Introduction
In our previous blog we outlined a few simple scientific principles that explain a reaction called sublimation.
If you haven't heard of sublimation, you might be more familiar with evaporation. Of course this is a thermodynamic reaction where water changes state from liquid to vapour. Sublimation is a similar reaction except it describes a reaction where water changes state from frozen ice to vapour without melting and becoming a liquid.
Sublimation is the process by which we achieve Freeze-Drying and it only occurs under specific temperature and pressure conditions. This blog will tell you how we go about creating and manipulating those conditions at Lynch’s Pantry to create the unique products that we are known for.
What Are We Trying to Achieve – A Quick Recap
If you are interested in understanding Freeze-Drying and haven’t taken a look at our previous blog then we recommend taking a few minutes to review it. In that post we outline the science that is behind the technique of Freeze Drying. Having said that, we'll just do very quickly recap here as well, the diagram below is what is called a phase diagram for water. You will see the “triple point”, this is the combination of temperature and pressure where water can theoretically simultaneously exist as ice, liquid and vapour. At this point any minuscule shift in either temperature or pressure will result in the water changing state, or “phase”.
The temperature and pressure conditions that we need to create for Freeze Drying techniques fall in the range on this phase diagram below the triple point. In this area we have the sublimation curve which is where we are able to drive a change in phase from solid to vapour, without the extra steps of melting and evaporation.
Creating the Sweet Spot – How we Achieve the Conditions for Sublimation
So in developing an apparatus and processes to achieve Freeze Drying, there are five critical things that our system needs to achieve:
1. Freezing:
The product is chilled until all moisture inside turns to solid ice. At Lynch’s Pantry we use a blast freezer to pre-freeze products for freeze drying to -35°C. This might seem excessive given the triple point occurs at 0.01°C, you might be asking yourself if we couldn’t get it to happen faster by not taking the initial temperature down so low. The fact is that we need to slow the phase change down in order to control the next part.
2. Establish the Vacuum:
The product is placed in the drying chamber which is sealed and air is pulled out via a vacuum pump, dropping the pressure dramatically. The vacuum pump that we use at Lynch’s is quite large and can achieve a speed of 48m3 per hour. Standard atmospheric pressure at sea level is just over 100,000Pa. The chamber pressure when we start the first phase of a drying cycle is under 4Pa. So just under 0.00004% of the atmospheric pressure that you are experiencing right now.
3. Trap the Vapour:
Now we have established the conditions for sublimation and the reaction starts. As the ice changes state to vapour, we need to do something with it for several reasons. Firstly, the vapour in the chamber will go into our vacuum pump and contaminate the oil and we risk losing our vacuum, this would mean that we risk going past the triple point. This would be bad because at low pressure the water would “boil-off” at very low temperatures which could ruin the product and the vacuum pump.
The other reason we need to trap the water vapour is that if we don't it will destroy the product when we open the chamber. You will notice that our products come with food grade moisture absorbers in the package. This is because freeze dried food will absorb water right out of the air if exposed to too much atmospheric moisture. We need to trap the moisture away from the product so that the product isn’t destroyed the moment the temperature are pressure in the chamber increases. We do this using a cold trap.
Our cold trap runs at -85°C so as the water turns from as to vapour it then hits the cold trap away from the product and turns back into ice where it is locked up before it has a chance to go through the vacuum pump and cause us problems.
4. Stepping up the Heat
4. So now we have a system that can achieve sublimation and trap the water away from the product we can freeze dry our products. Unfortunately this process would take a completely unviable amount of time to achieve. We need a way to make it happen faster. Even an oversized vacuum pump like we use and a beastly two-stage, 3 phase refrigeration system driving our cold trap, a full freeze drying cycle takes 36-48 hrs to do properly, depending on the product.
As we explain in our previous blog, sublimation (like evaporation) is an endothermic reaction. That means that the process consumes heat, if we want the process to happen faster we need to feed it heat. So we start to gradually increase the temperature of the heat product shelves and this increases the rate at which the product freeze-dries.
5. The Balancing Act
Nothing in this process is ever that simple though as again we’ve got a bit of a balancing act to pull off here. We want to feed the phase change heat to speed it up. But, If we go too fast then there will be too much vapour for the system to manage, the cold trap won’t catch enough of it and we will lose our vacuum resulting in a boil off. So we step things up in a controlled way. As the product loses moisture we can introduce more heat because the less water in the product the less vapour that the system has to deal with even at higher temperatures.
We continue like this, slowly ratcheting up the temperature of the shelving in our vacuum chamber to drive sublimation until all the water has been removed from the product and trapped as ice on the cold trap. At this point the food is freeze-dried. We then remove it from the chamber and package it, working like lightening in small batches to avoid any atmospheric moisture contamination.
Final Thoughts – Is it all Worth It?
So that brings us to the end of this little series of blog posts about our foray into freeze-drying. We have covered how and why we got into freeze-drying, what we make with it, how freeze-drying has changed the way we can work with Australian Native Foods, what the science of freeze-drying actually is all about and how we use equipment and technology to achieve it. If you've come into it part way then each post has forward and backward linking to the related articles and also to the relevant products in our webstore.
So the big question is has it all been worth it?
Well that depends on what you want food to be. If you are like us, you want it to be exciting, innovative, surprising, playful and intriguing. You want it to be about science and craft in equal measure. You want it to connect you to the place you are from and to your family, friends and neighbours. You want it to connect you to the artisans who produced it, to their knowledge and to their craft. You understand that the true value of food isn't in the nourishment, its in the sharing and in all of the connection, culture and little rituals that go with sharing food. Above all, you want your food to have a story. So if you are like us, if you want all of that in your pantry, then yes it has definitely been worth it
Up Next – Freeze-drying isn't all that we do
Somebody told me once that to be successful in business you have to "do one thing, and do it well". If that's what it takes then I hope we are very unsuccessful always because to us, food isn't about that. It's about having a crack at everything and anything that excites you. Its about trying things you've never tried before and learning things you never knew before. In our next couple of blogs we'll touch on some of the other foods that we produce at Lynch's Pantry which don't involve freeze-drying techniques. Up next is a really old fashioned product that has recently come back into vogue but is still quite unfamiliar to many Australian's. We'll talk about Shrubs (no, not the little tree. The kind you drink), what they are, where they come from and how to use them.