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Netphilosopher
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Joined: 14 Jan 2011
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Location: USA
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Posted Fri Mar 16, 2012, 5:02am
Subject: Re: coffee refractometer
 

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mitch236
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Joined: 7 Nov 2003
Posts: 64
Location: Delray Beach
Expertise: I love coffee

Espresso: Linea Single Group (PID,...
Grinder: Mazzer Robur E
Posted Fri Mar 16, 2012, 8:41am
Subject: Re: coffee refractometer
 

A couple of tidbits.  I asked Vince about why I can't just drop the filtered coffee right on to the meter from the syringe and he replied that transferring the coffee into a room temperature vessel allows the coffee to reach room temperature before pipetting it to the meter.

On my underextraction front, this morning I did an experiment where I ground 12 gm of espresso beans at espresso grind and put them in a mug.  Then I added 197.2 gm of brew water from the grouphead of the Linea.  I allowed this to brew for 4 minutes after an initial stirring.  I tested the TDS and it was 1.47 which equates to an extraction yield of 21.48% so I don't think the water is the problem.  It is either the Linea or my technique!

I'll find out in a couple of weeks when the Strada is installed!
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Netphilosopher
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Joined: 14 Jan 2011
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Location: USA
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Posted Sat Mar 17, 2012, 9:40am
Subject: Re: coffee refractometer
 

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

Espresso: Speedster, Londinium 1
Grinder: EK-43,Robur, HG One, M3
Vac Pot: Yama
Drip: various
Roaster: PIDed Popper
Posted Sun Mar 18, 2012, 8:45am
Subject: Re: coffee refractometer
 

Netphilosopher Said:

Found a new use for the paper discs for the Aeropress. ;^D

Fold to quarters, use as a mini gravity drip filter for sample filtering (like a Chemex filter) when you run out of sample filters from VST.  Very effective in removing fines from a sample, even espresso

Posted March 17, 2012 link

For quite a while I too used the Aeropress filters to prepare espresso samples for refractometer measurement. But when I compared the results to syringe filter measurements, the AP filtered measurements read high by an inconsistent margin.

The price is right when AP filtering, but unfortunately the results can't be trusted. IMO.

andys: filter_holder_7093b_L.jpg
(Click for larger image)

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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eMoJo
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eMoJo
Joined: 19 Sep 2010
Posts: 5
Location: Boston
Expertise: I love coffee

Posted Sun Mar 18, 2012, 10:20am
Subject: Re: VST LAB Coffee Refractometer
 

For the benefit of others reading this thread, I've pulled the calibration data, and run some tests in the lab on two other instruments from the same production date code that measured w/in +/- 0.02% at several REF points we use in final QC testing. Some measurements and data are shown, below (sorry about the image quality, limit is 76kB).

Four separate measurements were taken of the same sample using two VST Coffee Refractometers, a laboratory coffee refractometer with Peltier sample tray controlled to +/- 0.02 Deg C and a mono-modal microwave dehydration oven. All four measurements agreed to w/in 0.01% of each other.  The general brewing conditions were set up to be similar to what users might use at home.

Coffee:    75g MADCAP San Sebastian, Colombian
Water:     1000g; TDS = 157ppm, Hardness ~ 70ppm (mostly CaCO3), pH = 7.1, TEMP = 94 Deg C
Grinder:  Baratza Virtuoso, set at 24 (nominal-auto Drip Grind)
Filter:      Filtropa #4 paper, rinsed in hot water
Brewer:   Technivorm, 60-seconds of manual pre-infusion, 5:31 mm:ss
% TDS:   1.68
% Ext:    19.4

In a separate post I'll outline what I think might be happening to create the differences you're seeing.

Kind regards,

~Vince@VST

eMoJo: VST Measurement Summary.jpg
(Click for larger image)
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eMoJo
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eMoJo
Joined: 19 Sep 2010
Posts: 5
Location: Boston
Expertise: I love coffee

Posted Sun Mar 18, 2012, 11:26am
Subject: Re: VST LAB Coffee Refractometer
 

Further to the measurements summary provided in the previous post, some comments to address your measurements.

Protocols are absent from any of your posts as are some of the raw data so it’s hard to pinpoint a specific error–but what’s obvious is that you have many variables changing simultaneously, throughout your experiments, such as the coffee roast, water, brew method, grind, etc.  

The difference you're experiencing could be either a faulty instrument (which is always possible) or a faulty protocol (or both).  While you suspect the former, I suspect the latter.  

My guess is that, despite your observations so far otherwise, you’re making  an apples/oranges measurement. You're dehydrating a sample with total brew solids, while the refractometer is measuring total dissolved solids.  The differences are always in the right direction; your dehydration measurements are higher.

Some general background
The AP is not the brewing apparatus of choice to make this comparison, because the beverage is not well filtered. The forces are too high, and the coffee is frequently too fine (for the filter). The AP filter is just standard coffee filter paper, die cut to size.  Those filters are designed for gravity-drip and coarser grades of grind. While hot water causes the coffee paper filter fibers to swell, making them a better filter, the filter is also weakened when wet.  The cellulose fibers of the paper filter separate, deform and distort under higher than gravity-percolation forces. This is one of the reasons AP coffees are often cloudy, not clarified with sediment easily detected on the palate as well as by eye when compared side-by-side to other gravity-drip methods using similar filters, normal drip-grind, and brew formulas. Most paper filters are specified with a burst pressure rating in kPa, PSI or BARs, tested using a Mullen Burst Strength Tester in accordance with JIS P8112.

Filter strength isn't a problem when gravity based drip is used with nominal drip grinds. However, when you use the press with finely ground coffee, i.e., the “near espresso grind” used in your experiments, can clog the paper filter. This creates higher resistance, and the higher forces needed to press end up pushing even more non-dissolved solids through the paper and into the beverage you're dehydrating.

Side bar:
Some of those non-dissolved solids are soluble, but because of the typical/limited time to extract, end up in the finished beverage.  This is why taking a measurement a few days later will sometimes produce higher soluble solids measurements than initial ones done at brewing time. The best way to minimize this error is to filter using a syringe filter prior to storage in a sealed sample tube w/o air.

Finally, the generally lower temperatures typical of the AP contribute to a tendency to under-extract what most people would consider nominal levels of soluble solids for typical coffee. So, when measuring AP coffees, I always expect under-extracted results, and plenty of sediment, and unfortunately, frequently get them.  

The result of your dehydration measurement, therefore, likely includes total brew solids, not just the total dissolved solids (as measured by the VST Coffee Refractometer), which is the valid measurement we're after in order to determine the extraction yield.

I'll post a few suggestions in a final post for you to consider, regarding both the measurement method and also extraction yields in general, that I hope you'll find useful.

Regards,

~Vince@VST
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eMoJo
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eMoJo
Joined: 19 Sep 2010
Posts: 5
Location: Boston
Expertise: I love coffee

Posted Sun Mar 18, 2012, 1:09pm
Subject: Re: VST LAB Coffee Refractometer
 

Final comments and suggestions

Please allow me to suggest a few simple adjustments to your experiment(s) you can try, that should more closely align the two measurement methods.

a)
First, try a brew using the filter paper as it was intended, gravity filtered, using a normal Drip or Auto drip grind, versus an espresso or very fine grind. Pre-rinse the filter with hot water. Don't do any other filtering. I suggest 75-80g / 1000g brew water, to end up in the same 1.75 - 1.85% TDS range of your previous brew batches.

Then dehydrate.  Without regard to the level of extraction, just compare results until you are at least closer, w/in +/- 0.1% TDS with both methods.  Getting closer will require some investment in more expensive scales and refinement of your dehydration apparatus.

b)
Repeat your AP brew, being careful to limit the pressure and extend the time to press. Then, to be certain you're comparing apples//apples use a VST COF/ESP Syringe Filter to filter the coffee beverage prior to dehydration.

Notes:
Filtration
If you’re using an espresso grade of grind, you must use a syringe filter prior to measurement with either refractometer or dehydration methods. Regular coffee or AP paper is not suitable for filtration needed for accurate refractometer measurements (filtrate non-dissolved solids must be below 0.589µm in size).

Scale Accuracy
The scale we use resolves to 0.0001g with an accuracy of +/- 0.0001g, and a precision of +/- 0.0002g, which will allow you to use smaller samples, maintain accuracy of +/- 0.01% TDS, and do so with a lower carbon footprint (faster drying times). Scales are calibrated before each measurement session using the appropriate NIST traceable class weights.

3) Your experiment of scraping the total brew solids (good idea!) residue should/will show the same result, because brew solids = dissolved plus non-dissolved.  Therefore, your readings will appear low by exactly the same amount as the dehydration versus refractive index method. No surprise there, same problem, same result.

4)  As Glenn recommended, you can create a simple sugar solution using Distilled Water and pure cane sugar, at the ratio of 2.49g/100g to create a close estimate of refractive index to 2.00% mass fraction of coffee, and take a measurement at 20.0 +/- 1.0 Deg C.

Just use 7.50g sugar and 293.70g DISTILLED (not tap) water. This should read ~2.00 +/- 0.05% on the VST Coffee Refractometer at 20.0 +/- 1.0 Deg C.  

Be certain all of the sugar is completely dissolved into solution. Many granulated sugars use an anti-clumping agent so are not pure, but this will give you an idea if the instrument is faulty or was damaged during transit to you, for example.  If it reads 2.00 +/- 0.05% @ 20 +/- 1.0 Deg C, it's OKay, and you should suspect your protocol, per the above and previous post.


5) Finally, as a clarification, VST does not recommend a specific range of TDS or Extraction Yield you “should get” for a valid brew or measurement. We simply provide the instruments and software needed to tell you where you actually are, and you're free to edit onto the chart what you prefer as your taste preference.

Both eMoJo and MoJoToGo offer to show, hide or create your own “regions of interest” on the universal brewing control chart, such as the SCAA, SCAE, NCA, and custom regions. You can set those preferences wherever you wish. That said, I have always preferred coffee and espresso extracted to a range of 19.0-19.5% extraction yield, at various concentrations.  

What I have learned over thousands of measurements, hundreds of installations, lots of trouble-shooting, and a lot of cupping and tasting, is that most folks do not care for grossly under-extracted coffees/espresso and associate those taste defects as sour. Similarly, most don’t care for over-extracted coffees at greater than 21-22% extraction yield, and associate those taste defects with bitter (there are exceptions).  Interestingly, both defects can be offset, or treated with sugar or milk and sugar, but if you extract to roughly 18.5-20.5 % YLD, most coffees taste naturally sweet w/o adding sweeteners.  

Likely Causes for Severe Under-Extraction
Regarding extraction yields, we can achieve nominal yields in virtually the entire range of from 16-26%, without much difficulty, when the brewing equipment, water and coffee are all nominal.  

In our experience, the most common reasons for cases of severe under-extraction (<= 17% Extraction Yields) have been more than one of the following conditions present at a particular site, as follows:

- Coffee was not fully developed at roast, the inner-most core of the bean is under-roasted.
Try some coffees others can normally achieve 20%+ extraction yields w/o jumping through hoops to get there.

- Brew water is soft, has very low TDS and little or no hard mineral content. Invest in a low cost conductivity meter (see SCAA.org member's store) for water TDS measurement, and use Hach strips or similar for Hardness and pH testing to get a an idea of the condition of your water chemistry. I recommend the new SCAA publication on water quality.

- Coffee is too fresh off roast, usually accompanied by excessive out gassing during brew (either coffee or espresso). Some high density, high elevation grown coffees take 2-3 weeks to out gas in the whole bean state before they extract normally. Out gassing continues for more than 100-days in such whole bean coffees. If you suspect this might be an issue, try grinding the coffee into a one-way valved bag, seal it and wait 2-3 hrs and then brew again using identical protocols, noting differences in measurements.

- Water temps too low, coffee too coarse, brew time too short, lack of a pre-infusion process to ensure that the entire “batch” is uniformly extracted. Try the Clever Dripper, using drip-grind, 60-seconds of pre-infusion and a 3-4 minute total brew time with "coffee-grade" water at 94 Deg C.

~Vince@VST                                                               Edit 3/19: corrected typo on temp
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gt
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Joined: 29 Jan 2007
Posts: 210
Location: Mpls/St Paul MN
Expertise: I like coffee

Espresso: None
Grinder: Virtuoso
Drip: Cones & CCD
Roaster: P1 w/ variacs
Posted Sun Mar 18, 2012, 4:52pm
Subject: Re: VST LAB Coffee Refractometer
 

eMoJo Said:

Final comments and suggestions

Just use 7.50g sugar and 293.70g DISTILLED (not tap) water. This should read 2.00 +/- 0.05% on the VST Coffee Refractometer at 20.0 +/- 0.10 Deg C.  

~Vince@VST

Posted March 18, 2012 link

Vince thanks for posting here, I've been wanting to ask about a calibration/check solution for a long time.

Could you add what the reading would be (theoretically anyway) if the solution temperature was say 19C and 21C.

Thanks
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eMoJo
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eMoJo
Joined: 19 Sep 2010
Posts: 5
Location: Boston
Expertise: I love coffee

Posted Mon Mar 19, 2012, 2:21pm
Subject: Re: VST LAB Coffee Refractometer
 

Hi gt:

You caught the typo's which have been corrected, thank you.

You can read that formulation at 20.0 +/- 1.0 Deg C.

Best ~Vince@VST
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Netphilosopher
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Joined: 14 Jan 2011
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Location: USA
Expertise: Just starting

Posted Sat Mar 24, 2012, 9:44am
Subject: Re: coffee refractometer
 

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