mobeets Senior Member Joined: 28 Jun 2012 Posts: 3 Location: Melbourne, Victoria Expertise: I love coffee
Posted Thu Jun 28, 2012, 11:11pm Subject: Chemex variables...and confusion
I was thinking last night about how a pour-over coffee forces a relation between coffee strength and extraction level that doesn't necessarily exist when making, say, a french press.
This is because a filter will let a fixed amount of water pass through the coffee in the brew time necessary to reach your desired extraction.
A filter lets water through at a certain rate that is dependent (I'm assuming) on grind size and the amount of coffee you put in the filter. Each day I make a coffee I'd like to be drinking the same amount of caffeine, so all that leaves me room to adjust is the grind size.
Assume now though that I want to make coffee for me and a friend. By doubling the amount of coffee, I'm decreasing the amount of water that will get through the coffee in my fixed brew time. So I need to increase the grind size to compensate, correct?
The reason I'm asking is that I've read other people suggesting that you both overdose and underextract when making a Chemex coffee. Why would they not just increase their grind size instead of wasting coffee and underextracting?
Looks like I'm not the only one who has been cogitating on the differences between a press pot and a drip.
Here's what I've integrated so far:
Pourover and drip methods rely on a delivery rate and a percolation rate - these together will determine how the extraction goes. Then, to control the coffee produced, you have the amount of water you use to produce the desired amount of coffee.
If you've done this in a reasonable timeframe, then you won't have an issue with extraction. The goal is to deliver the water at a rate that matches a saturated grounds at approximately the percolation rate. The perc rate CAN change throughout the pour, but it doesn't necessarily have to. Changing the grind, or even the composition of the grind or how close to the filtering media you pour can change the percolation rate.
Coarse grind makes it hard to saturate the grounds because even at very slow delivery rates it is difficult to saturate the grounds. Non-saturated grounds will extract at a lower rate. Pour close to the filter media and you wash the filter and get water with a bit of coffee headed into your cup instead of a decent extraction.
Fine grind can cause the percolation rate to slow so much that you have to deliver your brew water over a very long period of time.
Then, with manual pourover methods - don't forget that while you're pouring, the temperature of the brew water is dropping (which slows the extraction rate). This may not be a bad thing - one theory is that the initial extraction should be high temperature and a lower finish temperature to avoid extracting a lot of the heavier, larger chain molecule compounds that cause bitters in the cup.
Auto drip machines basically fix the delivery rate - so they have a fairly small range of brew water (which controls the delivery rate) and grind (which controls the percolation rate).
That doesn't mean you can't make adjustments - for non-preheated reserve tank brewers (the traditional automatic drip $30 12 cup thing) you can use very cold water to slow the delivery rate, or use hot water to speed it up. But brewers like a Bunn (which have a pre-heated tank like a hot water tank in your house, and the delivery is initiated by putting water in to displace the preheated water) usually have a fixed delivery rate.
The difference between percolation methods and a method like a French Press, CCD used traditionally (steep and release), AeroPress is the fact that there are two different methods of brewing. The difference in methods is why the brew ratio should be adjusted.
For the most part, extraction is governed by grind, temperature and contact time. Strength is governed by how much water you use to perform the extraction on a given amount of coffee (the Brew Ratio). For most manual pourover methods, you control the delivery rate (i.e. it's independent of how much brew water you use), so your brew ratio (and therefore strength control) is theoretically independent of brew ratio.
Since an auto drip has a fairly fixed delivery rate, contact time is governed by how much water you use, and for a given brewer the percolation rate is somewhat dependent on how much coffee you use (as well as grind level, filtration, etc.), so these control variables are interconnected.
Regarding caffeine, it's generally agreed that it is one of the first compounds to be extracted - so even severely underextracted coffee has about 98% of the amount of caffeine that an overextracted coffee will have. That means the caffeine is dependent on how much is in the coffee (1.0%-1.6% by memory for Arabica with an agreed average around 1.2%, about double that for Robusta, and triple that for Liberica).
So, if you have a shot of espresso made from 8g of coffee, you're getting 80-128mg of caffiene.
If you have a 350mg cup of coffee at 1.25% strength extracted from 21.9g of coffee, you're getting about 220-260g of caffeine. Even if the strength is only 1.01% or 1.5%, since it's made from the same 21.9g of coffee and caffeine is basically extracted first.
Note that this is higher than what you hear in the media - they say "a cup of drip coffee has about 115-135g of caffeine - read the fine print. This is usually for a 6 or 7 oz cup (or 175 - 200g serving size). This doesn't say how much coffee was used to make it, which is basically what governs the caffeine content, and much of normal gas-station coffee is underdosed. But, if we assume they are normal extraction and strength, estimates for a 6oz cup would be 105-130mg, and a 7oz cup about 125 - 150mg.
Hopefully you found this more helpful than confusing.
Cheers!
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
Yes, that was very helpful! I have some clarifying points about my initial question...
First off, I should have said that I'm looking for a constant volume of coffee at the same strength--not the same amount of caffeine.
My problem is that I've noticed my brew water is taking WAY too long to get through the grounds, i.e. the percolation rate is way too low and so the coffee isn't tasting too good once I get all the water through it. It tastes weak, actually, though I'm using a 17:1 brew ratio.
Based on the reasoning I described in my first post, it seems like all I need to do is increase my grind size as this will increase the percolation rate and get the same amount of water through the grounds in nearer the ideal contact time. But I've read other people saying that when they make a Chemex they prefer to overdose and underextract?
My question is, what are the differences between increasing grind size vs. the overdose/underextract approach? And why would underextracting possibly be a good idea?
Yes, that was very helpful! I have some clarifying points about my initial question...
First off, I should have said that I'm looking for a constant volume of coffee at the same strength--not the same amount of caffeine.
My problem is that I've noticed my brew water is taking WAY too long to get through the grounds, i.e. the percolation rate is way too low and so the coffee isn't tasting too good once I get all the water through it. It tastes weak, actually, though I'm using a 17:1 brew ratio.
Based on the reasoning I described in my first post, it seems like all I need to do is increase my grind size as this will increase the percolation rate and get the same amount of water through the grounds in nearer the ideal contact time. But I've read other people saying that when they make a Chemex they prefer to overdose and underextract?
My question is, what are the differences between increasing grind size vs. the overdose/underextract approach? And why would underextracting possibly be a good idea?
Last Q first: Underextracting reduces the chances of extracting the longer-chain molecules that are responsible for bitter and astringent tastes. Mild underextraction for many people is preferable to mild overextraction. My experience with playing with drip (manual and auto) is cooler temperature underextraction is more toward underdeveloped - it just tastes like bland, fairly flat flavor profile coffee, with some mild tea-like notes. Warmer temperature brewing but pouring through coarse grounds quickly seems to trend toward a bit of sourness, and sometimes an off-flavor sort-of-but-not-exactly-bitter taste. It seems to be a somewhat mild transition from a good cup, to "enh" to "ah, this is probably underextracted", so long as it's tasted all at the same strength.
For me, the first tinges of bitterness are 208°F strike temperature fine grind on a manual pourover with an overall calculated extraction around 20.5%. If I drop to 200°F strike temperature, these creep in more around 21% or a bit higher extraction. However, at a calculated extraction above 22%, it is easily identified and classified as bitter, smokey, astringent, boiled, or any other character normally associated with overextracted. The transition toward overextraction seems to be, for me, more rapid.
Extraction is calculated by = (grams coffee produced * strength) / grams of coffee used to make the brew.
I'll show you how to backcalculate the amount of coffee and brew water to deliver a given amount of coffee in my next post.
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
For all my brewing methods, I measure what is left behind in the spent grounds. I find this fairly repeatable, and I define this as "absorption", calculated by:
(Spent Grounds Mass / Mass of coffee used to make the brew) - 1
It may seem odd, but this absorption term is useful in predicting how much coffee you will obtain from a given amount of brew water and coffee.
My Melitta BCM-4C has an absorption of 1.7, and when I use a Clever Coffee Dripper and terminate the draindown when the drips are pretty slow the absorption for that device is around 2.2. On average, my AeroPress is around 1.25.
If you know how much coffee you want (in my case, about 350g, pretty much the same as 350ml), and I know the desired strength and desired extraction, then the amount of coffee is pretty straightforward to determine.
For a Chemex, I'm not sure what the absorption is, but I bet it is right around 2, and since you said you use a brew ratio of 17, you can figure this out as:
Desired Coffee = Brew Water - (Absorption * Dry Coffee),
but if you manipulate this around, and since you already know your brew ratio (17g water per g of coffee), you can get:
Coffee to Use = (Desired Coffee) / (Brew Ratio - Absorption). In my example and your brew ratio, along with my estimate of a Chemex absorption:
350g / (17 - 2) = 23.3g coffee, and since we already know we're using a brew ratio of 17, the amount of brew water is 23.3*17 = 397g of brew water.
Now, since your percolation is taking way too long, it's probably an issue with grind coarseness. But the curious thing is you say it tastes weak, which is contrary to what long percolation times will do. What I've found is that if it sits in a pool and takes forever to drain, then you're not actually getting percolation, you're getting something approaching a contact brew - which will do exactly what you're saying. For a given brew ratio, a contact brew will deliver a lower strength coffee under similar grind and overall contact time. Additionally, this is longer time for the brew water in your kettle to continue to drop in temperature, which will also slow the extraction.
So - coarsen up the grind just a bit, and strive to DELIVER the water at a rate that doesn't submerge the grounds, but keeps them saturated. If necessary, if not just for the learning, put water in, let it drain until you definitely see the grounds settle and peek up above the liquid, then do it again - just putting enough to wet the top surface and let the top surface emerge from the liquid. Pour from moderately high in a circle. You may end up putting the water in as a series of a dozen dollops of water.
Don't pour "through" though. If you coarsen the grind too much, you'll know - the water immediately drains out, and you can't get enough contact time.
By all accounts, your brew ratio should deliver pretty much 1.25-1.35% strength at a normal extraction, and if your desired amount of coffee is around 12oz (350ish grams) the above suggestion should work.
Overdosing is a way of obtaining normal strength because the way the brew is being done is actually underextracting - it isn't a matter of attempting to underextract, it's that the coffee is underextracted and near 17 brew ratio provides an underSTRENGTH coffee. So, instead of fixing the extraction, it's easier to stay consistent with the brew and up-dose to achieve more normal strength.
Make sense?
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
And thanks for the equations, that seems like a good thing to measure. I'm curious to see what a Chemex's absorption is, but I'll have to wait until the weekend...
I'd definitely agree that I'm getting a "contact brew," because the water doesn't drain at all near the end. I'll loosen up the grind and see if that helps. Plus, my water temperature drops quickly (when making coffee at work) because I've got water coming out of one of those "insta-hot" faucets--so I've got nowhere insulated to keep my hot water! Agh.
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