A refractmeter, if you aren't a major coffee geek, is a tool for figuring out extraction rates in coffee. (Actually they do that even if you are a major coffee geek.) It measures the total amount of stuff in a solution that isn't water. Coupled with your brew recipe (how much water, how much coffee), this will give you an accurate picture of your Extraction (how much of the dry coffee wound up in the drink) and strength. These two factors are a great way to wrap your head around the way the coffee actually tastes.
For a geek like me this theoretical approach is super-useful, regardless of the tools. I'm not afraid of a little applied math, and while really into developing my sensory skills, I'm not always the best at intuitively using those to change technique. I need to have a bit of framework--although eventually that logic-train gets deep enough to look like some kinda "gut" instinct, like when I speed-dial espresso.
Anyway! Every time I taste my coffee, I think about extraction in my head, and use what I'm tasting to adjust my technique. With a little bit of practice, I think any barista can consistently identify under vs. over-extraction by taste, and these are really the two ways coffee winds up tasting bad--assuming the coffee, water, and equipment are good (fresh, clean, etc.)
Lacking a refractmeter, I decided to figure out my approximate extraction this morning, using the "weigh everything" approach, and see if I learned anything.
Method: Chemex ("8 cup" model).
Ratio: 30g coffee to 450g water. (1:15 ratio, about 67g/liter). This is a small dose for this size brewer, but I've been honing my grind/technique to it, and it makes the perfect amount of coffee for me in the morning.
Coffee: Bufcafe Natural Sundried Microlot, from Gikongoro, Rwanda. Roasted by Counter Culture. Sent to me by the gracious Ben at Peregrine Espresso. Most exciting coffee I've had in some time!
Technique:
-Rinsed Filter (about .75L hot water).
-Added ground coffee (Skerton Mini).
- Thorough preinfusion (38g water, 45 seconds. There's a little bit of drip-through but I'm not sweatin' it.)
-Circular pour until the water level's where I want it, then finish pouring to the center.
-Finish pouring a little past 2:00.
-I allow a little draw-down, then give the Chemex a gentle swirl to settle the grounds in the slurry. Adding agitation makes me nervous (difficult to control/replicate), but it eliminates the "high and dry" problem, and slows the draw-down by settling some of the finer particles--matches well with the (unfortunately large) particle distribution I'm getting at my preferred setting.
-Coffee finishes dripping right around 4:00.
I weighed everything as I went along, and find that I now have brewed coffee weighing 400g. Now I just need to dry and weigh the remaining grounds.
I spread the grounds with a knife so they'll dry more quickly.
I set the oven to 400F, assuming that won't catch on fire if I get distracted (Bradbury better not be pulling my leg), and leave it open a crack to let humidity out. I check on it in about 20 minutes, and finally pull it around 45 minutes later, using my very scientific method of "touch it and feel if it's wet at all". It looks/feels powdery-dry, so I'm reasonably certain this will get me in the ballpark. Old Buffalo house in winter= dessicating atmosphere, which helps. Probably I could have left it for longer to be certain.
Then, I weigh this, and subtract the original weight of the filter, giving me a weight of 25g-- so, by these measurements, I've extracted 5g of the original 30g of dry coffee.
Areas of inaccuracy: I'm using a $20 scale with only a 1g resolution. The more I think about it, the more I should have let the coffee dry longer, just to be safe. Finally, I don't know how much material is actually washed off the filter in rinsing--I'd have to rinse and dry one for a comparison. Given that the filter only weighs 5g to start with, I'd be surprised if I'm rinsing off enough to notice with my apparatus (as that would have to be 10-20% of the paper's mass).
Some quick math and laborious sketching later, and I can plug this onto a brew control chart.
So! As I measured it, this cup has an extraction percentage of 16.67%, and a solubles concentration of 1.25%. That actually...matches up pretty closely with how I would ball-park it from taste. If anything, I probably extracted a little more than this (not caught by my scale/failure to dry grounds totally/failure to weigh rinsed filter), which would shift that percentage and strength up a little bit. It also matches up pretty well with the mysterious Brew Control Chart Rays. (Have you heard my new band?)
How'd the coffee taste?
Freaking delicious. Beans are getting a little on the old side for something this phenomenal (11 days past roast), but it barely shows. Buckets of perfumey, floral aromatics, really juicy, sweet body, with tons of citrus & berries. "Jelly Man Kelly" starts playing when this coffee is brewed right. I did think it tasted slightly on the under-extracted side--but in my experience, that's a much more forgiving side to be on, sometimes even desirable.
What will I change, knowing this data?
Well, I'm quite happy with this cup, but it might be possible to get even more flavor, sweetness, clarity with a better extraction--this cup isn't as developed as it could be. So, I will look at getting my dosing ratio closer to 60g/Liter, and fining my grind a little more. Taste remains the ultimate arbiter, but this little experiment was useful in confirming my "under-extracted" concern about the coffee. We'll see if I like the next batch better!
Science!
*btw if you're wondering about the post title, it's in reference to the creator of the brew control chart.
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