As the Summer weather bears down with all the force of a faulty two-bar fire, people get excited about ice in drinks. Coffee does not escape. Especially if the coffee has to undergo a brewing process that resembles chemistry and can only be performed by the professional coffee person at the specialist independent coffee place. A barbarous fashion, innovation produced without reason? Perhaps it's more. A lot more.
Describing Cold Brew Coffee is a mysterious task, the taste seems to present as opposites, some say boozy and sweet, others say clean and dry. At first impression a deeply mysterious elixir, but reveals itself to be the simplest of brew methods. It excites and then it disappoints. It almost always leaves you expecting more. Like a bass solo.
A simple definition is coffee prepared using cold water instead of hot, as oppose to Iced coffee which tends to mean coffee brewed hot, then cooled. The result of cooling down espresso tends to be something quite sharp in need of sweetening or in the case of filter coffee, something quite diluted.
Brewing cold only creates a certain set of flavours, low in the acidic tasting oils, the dry cocoa-sweet notes prevail, and the long extraction time creates a concentrated liquor that doesn't get washed away by ice.
Like all brew methods, time and agitation have a part to play. Cold molecules move slower, so extraction takes longer. Introduce a little more energy by shaking or stirring and it speeds up again. (I got an A in science in year 7.)
But what to expect from those flavours is where things get a little more intriguing. Some research was needed, some idea of what cold brew should be.
But each trip last year to London would be fruitless, the few places that served it had invariably run out by the time I'd arrived on a delayed FCC, it's scarcity only adding to it's high brow mystique. this could have been some sort of marketing ploy- the member's club to which you can never find the door. Deep black waters that would not reveal their depths.
(It wasn't. It was just a matter of logistics, a slow drip brew method producing a small amount means a limited supply.)
Trawling the internet for ideas also resulted in as many contradictions as the beverage seems to embody- some say it lasts 3 days, some 3 weeks. Some sweeten, some didn't. Some warned of the caffeine levels, some say it contains hardly any. There are no rules. Just misinformation. Whispered through the internet and easily inflated egos of those pretend experts I waxed about in the last blog (all those months ago. Apologies for the wait.)
So with no benchmark to work to we had to do our own tests.
The experiments began last year in small tubs, altering grind size and steeping time to see what effects those variables might have, varying from fine ground to merely cracking the beans. Asides from learning just how astringent those fine particles can be, the tests only ever seemed to yield one flavour.
So when we began to think seriously about selling the cold brew this summer a few more tests were needed. The first batches we made were good, they had all the fragrant chocolate notes, but I couldn't help but expect more. Single origin coffee is a celebration of character, of flavour profiles that differ between varietals so it's somewhat frustrating to produce the same result regardless of bean. To me it suggested under extraction, something extra needed to happen to get the most of the bean, to get that taste that everyone was raving about. Another method was needed.
------ Enter The Cold Drip -----
Rather than just steeping coffee in cold water, most places were slowly dripping cold water through a bed of coffee. Rather than the soluble parts of the bean diffusing into a body of water, the cold drip effectively draws the flavours out. Or so I imagine- I spoke to a scientist friend and they mentioned something about potential gradients. I nodded sagely, thinking about that A I got for science in year 7.
The only real way to find out was to taste test. With a vague idea of how the expensive looking drip machines at this year's coffee festival worked, some antipodean knowledge gleaned from a brief chat with Ivanna of Afternoon Tease, and my best impersonation of Heath Robinson, I dismantled an Evian bottle and found another good reason why every household should own a cocktail shaker:
Upon her return from a trip to Australia Ivanna had raved about the cold brews out there, mentioning their use of Ice to control the drip rate. Much simpler than my idea of puncturing a hole in a bottle cap with an exact diameter to ensure a steady drip rate.
"Whilst on my travels in New Zealand and Australia I got to try a lot of different cold drip coffee. Cold water is slowly dripped over ground coffee for anywhere between 2 to 6 hours. Because of the gentler extraction the outcome is a much sweeter coffee concentrate."
If you want to do this at home, you can. I was surprised by how simple it all worked out. Brewing at a ratio of 500g water to 35g of coarse ground coffee. (The Bolivian Uchumachi we used in our first cold brew batches.) This was in line with our 70g coffee per litre of brewing water that had worked out well for the immersion batches.
In the cold-drip method I rammed 400g of ice into the cocktail shaker, placed a filter cone in the upturned top of the water bottle (with a hole punched in the cap to slow the exit of the coffee as much as possible) 35g of coffee in the filter and another piece of filter paper on top of the bed to ensure the water dispersed evenly across all of the grounds, rather than puncturing a channel through the grounds. I also added the remaining 100g of water to coffee bed, thereby ensuring all the coffee is saturated with water from the start, ensuring an even distribution of water during the brew.
I admit the resulting apparatus wasn't quite as elegant as I had envisioned:
But it was a little more complex than the immersion control:
Again 500g of water to 35g of coffee. (Other pickle varieties available.)
Even after a few hours of brewing the difference was becoming excitingly clear. The drip was pooling a deep ruby coloured liquid with all the glossy clarity of a good port. the immersion on the other hand was looking muddy and uninteresting. But looks aren't everything. Or so I've been told.
The taste results:
Drip cold brew produced a markedly brighter fruitier coffee, the cocoa dry sweetness that dominated all the other test brews finally had some company in berry and citrussy notes, not massive, but enough to notice a much more balanced beverage.
Movement is probably the key here, whilst the immersion brew was stirred, it's still a matter of swirling coffee around in water already partially loaded with soluble compounds. With the drip brew on the other hand, fresh water is constantly being introduced restarting the extraction process and pulling more from the coffee. And gravity ensures that there is a continual even flow through the coffee. (Also a demonstration of what happens in the draw-down stage of pour-over filters.)
And the immersion method?
The aroma reminded one person of MacDonalds. Which apart from being a failure, is an exemplary reminder to thoroughly clean everything you intend to brew with. So if you do try it at home, find the bleach or don't use an ex-pickle jar. As the photos show though, immersion needs agitation, otherwise the coffee separates out. I imagine the extraction would stop completely if you allow the coffee to settle completely, the fines fall slower and form a sediment, effectively sealing the bed from hitting any more water. See scientific diagram. >>>>
Those fines also make filtering necessary at the end. Without it the whole thing is chalk dry and astringent, masking the cocoa flavours.
So it would seem from the tests that the drip method would produce a better drink, but it's not exactly practical to make a large batch with that method. (Suspending 10 litres of water, or ice, is an engineering task and possibly a personal injury case waiting to happen.) So we had to improve the immersion method, with this new knowledge, stirring periodically and now with extra filtration (creating large cloth teabags to restrict how many fines enter the water.) So far so good. But still I want more. As with everything I suppose.
So even more experiments have followed; double concentrations, using frozen cold brew to drip through fresh coffee to produce a darker concentrate. But really all it produces is the same taste, just stronger. And subsequently learned that you can possibly freeze distill cold brew to concentrate it further. But that's a whole other blog. And there are methods still to try that include initial hot blooms and experiments with pressure.... the next rounds of experiments will begin.
But, if anything, despite it's mysterious facade and it's cachet, cold brew is a rather elegant reminder that sometimes, things do not have to be impressively complex in order to be good.
So go ahead, give it a go at home. And if you do, here is another use for that cocktail shaker. Reposado Mezcal, juice of an orange, egg white and a cold brew float.
What I like to think of as a breakfast smoothie.
Words and 'Science' : Shaun Lynch @SJLillustration