Timetable for Melting Ice with various starting points

[maverick3n1]

This may be a bit more rudimentary than other topics, but I figured I'd ask anyways.

I own my own business as a caterer, and some of the various things we have to pay attention to is temperatures with food transport.  If it's cold, I need to maintain cold, often times for hours prior to service.  If it's hot, the same thing goes.

My question is; does freezing ice, or other items such as freezer gel packs and so on, to -20F vs 0F make a substantial difference, or is it merely a mental illusion, and the difference is very little?  We prepare all of our food out of a Commercial Kitchen that has a commercial freezer set to 0F, but I also have a deep freezer at home that I can set to -20F.  If I'm freezing something like half of a watermelon rind I'm going to use as a fruit cocktail bowl, will freezing it to -20F make it last much longer than freezing it to 0F, or will the difference I see be mere minutes?  I know it takes a lot longer to get to -20F once you get past that 0F threshold, so I'd assume the same would be for bringing it above that 0F threshold.

I have noticed a big difference using gel packs and other various cooling devices sealed inside a food grade storage device, but that device is sealed with only 4-6 cu ft of airspace inside, so provided it's not opened often, the -20F units last much longer than 0F.  Out in open air however, I'm wondering if I'm wasting my time trying to freeze things for 3 days in a deep freezer rather than 1 day in a commercial freezer and just hope it holds.

I'd love some more details, both simplistic and scientific as I'm always down to learn

[swansont]

The specific heat capacity of ice is a little over half that of liquid water (2.108 J/g-K vs 4.187 J/g-K) i.e. for ice it's 2 joules of energy required per gram per K (or ºC) of temperature rise so ice heats faster than water. (this is physics and we use metric units). Colder is better, since that represents more energy to be absorbed.

However, water's enthalpy of fusion is 333.6 J/g — that's the energy needed to melt ice and turn it into water, all at 0ºC That's a much larger number. It takes as much energy to melt ice into water as it does to raise it to ~80ºC. That's where most of your effect is. Your frozen material, to the extent that it is water, is going to sit at freezing for a while before it heats up any more.

So the question is this: how long does it take for the ice in the frozen rind to melt in the conditions you have? If it's an hour, then every degree C below freezing buys you another ~22 seconds. So an additional 20 ºF, which is 9 ºC, gets you ~3.25 minutes in this scenario. That doesn't sound to me like it's worth much effort.  

[maverick3n1]

That definitely doesn't sound like it's worth it.  I found our last event I had the frozen rind with ice cold fruit in it, and the rind started to get soggy after about 2 hours.  I was thinking that if I were to instead, deep freeze it in my -20F freezer, it may last longer.  That said, the majority of the rind's internal makings is water, so it doesn't sound like it would make much of a difference.

What about things like freezer packs?  I've not been able to find any resources as to what's inside them, but I have had one leak once, and it leaked a blue gel like substance.

https://www.webstaurantstore.com/cambro-cp1220-full-size-camchiller-for-cambro-food-pan-carriers-camkiosk-and-camcruiser-carts/214CP1220.html

Would something like this benefit more from a -20F since it's not purely water, but some sort of coolant, or am I likely to have similar results as the water rind?  So far I've noticed that they work better in my -20f freezer, but to be fair, when I have had to store them in our commercial freezer, it was because I had forgotten to freeze them for 3 days ahead of time in the -20f, so they were typically room temperature when they were put in the 0F freezer, and they were likely only in there for 16 hours or less.  The only thing we've tested on these is I had some chicken that had been frozen and defrosted too many times, to where even though it was raw, it would effect the flavor some, so we decided we were going to toss it.  We left it in one of our cambro's with this ice pack, and 48 hours later, the chicken still had an internal temp of 37F.  These things are quite useful in the food industry!

One other question.  You were referring to how long it would take ice to melt, and that -20F would only give it a few minutes longer to get up to 0F.  But what about a large block of ice (like what an Ice Sculptor would use)?  If you have a large, say 1ft thick block of ice that's all -20F, wouldn't the internal temperature of the core, help to slow the process of the outside from reaching 0f and beginning it's transition into water?

[swansont]

More ice is better, and minimizing the surface area slows the melting. As you no doubt have observed, ice chips melt faster than cubes.