Monthly Archives: August 2012

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?

Advertisements

Introducing FlexiGel

We’re finally offering our ingredients to the public. More and more will become available as time goes on, and they will come sooner than later. Click on the Available Ingredients tab on top.

First round of ingredients are 3 kinds of carrageenan – iota, kappa, and a gum blend. Flexigel is predominantly kappa and will have it’s characteristics, but with the added benefit of setting at slightly higher temperatures when in a mixture with a high amount of solids, and also producing very strong but flexible gels at low dosages. These are it’s main characteristics:

– in fruit or vegetable purees, mixtures can set at temperatures between 45c – 50c (0.8% – 1.5% concentration of Flexigel)

– can produce very strong, but flexible gels at concentrations between 0.4% – o.8%. Think making noodles with apple juice.

– Can be used to make fluid gels

– Extremely easy to disperse

– gels are completely clear

– Needs to be heated to 75c to completely dissolve the FlexiGel into the mixture.

 

Here’s a few things I’ve done with it. This is a fairly old recipe, but I think it demonstrates the versatility of FlexiGel.

Pumpkin Pie: The pie itself contains roasted pumpkin puree, yogurt, nutmeg, cinnamon, maple syrup and sugar. The amount of FlexiGel is 0.8% of the total mixture.

Orange Caramel fluid gel: The fluid gel was made with a 0.6% concentration of FlexiGel. The mixture is first heated then allowed to set into a firm gel. Afterwards you puree it and turn it into a fluid gel. The mixture contains about 50% sugar.

 

For the  orange caramel, before I pureed the gel, I decided to slice it into shapes. One of which was a long rectangle to demonstrate it’s gel strength and flexibility.

 

 

And lastly, from the slab of solidified pumpkin puree, I sliced a “blanket” shape to again demonstrate the strength and flexibility of the gel. Of course, you can make the sheet even larger. It would still be able to hold.

 


A better steak: partial dehydration vs sous vide

Many of us are now quite familiar with sous vide, and many have accepted it as a superior way to cook a steak when compared to the conventional method of cooking directly into the grill or skillet. It is indeed better. Though, what if we can make it even better? I wanted to test out a theory, which I just stumbled upon while doing an experiment that compares the differences between partial dehydration and freeze drying (this experiment should be done tomorrow). I was checking on the samples in the dehydrator when I noticed that the steak had developed a dry crust. I thought to myself “wouldn’t this brown better”? So I set out to test this thought.

Results

I’ll share the results first, and delve on the details later below. In a nutshell, the dehydrated steak was much much better than the sous vide steak. The browning was absolutely even, and is a thicker layer. As a result, the steak had a beautiful thick, crisp crust. Also, with deeper browning there is more flavour. In the sous vide steak, the browning was really just on the surface. Also, in the sous vide steak, there are pockets of steam that formed under the steak, so some of the steak’s surface did not come in direct contact with the skillet.

Sous vide steak

Notice the various areas that are not browned, including the veined pattern.

Partially dehydrated steak

Almost no veined pattern, but the browning appears richer and deeper.

Moisture loss

Now, this was something I knew should be considered between the 2 methods. Sous vide is hailed as the best way to keep meats juicy and tender due to the moist environment and the temperature control. Dehydration is obviously cooking in a dry environment, but the temperature is also well controlled. In fact, most dehydrators work between 55c – 60c. As expected, the sous vide steak held more moisture, but surprisingly, not by much. Weights were measured before and after dehydration and sous vide. The difference was only 2.5% with sous vide having 3.5% moisture loss, and 6% on the dehydrated steak.

Explanation

So, here are my initial thoughts on why I got these results. But please keep in mind, these are my initial thoughts. I could be proven wrong. I welcome any discussion on the matters at hand.

First is the browning. The reason I think why partial dehydration results in better browning is that you are removing the moisture from the surface. Moisture prevents browning because water only goes up to a temperature of 100c, and you need a much higher temperature to start the maillard reaction (browning). Also, in the partially dehydrated steak, less steam is created between the steak and the pan, resulting in much more even browning, as well as more contact between surfaces. In the sous vide steak, surface moisture first had to be converted to steam before browning can occur. The production of steam results in another effect that hinders browning. Steam ends up being a layer that forms between the steak and the pan.

The second explanation I’d like to provide is with regards to the moisture loss. I spoke to my colleague about it and he gave me quite a simple explanation. He does have a masters degree in food science and is working on his PhD, so his explanation is quite likely, valid. He said that it was because of case hardening. When certain foods are being dehydrated, they form a hard outer layer, which acts as a barrier for moisture to escape. This may sound similar to the old myth that searing your steak locks in the juices, but it is different. Also, because the dehydration process is quite aggressive, you don’t have to leave it dehydrating for a long period. You only want to dry the surface.

It would be interesting to see what happens if you smoke a steak for 2 hours before cooking it. Configure the smoker to be dry, and at 60C.

Method

Both steaks were not salted or marinated. Just bought from the store. I used common settings for the sous vide steak – 55C for 30 minutes. The partially dehydrated steak was dehydrated for 2 hours at 55C. Both steaks were finished on an iron skillet with approximately the same heat setting on a gas stove.

A closer look at both steaks.

Sous vide