The only way to accurately describe the roast level of a coffee bean is to use a color densitometric analyzer. There are several color densitometric analyzers that will measure the color of roasted coffee beans. They then produce a number that relates to just that degree of roast. The most common numbers in use for coffee are the Agtron numbers.
The Agton Gourmet Scale ranges from 0 to 133. A score of 0 is pure carbon, a score of 133 is slightly roasted. Agtron numbers are used to measure the outside of the roasted bean as well as the roasted grounds. The two numbers are then averaged. Sometimes both numbers are reported, so a 55 medium roast coffee might have an outside number of 52 and an inside number of 58 which could be reported as either 55 or 52/58.
An Agtron number lets the consumer know what to expect when they open the bag. Here is a list correlating the Agtron numbers to the standard roast terms.
70 Light Roast
61-70 Medium Light Roast
51- 60 Medium Roast
41-50 Medium Dark
25-34 Very Dark
>24 Extreme Dark
If you like light roasted coffee, you should look for an Agtron score over 61. If you want a dark roast coffee, you should pick an Agtron number below 41.
You don’t often find Agtron numbers on coffee bags. Sometimes you can ask the roaster. Laura has decided to begin putting the Agtron numbers on each of our coffee bags. You will see a handwritten code like 52/54 on each bag. The first number is the roast level of the outside of the bean. The second number is the roast level of the grounds.