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jpender
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jpender
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Posted Sun Feb 26, 2012, 6:40pm
Subject: Re: coffee refractometer
 

Netphilosopher Said:

Not only the fact that we can't measure it because we haven't pulled it out yet, at a certain point we CANNOT pull it out.  Some of it is absorbed in the cellulose of the grounds and only evaporation will pull the water out.  It's why you can't "wring" a washcloth or paper towel 100% dry.

Posted February 26, 2012 link

Suppose you rinsed the grounds in cold water? By dilution you could wash away most of the coffee that is in solution. Of course even cold water has some capacity (how much?) to extract additional solubles from the grounds, but it might be a small enough effect that you could get a better approximation of what is left. Or maybe like Andy is suggesting no matter what you do will change the answer: a kind of Heisenberg Uncertainty principle for coffee extraction.

Ultimately we want to be able to relate some metric for measured extraction (even if it isn't "correct") with flavor. Are you saying that this can't be done consistently with the AP or press pot because people press out different final amounts out of the grounds? Or are you just incurably curious?
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andys
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andys
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Posted Sun Feb 26, 2012, 8:41pm
Subject: Re: coffee refractometer
 

Steve, you have given me a lot to think about, thanks. I need time to digest all this. Meanwhile, I have a comment about your comment:

Netphilosopher Said:

I, my wife, and anyone I've done presses for (normal ones, 2.5% strength, diluted to normal strength) cannot distinguish whether I press that last bit out or not - so even if that last bit is slightly different in composition, it doesn't affect the end product.

Posted February 26, 2012 link

I think what you say here was confirmed by an experiment I did today. I made two separate aeropress coffees using an inverted technique and 2 min of dwell time before pressing. Brew ratio was 11.8g of coffee with 200g of water. Each coffee was pressed out in four sequential portions. I made no effort to make each portion exactly 1/4; the amounts were approximations. Then I measured the TDS of each "quarter" with the VST LAB refractometer. Here are the results:

Aeropress #1:
1.10%
1.22%
1.19%
1.18%

Aeropress #2:
1.14%
1.21%
1.22%
1.19%

The coffees were underextracted (~18%) but aside from the watery first portion, there was only a very slight tailoff in TDS. So using this technique and dwell time, the AP is almost entirely a full-immersion brew (with a relatively flat extraction curve) and not a "wash-out" method like drip or espresso (which have sharply decreasing extraction curves). It's possible that very low dwell times or no dwell times before pressing would change the AP curve more in the washout direction.

(edit) I think the first portion is weaker because it consists of solution that was in static equilibrium with little agitation. The subsequent portions were of solvent that moved through the full bed of grounds, and that washed out more solids.

Netphilosopher Said:

Such are the mysteries of coffee, and they demand answers!  LOL  ;-)

Posted February 26, 2012 link

That's what keeps it interesting. The cool part is that while we'll never know everything, the system is simple enough so that we can unravel at least some of the mysteries.

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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andys
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Posted Sun Feb 26, 2012, 8:55pm
Subject: Re: coffee refractometer
 

jpender Said:

Or maybe like Andy is suggesting no matter what you do will change the answer: a kind of Heisenberg Uncertainty principle for coffee extraction.

Posted February 26, 2012 link

That is funny because in composing my post, I initially used the same phrase you thought of: a Heisenberg Uncertainty Principle for Extraction. But not being much of a Quantum Mechanic, I chickened out and deleted that verbiage.  :)

 
-AndyS
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Netphilosopher
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Posted Mon Feb 27, 2012, 8:32am
Subject: Re: coffee refractometer
 

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Chang94598
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Posts: 213
Location: SF Bay Area
Posted Mon Feb 27, 2012, 1:31pm
Subject: Re: coffee refractometer
 

Just some tidbits regarding coffee water content....

When caffeine is used as the principle compound extracted, it is extracted more efficiently in hydrated compared to non-hydrated coffee ground. I surmise this is the reason in hand drip coffee, the coffee is initially wetted and water poured in circles. In the siphon, the appearance of different air bubbles is used to determine when to stir.

To my knowledge, there are not that many studies that address the wetting of coffee ground and extraction. The closest I've found was from a study in 2007, which used SEM, gas pycnometry, and endothermic peak of water to determine the water in ground coffee.

The bound water which is not available for typical brew coffee extraction is about 0.15g/per gram of coffee. In the study, the coffee cake was dehydrated in the oven at 103C following the ISO method and water content in the coffee cake was determined by measuring the endothermic peak of melting water.

Another interesting observation is that coffee particles >1mm the center never become hydrated. So for example in French press, if the ground particle sizes are >1mm, it can taste both under and over extracted; ie only the peripheral region is wet, and can be over-extracted; the center is still dry, and therefore underextracted.

The above study was done with distilled water. A few years later another study examined espresso coffee brew with different brewing water TDS including non-softened, softened and distilled water and flow rate. The espresso flow rate is significantly different, even with the final coffee TDS similar.

From personal experience and these various studies, I've come to the conclusion that while TDS measurements is helpful, it is a partial picture of the entire taste/aroma experience, and will be true for only that particular brew system/water/grind/equipment/operator, etc, etc, as there are many links to connect to the final coffee brew.

Again, I want to emphasize this is only "my" experience, and mine only.
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jpender
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Posted Mon Feb 27, 2012, 4:50pm
Subject: Re: coffee refractometer
 

andys Said:

That is funny because in composing my post, I initially used the same phrase you thought of: a Heisenberg Uncertainty Principle for Extraction. But not being much of a Quantum Mechanic, I chickened out and deleted that verbiage.  :)

Posted February 26, 2012 link

I almost deleted it too. But it is pretty common to (mis-)use the term in everyday speech or the popular press. I don't think anybody gets too upset by that. Unlike the real UP it is possible in principle to measure what we're interested in here. We just don't have the tools.

By the way, my "rinsing" idea was a really bad one. I tried it and what I washed out had a great deal more dissolved solids (about 3x) than what Netphilosopher is expecting.
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andys
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Posted Mon Feb 27, 2012, 7:12pm
Subject: Re: coffee refractometer
 

Chang94598 Said:

From personal experience and these various studies, I've come to the conclusion that while TDS measurements is helpful, it is a partial picture of the entire taste/aroma experience

Posted February 27, 2012 link

Certainly true. When you consider how powerful some of the aroma/flavor components can be, a tiny amount of one or another may have a very large effect on flavor. We've probably all experienced how one rotten bean can easily ruin an entire cup; these stinky flavors cannot be measured via TDS.

And TDS measurements have no way of distinguishing Ethiopians from Kenyas from Sumatras, etc.

Despite that fact, when one prepares a well roasted coffee in such a way as to evenly extract around 19% of the dry coffee mass, one usually gets a balance between sours and bitters. The coffee's natural sweetness becomes apparent. In that subtly sweet environment, pleasant, fleeting flavors can often be appreciated. In a less balanced environment, the subtle pleasant flavors would be overpowered.

So whether you get a nicely balanced cup via extraction yield measurements, or simply by experience and a practiced palate, you set the stage for a cup that can really sing.

IMHO, of course!

 
-AndyS
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andys
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andys
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Posted Tue Feb 28, 2012, 7:31pm
Subject: Re: coffee refractometer
 

Kafeman Said:

I'm wondering btw, what the VST coffee refractometer instrument accuracy is on the raw refractive index

Posted February 18, 2012 link

I guess Kafeman has left this thread, but I did some calculations and asked some questions. A ballpark answer to Kafeman's question is posted on Dave Walsh's site.

 
-AndyS
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Netphilosopher
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Posted Wed Feb 29, 2012, 10:13am
Subject: Re: coffee refractometer
 

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andys
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andys
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Posted Wed Feb 29, 2012, 6:03pm
Subject: Re: coffee refractometer
 

(repetitive disclaimer) As a VST beta tester, I received test refractometers and copies of Mojo software free of charge. I do not profit from sales of any VST products.

Netphilosopher Said:

Ok, so talk to me about the VST.  I'm starting to think about getting one of these.

On the site, it's not completely clear whether the software is critical or nice-to-have.  I see the only device available is the VST LAB, at $599.  There's also the bundle at $799, includes the extractmojo software.

What does the software do?

Posted February 29, 2012 link

Neither software is a critical necessity to do this work if you always weigh your coffee dose and also your finished beverage mass. But either software (Mojotogo or Extractmojo) can be hugely helpful in other situations.

I use Mojotogo (on an iPod) regularly since it's easier and faster than a hand calculator. Once or twice a week, when I want to do something more with more complex data entry, I switch to Extractmojo (on a PC). Extractmojo displays the data graphically, which is really nice when comparing the results of multiple brewings.

I also use ExtractMojo when I'm mixing mass and volume units, like making a single serve coffee to fit in an 8 oz mug or running a Technivorm to fill a certain-sized thermos. All kinds of mixed mass/volume and Imperial/Metric conversions are preprogrammed and really convenient to use. In summertime you can easily create recipes for iced coffee that brew a concentrate that you later dilute with a measured amount of ice.

Cafes that have to vary their batch sizes due to varying demand or different size vessels to fill obviously can use these features to calculate accurate recipes and avoid mistakes.

One topic of continuing interest among geeks is the exact amount of water retained in the grounds. This parameter can be left at a standard value or you can collect your own data and enter custom values. So if you are used to measuring brew water input rather than beverage mass output, the software does the output calculations for you.


Netphilosopher Said:

All I'm looking for is an instrument to give me a very accurate %TDS from a small sample.  I know what to do with that information from there.  

The refractometer by itself does that, doesn't it?  
Does it also give an actual refractive index, or just the converted %TDS?

Posted February 29, 2012 link

Definitely, the refractometers read % coffee TDS by themselves. I don't see the older, somewhat less expensive refractometer on the site. I'm not sure if it's still offered. But having used them both, buy the LAB if you can possibly swing the bucks for it. It's really precise, easier to keep clean and using it will bring a smile to your face.

(edited to remove erroneous statement that the refractometers read raw refractive index)

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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