Isochoric Freezing and Emerging Applications in Food Preservation (2020) [pdf] | Hacker News Reader

Isochoric Freezing and Emerging Applications in Food Preservation (2020) [pdf]

gwern.net

45 pointsrfreytag20 comments

The paper refers to "volume" frequently (I quoted some examples), but I'm unclear on what "volume" means. Is it the thing that's being frozen? The space that it's being frozen in? Something else?

> In recent years, taking its roots from the biomedical industry, isochoric (constant volume) freezing is gaining both research and commercial interest as an effective method of food preservation.

> During isochoric freezing, the volume of the system remains constant while variables like pressure and temperature vary in tandem.

Edit: Having read a bit further, I think "volume" is the space that the thing is being frozen in. I'm don't really understand how that's different to normal freezing though: my freezer doesn't change size when I freeze something in it.

> "In a typical isochoric process, the food material is immersed in an isotonic solution inside a rigid container that is capable of withstanding elevated pressures."

Most foods you'd want to freeze are high in water, which expands by 4% when it freezes, so they are talking about placing the food product in a vessel that prevents that expansion (and increasing the pressure instead)

The expansion is actually more than double that, 9% is the figure I've commonly seen. See for instance Wikipedia [1]:

An unusual property of water is that its solid form—ice frozen at atmospheric pressure—is approximately 8.3% less dense than its liquid form; this is equivalent to a volumetric expansion of 9%.

The "4" might be coming from the temperature at which water's density is the largest, which happens at 4 degrees Celsius.

Water is weird. :)

[1] https://en.wikipedia.org/wiki/Ice#Physical_properties

Water expands when frozen under constant pressure. Since meats and produce contain a lot of water, they normally expand when frozen, too. This expansion damages the cellular tissues, ultimately making the food less pleasant to eat. They are using rigid containers able to contain high pressures to prevent the food from expanding at freezing temperatures.

I will say I've met a chef whose secret meatball recipe leveraged the cellular destruction caused by freezing along with salt to enhance the flavor.

Freezing under high pressures makes for a different type of ice, which doesn't expand like "normal" ice we know at atmospheric pressures we're accustomed to.

The point of the article is that no phase change happens, meaning water remains liquid, and so different ice types do not matter.

Not crystal clear after reading the paper - does this result in a glassine (no ice crystals) frozen food that can be maintained as such in an ordinary freezer, or does it require specialized storage (i.e near the triple point)?

pretty sure the ice would change phase as you released the pressure. I'm not as confident as the paper that the pressure changes themselves won't be affecting the foods; that's one of the knobs we twiddle to make chemistry go different in many ways.

So, in the future, along with pressure cookers, we'll have pressure freezers?

Pressure cookers are dangerous because water expands to 1700 times its volume as steam, and thus water acts like a big sponge of energy, and steam explosions can kill. A typical steam pressure cooker is run at 15 PSI or so.

A pressure freezer will operate with 9000 PSI or more, requiring a far more robust vessel. However, as water isn't compressible, a leak should be far less dangerous.

Regarding the water acting as a sponge of energy, that phenomenon is reversed compared to the pressure cooker, but the analogy should not be taken further.

When comparing with isobaric(constant pressure) freezing the paper mentions that it consumes less energy because for food preservation you only need to freeze to -5 degrees, and most importantly there is no phase change process. Thus, you do not need to spend energy in that extremely energy hungry stage. They state a 70% thermodynamic efficiency improvement vs conventional isobaric freezing. That is quite a lot. If you consider that most of a house's latent energy consumption is used for the fridge/freezer I can see important impacts for energy efficiency, although the paper seems to target industrial freezing and not home freezing.

Really nice business cases for industrial uses. 70% energy efficiency increase would probably lead to tax rebates being available for potential customers, making them more likely to upgrade their systems. Improved food preservation would also be a good pitch to make to premium food producers, think fishing boats plus saves in fossil fuel. With expertise gained in my non-critical food preservation industry I would later pivot into the bio-sciences selling lab and transplant freezers.

But you have to seal the food tight in pressure containers and ensure there are no air bubbles. So it isn't so clear cut to me the energy saving is worth it.

Also the containers need to be eventually opened. Either this is extra step for processing, or, if that step is left to consumer, with all the complications of consumer friendly packaging requirements and recycling.

Those are probably the reasons why they target industrial users.

There's no requirement for the pressure vessel to be the freezer itself. I'm imagining stainless-steel cylinders of wagyu steak or other similarly high-valued items lined up in a freezer, each with its own internal thermometer, to be taken home by consumers and returned to the retailer for reuse.

For some foods, though, isobaric (regular) freezing improves the product.

e.g. McDonald's french fries.

https://www.mentalfloss.com/article/25124/mcdonalds-fries-on...

Wait they aren’t freezing I.e. doing a phase transformation to solid.

They’re using pressure to reduce the freezing point and then lowering the temperature to beyond normal the 1atm freezing point, it’s still liquid due to pressure

Could you use super high air pressure around the food to accomplish this? Or does the food need to be in some kind of shape matching pressure vessel with no air?

You theoretically could, but since water is incompressible, you get arbitrarily high pressure for free in a sufficiently strong pressure vessel assuming zero air content. Doing the same with air would require a lot of energy and an intricate feedback loop. Beyond that: compressing air causes it to rise in temperature, which further complicates things.

Thanks. It seems like the issue would be fitting a pressure vessel around a food? We’d need a custom shape for each item?

Also why is the pressure free?

Can this be used for suspended animation?

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