Sous vide, liquid nitrogen, and deep frying

[Robert Jueneman]

There are several other liquid nitrogen topics, including a rather old one that addresses the safety issues, at

But other than making ice cream, there aren't many recipes or techniques discussed.

Modernist Cuisine has a fairly extensive discussion about using sous vide, liquid nitrogen, and a deep fryer to first cook the food, then freeze it, and finally deep fry it to get a nice crust, e.g., on an over-the-top hamburger.

Since I like doing this kind of experimentation, I tried it, but soon found myself floundering, because there didn't seem to be any underlying principles to guide me. The essence of sous vide cooking is the precision temperature control, but now I found myself wondering things like how much LN2 to use, how long the burger (for instance) had to soak in it, how long to leave it in the deep fryer, and what difference, if any, did the thickness of the meat make. My first attempts at the hamburger recipe ended up being overcooked.

So I turned to my friend Douglas Baldwin for some advice, since it's been over 50 years since my last class in thermodynamics, and I hoped some of his sous vide heating equations might be applicable.

He did a back of the envelope calculation of an impulse heat transfer, using the thermal diffusivity of water, which results in 1.4*10^-7 m^2/s, or 0.14 mm^2/s. So if you wanted a 2 mm thick crust, you'd soak it in LN2 for about 14 (=2/0.14) seconds, and then deep fry it for roughly the same length of time.

But that didn't seem to take into account the temperature of the liquid nitrogen, or the temperature of the oil. When I tried it, I SV'ed the burger at 55C, then froze it for 14 seconds, and then fried it for14 seconds at an indicated 380F or 194C, but in fact the measured temperature was only 174C. That burger was the best of four, and still medium rare. For the other burgers, I increased the time in the LN2 and the oil, to 30 and 60 seconds, as recommended by Modernist Cuisine.

However, because I was being super careful, I hadn't filled the double-walled bowl with enough LN2 to completely submerge the burger, and had to flip it. So instead of freezing it for 14 seconds, each side only got 7 seconds, and then the longer cooking time caused the burger to be overcooked. And it still wasn't clear what the ratio of the times (freezing vs. frying) should be.

So back to Douglas, who said we could estimate the ratio of the time in LN2 by comparing the difference in heating, including the latent heat (the energy required to melt the ice, before the temperature can start to go up) as follows:

(meat temp - LN2 temp + latent heat) / (oil temp - meat temp)

Since the latent heat of water is 80 calories per gram, the temperature of the LN2 is -196C, and assuming the oil is at 200C, we have

(55 - -196 + 80) / 200-55) = 2.3 (That doesn't take into account the surface heat transfer coefficients of LN2 and frying oil.)

So this agrees reasonably well with the recommendations in MC to fry for about twice as long as you freeze.

In my case, with a badly calibrated fryer, I would have had (55 + 196 + 80) / (174-55) = 2.8 as the recommended ratio.

(It is unfortunate, but I can't find a single electric fryer that will go over 375F/194C. I don't know if that is because some oils would start to smoke or break down at a higher temperature, or they are concerned about fires or liability, but the only way to cook "chips" or fries at the desired 425F/200C temperature is in a pan or dutch oven on the stove, which is a lot more dangerous, as I proved by having the oil spill over and being ignited by the gas flame. I quickly turned off the gas and blew out the flame, but I then had a lot of oil to clean up.)

I tried this again, using the last remaining burger (which I had rewarmed after the cook/chill) . I froze it in LN2 that was about 3 cm deep for about 14 seconds, flipping it a couple of times, and then frying it for 32 seconds. (This was before I discovered that the fryer wasn't getting hot enough.)

This produced a very nice, medium rare burger with a deeply seared crust, but one that was only about 1 mm thick. I think that if I had cooked it for the 14 * 2.8 = 39 seconds, the crust would have been slightly thicker.

My impression is that this technique produced a significantly better burger than I could have achieved by searing it with a torch or hot pan. But whether it is enough better to justify the cost of the LN2 Dewar and safety apron and gloves, plus the fact that I'm probably going to have to replace the deep fryer now that I know how inaccurate it is, I'll let others decide.

I hope this had been a useful discussion, and I hope that others will contribute new recipes and techniques to this thread.

[slkinsey]

I don't have my copy in front of me, but I believe the Modernist Cuisine burger is cooked to 54.5C/130F, then fully immersed in LN2 for 30 seconds, then deep fried at 235C/450F for 60 seconds.

Using the small amounts of LN2 you used, as well as the low temperature of your fryer, appear to be major limitations.

Edited October 26, 2011 by slkinsey (log)

[Robert Jueneman]

Have you read the stuff in Modernist Cuisine on using LN2 this way? I believe they are pretty specific as to the length of time to leave the meat in the LN2 and then the length of time to deep fry and at what temperature. I don't have my copy in front of me, but I believe it is cooked to 54.5C/130F, then fully immersed in LN2 for 30 seconds, then deep fried at 235C/450F for 60 seconds.

Yes. As I said, MC has some recipes, but didn't provide any explanation as to WHY they worked, or how you might modify them for other applications. That was what I wanted to develop.

[slkinsey]

What generalities have you observed as to the LN2 freezing times, frying times and frying temperatures in MC? I would think that if 30 seconds LN2 freeze/60 seconds 450F fry provides the crust they want while protecting the interior from cooking, then that's that. Maybe they figured it out via trial and error. But I'm not sure why these times and temperatures wouldn't work for everything. Needless to say, thicker or thinner crusts would mean a reduction in the freezing and frying times.

[Robert Jueneman]

Using the small amounts of LN2 you used, as well as the low temperature of your fryer, appear to be major limitations.

I agree. I'm still learning. I have since found a way (using an iSi funnel) to safely pour the LN2back into the Dewar after using it, so I can be a little more liberal in its use.

And I tried to recalibrate my deep fryer, and it came up to temperature nicely and then quit entirely! I may rewire it and try to use a PID controller to control the temperature more accurately, if I can find a high temperature probe. Or buy another unit, such as the Krups, with a carbon filter to keep down the grease smell .

[slkinsey]

Might it work better to do it stovetop with 3 inches of oil in a dutch oven or stock pot? Granted, you don't have the "set it/forget it" temperature control feature of a dedicated fryer.

[Robert Jueneman]

What generalities have you observed as to the LN2 freezing times, frying times and frying temperatures in MC? I would think that if 30 seconds LN2 freeze/60 seconds 450F fry provides the crust they want while protecting the interior from cooking, then that's that. Maybe they figured it out via trial and error. But I'm not sure why these times and temperatures wouldn't work for everything. Needless to say, thicker or thinner crusts would mean a reduction in the freezing and frying times.

The only recipe I found was for the sous vide burger. It wasn't obvious, at least when I started this, that similar times would apply to a steak, a leg of lamb, or whatever. At this point, I think it probably would, but I haven't tried anything else yet. That's why IO'm hoping that others can contribute.

[slkinsey]

I think there is a recipe in there where a pork roast or some such thing gets the LN2/deep fryer treatment.

[Robert Jueneman]

They mince the thick pork skin, then effectively reattach it with with Ultrasperse starch and methylcellulose until the the pork "skin" puffs. There is also a recipe for ultra crisp chicken crown,but it calls for a Pacojet.

[Kouign Aman]

This burger was warm all the way thru?

[Robert Jueneman]

This burger was warm all the way thru?

Yes, medium rare (or however the SV cooking does it).

Basically, the relatively short time in the LN2 is not enough to chill the whole burger - just the surface. Likewise, the time in the hot oil is not (or at least should not be) so long to overcook it.