The impact of filtering on refractometer TDS and extraction yield measurements - Page 2

Beginner and pro baristas share tips and tricks.
CwD

#11: Post by CwD » Feb 14, 2019, 4:08 pm

catalinH wrote:What are the chances when I take 3 readings of TDS and they all vary less than .05%(eg 9.72,9.73 and 9.76) and I pick the lowest one each time that all of them are wrong every time?
This means that all my samples are murky, filled with fines that artificially increases TDS seriously overestimating EY.
Almost certain. The variation within unfiltered samples is much smaller than their distance from the "real" readings. Even the very lowest unfiltered samples taken were nowhere near the highest filtered readings. Your actual distance may vary depending on grind (mine is extremely fine, which may push my filt/unfilt difference to the higher side) and coffee, but it's probably not what anyone else is measuring as 25%.

Also reading the same >25% consistently doesn't make it any more real at all if you're consistently failing to measure it with the correct processes, because the error is consistently in the direction of mensuring far too high. And other people measuring 25% or 27% or whatever percent shots while using proper methods just isn't relevant to what yours measure in any way.
namelessone wrote: Centrifuge seems good enough for espresso, perhaps not for cupping QC or similar activity (0.05% TDS is still significant at immersion ranges).
Not really seeing that. The precision and accuracy wasn't any worse than VST filters, and I don't see any reason to think we'll be getting the same actual "numbers on the display" systematic difference rather than similar margins as a percent of the actual reading, which would put it below the resolution of the .01% screen. I'll probably do a whole experiment for it eventually, but I'm not planning it very soon since it's going to take like $80 worth of VST filters and I don't really like any immersion methods in the first place.

Maybe in the next few days I'll do a quick one with just like 3 trials instead of 20 just to see if it holds up to basic scrutiny, and can set up a big test later.

catalinH

#12: Post by catalinH » Feb 14, 2019, 4:55 pm

I'll start filtering from now on and see how it goes.
Another thing worth mentioning is that my high extracting shots are pulled with crazy long PI and the total shot time is somewhere between 60 to 75s.

CwD

#13: Post by CwD » replying to catalinH » Feb 14, 2019, 4:59 pm

Most of mine are similar. I did a 15sec preinfusion followed by a 45sec bloom for every shot in my trials. It does help extraction, but I think this kind of espresso is also especially susceptible to problems from not being filtered as there's more sediment that can settle on the lens vs a shot that's ground coarser for a 25sec total brew time. I didn't switch to more standard shot styles than my usual for the experiment, as I feel a good filtration method should work even when I'm throwing the most difficult things at it.

nuketopia

#14: Post by nuketopia » Feb 14, 2019, 6:23 pm

I strictly filter refractometer samples, as VST recommends. I find this the best way to normalize results across different preparation methods, such as filter drip, Aeropress, French press and espresso.

The presence of particulates varies by prep method. Different types of espresso baskets may have somewhat different holes. Paper filter methods of generally remove a great deal of fines, where metal filters not as much and press-pots not at all. Different grinders produce different distributions of particles, even the same grinder at different settings, and of course, beans also very tremendously.

It is user's choice whether to filter. But one can't compare filtered readings to unfiltered readings. There's no way to "scale" them as there's no way to determine the presence or distribution of undissolved macro/microscopic particles nor the effect of such on any particular measurement device.

I know if I take readings that are prepped and filtered that most of the variability is removed and I'm measuring more of what is changing in terms dissolved solids and anything over the micron size of the filter is eliminated.

jpender

#15: Post by jpender » Feb 14, 2019, 7:13 pm

nuketopia wrote:It is user's choice whether to filter. But one can't compare filtered readings to unfiltered readings. There's no way to "scale" them as there's no way to determine the presence or distribution of undissolved macro/microscopic particles nor the effect of such on any particular measurement device.
It is possible to determine the ratio of dissolved to undissolved solids by using gravimetric methods coupled with filtration/centrifugation. I've used this approach to measure the undissolved solids in some Aeropress and moka pot coffee. Of course the specific value isn't constant for a given brewing device so it's not possible to have a fudge factor by device that can be depended upon to high accuracy. But within very limited contexts it would work reasonably well, at least with dehydration. There would still be the uncertainty of exactly how the refractometer would behave though.

Microfiltration is expensive. It's good to read once again that centrifugation works at least very nearly as well.

nuketopia

#16: Post by nuketopia » Feb 14, 2019, 7:19 pm

jpender wrote:It is possible to determine the ratio of dissolved to undissolved solids by using gravimetric methods coupled with filtration/centrifugation. I've used this approach to measure the undissolved solids in some Aeropress and moka pot coffee. Of course the specific value isn't constant for a given brewing device so it's not possible to have a fudge factor by device that can be depended upon to high accuracy. But within very limited contexts it would work reasonably well.

Microfiltration is expensive. It's good to read once again that centrifugation works at least very nearly as well.
The point being that one simply can't take an unfiltered reading, apply some sort of scaling factor to it, and proclaim it the same as a filtered reading.

Yes, additional laboratory means can determine how much undissolved solids are present. The effort and expense to do so, would likely be far more than snapping a filter onto a syringe. :)

I should think reasonable centrifuging would also be an acceptable substitute for micro-filtering.

The little disposable VST filters are simpler to deal with in a food service or home setting.

But we can't reasonably apply some simple numerical adjustment to convert filtered/non-filtered readings.

CwD

#17: Post by CwD » Feb 14, 2019, 7:39 pm

nuketopia wrote:I strictly filter refractometer samples, as VST recommends. I find this the best way to normalize results across different preparation methods, such as filter drip, Aeropress, French press and espresso.
I should note that VST explicitly recommends that you do not use their filters on paper filter methods. Page 8 of the manual in bolded text.

It doesn't seem to change the reading anyway, but it's wasteful and VST does not recommend it.

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AssafL

#18: Post by AssafL » Feb 14, 2019, 8:44 pm

I think filtration to achieve precision is very clever. Too bad it is at a cost of accuracy. So much spent on accurately measuring RI... accuracy because A dehydrator can rarely ignore particulates from a suspension by size. So it will measure all that is in the cup (for filter and espresso).

Probably a better way to achieve both accuracy and precision would be to use a homogenizer. Then one would get a similar figure to what one would get with dehydration (since dehydration does not readily negate larger particulate that a filter does. )

I do not know if lab homogenizers can work with low espresso volumes.

I always wanted one, though. It is my understanding they disperse hydrocolloids perfectly.
Scraping away (slowly) at the tyranny of biases and dogma.

jpender

#19: Post by jpender » Feb 14, 2019, 9:38 pm

CwD wrote:I should note that VST explicitly recommends that you do not use their filters on paper filter methods. Page 8 of the manual in bolded text.

It doesn't seem to change the reading anyway, but it's wasteful and VST does not recommend it.
The recommendation is for drip grinds and implicitly refers to gravity fed paper filtered coffee. The Aeropress, when used with paper filters, often passes a significant amount of undissolved solids, depending partly on grind size but even more so on how hard you press. Whether this shows up with a refractometer I can't say. But I've found that the espresso syringe filters are overkill for clarifying AP coffee. Less expensive 0.7um work well enough.

jpender

#20: Post by jpender » Feb 14, 2019, 9:41 pm

AssafL wrote:Probably a better way to achieve both accuracy and precision would be to use a homogenizer. Then one would get a similar figure to what one would get with dehydration (since dehydration does not readily negate larger particulate that a filter does. )
The standard dogma is that taste correlates with undissolved solids, not total solids. And I'm pretty sure that VST validates their refractometers with coffee that is filtered first and then dehydrated.