How much can you re-extract cold brew coffee?

I dislike waste, so I decided to fuck around and find out a bit on a Sunday night to see exactly how much good bean juice I could get out of some beans by making cold-brew coffee.

The plan was to just keep consuming the coffee, topping up the water, and see how many "days" I could keep it going for before it became too weak, and then see how much additional coffee was needed to bring it back to life.

So the first step was to grind a fairly arbitrary amount of beans - I filled my Hario hand grinder three times with beans, ground them roughly, and chucked them into the 1L Simax media jar that I use for this purpose. I did not bother weighing, an oversight I might correct in the future.

The Hario hand grinder in question.
Real roughly ground, almost random grind sizes, not fine. Like sea salt or kosher salt?

Then I simply filled it to near the top with cold water, shook it up, and put it into the fridge.

weak bean juice.

At this point, it is entirely undrinkable as the extraction takes quite a while to happen in the cold.

There is some shite that coffee nerds bang on about how the cold extraction only pulls out the shorter chain, sweeter acids, and the longer chain, bitter ones remain behind that may even be true, but I can't be bothered and don't have the analytical chemistry equipment to validate this claim.

What I do know, is that the stuff that comes out tastes good and is fucking rocket fuel.

After 12 hours, I woke up from being asleep, and was in dire need of caffeine. That tepid, unappealing bean soup had improved dramatically.

its a lot darker now. good extraction.

Some of this was decanted and filtered in a Chemex, to yield a nice two mugs of cold wakey-wakey liquid, about 500 millilitres. The container was refilled with water, and put back in the fridge for the next day.

This process was repeated - Monday through Wednesdays extractions were fine, but Thursdays felt a bit "muted" in flavour. Adding in one "grinder" of material brought it back, and got us through to Saturday, by which time I figured adding more would be a waste of time - probably diminishing returns. I suspect had I added the extra "grinder" on Wednesday night, the result would have been better on Thursday, and the same on Friday/Saturday.

However, this did effectively mean we got 6 days of perfectly good coffee out of an amount of coffee that usually would make 2-3 days at best as cold brew, and probably less if treated as pour over or similar (one full "grinder" load seems to fill a Moka pot basket, if ground finely, which makes exactly one cup of coffee for non-Italians, or three tiny little cups for Italians...).

Something I'd like to do sometime is get one of those things that measure total dissolved solids in coffee - how "well extracted" it is - some various mesh sieves, some scales, and other stuff and do a series of measurements to determine the absolute best way to make cold brew. How long to leave it? What ratio? What grind size? Is mixed grind sizes better? How much extracted is good? And so on.

Now, the next post is "what do we do with the grounds?". We have options. You could compost them, or you could further extract them (with a bit of fresh coffee) with vodka to make a coffee liqueur... I think I'll make coffee liqueur.