TDS, Agitation, and the Good and the Bad


(j l) #1

Assertion 1 : Refractometer useful as an overall gauge of extraction, but not so much for the ideal “perfect” cup.

Experiment : Pick a brew method, strong agitation at the beginning of the brew and “none” at the end. Brew again with same recipe, however with “none” at the beginning but heavy agitation at the end.

Goal is to have TDS with the same result of both approaches, then compare flavor profiles. Good or bad not relevant, if different, then what? How is this information used to guide refined brew recipes?

If not different, then Assertion 1 is wrong?

At issue, is extraction of the good vs. bad vary throughout the brew or just an end brew concern?


(j l) #2

The reason issue came up, is “pulling” the coffee cup while water is still flowing through. Good or bad practice? Very dependent on above “Experiment”, if “Bad” is at the end of the extraction, then pulling the coffee cup while water still flowing is largely OK. If however, “Bad” can happen throughout, then this means “Good” can as well. Which means the “Good” is mixed with the “Bad” even at the end so pulling the coffee cup away while brew water still flowing is essentially, a bad practice. In other words, a last ditch attempt to fix a bad brew recipe.


(Mark Burness) #3

Assertion 1 is essentially correct, but also a straw man fallacy.

Extraction yield (whether measured via dehydation, or with a more portable & faster coffee refractometer) is an objective measure of brew efficiency & can then be used to determine consistency brew to brew. From the earliest days of extraction yield, brew charts carried the caveat that it was not an indicator of quality…in the same way grinders, thermometers & scales cannot indicate the quality, nor taste of a brew, they can only work within their remit. You can ruin a brew with over agitation, even at a ball-park extraction, as tiny particles (not counted towards extraction) can get into the beverage & interfere with sweetness & clarity.

Your second post talks about “good” & “bad”. Extraction yield measures how much total & makes no distinction between good/bad. That’s up to the barista/customer and researched targets that are used as a guide, but not as a limiting factor (you can be happy outside ‘the box’). If your brews always taste better by leaving some liquid in the brewer, go ahead, I’d be keen to know how you determine how much to leave. You could also try grinding coarser & letting the brew drip out, this will be highly repeatable for a given brewer & recipe if the brew inputs are also consistent.

Do you have some way of observing & reliably identifying the “good” & “bad”, in a way that we can easily follow & replicate? It would be wonderful to make every brew ideal & perfect, irrespective of quality & roast, if anyone is actually doing it, they’re not letting on. If someone can do this, but not pass on how it is achieved to others, then it seems a bit pointless.

Very few people reading this will have been brewing coffee in a pre extraction yield world, the coffee refractometer is relatively new, but the paradigm is not.


(j l) #4

Awesome Mark thanks for the response! To continue.

Given assertion 1, then I don’t really see the point in competitions taking TDS measurements.

My own experience has been that agitation in the beginning is a “good” thing, agitation at the end is “bad”. Anytime I have lifted the dripper with water remaining, has always been a sub-optimal experience. For anyone who has a Sette grinder, you know what I’m talking about.

My own non-scientific “feely” argument has been that toward the end, the coffee grinds have been fully penetrated and expanded by the water. So any agitation, has a stronger impact on extraction meaning that the timing “window” for the optimal brew becomes much shorter with agitation at that time.

Ilustration by extreme, would be to heavily agitate the entire brew continuously, with same TDS as above experiment and determine “quality” of the varying cups. While certainly subjective, might provide some hints of agitation extracting the “better” at the right time and possibly minimizing the “bad”.

Of course, without the water fully penetrating and expanding the grinds, efficiency of extraction likely affected. Flip side, agitation might accelerate water penetration and expansion so that further slow and controlled flow rate efficiency is maximized. For Good or Bad… dunno.

For me, my brews have greatly improved by stronger pours initially and low force pours at the end. What doesn’t work as well, is maintaining a constant rate of pour for the different pulses. This method assumes a certain inherent “goodness” around uniformity, which while helpful for hitting the mark for consistency(a good cafe goal), is a meaningless goal for the home if the opportunity for the “perfect” cup is sacrificed for cafe efficiency goals.

Also, with the Barista Hustle subscription, I am now exposed to coffee that can really push extractions with great results so the rules for those types are different for the usual (but Great!) coffee roasters I have access to in SoCal.


(j l) #5

To add further with a completely non-related example of life :grin:

Where I live, you have to drive up to a gate that will let you out. For FIVE years, I knew that for the gate to open, I had to pull up very close for it to trigger, within 3 feet.

Then one day, epiphany happened. I pulled up to the gate 10 feet away and just waited. After a small delay… the stupid gate opened… :anguished:

So what I thought, wasn’t. Though I had FIVE years of PROOF… I think, that something like that is going on with coffee brewing, I just don’t know where this is happening.


(Karl Norman) #6

@homecafe “Assertion 1” is almost certainly true because the refractometer doesn’t measure the contents of the coffee, just the light passing through it. You can achieve the same TDS on two different brews, but it doesn’t mean the same compounds were extracted in each. If you look at agitation as the application of kinetic energy, then doesn’t it make sense that the product of agitation is dependent on the relative extraction level of the coffee. i.e. agitate at the beginning, when the coffee is relatively un/der extracted, and you yield one thing; at the end, when the coffee has had more time to extract, and the effect of kinetic energy is different.

So, is it like hitting the boost button in a race? Do you punch it right at the beginning to get off the line faster, or do you hit it at the end to pull ahead?


(j l) #7

What I use when guiding my brewing, the act of agitation is more to introduce uniformity to the bed for more even extraction. This is particularly important at the bloom stage when the water is not evenly distributed through the grinds. Now having said that, an aggressive bloom pour can do this, however the water tends to filter quickly through the coffee grinds which sort of reduces the value of the bloom.

Agitation is only performed to achieve bed uniformity, anything more is non necessary variable.

I would change your analogy a bit though. Introduce the finish line(optimum brew) as a brick wall. Slower extraction times at the end mean easier time of stopping just before crashing through the wall. Agitation at the end means breaking through the wall to bitterness on the other side.

The completely invented in my head model, is that the coffee grind has an initial tight structure which gradually expands AND loosens, allowing for faster extractions of whatever is there. Hence a very small window at the end for maximum extraction before hitting bitterness.

I brew to have maximum “forgiveness” at the end so to have a wider window of brew time before getting smacked in the face by bitterness. Cooler water middle toward the end, very light pour as well. More aggressive at the beginning.

Anyways. the grinds ability to allow extraction of whatever is there, I don’t think is uniform due to cellular wall structure loosening throughout.

Again, fair warning, is completely invented. Just a way of describing the approach to how I brew. :smiley: