Cryo Searing

Matt_H_63094

Hi folks,

I run a street food business and over the summer I want to be cryo searing at one of the night markets in london - it's all about the show, but should put out an epic product as well. 

I've not yet used LN and I'm wondering whether my idea is actually realistic.  Basically I want to be cryo searing meat in a gastro straight from the water bath, frying, dipping in sauce and blow torching. There's a few things on the menu I want to try, steaks, meat lollipops etc.

But, what I'm trying to work out is if I'm doing 300 to 500 small portions (e.g. 100 to 150 grams each) per night, will I need an obscene quantity of LN to do it?  I have no idea how to factor in evaporation at the moment so it would be useful to get some advice from someone who's done this a few times.  I know it's all relative to ambient temperature, temperature of the meat etc but just a rough idea would help. Dewars are pretty damn expensive, and that's my biggest issue right now.  My budget will stretch to a 35 litre one at the moment...

Commercially viable or just a cool kitchen trick?

Chris_Young_80640

Well, let's try to do a rough lower-bound estimate on the amount of LN2 you will need...

I'll assume that your liquid nitrogen is in a very well-insulated container that has been filled and quenched so that the vessel is at an equilibrium temperature with the nitrogen, and then topped up. So, we'll pretend that while it's just sitting around you are not loosing any liquid nitrogen. That, of course, is unrealistic. But we'll pretend it's true for a moment.

I'll also assume your food is a sphere, since that geometric shape minimizes surface area relative to volume. I'll assume that when you plunge your spherical meat into the perfectly insulated Dewar of liquid nitrogen, you will leave it their long enough to freeze the outer 5 mm. Since food is mostly water with a bunch of impurities, let's assume that we're freezing pure water. How much water is this? Assuming your sphere weighs 150g, and has the density of water, it works out that the first 5mm accounts for 59g of water that needs to be frozen. You have to remove an enormous amount of heat energy from water to freeze it, so we can pretty much ignore the energy required to cool the surface from it's initial temperature down to 0C, and just focus on the fact that we need to remove 334J of energy per gram of water to turn it into ice. So that's 19,706 J of heat energy that has to be removed from each portion by boiling away liquid nitrogen. Each gram of liquid nitrogen that boils away will remove 199 joules of heat energy. So we're going to boil away 99 grams of liquid nitrogen with each portion You want to do 300 to 500 portions per night, so your theoretical minimum is around 30,000 to 50,000 grams of liquid nitrogen. Liquid nitrogen has a density of around 0.807g / mL, so you're going to need 37 to 62 liters of liquid nitrogen. 

Of course, you're going to actually do much worse than this. In my experience, doubling this number is an absolute minimum. So 80 to 125 liters per service. Is that a lot? Yes and no. We have our liquid nitrogen delivered by an industrial gas service (this is the norm everywhere) in 140 liter tanks for about 160 dollars. We have regular deliveries, so our cost is probably a bit better than one purchase. But you're going to go through a tank per night. So working out a contract in advance for a delivery of two tanks every two days will probably get their attention and you'll get pretty good pricing. Don't buy your own Dewar, you just want the tank delivered as a rental and you want them to provide you with a dispensing hose. 

Hope this helps.