Freeze dryers might sound like cool (groan!) new technology, but they’ve actually been around for a while. Modern freeze-drying has existed since at least the early twentieth century, where it was invented as a food preserving method, but some forms of these processes date back even further, with the Incas using some form of this as far back as the 13th century.
Freeze drying became more popular in the Second World War as a great way to preserve sensitive biological materials such as antibiotics. Once freeze-dried, these could be transported without a need for cold storage. The dried goods were also lighter due to their drastically reduced water content. This made the logistics of transporting freeze-dried goods, particularly to fight a war thousands of miles away, much easier!
Fast forward to today, and freeze-drying doesn’t just have medical applications but is also a great way to store food. Dedicated freeze dryer units are now used not only commercially but in the home. This is not just for the reasons above (that freeze-dried food is light and easily stored), but also because the process retains the taste and texture of the food much better than other conventional food storage solutions.
Now that you know a little about the history and reasons behind freeze-drying, you’re probably wondering about what the process actually involves! Freeze-drying involves a few different steps and physical processes, so read on to find out more about how dryers work and the science behind them…
How Do Home Freeze Dryers Work?
Freeze dryers work because of the process of sublimation, whereby ice turns directly to gas. This process allows freeze dryers to remove almost all of the water content of food without having to apply high temperatures.
This has several advantages, notably that without applying high temperatures the food retains the taste of texture it had before. Sublimation also only removes water content, meaning that tasty oils and spices can stay held in the food, in contrast with other drying methods.
Before we get on taking a look into what’s inside a home freeze dryer, and how it’s used, let’s first consider the science behind how a freeze dryer uses sublimation to remove water from your food.
The Science of Sublimation (Nerd Alert!)
Warning: if you’re a science-phobe and looking for a technical description of your dryer, you may wish to skip this section. If you’re fascinated by how your dryer works, you’ve come to the right place!
Everybody knows that ice melts into water, which evaporates into steam, right? Well, not always.
Sublimation is a process that happens naturally to many chemicals under certain conditions, whereby they skip past being a liquid from the solid-state and become gaseous. A famous example is ‘dry ice’, which you might have seen used to make clouds in the theatre. This is the result of carbon dioxide, a substance normally a gas at room temperature, sublimating from its solid state to become gas.
Ice can sublimate to become steam in the right conditions, also known as water vapor. Naturally, this sometimes happens when layers of snow are hit by the right combination of wind and direct sunlight, making them cold enough (below a temperature known as the ‘triple point’) and giving enough energy to vaporize the snow. It’s more difficult to create this effect artificially, however, but this is the beauty of the freeze dryer!
The real genius of the invention of the freeze dryer depends on the discovery that the ‘triple point’ (conditions under which ice sublimes rather than melts) is related to the pressure in which the ice is held. If ice is held in a vacuum with pressure below 0.6 kPa (over 100 times less pressure than normal atmospheric pressure), then at a temperature of above 32 degrees Fahrenheit it will sublime rather than melt.
Therefore, to sublime the water from your food, your freeze dryer needs to do two things; first, freeze the food (or keep it frozen), and second hold a vacuum with pressure low enough to keep the ice below the crucial ‘triple point’.
Why is Sublimation Important?
Sublimating the ice from foods rather than using dehydration methods has several benefits. It removes far more water than these methods (which contributes to a lighter end product) but also preserves the taste, texture, and biological structure of the food.
When ice is sublimated from food, the food doesn’t have to be exposed to high temperatures. This saves the food from being cooked or dried — in theory, it’s almost identical to how it was before, just without the water in it. As most oils are more difficult to sublimate than ice, these are also retained within the food, which goes a long way to helping keep the taste and textures as similar as possible.
The sublimation process removes up to 99% of a food’s water content, which drastically reduces the food’s weight and size. One study which involved freeze-drying sliced apples found that they lost over 85% of their initial weight during freeze-drying (source). This is hugely significant for hikers, the military, and even astronauts, who often use freeze-dried foods because they can be transported much more easily than other preserved foods, while still retaining taste and texture.
Now that you’ve got to grips with the science and benefits of the process, let’s delve into the mechanics of a home freeze dryer.
How Does My Freeze Dryer Do This?
Freeze dryers are generally made up of two key parts; a freeze-drying chamber and a vacuum pump. Both of these have important roles to play in the freeze-drying process. Let’s take a look at the mechanisms of your freeze dryer.
Freeze dryers operate in two roles, freezing foods and creating a vacuum inside their chamber. The freezer part of the dryer turns the water content of foods to ice, and the vacuum both allows ice to sublimate and removes water vapor from the environment.
The most substantial part of the freeze dryer is its main chamber. Here, the user inserts food on trays, before sealing the chamber, switching on the appliance, and leaving it to work its magic in a process that can take as much as 15 hours.
The chamber is built for two things; to freeze the food, and to maintain a solid seal from the outside world that allows the vacuum pump to lower the pressure as much as possible. The most up-to-date units also contain interfaces and software that allow the dryer to automate the process, meaning you can just press start and leave it to work.
The freezer part of the dryer works just like a normal freezer you’d find in your house. This involves coolant liquid being passed through a coil of pipes at two different pressures. The pressure lowers inside the freezer, which allows the liquid to turn to gas. This process of evaporation requires energy, which it takes from any warmth inside the freezer.
Once the liquid leaves the sealed part of the dryer, it is recompressed into liquid, which regenerates the warmth (which is why you’ll find that the back of your freezer is generally actually quite warm). This process might be thought of better as a clever way of moving heat away from the chamber than creating cold inside it.
The vacuum pump is something of an independent part. While the freeze dryer is an expensive piece of kit, mostly due to the freezer coil and computing elements involved, the pump is relatively cheaper, but that doesn’t diminish the importance of its job!
The vacuum pump is responsible for creating and maintaining the low pressure inside the chamber. Broadly speaking, they work by removing the gas molecules from the chamber of the dryer. Large-scale vacuum pumps, which may even be used in a commercial freeze dryer, can work by capturing gas with chemical processes (even without mechanical parts!).
However, in a small unit like a home freeze dryer, this is an unnecessarily complex process, so these work literally by trapping gas and manually pumping it out of the system. Mechanical vacuum pumps come in two types; oil-based and water-based (also known as oil-free). If you’re intrigued by the difference between them, I’ve got a complete guide to oil pumps planned and coming soon!
Tips to Improve Home Freeze Dryer Efficiency
As we considered earlier, the sublimation process is dependent on two factors; temperature and pressure. This means that the parts of your freeze dryer that are working when it’s turned on are the freezer and vacuum pump.
If you want to improve the efficiency of your freeze dryer, you can do so by helping these along. There are a few ways to do this;
Tip #1: Keep it cool
A freeze dryer placed in a cool environment, for example, will have to work much less hard to keep the temperature from rising compared to one in a hot place, so will consume less electricity.
Tip #2: Keep it sealed
Any breaks in the seal of the drying chamber will mean that your pump will have to work much harder if it is even able to create a vacuum at all.
Tip #3: Pre-freeze your foods
Many people choose to pre-freeze foods before putting them in the freeze dryer. This is a great idea, as it will not only reduce the time taken inside the dryer but help you make the most of your space (because your conventional freezer is likely to be much larger than your freeze dryer).
Tip #4: Invest in a more efficient pump
A more powerful vacuum pump might be a good investment, as it will be more efficient and reach the optimum pressure for sublimation more quickly. A more efficient vacuum pump is also something that can decrease the noise that your freeze dryer will make.
Want more information on freeze dryer pumps? Read our complete guide to pumps of all kinds right here: Complete Guide To Freeze Dryer Vacuum Pumps (Oil And Oil-free).
If noise is of concern to you, check out our guide to freeze dryer noise in this article: How Loud Are Freeze Dryers?
Does the freeze dryer pump affect overall freeze-drying time?
A more powerful freeze-drying pump can affect the overall freeze-drying time, as it will reach the optimum pressures required for sublimation faster than others.
However, the process sadly isn’t as simple as “the lower the pressure, the faster the process.” When a vacuum is too strong, there aren’t enough molecules around in the chamber for heat transfer to occur. For this reason, freeze dryers are generally configured to maintain a pressure of 500 mTorr. The time that you will save is just the time taken to reach this pressure, rather than speeding up the process itself.
There are however other benefits to having a better freeze dryer pump, such as more efficient use of electricity, and quieter noise. Oil-based pumps can make a mess, and occasionally require the oil to be changed.
Pumps are the most likely part of a freeze dryer to break down due to the mechanical parts. For this reason as well as that of having to change the oil, it can be a good idea to have more than one pump – if you can afford it. This way, if you’re after preparing a lot of food with your dryer, you’ll be able to alternate between them and minimize the time in which the freeze dryer can’t be working.
If you’d rather not have two pumps, though, here’s a cool way we worked out (for under about $35) to never have to change our vacuum pump’s oil again – and, as a result, minimize downtime between freeze dryer loads. Give it a watch here – and consider subscribing to our channel.
Key Takeaways on the Science of Freeze Drying
Freeze dryers are awesome machines. They really work miracles in food storage, so that we don’t have to live in a dry desert to preserve viable wheat in a pyramid for thousands of years.
Okay, so a freeze dryer’s upper limit for food storage is a fraction of a thousand, but a 25-year shelf-life for most foods is still amazing! Especially when you consider that the food would never last that long on its own.
In any case, having a freeze-dryer is a great addition to any food storage enthusiast’s pantry. And it doesn’t have to cost as much as you think – here’s exactly what it will cost you to run and maintain a freeze dryer of your own. Go give that article a read next.
Learning from your own experience is important, but learning from others is also smart. These are the sources used in this article and our research to be more informed as homesteaders.
- “Food Storage.” Backyard Homestead HQ, backyardhomesteadhq.com/category/food-storage.
- Millrock Technology, Inc. “Freeze Dryer Basics: What Is a Freeze Dryer and How Does It Work?” Millrock Technology, Inc, 9 Apr. 2021, www.millrocktech.com/lyosight/lyobrary/what-is-a-freeze-dryer.
- “Sublimation and the Water Cycle.” USGS, www.usgs.gov/special-topic/water-science-school/science/sublimation-and-water-cycle?qt-science_center_objects=0#qt-science_center_objects. Accessed 23 July 2021.