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The Perfect Cup?
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So much of coffee’s taste is subjective.  While I may love the crisp bite of a Brazilian espresso or the fruity notes of a washed Ethiopian, you may prefer the earth and spice of a Sumatran.  One thing that is less subjective is brewing extraction and strength.

In the 1960s, Dr. Lockhart of the Coffee Brewing Center developed the Coffee Brewing Control Chart, giving a graphical representation to your coffee’s strength & extraction.  This chart measures the percentage of the beverage that is coffee vs the percentage of the coffee solids that are extracted, with an ideal range of 1.15% - 1.35% strength and 18% - 22% extraction.

Download a .pdf version here

A coffee bean is about 30% soluble by weight.  Coffee (the final beverage) is made up of roughly 1-2% coffee solubles.  It is possible, with a little bit of work, to map your brew on the Brewing Control Chart.

Quick note:  In this post, I will be using the terms Strength & TDS, they can be used interchangeably.


Equipment Required
You’ll need some way of measuring the coffee’s strength, measured in TDS (total dissolved solids).  There are three common ways to do this:


Conductivity meters give an approximate reading of the dissolved solids content of a fluid by passing a small electric current through the sample.  Most display their readings in parts per million or milligrams per liter.  They are inexpensive, and the least accurate.  They can only measure some components of coffee, so their readings must be multiplied by a factor of 10.

  
Handheld Refractometers measure the refractive index of a fluid to determine its non-water content by passing light through the sample into a prism.  Most refractometers display readings in Brix units, and must be multiplied by .85 to reach a TDS reading.  Refractometers are very accurate, and inexpensive.  Readings can be greatly affected by temperature, so opt for one with Automatic Temperature Compensation.


Digital Refractometers work like handheld refractometers, but are more accurate, and much more expensive.

Preparation:
Before you brew, it is important to take some notes along the way.  You’ll need to measure and record:

  • the mass of the coffee you’re using (in grams)
  • the mass of the water you’re brewing with (in grams)
  • your grind setting and brewing method

Measuring Strength
After you’ve brewed your coffee, you’ll need to determine the strength or solubles concentration.  This is the percentage of the final beverage that consists of coffee solubles.

With a Conductivity meter:  Turn on the meter and wait for it to read 0ppm.  Insert it into your sample, and record the reading.  Multiply the reading by 10, this is your actual TDS, and divide this number by 1,000,000.  This is your strength in percent.

With a Handheld Refractometer:  Place a few drops of your sample onto the glass, and place the clear flap ove the glass.  Look through the eyepiece and note the reading.  If your refractometer displays readings in Brix, multiply the reading by .85 to get % strength.

With a Digital Refractometer:  Turn on the refractometer and wait for it to read 0.  Apply a few drops of your sample onto the glass, and record this reading.

Doing the Math

  • Multiply your TDS % by the mass of your water to find the mass of the extracted solubles.
  • Divide the mass of the extracted solubles by the mass of ground coffee to find the extraction percentage.
  • Plot your Strength % and Extraction % on the chart.

Conclusions
Mapping your brew cannot replace your taste buds.  You may find that what you like doesn’t fall into the ideal range.  Thats okay.  This is just another way to measure our brew against what has been determined to be the ideal cup.