Joejoe wrote:A) Put in 15 grams of beans in a basket then grind it to a certain grind that let's you pull out 20% TDS of the beans in say 30ml of water, 50% ratio
B) Then put in 10 grams of beans in the same basket and grind it to a certain grind (finer) that would also give you 20% TDS in 20ml water, same 50% ratio. (granted this would be difficult I suspect as it would have to be perfect)
You should have identical drinks, just drink B would be shorter, but this is not the case.
So I am trying to wrap my head around why 50% ratio of 20% dissolved solids can taste different.
First, your example mixes apples and oranges. Brew ratio is the weight of the grounds divided by the
weight of the beverage, not the volume of the beverage.
Second, it's not correct to say, "pull out 20% TDS of the beans". %TDS (total dissolved solids) measures the percentage of your beverage that consists of dissolved solids. 20% TDS would mean that 20% of the weight of the beverage consists of dissolved solids from the coffee. Incidentally, this would be a *really* bad cup of espresso: typical %TDS measurements for espresso should be in the 9%-14% range, give or take a little. By way of comparison, typical %TDS for brewed coffee would be in the 1.2%-1.5% range.
%TDS tells you how strong the coffee is -- i.e., the ratio of dissolved solids to water. But it doesn't tell you whether the dissolved solids in the cup represent an optimum extraction from the grounds (i.e., how much of the bean weight was extracted.)
You may be confusing %TDS with % extraction yield. The latter is the percentage of bean weight that has been extracted. Research studies have indicated that most people prefer coffee made with an extraction yield of between 18%-20%, give or take a little. That's where the sour and bitter flavors will be in the best balance. In other words, you want 18%-20% of the weight of the grounds to end up in the cup.
The % extraction yield tells you whether the coffee is going to taste balanced or not. The % extraction yield can be determined in the lab by dehydrating the espresso or coffee, then weighing the remaining solids to determine their percentage of the starting dose weight. This is a tedious and difficult (and smelly) undertaking, but there's a better way: You can compute the % extraction yield if you know the %TDS, which can be measured with a refractometer, along with the starting dose weight, the starting water weight and the ending beverage weight. VST has computer programs that do this: ExtractMojo and MojoToGo.
Back to your example. Let's restate it using the correct terminology:
Peppersass wrote:A) Set the grind so that 15g of grounds produces 30g of beverage in 25-35 seconds and produces an extraction yield of 19.5%. This will be a brew ratio of 50%, right in the middle of the Normale range. According to MojoToGo on my iPad, the %TDS for that pull will be about 9.7% (beverage weight will actually be 30.2g).
B) Then put 10 grams of beans in a basket with the same properties, adjusting the grind so that you still have a 50% brew ratio (20g of beverage), %TDS of 9.7% and an extraction yield of 19.5%.
In theory, these two cups should taste exactly the same. The only difference is that there's more coffee produced with brew formula A. This shouldn't come as a big surprise. When you brew coffee, you adjust the amount of grounds based on the amount of coffee you want to produce (full pot, half pot, etc.) Typically it's not necessary to adjust the grind because a dose change from a full pot to a half pot doesn't change the flow rate all that much. You might have to make a small grind adjustment between a full pot and for a single-cup pourover, though. The point is, with espresso these adjustments have much more impact on the flow rate and extraction yield.
I said "in theory the two cups should taste the same". In some ways they will. Because the brew ratio and %TDS are the same, they should be of exactly the same strength. Further, because the extraction yield is the same, the acid/bitter balance should taste about the same. But just because the strength and acid/bitter balance are the same, it doesn't follow that the varietal and roast flavors have emerged to the same degree, and therefore the cups could taste quite different to a sensitive palette. This is because these flavors are highly sensitive to the grind setting, which is different for the two different dose sizes. It's also possible that the different thicknesses of the puck, and resulting longer or shorter travel for the water, cause somewhat different extractions.
This suggests that a good extraction depends not only on getting the right percentage of dissolvable solids out of the beans, but also getting the right compounds out of them. While we have a pretty good understanding of the former and how it affects balance, I don't think we know very much about the latter -- i.e., for any given coffee, which compounds in which ratios produce the unique and sometimes subtle varietal and roast flavors we seek.
On your second post (the one after the one with the A and B examples), it's not just grind that determines the extraction yield. It's also the dose. Together, they determine the flow rate. The flow rate has a big impact on the acid/bitter balance.
Grind affects the extraction in multiple ways, which is why it has such a big impact on flavor. First, the amount of surface area affects which compounds will be extracted and in what quantities. Second, the density of the grind, and particularly the percentage of fines produced, affects the flow rate. Dose has a very direct impact on flow rate (thicker puck, slower flow rate), and in turn the acid/bitter balance, but it also can have an impact on the more subtle flavors, perhaps because of the water path through the puck.