Dehydrating – Freeze Dryer vs Dehydrator

Hello everyone! This is a post I’ve been wanting to do for a long time, and I’m finally doing it!

The reason for this post is that I wanted to share with everyone what the difference is between these 2 dehydrating methods. Not everyone has a freeze dryer, but lots of people have heard/seen what it can do and are curious about it. I’ll show the comparisons later down the post. I’d like to give some information on how the freeze dryer works. I think most cooks already know how the conventional dehydrator works so I’ll skip that to keep this post as short as possible.

There are 3 main components to a freeze dryer – temperature controlled vacuum chamber, condenser, and a vacuum pump. A vacuum is exerted in both the chamber and condenser. The freeze dryer works by applying the concept that at lower pressures (ie vacuum), water boils/evaporates at lower temperatures. Similar concept to how the rotary evaporator works, however, the vacuum pump of the freeze dryer is much stronger, therefore allowing evaporation at much lower temperatures.

The food must first be frozen, and the temperature of the chamber should also be below freezing. When there is sufficient vacuum being applied, the chamber will slowly increase the temperature until it reaches higher temperatures (usually around 20c – 25c). These temperature settings can be changed. As the temperature increases, the frozen moisture in the food instantly evaporates due to the vacuum. This process is called sublimation. It is when a substance goes from its solid phase, to a gas phase without going through its liquid phase. The evaporated water then collects in the condenser, which is usually at -80c. It needs to be this cold in order for the water to condense and freeze at such negative pressures.

I must also point out that the freezing method is very important. Best results will be from foods frozen using a blast chiller or liquid nitrogen. The faster freezing will reduce ice crystal formation, therefore resulting in better preservation of the food.

So the benefits of freeze drying are:

– no oxidation since the process is under vacuum

– much more moisture is removed from the food

– the food does not get exposed to as high temperatures as a conventional dehydrator.

– colours and shapes of (most) foods are preserved.

Some clear disadvantages:

– expensive $10,000 and upwards

– process takes longer (average of 2 – 3 days)

– very high power consumption from the constant vacuum and extremely low temperatures of the condenser.

Before we proceed, I just want to point out that during this trial, the freeze dryer had a bit of a hick up, and thus may not have optimal results for this particular trial. Though, results are still indicative of what freeze dryers can do.

I measured the temperature of the conventional dehydrator and got a reading of 60c.A  bit high, but there were no temperature settings for the one I borrowed.

Foods dehydrated are – rib eye steak, sliced portobello mushrooms, blueberries, raspberries, asparagus. Foods were raw, then frozen overnight in a regular freezer (wish I had a blast chiller).

Photos on the left side are from the dehydrator, photos on the right are from the freeze dryer.

First off, blueberries.

Here you can see that colours are different, and there is less shriveling on the blueberries from the freeze dryer.

Flavour: Freeze dried(FD) berries had very strong aromas of rose. Not as much in from the dehydrator(CD). The CD blueberries still had blueberry notes in the flavour.

Texture: Both were crispy, but CD bluebrries had more body to them. the FD ones felt lighter.

Raspberries

The results with the raspberries were quite similar with blueberries, except with different flavour notes of course.

Asparagus

Again, colours are better preserved when the food is FD. Flavours were similar, both had crispy textures.

Portobello Mushrooms

Here you can see that there is absolutely no shriveling in the mushrooms from the FD. There is however the cracking. With the CD mushrooms, they really do shrink to a much thinner thickness, but have no cracking.

Last but not least, rib eye steak.

Here the differences are very obvious. Essentially, the steak is already cooked when processed in the CD, as the temperature was at 60c. Also, there was still plenty of moisture left in the interior of the CD steak. For the FD steak, colour is still very red, and preserved. Again, no cooking or oxidizing of the fat due to lower temperature and pressure.

I still have the FD steak in the freezer. I’m going to see what happens when I let it soak in mushroom stock. Obviously the texture won’t be like a freshly cooked, properly dry aged steak, but hey, lets just find out! Why not right?

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About torontofoodlab

Chef of Toronto Food Lab, a GPI division. View all posts by torontofoodlab

14 responses to “Dehydrating – Freeze Dryer vs Dehydrator

  • Renée S.

    I like that you’re getting more use out of that dehydrator than I ever did. Are you going to post the flavour diff of the steaks? Would you consider comparing your lab “dried” steaks to store bought dry aged meats of the same cut? Curious minds want to know. 🙂

    Re: controls. Were the CD dried foods also done overnight? Would it be more appropriate to standardize time to exposed surface area or water content? Just wondering (even though it might be a tad onerous).

  • Renée S.

    I like that you’re getting more use out of that dehydrator than I ever did. Are you going to post the flavour diff of the steaks? Would you consider comparing your lab \”dried\” steaks to store bought dry aged meats of the same cut? Curious minds want to know.

    Re: controls. Were the CD dried foods also done overnight? Would it be more appropriate to standardize time to exposed surface area or water content? Just wondering (even though it might be a tad onerous).

  • torontofoodlab

    I thought of controlling for time, but it would be questionable as both methods have different recommended times (8hrs vs 72hrs). But CD was done overnight.

  • Tidbits

    […] Toronto Food Lab guys photograph a few comparisons of freeze-drying vs. traditional dehydration. Useful if you’ve ever thought about exploring the […]

  • Nagarkar

    Please let me know where I can buy the FREEZE DRYER. I am frm Dubai and am interested is using the technique for setting up commercial food production unit

    • torontofoodlab

      A quick google search will yield several manufacturers and suppliers of freeze driers. I think it is best to contact them with a phone call and ask for their nearest distributor where you can go for a demo.

  • ericbescak

    Great info on the freeze drying method. Do you know if there’s another “vacuum-drying” method that doesn’t involve extreme temperatures? I want to be able to preserve the nutritional quality of the foods, and any time heat or cold are used, the quality degrades.

  • highly curious

    I know at this point the article I viewed is old but I really enjoyed it. Thanks for taking the time to make it. I just had to ask how your results were on cooking the freeze dried ribeye? Was wondering what all changed ie: taste, texture, preparation? A reply would be greatly appreciated.

    • torontofoodlab

      Hello! It has been a while indeed so my memory might be hazy. Have you opened one of those ramen cups with the bits of meat/veg in them? Those are all freeze dried. The meat, before rehydrated is very flaky and fibrous. Also very brittle. One its rehydrated it becomes spongy, and to be honest, quite unappetizing in appearance. My opinion is that freeze drying steak should only be done for absolute preservation of the nutrients – would be great for long expeditions, the space station, etc.

  • Dave

    I know quite a bit about Vacuum. If you pull enough torr you can watch water freeze before your eyes. If you vacuum seal mushrooms using a food saver to pull a vacuum in a mason jar the freeze will that be like freeze drying? or could you vacuum the mason jar & then dehydrate them using heat. Just curious and looking for your input

    • torontofoodlab

      That’s an interesting idea. I think the main issue is that the vacuum in a mason jar is fixed, whereas in a freeze dryer there is a pump constantly exerting a vacuum as pressure increases when temperature is increasing. What I am thinking is first freezing the mushrooms on the mason jar. Once the contents are frozen, place it in a vacuum then leave it in room temperature. As the temperature goes up, return it to the vacuum. Please let me know the results. I too am curious of this method.

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