What is the best way to store stock in the freezer?
I can think of ice cubes and plastic bags, but am looking for other ideas.
Also, what would be the optimal portion size?
When I want to freeze portions of almost anything liquid (including stocks and sauces), I use a covered ice cube tray like OXO Good Grips. Just spoon it into the tray and throw it in the freezer. Easy! Keeps very well, and makes it very easy to portion out later.
The OXO product is my favourite, but it's not too hard to find generic substitutes at any superstore. Try to find one with a soft bottom so you don't struggle for 5 minutes trying to get the cubes out and then scraping the sides for all the bits you left behind.
I don't like ice cube trays as the only freezing method, simply because of portioning -- I typically make large batches of stock, and I only have so many ice cube trays. So I make a few different sizes, which are mostly just based on things I have, and so I have a variety of sizes when I need it:
I've also been known to use muffin tins for freezing things that I'll be using about 3/4c. at a time. (eg, pesto)
Oh -- and of course, for the plastic bags, you'll want to cool the stock down first. I don't tend to refrigerate it first, as I use enough bones that it'll go gelatinous on me if I do. I cool it down to near room temperature through use of a cold water bath (I put my stock pot in the sink, then fill the sink with cold water and ice, and stir every few minutes)
I've never checked the freezing time of stock from hot / cold (Mpemba effect), but I'd be reluctant to put large volumes of hot items in my freezer .. so maybe ice cube trays, but the rest I cool before freezing.
I use quart ziplock bags. I fill them fully, and freeze them on their side on a flat surface. I also often use 1 pint plastic takeout containers. I have found that those two sizes nearly always are the right amounts for me.
I store my stock in 250ml one-use rectangular dozes with a cap.
My friend - a professional chef - has another method. But it requires some equipment:
He puts the stock into a plastic bag. Then he uses a machine that sucks all the air from it and seals it. He freezes each flat bag individualy. Once they are frozen, he stacks them. This allows optimum use of freezer space.
When he needs to use the stock, he just cuts the bag open and break the frozen stock into pieces. It's easy, because the slices of frozen stock are very thin.
I usually put 500 ml portions in a quart zip-lock bag and put the bags in a bread pan to help retain their shape while I freeze them.
But I recently bought some "tovolo king" silicone ice cube trays for freezing portions of baby food (about 100 mL per cube) and found that they're also good for freezing small portions of stock or broth. Since they're silicone, it's easy to peel the mold off of the cube after freezing. (I freeze them and then bag the cubes.)
The 500 mL bags are nice when I need a few cups of broth and the 100 mL cubes are nice when I need smaller portions.
They also make a smaller tray of 1 oz cubes, which are useful when you need a small amount of stock to finish a sauce. (I also use them for leftover coconut milk.)
Plastic quart and pint containers....the kind frequently used for Chinese take-out in the US. Inexpensive. Reusable. Microwave safe. Dishwasher Safe. Can write on with Sharpie...Sharpie wipes off for new label.