How does it work? I do not get it my brain won’t comprehend how you can freeze something and also remove moisture without heat
In: 209
You freeze it quickly, reduce the pressure (pump air out), heat a bit. This makes frozen water sublime (goes from solid to gas skipping liquid state).
https://www.barnalab.com/en/what-about-freeze-drying/
The lower the pressure, the lower the boiling temperature of water. You freeze something in a vacuum, and the ice sublimates; meaning it goes directly from solid ice to water vapor, which is then removed by the vacuum pump.
This leaves the food extremely dry, which is good for long term storage. Bacteria, mold, fungi and yeast, all the things that make food go bad require water to live. No water, no spoilage.
It doesn’t do much for the flavor or texture, though. Freeze dried foods tend to be crunchy and near flavorless unless rehydrated.
Astronaut ice cream is freeze dried normal ice cream. But it barely has any flavor left.
There’s 2 ways you can do it. You can make something really cold so it takes out all the water, or you can take out all the air, causing the water to evaporate and, as a result, freezing the product.
1. When you make the air cold, it can’t hold as much water. If you cool the air down enough that it drops all of its water, and then warm it up slightly, you have really dry air. If you expose that air to something with water, that air will want to absorb the water from that object. Rinse and repeat until you take out all the water.
2. When you reduce the air pressure, the boiling point of water goes down. [Water boiling in a vacuum.](https://youtu.be/glLPMXq6yc0) This lets the water in the object evaporate, and when water evaporates, it’s absorbs energy and cools the object down.
These are both freeze drying. It’s useful because bacteria can’t work or eat without water, so you can preserve food for a very long time.
Why is the freezing step required? Would the water turn to vapor at room temp given the pressure is low enough?
[deleted]
Just to supplement the top comment.
[https://en.wikipedia.org/wiki/Phase_diagram#/media/File:Phase_diagram_of_water_simplified.svg](https://en.wikipedia.org/wiki/Phase_diagram#/media/File:Phase_diagram_of_water_simplified.svg)
That is a phase diagram for water. The vertical axis is pressure, the horizontal is temperature. If you pick 100 Pa on the left and go over to say -40 C then you would see you are in the blue so water would be ice.
If you keep the temperature at -40 C and reduce the pressure then you finger would move vertically down across the black line and in to the brown area and water would be a gas, never passing through the liquid phase.
Crossing that black line from a solid to a gas is know as sublimation.
I don’t want to be an ass, and I do like reading the answers… but I just want to ask..You do know that google exists right? You could have had your answer in seconds. Maybe not ELI5 explanation, but I doubt you are actually 5. If you start there, then you’re unlikely to need to post here. Again it was a good question, but one easily answered by a quick search.
Imagine you have some amazing toy food, like little toy strawberries. Freeze drying is like a superpower that grown-up scientists use to keep those toy strawberries fresh and fun to play with for a long time.
Here’s how it works: First, they make the toy strawberries super cold, just like when you put a toy in the freezer to make it chilly. Then, they put the cold toy strawberries into a special machine, like a superhero lab.
Inside the superhero lab machine, something super cool happens. It’s like having a toy ice castle that slowly turns into invisible mist, similar to the steam that comes out when you’re taking a warm bath. But guess what? This superhero lab doesn’t use hot things like a toy oven. It uses magic science to turn the ice into mist without making it wet.
So, the cold toy strawberries become mist and the water goes away, leaving only the yummy strawberry part. It’s like turning toy ice into a super fun toy snack that stays fresh for a really, really long time. Freeze drying is like a special superhero power that helps keep toy food fresh without needing heat. How awesome is that?
I use freeze driers (aka lyophilizers) in my work from time to time. Water moves from warm areas to cold areas, like what happens when your glasses fog up if you come into a warm house after being outside when it’s cold outside. In the cold glasses case the glasses are considered a “cold sink” and the hot area is the room. The water goes from the hot area (air) to the cold sink (your glasses).
The process of freeze drying includes the following: The sample (thing to be freeze dried) is first frozen. Once it’s frozen everything is locked in place–sort of. In this case let’s say the sample is at -20 Celsius. The sample is placed into a vacuum chamber which is pumped down to remove the gaseous nitrogen, oxygen, CO2, and some *water vapor molecules but not all*. The vacuum chamber is now connected to a cold sink, usually it’s at -80 C. So now the system consists of a frozen sample (at -20C) in a vacuum with a cold sink (at -80). Since the frozen sample is not at absolute zero the water molecules are actually still moving! The water molecules will tend to fly off of the hot thing (in this case the “hot thing” is merely the hottest thing in the system which is at -20C) and bounce around until they encounter something to make them settle down, a cold sink. If the vacuum is low enough the water molecules will bounce around a few times and find their way to the cold sink (at -80C). The water molecules will tend accumulate on the cold sink and literally evaporate from the “warm” -20C frozen sample without going through a liquid phase. The vacuum is essential for avoiding the liquid phase*. So in essence freeze drying is no different than water condensing on a pair of cold glasses after you come in from the outside, water is merely going from a warm place to a cold place. What’s odd about freeze drying is that ALL the temperatures involved are below 0C and it messes with our everyday experience. In the end it’s water going from something hot to something cold. I hope this helps a little bit.
*Technically the role of the vacuum is to lower the number of air molecules in the system so that the mean free path from the “hot” sample to the cold sink is lower. If there is too much air in the system then the water molecules bounce off of the air molecules and can’t find the cold sink quick enough; this causes the sample to melt before the water sublimes. Most freeze driers will use a single rotary vacuum pump and a cold trap. Ultra high vacuum systems will often use multiple types of vacuum pumps coupled together and always use a cold trap called a “cryo pump” to remove residual water molecules.
If you want to see water boiling without heat…
Take a syringe (without the needle, of course) and take a small amount of maybe 50 degree Celsius or as warm as you can get from a tap. Put your finger over the opening of the syringe and pull it.
The reason the water is boiling is as others have said, water only boils as 100 degrees Celsius at normal pressure. The pressure on mount everest causes water to boil at 68 Celsius and in the deep oceans where the pressure is high, the boiling point can be as high as 400 degrees
Latest Answers