carsonbdavis Senior Member Joined: 7 May 2014 Posts: 1 Location: Tampa, FL Expertise: Pro Barista
Posted Wed May 7, 2014, 10:56am Subject: TDS Meter
Hey! So a couple of shops in my area have been bragging about their TDS readings, I have hardly heard of anyone doing this before so I tried doing some research. I can't find much accept one article on coffeetimeuk. It said that TDS meter is a waste because it reads the ions not the actual coffee as it's made up of sugars, lipids and organics which the meter won't register. So my question is this. Is using a TDS meter a viable practice or is it mainly a fad right now.? As quality control control supervisor and trainer for a roaster should I use it?
MWJB Senior Member Joined: 1 Jun 2013 Posts: 178 Location: UK Expertise: I like coffee
Grinder: Rocky, Lido, Porlex, Hario... Drip: Not enough room to list...
Posted Wed May 7, 2014, 1:38pm Subject: Re: TDS Meter
Firstly, I think it's worth identifying what it is, in terms of the device, we are talking about.
A TDS meter is usually a conductive meter that people use with a multiplier on the reading, in order to get a coffee %TDS based reading. The problems I experienced with a TDS meter were a large variation in reading depending on beverage temperature (despite claims of being accurate from 0-50C). Some claim to be able to cope with undissolved solids in the brew, but typically you need to make sure the samples are filtered (as you do for all methods of yield analysis). I also found that for a consistent protocol, the readings drifted considerably, over several identical brews (or as close as I could manage) you had to average out readings. The meter needed a significant, if not huge sample to work & readings could vary depending on the depth of the liquid below the meter.
Additionally, the TDS meters are not supported by any software to extrapolate the reading into an accurate extraction yield (the whole point of taking a TDS reading in the first place, a nominal extraction could be anywhere from, say, 1.15% to 15%TDS depending on the brew method). There are paper charts that can be used for a ball-park reading, for certain manual brewing conditions.
If you make 2 similar brews, you might be able to determine which of the 2 was more concentrated than the other, but your reading may only be relevant to your immediate environment & not directly comparable to other readings you see elsewhere.
It is more likely the shops that you are discussing are using the VST coffee refractometer & software, which determines extraction yield, from a TDS reading. It is based on the previously established concept of yield analysis, but updated to reflect modern brewing practices and with certain innaccuracies/assumptions corrected.
From the 50's onwards (so, accepting it's a 60yr old paradigm, not so much of a fad?) they use(d) dehydration ovens & dessicators to drive off the liquid in the filtered beverage until they just had the dry mass of the dissolved solids & could weigh them on lab scales to determine extraction yield. This took time and a great deal of care. The VST coffee refractometer takes a few drops of coffee & a minute to get a reading, the accompanying software translates that reading to a yield. It's more accurate & expensive than a TDS meter, but cheaper & much quicker than lab standard dehydration.
Extraction yield is the amount of dry coffee dose that is dissolved into your beverage, many people find they have a preference in this respect, somewhere between sour/acidic through sweet/balanced, to bitter/drying (normally, but not exclusively, 18-22% yield). Knowing your yield can be useful in determining a preference region, or by suggesting a path for corrective action, if that is not obvious by tasting alone.
TDS is the mechanism by which we establish this - on it's own, TDS doesn't tell you much. Extraction yeld is an objective measurement that can be applied accross different coffees, in terms of desirable flavour balance. It's not a substitute for tasting, you have to corroborate the reading against taste. Think of it like many other brewing tools like scales, thermometers, etc.
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