Freeze Dried

Today's trivia covers two subjects in one thus making it rather exciting.

What is freeze drying, and what does it do to coffee?

First developed as a method of preserving medical treatment and pharmaceuticals during WWII. Freeze drying is now often applied to food-stuffs as an effective method of preservation that increases the shelf life of food.

In the case of a jar of instant coffee this will extend its life well beyond that of the original bean based product.

The process of freeze drying food removes almost all the water from the food. In doing so the food is preserved by preventing the usual spoiling agents of bacteria and mold from affecting the food stuff.

However as you might expect this also has an effect on flavor in some cases as water is not the only component dried out of the food stuff. Volatile compounds like acetic acid (vinegar) and alcohols are also prone to loss in the drying process.

Another side effect of freeze drying is that it makes the resultant material quite water soluble afterwards as the process leaves microscopic pores in the material. In the case of pharmaceuticals this is quite the benefit as it allows quick re-hydration. We have Clarence Frank Birdseye II to thank for that, yes the same Birdseye as the fish fingers.

Wet books can be recovered using freeze drying as well.

Sublimation

The principle behind the freeze dried process is sublimation, which brings us nicely to the triple point of water. This is the temperature and pressure where three phases (gas, liquid and solid) of that substance coexist in thermodynamic equilibrium.

Graph showing the relationship between the three states of water against pressure and temperature full sized

When water is cooled below its triple point, we can then use pressure changes to force the water to transition from a solid to a gas by a process known as sublimation. In this case the water literally transforms from frozen solid to gaseous form without passing into a liquid in-between.

Industrial Process

So with this useful knowledge we can understand how it might work in an industrial process like the manufacture of instant coffee.

The coffee beans are roasted, ground and brewed as you might expect. However before the freezing process takes place the coffee is reduced to a syrup to ensure it retains the most of its flavour once dried.

Coffee syrup is moved onto a flat conveyor to ensure it is a consistent depth

Then the coffee is moved to freezer room kept at -50C. At this temperature the coffee will freeze very quickly. The speed of the freezing is important to reduce water crystal size and thus damage to the food stuff being freeze dried.

Factory staff dressed in thermal insulation when entering the industrial freezer

Once completely frozen the coffee is then broken up into granules, before being moved to a pressure chambre.

Sheet coffee is broken up into smaller pieces whilst at -50C

From here we move onto the pressure chamber which will reduce the pressure whilst warming the coffee to speed up the process of sublimation. In the image below the coffee takes 5 hours at 60C to sublimate almost all of its water.

The coffee granules are placed on trays and moved into a vacuum chamber

Then as you would expect bottling and packaging commence afterwards.

The factory in this video produces 420 tons of coffee each week.

References

For those that want to see the process in action, this video is informative.

Wikipedia:

• Triple Point
• Freeze Drying