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can we use a phase change material (PCM) for sous vide cooking?

Sous vide is a cooking technique where you hold food at a constant temperature in a water bath.

In all cases the principle is to keep the water at the exact temperature you want the food (usually meat) to cook. That way it’s impossible to over-cook.

There are three major temperature phases:

  1. charging¬†– the water bath needs to be brought up to temperature – let’s say 20L (~5gal) taken from 7C (45F) to 50C (122F). According to this calculator we need 1,788,800J
  2. stabilization¬†– the food needs to be brought up to temperature – let’s pretend that’s 1L of water taken from -23C (-10F) to 50C (122F). That would be¬†151,840J
  3. maintenance¬†– the whole system needs to maintain temperature despite losses to the environment – let’s assume a 1C change to the 21L system each hour. That would be 43,680J

The¬†charging phase is unique. It calls for crudely dumping heat into the system as fast as possible until it reaches a (relatively) broad temperature range. This phase can easily be monitored by the cook and doesn’t benefit from automation.

The stabilization phase is largely irrelevant as long as it happens fast enough to meet food safety standards, which is easily accomplished by adding a little bit more energy during the charging phase.

It’s the maintenance phase that’s challenging. All of the sous vide techniques revolve around keeping the water bath within a specific temperature range for as long as possible.

The expensive approach is to throw $300 worth of computers at it. The cheap approach is to dump hot water into an insulated cooler and check it every 30-60 minutes. There are various ways to split the difference by re-purposing cheaper low-heat or temperature-controlled electronics such as crockpots and aquarium heaters.

However, there are a variety of substances that maintain a relatively constant temperature automatically when they change phase. Based on the Wikipedia page for phase change materials (PCMs) it looks like several commonly occurring fatty acids and paraffins change phase in the ranges we’re interested in for cooking 49-88C (120-190F).

That means that, theoretically, if we dropped an appropriate quantity of appropriately hot palmitic acid into the water bath along with the food, it should take care of the stabilization and maintenance phases for us. With the right mixture of fatty acids we could target specific temperatures for different foods (EX: 135F for beef and 160F for hard eggs).

Since several of the fatty acids in the target range are the primary components of animal fats (palmitic acid, stearic acid, myristic acid, etc) there’s a chance we can just use lard or tallow. Unfortunately, naturally occurring fats have a significant percentage (50%) of monounsaturated fats (oleic acid) that throw off performance.

This resource listed the latent heat of pork backfat as 26,000J/kg and butter as 60,000J/kg.

So that implies if you had 1kg of butter sitting in the water bath it could theoretical maintain a temperature for over an hour (1.3hr) if the bath only lost 1C per hour. That won’t happen in practice because mixtures don’t have sharp latent heat cutoffs and even holding a small temperature range wouldn’t help if it’s the wrong temperature range anyway.

What we need is a tailored¬†fatty acid with a sharp phase change that would hold between¬†54-60C (130-140F). I’ve found a lot of research on PCMs for buildings and solar arrays. So far nothing in the cooking temperature range.

The wikipedia pages lists the melting points for myristic acid (54C) and palmitic acid (63C).

Myristic acid is available in bulk. Seems to be about $8-10 per lb (double that on Amazon).

This abstract says a mixture of 58% myristic and 42% palmitic has a melting point of 42.6C, which is lower than both of their individual melting points.

 

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