3 favorites]
posted by backseatpilot at 4:04 PM on January 29, 2019 [3 favorites]
Looking around, I'm seeing vacuum sealer ratings anywhere between 5 and 50 mbar. At those vacuums, you could boil water anywhere between 28 and 90 Fahrenheit. Adding stuff to water increases its boiling point to varying degrees, but if you wanted to you could boil soup in your freezer. Why you would want to do that, I don't know.
posted by backseatpilot at 4:09 PM on January 29, 2019 [3 favorites]
posted by backseatpilot at 4:09 PM on January 29, 2019 [3 favorites]
Response by poster: So what I'm getting from this is that my popcorn will also start to pop sooner and with less steam because the oil will react as if it was hotter than it actually is.
my food saver vac stops pumping air once it feels the air is all out of the container, so it wouldn't keep sucking in the steam as it cooks. It just stops. Then the lid becomes impossible to take off the container until I press the valve and air comes out.
posted by fantasticness at 5:32 PM on January 29, 2019
my food saver vac stops pumping air once it feels the air is all out of the container, so it wouldn't keep sucking in the steam as it cooks. It just stops. Then the lid becomes impossible to take off the container until I press the valve and air comes out.
posted by fantasticness at 5:32 PM on January 29, 2019
The popcorn is sort of a special case because of how popcorn... pops. Making the kernels pop in a low-pressure environment causes them to get "fluffier".
But if you put soup in a vacuum chamber and started dropping the air pressure, what will happen is the boiling point of the water in the soup drops, but the soup doesn't necessarily get hot or anything. Eventually you'll bring the pressure low enough (with a good vacuum pump setup) so that the water in the soup will start to boil, just at ambient/room temperature. But again, the soup isn't hot; it'll be boiling, but only room temperature. If you do this delicately enough, you might be able to make soup concentrate... but it probably won't be terribly interesting.
There are cool things you can do with a vacuum pump in a kitchen. Mostly flavor extractions: you can soak stuff in alcohol, then use a vacuum chamber to remove the alcohol (decrease pressure until the alcohol is above its boiling point at room temperature), leaving the other flavor components behind. There are some modernist cuisine type recipes that do stuff like this.
But anyway, if you heat soup under vacuum, you would just end up with lukewarm soup. Because the boiling point is depressed due to the low pressure, you won't be able to heat it without it boiling. This is also the reason why tea in Colorado Springs sometimes tastes slightly different than tea in, say, San Francisco. In Colorado Springs, due to the 6000' elevation, water boils at 201F instead of 212F, leading to slightly different flavors from the tea. (Personally I think tea made with boiling water is garbage, but whatever.) The point is: no matter how long you run your kettle, it won't go above 201F in Colorado Springs, unless you let the kettle pressurize, which is a Bad Idea. You're basically artificially creating that situation.
Most of the time, what cooks are interested in is being able to drive the temperature higher without having water boil, hence why pressure cookers exist. (A pressure cooker at 15psig, in a room at sea level, will let you take watery liquid up to around 250F.) A "reverse pressure cooker" is not really as useful, because you can have the same effect by just not heating the water as hot, e.g. using a sous vide immersion heater.
posted by Kadin2048 at 5:36 PM on January 29, 2019 [6 favorites]
But if you put soup in a vacuum chamber and started dropping the air pressure, what will happen is the boiling point of the water in the soup drops, but the soup doesn't necessarily get hot or anything. Eventually you'll bring the pressure low enough (with a good vacuum pump setup) so that the water in the soup will start to boil, just at ambient/room temperature. But again, the soup isn't hot; it'll be boiling, but only room temperature. If you do this delicately enough, you might be able to make soup concentrate... but it probably won't be terribly interesting.
There are cool things you can do with a vacuum pump in a kitchen. Mostly flavor extractions: you can soak stuff in alcohol, then use a vacuum chamber to remove the alcohol (decrease pressure until the alcohol is above its boiling point at room temperature), leaving the other flavor components behind. There are some modernist cuisine type recipes that do stuff like this.
But anyway, if you heat soup under vacuum, you would just end up with lukewarm soup. Because the boiling point is depressed due to the low pressure, you won't be able to heat it without it boiling. This is also the reason why tea in Colorado Springs sometimes tastes slightly different than tea in, say, San Francisco. In Colorado Springs, due to the 6000' elevation, water boils at 201F instead of 212F, leading to slightly different flavors from the tea. (Personally I think tea made with boiling water is garbage, but whatever.) The point is: no matter how long you run your kettle, it won't go above 201F in Colorado Springs, unless you let the kettle pressurize, which is a Bad Idea. You're basically artificially creating that situation.
Most of the time, what cooks are interested in is being able to drive the temperature higher without having water boil, hence why pressure cookers exist. (A pressure cooker at 15psig, in a room at sea level, will let you take watery liquid up to around 250F.) A "reverse pressure cooker" is not really as useful, because you can have the same effect by just not heating the water as hot, e.g. using a sous vide immersion heater.
posted by Kadin2048 at 5:36 PM on January 29, 2019 [6 favorites]
There is nothing magical about lowering the pressure. Popcorn is popped by heating the kernels until the water within them boils off into steam and this “pops” the kernel. If the local pressure is reduced, Boyle’s Law tells us that the boiling point of that moisture is lowered. Thus, you can pop popcorn st a lower temperature if pressure is reduced. Whether this is good or not is a matter of opinion and taste. When it comes to reheating soup, you care about temperature not phase change. Thus lowering pressure is not worthwhile.
posted by slkinsey at 6:51 PM on January 29, 2019
posted by slkinsey at 6:51 PM on January 29, 2019
Thus, you can pop popcorn st a lower temperature if pressure is reduced.
I don't believe this is true. The popcorn is a bit of a special situation because you're essentially working with two systems. You can think of the outer shell of the corn kernel as (more or less) an impermeable barrier - what you do to the pressure outside of the kernel has no bearing on the inside of the kernel until that wall is breached. You'll still need to raise the moisture inside of the kernel to 212 F to generate steam, assuming that the initial interstitial pressure inside of the kernel was around 1 atmosphere. You also are not affecting the impact of the temperature of the oil on the cooking process, since oil does not boil to cook popcorn.
What the pressure change will impact, however, is twofold. First, you're "prestressing" the hull of the kernel by starting the cooking process at a pressure differential. This means that would need to generate less pressure inside the kernel to pop it; however, this doesn't mean you can cook it at a lower temperature, just that it will take less steam generation (therefore less time) to pop. Second, with no air resistance the actual popping mechanism has nothing "fighting" it which will allow the kernel to expand farther. If you threw a ball in a vacuum, you could get it to go a lot farther than in a normal atmosphere because you'd have no air resistance - same idea.
Thinking about it a different way, if you took a hollow metal ball, filled it halfway with water, and sealed it in your kitchen at 1 atmosphere and then dropped that ball in your vacuum soup pot and turned on the heat, the water inside that ball would still boil at 212 F. If you dropped it in your vacuum soup pot that was partially filled with water, the water in the pot would boil at some low temperature and the inside of your ball would never boil.
posted by backseatpilot at 7:03 PM on January 29, 2019
I don't believe this is true. The popcorn is a bit of a special situation because you're essentially working with two systems. You can think of the outer shell of the corn kernel as (more or less) an impermeable barrier - what you do to the pressure outside of the kernel has no bearing on the inside of the kernel until that wall is breached. You'll still need to raise the moisture inside of the kernel to 212 F to generate steam, assuming that the initial interstitial pressure inside of the kernel was around 1 atmosphere. You also are not affecting the impact of the temperature of the oil on the cooking process, since oil does not boil to cook popcorn.
What the pressure change will impact, however, is twofold. First, you're "prestressing" the hull of the kernel by starting the cooking process at a pressure differential. This means that would need to generate less pressure inside the kernel to pop it; however, this doesn't mean you can cook it at a lower temperature, just that it will take less steam generation (therefore less time) to pop. Second, with no air resistance the actual popping mechanism has nothing "fighting" it which will allow the kernel to expand farther. If you threw a ball in a vacuum, you could get it to go a lot farther than in a normal atmosphere because you'd have no air resistance - same idea.
Thinking about it a different way, if you took a hollow metal ball, filled it halfway with water, and sealed it in your kitchen at 1 atmosphere and then dropped that ball in your vacuum soup pot and turned on the heat, the water inside that ball would still boil at 212 F. If you dropped it in your vacuum soup pot that was partially filled with water, the water in the pot would boil at some low temperature and the inside of your ball would never boil.
posted by backseatpilot at 7:03 PM on January 29, 2019
Some popular literature on popcorn at low pressure. I used to work with Dr. Quinn some - it bugged him that this novelty project got much more press than this more serious research.
http://discovermagazine.com/2005/may/physics-of-popcorn
posted by teragram at 7:06 PM on January 29, 2019 [2 favorites]
http://discovermagazine.com/2005/may/physics-of-popcorn
posted by teragram at 7:06 PM on January 29, 2019 [2 favorites]
if you took a hollow metal ball, filled it halfway with water, and sealed it in your kitchen at 1 atmosphere and then dropped that ball in your vacuum soup pot and turned on the heat, the water inside that ball would still boil at 212 F.
I would expect it to boil at a substantially higher temperature than that, because being sealed there's nowhere for vaporized water to escape to; the pressure inside the ball would rise well above 1 atmosphere before boiling, just as it does in a pressure cooker.
posted by flabdablet at 8:04 PM on January 29, 2019
I would expect it to boil at a substantially higher temperature than that, because being sealed there's nowhere for vaporized water to escape to; the pressure inside the ball would rise well above 1 atmosphere before boiling, just as it does in a pressure cooker.
posted by flabdablet at 8:04 PM on January 29, 2019
As for what happens if you heat up soup in a vacuum: depending on how good the vacuum pump is, it might be feasible to engineer a well-boiled soup that still carries a substantial risk of food poisioning due to never having reached a temperature high enough to kill bacteria.
posted by flabdablet at 8:07 PM on January 29, 2019 [1 favorite]
posted by flabdablet at 8:07 PM on January 29, 2019 [1 favorite]
In the lower pressure, your soup is going to boil at a much lower temperature and quickly become vapor. If you have enough of a vacuum, you might even get below the triple point of water where water cannot exist as a liquid. This is the principle that freeze drying is based on.
In freeze drying, food is frozen about an inch deep on trays. These trays are loaded into chambers where the pressure is dropped. Lining the chamber are coils cooled below the freezing point of water at the target pressure. Then the chamber is heated. The solid water goes directly from solid to vapor, then re-condenses as a solid on the coils, thus drying the air and making more room for more vapor to leave the food. Pretty soon, you have the vapor out of the hot food and on to the cold coils. Freeze dried food.
I think you may find your boiling lukewarm soup interesting in the chamber but I doubt the taste will change and, yuck, lukewarm soup.
posted by Foam Pants at 11:57 PM on January 29, 2019
In freeze drying, food is frozen about an inch deep on trays. These trays are loaded into chambers where the pressure is dropped. Lining the chamber are coils cooled below the freezing point of water at the target pressure. Then the chamber is heated. The solid water goes directly from solid to vapor, then re-condenses as a solid on the coils, thus drying the air and making more room for more vapor to leave the food. Pretty soon, you have the vapor out of the hot food and on to the cold coils. Freeze dried food.
I think you may find your boiling lukewarm soup interesting in the chamber but I doubt the taste will change and, yuck, lukewarm soup.
posted by Foam Pants at 11:57 PM on January 29, 2019
A practical example of what many people describe above is that sometimes you'll see different cooking times for different altitudes. Lower pressure -> lower boiling point -> less heat -> longer cooking time.
posted by GenderNullPointerException at 6:33 AM on January 30, 2019
posted by GenderNullPointerException at 6:33 AM on January 30, 2019
> Since the steam had no where to be vented out I was surprised nothing bad happened. I mean, my vitamix blender comes with a warning that some of the blender containers are not vented and therefore shouldn't be used for hot items..
The other answers cover what will happen to your soup, but I think you have a misunderstanding here about "venting" and the dangers.
The usual problem with heating things in sealed or un-vented containers is a risk of a pressure explosion. If you put some liquid in a non-vented vitamix blender container and heated it on the stove, then the pressure would rise until the container was not strong enough to hold it in, at which point it would explode.
If you have attached a vacuum pump to a container, then you will be keeping the pressure low (assuming the pump can keep up with the steam being produced), so there shouldn't be any risk of a pressure explosion.
> the steam had no where to be vented out ... but in a vacuum there's no vent either
If you boiled soup in a container with a vacuum pump, then the soup would produce steam and it would be extracted by the pump (the water might damage your pump). So I think it is "vented" in the sense you mean.
posted by richb at 6:48 AM on January 30, 2019 [1 favorite]
The other answers cover what will happen to your soup, but I think you have a misunderstanding here about "venting" and the dangers.
The usual problem with heating things in sealed or un-vented containers is a risk of a pressure explosion. If you put some liquid in a non-vented vitamix blender container and heated it on the stove, then the pressure would rise until the container was not strong enough to hold it in, at which point it would explode.
If you have attached a vacuum pump to a container, then you will be keeping the pressure low (assuming the pump can keep up with the steam being produced), so there shouldn't be any risk of a pressure explosion.
> the steam had no where to be vented out ... but in a vacuum there's no vent either
If you boiled soup in a container with a vacuum pump, then the soup would produce steam and it would be extracted by the pump (the water might damage your pump). So I think it is "vented" in the sense you mean.
posted by richb at 6:48 AM on January 30, 2019 [1 favorite]
I think your concerns about "venting" are misplaced. In the normal use of the word, it means "releasing built-up pressure to atmosphere." That's pressure that would exceed atmospheric pressure. If you evacuate the space, you've lowered the pressure to below atmospheric, and lowered the boiling point in the process. Boiling will bring the pressure back toward atmospheric, if you stop evacuating the space. As it does so, it increases the boiling point. I doubt that you could generate dangerous pressures by doing this.
Like what richb said.
posted by Kirth Gerson at 6:56 AM on January 30, 2019 [1 favorite]
Like what richb said.
posted by Kirth Gerson at 6:56 AM on January 30, 2019 [1 favorite]
Thus, you can pop popcorn st a lower temperature if pressure is reduced.
I don't believe this is true. The popcorn is a bit of a special situation because you're essentially working with two systems. You can think of the outer shell of the corn kernel as (more or less) an impermeable barrier - what you do to the pressure outside of the kernel has no bearing on the inside of the kernel until that wall is breached. You'll still need to raise the moisture inside of the kernel to 212 F to generate steam, assuming that the initial interstitial pressure inside of the kernel was around 1 atmosphere.
The corn kernel will pop when the differential pressure exceeds the strength of the outer shell. This will be at a lower temperature under vacuum than it would be under atmospheric pressure, although I agree that the temperature difference is probably not meaningful. In any event, there is no conceivable circumstance in which it would be beneficial to reheat soup under reduced pressure. It does appear that popping corn under reduced pressure increases overall size and reduces the amount of unpopped kernels. A FoodSaver-strength vacuum pump seems unlikely to confer much benefit, however, as they aren't even strong enough to degas water.
posted by slkinsey at 7:07 AM on January 30, 2019
I don't believe this is true. The popcorn is a bit of a special situation because you're essentially working with two systems. You can think of the outer shell of the corn kernel as (more or less) an impermeable barrier - what you do to the pressure outside of the kernel has no bearing on the inside of the kernel until that wall is breached. You'll still need to raise the moisture inside of the kernel to 212 F to generate steam, assuming that the initial interstitial pressure inside of the kernel was around 1 atmosphere.
The corn kernel will pop when the differential pressure exceeds the strength of the outer shell. This will be at a lower temperature under vacuum than it would be under atmospheric pressure, although I agree that the temperature difference is probably not meaningful. In any event, there is no conceivable circumstance in which it would be beneficial to reheat soup under reduced pressure. It does appear that popping corn under reduced pressure increases overall size and reduces the amount of unpopped kernels. A FoodSaver-strength vacuum pump seems unlikely to confer much benefit, however, as they aren't even strong enough to degas water.
posted by slkinsey at 7:07 AM on January 30, 2019
The idea sounds similar to a rotary evaporator, this is used in chemical laboratories all the time to evaporate liquids at lower pressures and apparently some people have been using it in the kitchen too; I have encountered it in the context of quickly and efficiently reducing the volume of a water sample without raising it to a high temperature.
posted by each day we work at 1:12 AM on January 31, 2019
posted by each day we work at 1:12 AM on January 31, 2019
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My concern with doing this with a mixture like soup is that the dissolved proteins and starches in the liquid could cause bubbles to form and link together, causing a big mess. Think about what happens if you boil over a pot of cooking noodles or potatoes.
The reason the popcorn got fluffier is because the escaping steam from the kernels didn't have to fight the local air pressure. More, and more rapid, expansion means that the kernels could puff up bigger.
posted by backseatpilot at 3:59 PM on January 29, 2019 [5 favorites]