Posted Wed May 16, 2012, 8:23am Subject: Brew Ratio and VST Refractometer Measurement
Here's a re-vamp of the brewing control charts. Generally, we set up a brew ratio, brew our coffee, and then use the VST coffee refractometer to determine %TDS. From here, we can infer extraction based on studies, or we can calculate it based on assumptions.
It turns out that the thinking of brewing is essentially a set of closed form equations, where four dimensionless parameters can be used to get a very good representation of the process. In general, we're first concerned with consumption strength and also "extraction". We control extraction by the way we brew the coffee, and we control the strength at that extraction by altering the brew ratio.
Since the "optimum" extraction range seems to be fairly agreed upon, generally between 18% and 22%, with the "sweet spot" somewhere between 19% and 20%, I figured we're only interested if our brews are over, under or optimal range extracted - so I included the expected range for the strength of produced coffee over a range of brew ratio.
I'm attaching two charts, because immersion methods (like press pot, aeropress, CCD, cold brewed, or basically any method where there is end-strength coffee in contact with the grounds at the time of filtering) require an adjustment to how extraction is calculated.
Attached is the brew chart for percolation methods (methods where the brew water passes THROUGH the coffee only once, and filtering is continuous or semi continuous):
It is based on the relationship of
Strength = Extraction / ((1/Brew Ratio) - Absorption), where Absorption is estimated to be 1.76
(Click for larger image)
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
Posted Wed May 16, 2012, 8:33am Subject: Re: Brew Ratio and VST Refractometer Measurement
This all comes from the simple relationship of:
Produced Coffee = Brew Water - (Absorption * Coffee)
Absorption is an estimate of how much the coffee grounds gain in mass during the brew process. Brew water is the mass of the water used to brew the coffee, and Coffee in this context is the mass of the dry initial ground coffee used to produce the coffee.
Further Definitions:
Brew Ratio = Coffee / Brew Water
TDS = "Total Dissolved Solids", or the solids that are generally thought of as "extracted".
%TDS or Strength = TDS / Produced Coffee
and Extraction = TDS(in mass) / Coffee
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
Posted Wed May 16, 2012, 1:43pm Subject: Re: Brew Ratio and VST Refractometer Measurement
I like this, but I don't fully understand it. Where to start? Hmm, how about here: let's say I hold brew ratio constant at 6.25 (which corresponds to a 1:16 coffee-to-water ratio). Comparing both charts--which I'm not at all sure is kosher to do--it appears I'll get a stronger cup in terms of % TDS at 20% extraction using a percolation method than I would using an immersion method (~1.4% vs. ~1.2%, respectively). Is that right? Seems counterintuitive. Feel free to set me straight! ;)
Posted Wed May 16, 2012, 2:17pm Subject: Re: Brew Ratio and VST Refractometer Measurement
jbviau Said:
I like this, but I don't fully understand it. Where to start? Hmm, how about here: let's say I hold brew ratio constant at 6.25 (which corresponds to a 1:16 coffee-to-water ratio). Comparing both charts--which I'm not at all sure is kosher to do--it appears I'll get a stronger cup in terms of % TDS at 20% extraction using a percolation method than I would using an immersion method (~1.4% vs. ~1.2%, respectively). Is that right? Seems counterintuitive. Feel free to set me straight! ;)
Yep. You got it. Seems counterintuitive, so that's why I've spent the last several weeks trying to figure it out. I actually stumbled onto this because I was unable to achieve the strengths one would expect using immersion at brew parameters that should be (and taste like) decently extracted coffee.
With percolation, the brew water is constantly washing through the grounds, and at the end of the brew cycle the grounds in the basket mainly contain weak coffee with undesired dissolved solids. So, if you use an auto drip with 500g of brew water, and 40g of coffee - and you get about 420g of coffee at 1.9% strength - the liquid in the brew basket is weak-strength coffee with basically the nasty parts of the extraction that you didn't want anyway.
But, if you make a press pot with 500g of brew water and 40g of coffee, the moment you press the plunger - if you have extracted 20% of the coffee, it's all in the solution mixed with the grounds - 500g of water with 8g of dissolved solids - or a strength of 1.6%. When you press the plunger for the press pot - that's what you get - about 400-420g of coffee (properly extracted) but at a weaker strength of 1.6%.
In many ways, percolation methods are like a long, drawn-out version of espresso. You've got an extraction going on, and you want to terminate the extraction just before it really starts extracting the nasty stuff - with an auto drip you may control this with grind and the amount of brew water (less water means the brewing stops earlier, or coarser grind slows the extraction rate over the brew cycle).
With immersion methods, while I'm not done investigating it, indications are that we can control the extraction at very long contact times by controlling just the grind level.
------------------------------------------ ----------------------------------------- Le café doit être noir comme le diable, chaud comme l'enfer, pur comme un ange, et doux comme l'amour.
"There is no right answer with coffee. There is only the elixir in your cup at the moment you partake."
"...I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;..." - Lord Kelvin RECIPES thread => http://www.coffeegeek.com/forums/coffee/machines/585708
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