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andys
Senior Member
andys
Joined: 10 May 2003
Posts: 857
Location: NY
Expertise: Just starting

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Posted Sun Feb 19, 2012, 8:09am
Subject: Re: coffee refractometer
 

Kafeman Said:

I am wondering if coffee beans continue to generate CO2 in the days after the roasting process or if it is mainly trapped insided them (disolved, like in a bottle of cola, can probably hold a larger weight than imagining it to be just gas bubbles) gererated only during roasting and then slowly escaping.

Posted February 18, 2012 link

According to Illy, the CO2 is generated during roasting. For whole beans, it takes a very long time to diffuse out.

BTW, you should buy, beg or borrow Illy's book ASAP. I think you would find it enlightening.

Kafeman Said:

I'm half convinced since I feel like my understanding is still on thin ice, just a slightly different way of looking at your same calculation, say substitution in it for 14 g of beans in 230 g of coffee, the three percent variation you illustrated gives uncertainty between 21.36 and 22.02% for my cups.  

Posted February 18, 2012 link

Again, I don't think there's a crucial difference between cups extracted to 21.4% versus 22.0%. Both are likely to contain more bitter flavors than need be. But there's a big difference between brewed coffee (or espresso) extracted at 17% versus 20% (IMO). Among coffee's overly-hyped "third wave" cafes there had been a distressing preponderance of those 17% coffees served. Due in part to VST's making extraction science easy and accurate, and Scott Rao's practical guides, many cafes have been waking up and extracting in a tastier and less wasteful range.

Kafeman Said:

What I can do is another experiment I can put here although it may be old hat for you guys.  Maybe you already know this well, but I don't.  My house temperature is constant and I do measure the humidity.  I just got a new bag of coffee roasted three days ago.  I can open it and sacrifice 10.0 grams of innocent delicious beans aside as is and then just weight them daily until I get bored, to see what happens to the weight.  Of course, as you've already pointed out, I could be losing volatiles and gaining water, and just my luck that they cancel out for the measurement.

Posted February 18, 2012 link

On one of the forums somebody did that experiment and found very little weight change. I think the effects more or less cancel out, and I think that may be why the earlier experimenters were able to ignore it.

Kafeman Said:

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

Posted February 18, 2012 link

Although VST warrants accuracy in coffee TDS, I'm not sure if they quote an nD spec.

Meanwhile, how are you converting your refract reading to coffee TDS?

2/19/12 (edited for clarity and more information)

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


Joined: 18 Feb 2012
Posts: 24
Location: FL
Expertise: Just starting

Posted Sun Feb 19, 2012, 1:10pm
Subject: Re: coffee refractometer
 

First of order: Thank you AndyS.  I've been burning up the keyboard since my initial post and see you've been around posting no-nonsense information ever since refractometers became an interesting topic.  By now I would think it is tedious to keep it up every time some new guy puts his spin on it and makes a post which you are *so kind* to answer.  You are truly a star member, even if you decide to flame me in the future, your posts have been great to run across.  OK with the necessary preamble;

buy, beg or borrow Illy's book ASAP

Will get right on it.  Is the SCAA 60 page brewing booklet very technical, or would it not be so necessary if I got 'Illy'?  For example:

I am really wrestling at this point with trying to properly measure the extraction liquid volume to make the (initial coffee {slightly} concentrate for me) in a practical sense.  I've run across many comments about this, but at this point I'm not looking for opinions, but rather to know how the darned control charts were calculated.  Sure maybe they dried the puck, etc., etc., and I already have my firm opinions on a water calculation, all I want to know is exactly how the control charts were made regarding this so I can adjust my method as best as possible to mimic that.

Again, I don't think there's a crucial difference between cups extracted to 21.4% versus 22.0%. Both

Just to be clear, and I really know I'm preaching to the national choir here, but taste is obviously what it is all about in the end.  I'm still in the *geek* part, i.e., learning curve, in which I hope to develop a feel for the errors in measurements so that when I finally graduate, instead of hand waving for that 0.5% difference, I'll be able to be a better analyst, to form the basis of comprehension of what I'm doing and not just a sum of errors that are all within an experimental error envelope but will tend to push it bigger and defeat the purpose of being a better analyst.  I've seen your posts AndyS and I love your replies to folks that play down the importance of having a good analytical toolkit and removing the hokus pokus to the extent possible.  Let me just say that in the heat of making a single cup of coffee, after it all falls together, if I can control it to 0.5% up or down on the target TDS I would see the light at the end of the tunnel and do all kinds of geekish things I can't do now.

Although VST warrants accuracy in coffee TDS, I'm not sure if they quote an nD spec.

For anyone digging down this far in this thread, you deserve an answer on this and I think AndyS is right though the right thing to do would be ask the company.  In any case, after careful study of the technology and price point, if you are using a refractometer that is not this VST one, you will need to compete with something that is accurate to +/- 0.0002 on the refractive index.  That is the question I posed to AndyS which he was unsure had a public answer, and this is my answer, and I don't speak for the company and have nothing to do with them.  I would, however buy their product as the best value on the market if I didn't have a refractometer, and this is after much thought, I have decided that is a no brainer.

how are you converting your refract reading to coffee TDS

;-), well, this is grasshopper's strategy so far:  It is sub-optimal, but I will correct that.  Let me clarify, my refractometer has a specific gravity scale I've found most convenient, which I have several polynomial fits to get to brix from SG from the literature, all which are nearly the same in result for these purposes.  And then, I'm just in the same boat with the rest of the quick and dirty analysts: Multiply the fitted brix by a factor to get TDS (so far a dimensionless, temperature independent number).  That factor can lead to an introduced of +/- 2% from what I've seen, which itself is coincidentally about the same black box I complained about above: 22.0 to 22.4% when the dust clears.

But, this is a work in progress and I'm doing my best to attack each of the errors and diminish them to the extent possible.  I am starting to dry the pucks and will simply calibrate this directly to specific gravity or maybe refractive index for my methods.  Then, I will have my own calibration curve and the only thing that will matter is the accuracy of the detector of the refractometer and the stupid factor - that is if I overlook something that someone else who has spent more time playing with this hasn't ;-)

found very little weight change.

OK, thanks for saving my 10 grams of suicide beans; I'll put them in today's Jamaican Blue Mountain instead where they'll be delicious instead of getting stale.  I'll take the word of the other guy, it hurts too much to reproduce a simple experiment and watch coffee go stale!

Thanks for the references, maybe I need to look up what Scott Rao is all about while I'm at it.  Coffee is new to me regarding controlling the brewing process, purely for the personal fun of it, but I do have a little professional experience with analyzing vegetables so please excuse me if I'm all over the map and still a cinnamon roast behind the ears - I'm trying!
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gt
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Joined: 29 Jan 2007
Posts: 210
Location: Mpls/St Paul MN
Expertise: I like coffee

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Posted Mon Feb 20, 2012, 6:58am
Subject: Re: coffee refractometer
 

andys Said:

On one of the forums somebody did that experiment and found very little weight change. I think the effects more or less cancel out, and I think that may be why the earlier experimenters were able to ignore it.

Posted February 19, 2012 link



Here's the data Andy mentioned.

Post by gt on Fri Mar 26, 2010 1:53 pm

I didn't have a valve bag handy but here's what I did. As soon as cooled (2-minutes), I put a batch from my P1 into an uncovered bowl and left in on the kitchen counter. Here's the bean weights I've measured.

2-min 170.7 grams
1-hour 170.4 "
4-hours 170.5
18 Hrs 170.7
24 hrs 170.9
2 days 171.1
3 days 171.3
4 days 171.5
5 days 171.7
6 days 171.8
7 days 172.0

I would guess in the summer when the humidity is up there would be more of an increase in weight.
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Netphilosopher
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Joined: 14 Jan 2011
Posts: 1,602
Location: USA
Expertise: Just starting

Posted Mon Feb 20, 2012, 2:39pm
Subject: Re: coffee refractometer
 

...
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andys
Senior Member
andys
Joined: 10 May 2003
Posts: 857
Location: NY
Expertise: Just starting

Espresso: Speedster, Londinium 1
Grinder: EK-43,Robur, HG One, M3
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Roaster: PIDed Popper
Posted Mon Feb 20, 2012, 8:54pm
Subject: Re: coffee refractometer
 

Kafeman Said:

You are truly a star member...your posts have been great to run across.

Posted February 19, 2012 link


Dave:

I am glad that you liked some of my posts, but as far as I know, there's no "star system" on Coffeegeek -- so please let's not start one, OK? It's off topic and doesn't encourage free discussions.

Kafeman Said:

my refractometer has a specific gravity scale I've found most convenient, which I have several polynomial fits to get to brix from SG from the literature, all which are nearly the same in result for these purposes.  And then, I'm just in the same boat with the rest of the quick and dirty analysts: Multiply the fitted brix by a factor to get TDS (so far a dimensionless, temperature independent number).

Posted February 19, 2012 link

Specific gravity scale? Those scales are calibrated differently in instruments designed for sea water, calcium chloride, urine, etc. Even cat urine SG scales are different from dog urine SG scales. I hope you see the irony in making three conversions (nD to SG to Brix to TDS) to get where you want to go (with all the attendant errors) and then obsessing about a one or two percent uncertainty in moisture content.

Kafeman Said:

I am starting to dry the pucks and will simply calibrate this directly to specific gravity or maybe refractive index for my methods.

Posted February 19, 2012 link

Dissolved solids correlate well with taste; undissolved solids seem to correlate best with mouthfeel. You have to be careful what you measure. For instance, drying used espresso pucks tells you the TOTAL solids that were removed, but leaves you in the dark as to how much extract was dissolved and how much was undissolved. The same is true for metal-filtered spent grounds in brewed coffee. In Netphilosopher's press pot example above, 2.5g of extract was undissolved, and probably ~9g of extract was dissolved. That means that the undissolved material made up over one fifth of the total extract. If you're not careful to distinguish dissolved solids from undissolved solids, your measurements won't correlate accurately with taste.

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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andys
Senior Member
andys
Joined: 10 May 2003
Posts: 857
Location: NY
Expertise: Just starting

Espresso: Speedster, Londinium 1
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Roaster: PIDed Popper
Posted Mon Feb 20, 2012, 9:00pm
Subject: Re: coffee refractometer
 

gt Said:

Here's the data Andy mentioned.

Posted February 20, 2012 link

Gary thanks, that was it. Probably if the beans were in a valve bag instead of out in the open, they would absorb moisture more slowly. Then the weights might be even more consistent.

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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Kafeman
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Joined: 18 Feb 2012
Posts: 24
Location: FL
Expertise: Just starting

Posted Mon Feb 20, 2012, 10:45pm
Subject: Re: coffee refractometer
 

Hi Andy

there's no "star system" on Coffeegeek

Don't know where a "star system" came out of an attempt to qualify contributions I read as stellar, a common practice for the better or worse from appreciative newbies; ;-) duly noted and moving right along ...

I hope you see the irony in making three conversions (nD to SG to Brix to TDS)

I don't see any *irony* for using the best tool I have at the moment! (aw, a chuckle though...)

obsessing about a one or two percent uncertainty in moisture content.

'obsessing'?  Please - these are a bit strongly worded.  You asked how I was getting TDS and I admitted I had a clunky, sub-optimal process with which I was unsatisfied, but I am honing for my personal analytical use (that's precisely the reason I found and signed up in this forum).  For that reason I labeled it as "work in progress".  (And unexpectedly, in the it doesn't matter category: this multi-fudged TDS has been great so far in that I can make brews of different sizes and shoot for a reading - and I get the reading - over my concentration range.  At first I thought it was luck, now I just think it is because of the care in accounting for sample preparation.

I'll be receiving my Illy this week, and hopefully can up my game a little with that recommendation.  My hope is that Illy has good references as it covers a lot of ground and besides a general technical intro, it will be great if it is as chock full o' references as I hope.  At this point I hope you can see that I don't have the benefit or liability of any preconception on the relative sizes of these errors.  Rather than describe it as an obsession, it would be more accurately stated as building my analytical model, breaking down all the sources of error to minimize the size of the error bars, learn what those error bars are, and then move forward to the sensory correlations.

As for the refractometer, since I have a +/- 0.0001 uncertainty in my refractive index measurement, it would be better to just go directly from refractive index directly to TDS.  The extra effort wasn't worth it to find a reliable TDS vs. nD conversion (if it is published!) and within a few weeks it will be all moot since I'll make my own calibration curve directly, and if I do it right it will be more accurate for the ranges I calibrate than instruments in the class of the VST, since they typically are limited with detectors which have twice the uncertainty.  That is the hope anyway, unless I overlook or screw something up in the process.  It's all part of the geekery, so the effort is actually very satisfying this first time through it would be senslessly tedious afterwards.

Dissolved solids correlate well with taste; undissolved solids seem to correlate best with mouthfeel.

Thank you for that tip.  For the moment, it is irrelevant as far as I can tell since all of my "pucks" are (paper) filtered, though it wasn't mentioned in relation to the TDS estimation.  I suppose some very tiny particles can make it through the cellulose entanglement of a paper filter, but that particular source of error doesn't seem to be anything near a magnitude that would be of near a concern, though if you have any data to the contrary - that might formally put it to rest and it would be fun to know.   Once I can reproducably do this and move on to solutions containing particulates normally removed in a paper filter, for sure - it won't be overlooked.
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Kafeman
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Joined: 18 Feb 2012
Posts: 24
Location: FL
Expertise: Just starting

Posted Mon Feb 20, 2012, 11:02pm
Subject: Re: coffee refractometer
 

Hi Gary,

Thanks for putting the raw data here, while not definitive, it is extremely helpful and practical.  Let me add that according to some Czech reference I can't recall which analyzed this in a laboratory setting, it was determined (as Andy cited earlier) that the CO2 is only generated during roasting.

Apparently the CO2 is only generated above 180 C; furthermore the CO2 loss in mass is much less than the loss of moisture.  The implication arguably might mean the water gain you saw, however slight, is not being principally offset by CO2 loss, especially when one sees the 7 day span you looked at.  But that requires a bit more thought and referencing and I have enough problems in other areas to work out.

Further, we could look up the composition of a coffee bean in Illy (me, when I get mine) and see if the environment is mostly hydrophobic in there.  If so, it will be a pretty safe bet theoretically that they won't pick up water unless they are subjected to condensation or other soaking.  Just guessing, very interesting results, thank you for complementing this thread.

{edit 21Feb12: just a note of interpretation:  CO2 it said to be generated as a byproduct by the breakdown of the less temperature stable compounds in the green bean; Water evolved during and after roasting has two sources: 1- the weakly-bound moisture content of the bean and 2 - that which is generated during roasting.  Quantitatively, the latter during roasting was seemed to be overwelmed by the former 'free' moisture.  I would guess based on this, that because the CO2 breakdown product is only formed during roasting, we can probably safely infer the same can be said of the secondary evolved H20.  I.e., generally the initial moisture content of the bean is more significant than the breakdown products.  Be nice to know as a rule of thumb the relative weights of these two, just to picture better the roasting process.  If all the oxygen is from air, it might be on the order of 6:1, for example if one H20 is generated for one CO2, and I am sure this info is documented somewhere...if it could be found... still, there are quantitative results out there and all this is a bit fluffy until we could see the actual lab measurements}

--Dave
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Kafeman
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Joined: 18 Feb 2012
Posts: 24
Location: FL
Expertise: Just starting

Posted Mon Feb 20, 2012, 11:12pm
Subject: Re: coffee refractometer
 

Hi Netphilosopher,

Thanks also for the illustrative example.  It looks like what I'm doing isn't to far off some of your posted stuff.  So, may I ask you how you decided upon 215 F, was it simply by your gravimetric trial and error, or is there some vanished published standard that this is the temperature that SCAA, 1950's brew chartman, or some other official source sets that as the correct temperature.  

Any comment would be much appreciated and thanks again for the help!
--Dave
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Kafeman
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Joined: 18 Feb 2012
Posts: 24
Location: FL
Expertise: Just starting

Posted Tue Feb 21, 2012, 4:26pm
Subject: Re: coffee refractometer
 

Thanks Glenn, certainly very interesting!  Though I have trouble thinking a patent could hold up for scrutiny for a calibration, analytical chemists employ a variety  of techniques all the time, so hopefully the calibration is only incidental and anything patented is innovative.  I was under the impression that the work involved in developing VST refractometer (for which they really don't charge much given the refractometer, like buying one at list price and they throw in the software for $40) was to cover some innovations measuring unfiltered material (after thinking about Andy's comment).

Just in general to anyone reading about refractometers but with less experience than some of those kind people posting in this thread (I'm a forum newbie and am extremely happy to 'discover' such like minded people and feel like I'm on a coffee honeymoon at the moment with all the talent here):

Still, while that refractometer is fine for spot checks and casual use, if someone wants really to use a refractometer as an R&D tool, the best approach is to get a more precise instrument (for $1000 or more) and just make your own calibration chart.  It is really not hard if you've ever done intro to analytical chemistry as a college freshman or AP high school, to get a fine curve that is specific to your problem if you're ibnto the wet aspects of coffee.

The issue with that is when people use an instrument and don't report what the instrument measures (in this case refractive index), but rather report TDS or so other measure derived on a primary metric, but don't tell you that metric.  For example, if a lot of people start using the VST, it could become a defacto standard, even if the TDS it reports is not accurate, it would be standard (precise) for sharing data, especially casually.  If it were my product, that would be my motivation to publish the calibration curve.

On the brix correction, that's great to know.  I've been usuing 0.84 as the factor until I can make my calibration curve for my own refractometer and dispense with brix all together; and you can search to see others use numbers between 0.81 and 0.85, , especially 0.83-0.85 so it's not a surprise if you dig into it.  I think the 0.85 was a back of the envelope single point estimate by taking a brix reading of I believe a standard solution of around 1.25% TDS (Not sure who prepared the 'standard').

In any case, if it isn't in the literature already, what we really need is a non-commercial paper with a few good standardized curves for nD vs. TDS for a few different types of coffee beans just to be sure and as was pointed out, it should extend over the entire measured ranges.

--Dave
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