The truth is, there is no good way to do this. Taste is the best way to decide and, not only does everyone have a different opinion about what good taste is, but the chemistry of coffee taste is still in its infancy. We don’t even know what to quantify!

That said, it isn’t like we haven’t come up with some proxies to help guide us. After all, if all we’ve done is a simple chemical extraction, then there ought to be measures of how successful that extraction was. There are two ways to approach this problem. One is to find what percent of solutes were removed from the coffee beans. The other is to measure what percentage of the coffee brew is composed of solutes from coffee beans and not made up of water. Then, all we need to know to make it work is what defines success.

Thanks to Dr. Lockhart, we have this information!

Lockhart determined that of the original mass of coffee used for brewing, most people preferred the brews when the extraction yield—the amount of coffee removed from the grounds—was between 18 and 22 percent. So, if we just measure that, will we have a good idea of whether the coffee will taste good? Yes! In practice, though, it is neither very quick nor practical, so for most people, it is merely a fun idea to think about. The problem is that you can’t simply weigh the coffee before you brew, weigh it right after, then divide the former by the latter and subtract it from one. This is because the coffee grounds absorb a substantial amount of water and the added weight throws off the calculation. Instead, you must first slowly dry the coffee grounds in an oven (for about twenty-four hours).

Then you can weigh them and add the value into your equation. So, it is doable, but not terribly practical.

Measuring the amount of solutes in the brew is also tricky. To perfectly measure the total dissolved solids (TDS) in the brew, you would have to follow a similar procedure: weigh the total amount of brew, evaporate off the water, and then weigh the solids that are left behind. After some quick number crunching, you could see if your coffee fell into Lockhart’s range of 1.15 percent to 1.35 percent. 

Did you know?

Coffee is not the second most valuable or traded commodity behind petroleum, by any metric.

Fortunately, there are two quick ways to estimate the TDS in water/coffee. All that is required is the right instrument and the correct calibration. Of course, you probably don’t have a conductivity meter or a refractometer at home, but it doesn’t mean they wouldn’t work if you did have one! Actually, some of these kits aren’t too expensive and there are some made specifically for coffee. So, if you’re really keen on having such a toy, they are pretty easy to find.

Pure water conducts a very tiny amount of electricity. However, water that contains ions can conduct electricity quite well (standing in a puddle + lightning = bad). Ions are electrically charged particles that naturally occur. For example, table salt (sodium chloride, NaCl), when dissolved in water, dissociates into its ion components: Na+ and Cl−.

Because of their electrical charges, electricity can pass through them readily. The greater the ion concentration is in the water, the greater the electrical conductivity will be. Thus, by measuring the conductivity of the water, you can get a sense of how many ions are in it.

Note, if nonionic species exist in the water, they won’t register electrically. 

This works for coffee, of course. However, like with any such measurement, you need to have a calibration curve to translate the value for electrical conductivity into TDS. I, for one, don’t derive any meaning from a conductivity of 2 mS/cm! Doing this is fairly simple; you just have to plot a graph where the x-axis is conductivity and the y-axis is TDS. You create this graph by measuring the conductivity of several solutions (or brews) that are known to have different TDS (say, by making several cups of weaker and stronger coffee). Hold these values on the x-axis. Then, dry down the brews as described above and once the TDS is known, use the x-axis values to plot against these y-axis points. With three to five points, you’ll have a curve (which is actually straight for a good portion of the curve that interests us) that is represented by an equation. That equation is your calibration curve. For any x-value you measure, the equation will produce the y-value TDS!

Did you know?

In 1869, Hemileia vastatrix began the decimation of coffee production in Ceylon (now Sri Lanka), the third-largest coffee producer at that time.

Now, it is important to measure the TDS by drying down the coffee. As mentioned above, nonionic species won’t register. If we don’t measure the TDS accurately this way, we’ll never have a true correlation between conductivity and TDS because we won’t ever be accounting for those nonionic species!

The last little trick with measuring TDS via conductivity is time. If you leave a liquid in the open air (like most of us do with our mugs), it will absorb some carbon dioxide from the air. When this happens, some of the carbon dioxide molecules react with water molecules to become carbonic acid. As acids are ions, this changes the conductivity of the water. So, if you care a lot about the accuracy of your TDS measurement, then do it quickly! The other trick is temperature; conductivity of a liquid changes with temperature.

So, for readings to be comparable, you must either always take readings at the same temperature or use an instrument that measures and accounts for temperatures.

Refractometers can also produce values for TDS. They measure the direction in which light moves—its refraction—through a liquid. If you shine a light on a glass of water, it never comes straight out; it always bends a little. If there are dissolved molecules in the water, the amount of bending changes. You can use this bending to calculate the amount of TDS in the liquid. Of course, you need to have a calibration curve to make sense of the reading. Fortunately, refractometers aren’t influenced by the absorption of carbon dioxide in the same way TDS meters are. However, their readings are heavily influenced by temperature.

So, there you have it, the knowledge necessary to measure the TDS in your coffee brew. All you need is an instrument and calibration curve (which likely is already built into or calculated by the instrument). Of course, once you know you the TDS of your brew, you need to calibrate that number to your personal preference for the brew.

Otherwise, what the heck does TDS mean? 

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