The shot glass on the left is a much stronger coffee than the one on the right. Yes, you did read that right – the one on the left, that looks much weaker is in fact stronger. I could just be lying to you but I will explain.
For a little while now I have had a the goal of creating a cup of coffee that is crystal clear. I think, in the run up to the WBC I rambled on at a few people (sorry Zander) about my grand plans and in the end I did something totally different (about which I am very glad).
The original plan was to try and use a Rotary Evaporator, which looks a bit like this:
I had planned to use it by loading the bowl on the right with coffee. This is then lowered into the waterbath below and gently turned as it is heated. Whatever evaporates goes up into the cooling column which is kept nice and cool by pumping water through the coil so that it would condense into the bowl on the left. What particularly appealed to me about this is that it is a closed system pressure wise and you can see the large display showing you the millibars of pressure inside. This you could reduce, which would in turn mean that your coffee would boil at a very low temperature meaning you would damage it less and it would all go quicker.
The idea of a crystal clear liquid at the end of it, that tasted of all the volatiles of coffee got me quite excited. I spoke to Chris, back when he was at the Fat Duck, and he had done some tests and from a taste point of view suggested I look for a different route. In the end I dropped the idea completely and went back to the drawing board and went down the tasty/flavour marriage route.
Last week I was hanging out with Chris and we were pulling some shots and the subject came up again. He’d recently been chatting with a few people about ice gelatin filtration and suggested I give it a try. When I got home I googled a few things looking for a precise recipe and I came across Harold McGee’s article in the New York Times. It also links to the rather excellent blog Ideas in Food which has some great ideas for this kind of thing (Hot Dog Stock! I love it!)
So I began my experiments to try and succeed with this method. As you can see from above I have not yet been totally successful but I figure if a few of us try we might get there much faster.
You are going to lose a fair percentage of the liquid each time you do this, so start with quite a lot. I brewed about a litre in a press:
Herbazu was the coffee going through the transformation. The problem with press coffee is that is leaves a lot of solids in the cup and we want rid of those. I chose to filter it again through heavily rinsed filter paper:
You could of course just brew it in a Chemex but I can’t brew 1.2 litres in my Chemex so I did it this, rather odd, way around.
For the next stage you need some nice leaf gelatin. Prepare/soak it as per its instructions. I worked through a range from about 0.5% to 0.8% by weight. (So for 1 litre I used between 5g to 8g of leaf gelatin). Melt it in whilst the coffee is nice and hot and allow to cool and then put it in the fridge so it gels. This is very important. VERY. What is happening when it gels is that the gelatin is creating a nice coherent network woven through the liquid and this is going to do all the hard work later. After a few hours in the fridge you can then put it into the freezer. Leave it overnight. Nothing in this process can be hurried.
For the next part you need a bowl and some muslin and maybe a perforated tray. I didn’t have any muslin to hand so just used a sieve, though please understand that using a sieve with holes this tiny is total overkill (I was assured):
All you need the muslin to do is keep the block away from the bowl and stop any pieces of ice falling into it. I am sure you could just put the muslin on top and secure it there like the skin of a drum and pop the block on top and let it melt. Be patient. Nothing will happen for a few hours. But eventually the water crystals will start to melt and head south to your bowl. What is important is that the fridge keeps the gelatin solid so that its network filters out all of the solids. The whole melting process can take up to 48 hours if you want maximum yield. You will be left with a goopy and unpleasant feeling jelly afterwards. In the bowl below you will have a liquid that is a concentrate of the water soluble coffee flavours.
I have yet to manage a really great clarification in one step and the above shot glass of liquid was the result of a second clarification. Maybe I am doing something wrong, maybe it is unlikely to get any clearer without several repeat efforts. I will probably bug Chris (when he gets back from El Bulli – so jealous!).
One thing I haven’t covered: How does it taste?
As mentioned above this is very much a water extraction. All of th oils we hope to leave behind in the gel above, and this is certainly a goal if you are using this for meat stocks.
A few things surprised me: The strength of the chocolately flavours in the reduction, and there is a certain sweetness though often I was tasting it straight from the fridge which really caused the bitterness to pop out. The other interesting thing is the length of finish – I always associated the long finish of coffee with oil droplets coating the tongue and slowly releasing both tastes and flavours. It seems that some water soluble stuff likes to hang around for quite a long time too.
And finally – what is the point? Good question. I think it would be fun to serve people a cheeky glass of clear coffee, though probably slightly sweetened as part of a signature drink and I guess it would be kind of fun to do some work with gelan and do a vertical split of clear coffee on one side and normal coffee on the other. Perhaps have the clear one hot and the normal one cool and sweetened. I don’t have enough gelan to hand to play with this, and not enough time in the next few weeks but it could be cool.
Let me know if you have a go with this – it is not very labour intensive and just requires a little faith/patience. And if you have a more successful filtration then please, please share the recipe.
Hope at least a couple of you have fun. Question, as always, in the comments…
UPDATE:Â Seriously – the repercussions and possibilities of this for competition are just starting to dawn on me.Â This would have been a much cheaper and easier way to do the liquid donut, and this method opens up a load of foodstuffs for work with coffee.Â Prepping this stuff up obviously can’t be done on stage but it does open up a whole range of ingredients and possibilities.Â Be interesting to see if people take this where it can go.Â Almost makes me wish I could compete again just to put together another signature drink.