Introduction
In our last couple of blog posts, we told you about how we use freeze drying to produce specialty grade instant coffee and unique flavours in our jams and creative confections. We even talked about how the capability has changed the way we are able to work with the ingredients that we are most passionate about, Native Australian Botanicals.
What we haven't talked about yet is something that we are constantly asked by people discovering our products for the first time. How does it actually work? What does it mean to "freeze-dry" something. In this post we will give you a very simple layman's terms explanation of the science behind the technique.
What Exactly Is Freeze Drying Anyhow?
Freeze drying is just the layman’s term, the lab-coat types use the fancier term “lyophilisation”. Whatever you call it, freeze drying is basically a method of preserving products, in our case food, by removing the water in temperature controlled vacuum conditions.
There are three main scientific principles to get your head around to understand how this works and that’s what we will focus on for the rest of this blog, later we will create another article about the technology that we use to exploit these principles in our humble little kitchen to make some of the most special, innovative and unique products you will ever put in your pantry.
Just a quick note before moving on to the nitty-gritty. Some of the terms we are about to start throwing around sound really technical and complicated. If you have been interested enough to get this far, please don’t let that put you off reading further because we are going to really briefly and simply explain these principles in very straight forward layman’s terms and it really is very interesting stuff.
First Thing to Understand – The Clausius-Clapeyron relationship
The Clausius-Clapeyron relationship basically says that as the temperature of a liquid increases so does its vapour pressure. There’s some thermodynamic laws behind this regarding how a substance's molecules respond to temperature but you don’t have to worry about any of that. Just imagine an enclosed ball that is half full of water, at the surface of the liquid there will be some amount of phase transition (liquid to vapour) or evaporation which will mean there is a little bit of pressure inside the ball.
This pressure inside of the ball is the vapour pressure and it is being caused the phase change from liquid to vapour (evaporation) inside the ball. Evaporation is a reaction and like all reactions it requires energy. Evaporation is an “endothermic reaction” and this means that heat is the energy that drives the reaction. In order to change phase from liquid to gas the water’s molecules must absorb heat. So if you heat the water inside the ball up, then there is more heat energy driving more phase transition to vapour and so the pressure inside the ball will be higher.
Boiling a liquid isn’t just about heat, it is just as much about the pressure of the environment around the material that you are heating up. Boiling actually happens when a liquid reaches a temperature where its vapour pressure equals the pressure of the atmosphere around it and so the vapour freely bubbles out and releases. If you heat up water at sea level then we all know that boiling occurs at 100°C. But it isn’t 100°C everywhere, if you then carried that same water to the top of Mount Everest where the atmospheric pressure is less than one third the pressure at sea level and heated it up there, you would find that it would boil at about 70°C. This diagram shows how the boiling temperature of different liquids is related to the atmospheric pressure, each line is all the different combinations of temperature and atmospheric pressure at which each different liquid will boil.
There’s actually lots of food technologies that exploit this relationship. For example, pressure cookers help you cook food faster and more efficiently because the higher pressure means you can get the food hotter using less energy before your heat energy is stolen by the endothermic reaction. There are also vacuum cookers which are useful for making jams, sauces and preserves where you want to boil out the water to reduce and concentrate the product. These devices allow the chef to achieve that at lower temperatures which not only reduces the energy required and speeds the process up, but allows them to avoid higher temperatures that might destroy delicate flavours and volatile compounds.
The “Triple Point” of Water
The next thing we need to understand follows on from the Clausius-Clapeyron relationship and is known as “The Triple Point”. We’ve already seen how the thermodynamic state or “phase” of a substance (ie. liquid, solid or gas) is related to both temperature and pressure. The triple point is a very precise combination of temperature and pressure conditions at which a substance is theoretically able to exist in equilibrium as a solid (ice), liquid (water), and gas (vapour or steam).
The triple point of water is demonstrated in this diagram which is known as a phase diagram. This phase diagram is specific to water, other substances will look slightly different. For water, this unique state occurs at a combination of 0.01°C in temperature and a pressure of 611.657 Pascals. When water is at this triple point, any miniscule change to temperature or pressure will result in a change in the thermodynamic state of the substance.
So now you are probably thinking “Ah so all the fancy freeze drying equipment is about holding the conditions at the triple point”. Not quite. It’s the part of the phase diagram before the triple point that we want to exploit. This is where we can get water to turn directly from ice into vapour without ever melting to become a liquid and that reaction is called sublimation.
Where the Magic Happens – Sublimation
And here we are, the final technical term to understand before you can really get your head around how the magic of freeze drying works and blow your friend’s and family away with what a food science, big-brain boffin you are. Sublimation is the part of water’s relationship between temperature, pressure and phase that exists below it’s triple point. This is where water can turn from ice to vapour without going through a liquid phase. If we hold our product on this part of the curve in a temperature controlled vacuum without allowing it to reach the triple point then we can remove all the water from a frozen product without actually melting it and THAT’S what freeze drying is.
You’ve probably seen this actually happening before your eyes if you have ever seen dry ice used to make smoke in a stage production or a fancy culinary display. Dry ice is frozen carbon dioxide. Remember how I said the phase diagram for different substances would look a bit different, well dry ice just happens to sublimate at normal atmospheric pressure and temperature. So we can watch it turn from solid to vapour. Water does the same thing but at under different temperature and pressure conditions and that’s what we use the special equipment to achieve.
Next Up – Using Technology at Lynch’s Pantry to Exploit the Sublimation Sweet Spot
That’s probably enough to digest in one sitting but we haven’t actually explained how we do it yet. You now appreciate the science but probably still can’t really visualise how we exploit those principles to produce intriguing, freeze-dried foods in our humble little kitchen at Lynch’s Pantry. After all, this stuff wasn’t too hard to get our heads around so why doesn’t everyone do it.
Well that’s because there’s some very specialised equipment and know-how involved in being able to bring all this together and in our next little blog, we'll show you how we actually make all of this wonderful science-magic happen in our humble little kitchen.