When people discover that I’m a coffee roaster, they usually have one question: How about some free coffee? After that, I’m often asked how I can tell when the beans are done. The answer is a fairly complex combination of smell, sight, time, temperature, and several other factors. Today, however, we’re going to focus on the ‘sight’ aspect of bean development.
We spend a lot of time with all of the coffees that come into our warehouse deciding on the best roast profile for each bean. One visible mark of this roast profile is the outward color of the bean after it is roasted. This serves as one benchmark for the roasters to determine consistency and proper roasting. It is by no means the only factor we look at, but it is a quick and consistent way to make sure we’re on the right track.
As the bean is roasted, the color progresses through several stages the roaster watches to be sure that the process is proceeding smoothly. Once the bean reaches several parameters (time, temp and color are all factored in), it is removed from the roaster and the roasting process is stopped. We use a cooling tray for this with fans pulling cool air over and through the beans. Once the coffee cools, it is time to check it for color consistency, which is where the spectrophotometer comes in!
According to the Random House dictionary, a spectrophotometer [spek-troh-foh-tom-i-ter] is ‘an instrument for making photometric comparisons between parts of spectra’. Basically the instrument analyzes the color of the bean and assigns it a numerical value. We use this number as one way of checking the consistency of our roasts. It also allows us to discuss roast level using a common vocabulary.
She's a looker!
The specific instrument that we use is made by the Agtron company and thus we refer to a blends ‘Agtron number’ when speaking about the roast level. The analysis procedure is very simple and takes only a few minutes. First we calibrate the instrument using a set of specially colored plates. This way we can be sure that the Agtron is always starting with the same reference points. Once it is calibrated, we place a sample of coffee onto the analysis port. The coffee can be either ground or whole bean, but we usually use whole bean in day-to-day operations. It is important to check a few samples from the same roast, so we’ll check two or three to be sure. The Agtron then assigns a number to the roast; the higher the number, the lighter the coffee. For example, a very light roast that we would use for cupping would be in the mid 50 range, while a very dark roast (French roast or darker) would be in the 20s.
Small changes in the roast level translate to small changes in the Agtron number, but these can reflect very big changes in the character of the cup. While there is a bit of tolerance if the roast is not exactly the Agtron number we wanted, it is a very narrow band and most roasts are within just a few tenths of our target. If the roast falls outside of that tolerance then we know that we have to determine where the change originated: does the roaster need to be serviced, are the temperature probes installed correctly, is the airflow set correctly, is the weather affecting the roast times and so forth. Then we fix the issue and get back to roasting!
When roasting, consistency is king and we use all the tools at our disposal to ensure that each roast is as good as the last. The humble Agtron is an important aspect of that quality control and serves us well in achieving our goal. Now if I could just get it to give me some free coffee…
