When coffee is brewed, hot water removes most of the water-soluble material by a process that combines dissolving and extracting. The water first penetrates the grounds, dissolves some of the chemical components it encounters, makes a solution of these materials, and then exits the grounds to produce the familiar beverage.

We measure the solubles yield and solubles concentration of coffee because it relates directly to sensory studies of what constitutes acceptable beverage flavor. Solubles yield refers to how much soluble flavoring material is removed from the grounds. Solubles concentration refers to how much is present in the beverage. But measuring isn’t easy.

We measure the concentration of soluble coffee flavoring material dissolved in the beverage relative to the amount of water, and then graphically calculating the solubles yield given the specific coffee-to-water ratio used to produce the brew.

Good coffee is a delicate balance between strength (solubles concentration) and extraction (solubles yield). Strength and extraction are the two most important physical properties of a finished coffee drink. It is indeed important for an experienced barista to detect extraction and strength mishaps using their palate. But there are also tools available to help us process this.

TDS, which stands for total dissolve solids, refers to the amount of the soluble flavor material from the coffee bean that is extracted as part of the brewing process. Note that beans are comprised of roughly 70 percent insoluble material (like the cell wall fibers). The remaining 30 percent includes things that make coffee taste awesome, turn your water brown and deliver caffeine to you.

But there are also some things that can be dissolved but really shouldn’t, because it doesn’t taste all that great and can overwhelm the other stuff that you extracted that tasted really terrific. It’s usually bitter. On the other hand, extracting too little of the good stuff leaves a sour taste and doesn’t add any of the sweetness. The sweet spot is considered to be 18 to 22 percent.

Let’s further distinguish strength from flavor, or extraction. You obviously know the difference between strong coffee that puts hair on your chest and the weak stuff. Strong coffee has a lot of dissolved solids in it. Weak coffee, not as much. The ideal range for most people is a brew strength (TDS) of 1.15 to 1.35 percent.

Extraction is tied to flavor. This is what you pull out of the beans. When coffee is underextracted, it means that not enough of the flavor components are pulled out of the beans. When it’s overextracted, too many solids are pulled out. The best range of extraction is between 18 and 22 percent.

You can have a strong cup of coffee that’s underextracted, or a weak one that’s overextracted. That’s part of the secret behind some mega-chain coffee companies. They figured out that you can use less coffee by grinding finer and extracting more from the available grounds. The result, as you know, is pretty bitter.

Using the Coffee Brewing Control Chart

The coffee brewing control chart provides a simple, graphic representation of the inter-relationship between the three primary variables of strength, extraction and brewing formula. Together, these variables create the aroma, taste and body of the coffee beverage.

Strength is a measure of the solubles concentration. it is expressed as a percentage, and compares the amount of coffee flavoring material to the amount of water in the final beverage.

Extraction is a measure of the solubles yield. It is also expressed as a percentage, comparing the amount of coffee flavoring material in the beverage to the amount of coffee grounds used to prepare the beverage.

Brewing formula is expressed as the ratio of coffee to water used when brewing the coffee.

Strength (Solubles Concentration)

Coffee is a pretty potent flavoring agent. A typical coffee beverage contains about 1.2 percent coffee flavoring material and 98.8 percent water. The weakest coffee acceptable to the average person is about 0.5 percent coffee and 99.5 percent water. The strongest acceptable beverage (excluding espresso) is about 1.8 percent coffee and 98.2 percent water.

The left-hand side of the coffee brewing control chart indicates strength. On the coffee brewing control chart, strength can be expressed as both a percentage and in total dissolved solids. For example, if a measurement of the coffee brew resulted in a reading of 1.18 percent TDS, that would mean the brew contained 1.18 percent coffee and 98.2 percent water.

Extraction (Solubles Yield)

Approximately 28 percent of the organic and inorganic matter contained in roasted coffee will readily dissolve in water. The remaining 72 percent is cellulose bean fiber that isn’t water soluble under normal brewing conditions.

The water-soluble flavoring materials is caught up in the cellulose fiber of the roasted beans. Only after the beans are ground and immersed in hot water will the solid flavoring materials dissolve, changing into liquid and gases. The solid materials dissolve at different rates, which means that different solubles yields will contain different mixtures of liquids and gases. Each mixture of solubles yield will exhibit a different flavor.

Brewing Formula (Coffee to Water Ratio)

The coffee brewing formula is the weight of ground coffee to volume of water used in preparing the brew. On the coffee brewing control chart, the brewing ratio is often expressed as the weight of coffee required to prepare a standard-size batch. For example, grams of coffee per serving, assuming a 6 fluid ounce U.S. cup size.

Putting it all together

Once a coffee-to-water ratio has been established, one can draw a linear relationship between the strength of the beverage and the amount of soluble flavoring material extracted from the coffee. For any given brewing formula, increases in solubles yield will cause proportional increases in solubles concentrations; and decreases in solubles yield will cause proportional decreases in solubles concentration. Here’s an example:

Assume that one uses 10 grams of coffee and 6 fluid ounces of water to prepare a single serving. If 1.84 grams of flavoring material were removed (18.4 percent extraction), the reulting beverage would show strength of 1.18 percent TDS.

On the other hand, suppose it was determined the resulting beverage showed strength of 1.33 percent TDS. Then the yield of material removed from the grounds during the brewing process would be 2.08 grams (20.8 percent extraction) for this brewing formula.

Objective Measurement

It is possible to remove a large amount of flavoring material and dissolve it in a small volume of water. This results in a beverage with a very high concentration of solubles (espresso is a good example). Conversely, it is possible to extract a small amount of material and dissolve it in a large volume of water. That would result in a beverage with a very low concentration of solubles (tea is a good example).

Subjective Measurement

Coffee derives its flavor from two sources – aroma (gases) and taste (liquids). During the brewing process, aroma is extracted much more rapidly than taste. In fact, the gases are almost immediately driven out of the grounds when they come in contact with hot water. Although they are an important part of the overall flavor profile, volatile aromatic components don’t contribute to taste.

The taste components come from the soluble flavoring material removed from the coffee grounds that remain in a liquid form. Flavoring material that does not dissolve (i.e., transform into a liquid) cannot be tasted. It does, however, create mouthfeel and is referred to as the coffee’s body.

Compared to aroma, taste is extracted more slowly. Because body also slowly increases during the brewing period, taste and body are closely associated. That said, you can use the concentration of the dissolved flavoring material as a measure of taste and body build-up.

Balance Between Strength and Extraction

Solubles yield below 18 percent tend to have grassy and peanut-like tastes and are classified as under-developed tastes. Solubles yields above 22 percent tend to have unpleasant astringent and bitter tastes and are classified as over-extracted tastes. Extraction levels between 18 and 22 percent are ideal.

Concentration levels of soluble flavoring material have a broader range. Solubles concentration below 1.15 percent coffee TDS are considered weak. They tend not to present the flavor of the coffee at an intensity where all the components are above the taste threshold for the average person. Concentration levels above 1.35 percent coffee TDS are considered strong. They tend to present the flavor components at too intense a concentration to be easily perceived. For the average person , solubles concentrations between 1.15 percent and 1.35 percent are ideal.

Tools that Measurement Easier

Refractometers

Refractometers can be used for lots of things, like testing urine samples for medical purposes (or, fun? I don’t know) to reading the salinity of water in an aquarium or determining the potential conversion of sugars in alcohol before fermentation. It works by measuring the refraction of light shone through a substance as it sits on a lens. The refractometer can be used to measure the concentration of coffee solubles in both brewed coffee and espresso. The refraction can tell us precisely how much soluble coffee material a sample of the liquid contains.

VST CoffeeTools

The VST coffee refractometer is designed to measure coffee’s concentration by measuring its refractive index. When you plug that number and enter a few additional variables, like the amount of water and coffee, the VST CoffeeTools software indicates the brew strength and extraction yield. By plotting these on the brewing control chart, you know whether or not you’ve brewed within the appropriate range. You can also tell where you are in relation to the sour, bitter, weak and strong qualities of coffee. By knowing exactly where your coffee lands, you have an idea of how to get to the sweet spot.