Have you tried putting a thermometer in various places in your freezer? Not having the exact same temperature throughout the freezer, especially if you open the door on occasion, wouldn't be surprising. Plus with the warmer temperature of those bags, the one in the middle probably was the least solid at first. Is there room for the air to circulate around the bags or is it packed pretty tight, especially near the coldest areas?
I'm not a chemical engineer, so are there any reasons anyone knows of in the composition of stock that would make this happen?

Just throwing out there some ideas...

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Maybe you need air to be flowing around each item? I think I remember Alton Brown saying something about that... although my parents' freezer is so packed that I don't think air circulates...

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Yeah, I wonder if your freezer's just a bit warm? The bags are near the side of the freezer. If you don't have the thermostat at ultra-cold (0 degrees F), where it should be, and if there's a crack in the seal of the door, and if there's a little salt in the broth that drops its melting temp a few degrees, I guess you could get some melting. Try putting a little piece of paper various places in the seal of the fridge. If it's easy to pull it out with the door closed, you've got problems...

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A thermometer is a great idea. I've been advised by food safety experts to keep a thermometer in the refrigerator and the freezer at all times.

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Fishie, Robyn, Harlan, Lindsey -- Those are all things I considered. It's definitely not air flow or temperature or seals.

Let's put it this way.. I just checked again, where one bag was jammed in with containers on the second shelf (almost no air), and the container above the bags in the picture was put between the remaining two in the same space -- it's all frozen solid.

It seems to be something with storing three zip locks of stock on top of each other.

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Mechanically, by not exposing a large area of the upper bags, the air in the freezer isn't getting warm enough to trip the thermostat. Increasing the amount of surface exposed to the cold circulating air will increase the rate of cooling, though the one on the bottom is getting the best treatment, with its energy being conducted away.

thefishie's onto something with his suggestion of a thermometer and question about the composition of the stock-- if the stock is on the salty side, it will have a lower freezing point.

I always let items cool first on the counter, then in the fridge, and then put them in the freezer-- less work for the freezer...:)

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Boss -- It's stock, so no seasoning whatsoever save the few peppercorns and the like that the stock got made with. No salt at all. And, you're definitely right about cooling! I had them in the fridge for a few days.

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I was going to suggest that since the physical bottom of the freezer would be ice cold, it would contribute to a faster freezing of the lowest bag. Like putting it on a block of ice. There's nothing really around the other two, so they're just being air-cooled instead of having a solid frozen mass contributing to the effort.

But then I saw that the one that originally froze went all jiggly when it was removed from the bottom. I found this the really odd part. But, since it's poultry stock, I imagine it may have a high fat and oil content. Could this act as an anti-freeze? Perhaps the ambient temperature of the freezer interior isn't enough to completely freeze the stock, but when it's actually in contact with something that is already frozen solid, it's enough to keep it frozen.

Just a guess.

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Hi, I'm afraid you've run up to something called a change in colligative properties. Simply put, dissolving things in a liquid tends to decrease the freezing point, and raise the boiling point. Now this effect is a function of the concentration of solute, but this tends to be inexact. If your freezer has a gradient of temperature, where the lowest level is coldest (usually because the relatively warmer air will rise), then your stock's freezing point may just be at where your freezer is. I also noticed that your freezer is a frostfree type, which tends to cycle its temperature to keep it frost free.

http://en.wikipedia.org/wiki/Auto-defrost

But if your concern is inhibition of bacterial growth, I think you're fine. You just won't have a solid block.

http://food.drricky.net

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Two things are possibly going on here:
1. The freezing point of water is lowered when solids are added to the water (the stock proteins, fats, etc from the bones). Though the freezer may be cold enough to freeze everything else, the proteins and other components may preventing ice crystals from forming. Will it gel at refrigeration temps?
2. The freezing point of liquids raises when under pressure. The bottom one is freezing because it is under more pressure than the 2 bags above it.
You're at that magical point of pressure/temperature that water can be observed to act as a strange substance. Google "triple point of water" and you'll get the idea, even though this is stock we're talking about. Between the lowered freezing point, the protein/fat/solids mix of the water (this is why HFCS is sometimes thrown in ice cream mixes... to prevent ice crystal formation), and the pressure exerted on the bottom bag of stock, you've got your Twilight Zone freezer...
The bags then froze (from Comment #5) because you jiggled the cold stock enough that the ice crystals could start forming.

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hi

your freezer is broken

fondly

ann marie

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I think Michael is on to something with the idea about pressure and freezing points. If you can, try to lay them all across the bottom of your freezer to get them to all freeze solid. Then, once solid, place them against each other, but stand them vertically in the freezer and see what happens.

At this point in time, I think that it's time to say goodbye to the stock (Sad, I know...), but you can always say hello to some food science love....

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I don't know if this is the cause, but I know that when I make stocks or soups to freeze, I will lay them flat on a tray (or trays) and freeze them in a single layer. Once they're frozen, then I move them around and stack them and shove them to the back of the freezer (till I forget them....). Maybe that process will help here. The one time I tried freezing them in stacks I got a gloopy mess instead of a hunk of hard soup.

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Ok... time to geek out for a moment...

It has to do with among other things mentioned (freezing point of the stock is probably just below the the temp of the freezer. Another thing is that the home freezer is not a commercial or a deep freeze that has a lot of extra capacity to move heat from inside the chest to the air. It relies on things being fairly cold / frozen before being put in there and when you add 'hot' (above freezing) it has to do more than normal.

The bottom bag is right over the cooling coils in the chest so it freezes first and oddly enough acts as an insulator to the inner bags. Think of a baked alaskan, but in reverse or liquid water in a river under a layer of ice. The top freezes and protects the rest.

When you shuffled them around you allowed the new top and bottom bags a place to move the heat. Given enough time everything would reach thermal equilibrium but that may take some time. If you increase air flow (being able to draw out more heat) it would freeze faster (shelves, a fan, etc) would help. A more reasonable may be to juggle the yet to be frozen bags to the bottom of the freezer (under the mixed veggies an as close as you can).

Or just wait a few more days... since it's obviously under 40 degrees bacterial growth should be non existent. When enough energy is removed they will freeze... eventually.

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Jonathan - Thanks! That makes sense. They all froze once I jammed them into other areas.

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