March 4, 2019

Using UV Light For Quality Control in Coffee Roasting


Consistency is key in coffee roasting. Keeping all of your variables as uniform as possible helps maintain a level of quality. Ultraviolet (UV) light is one tool that you can use to help achieve consistency.

Read on to find out how to use UV light to identify damaged green coffee beans and improve the final quality of your roasted coffee.

Learn more in How Can Roasters Ensure Quality – In The Roastery & In The Café?

Green coffee with deffects readdy to be sorted

Defective green coffee beans. Credit: Ana Valencia

What Is UV Light?

The UV range of the spectrum is invisible to the human eye, but black lights allow us to use UV to highlight some substances.

Black lights are made in the same way as incandescent or fluorescent lights, but the glass casing or coating uses different materials. This means that most of the light emitted is UV light with just a bit of visible light in the wavelengths closest to the UV spectrum (indigo and violet). That’s why black lights usually appear dark blue or purple.

Under a black light, white clothing and other items glow in the dark. This is because they contain phosphors, or substances that absorb energy and re-emit it as visible light.

black lights are used for coffee quality control

Black lights. Credit: Rich Smith

How to Use UV Light For Quality Control

But what does UV light have to do with coffee? As a roaster, I’m always looking for ways to improve consistency and quality. One way I do this is by using UV light to find damaged and irregular green beans before roasting. I had heard of others using this technique and was curious, so I bought a US $2 black light and did an experiment. You can do the same.

  • Preparation

I started with two sample lots: 350 g each of the most expensive beans I buy and the same amount of our bestselling beans. Both were natural processed. You may want to compare two grades of beans or simply pick any that you’re curious about. Lay them out in a baking sheet or tray.

Now, turn the lights off and sit down. You don’t want to stumble around in the dark and risk injury. Take a few minutes to allow your eyes to adjust.

some coffee samples ready to be expoused with black light

Preparing the samples and testing the black light. Credit: Mary Beth Stephens

  • The Technique

Don’t look directly at the black light and shield your eyes from its rays. If you do happen to look at it, wait for your eyes to re-adjust.

Raise and lower the light above your sample. Slowly move the light up and down until you find the perfect spot. Different types of coffee will look different under the UV light but within a batch, all will appear the same apart from the bad beans.

When you have the right level, it will be obvious. Bad beans absorb the UV light differently and will stand out clearly. I believe that the change in color between a good bean and a bad bean is caused by variations in drying and processing. It seems that beans with less moisture glow more brightly. Pick out the bad beans.

some beans glow brighter under UV light

Flawed beans glow brighter under UV light than the rest of the sample. Credit: Mary Beth Stephens

  • Evaluation

When you’ve picked out all the bad beans, turn the lights back on and evaluate what you’ve got. I found that the ones I separated out had visible flaws. These included broken beans, undeveloped small beans, ones infested by boring beetles, and some that were cut or split by machinery.

In daylight, I would not have seen these flaws without careful inspection. Using the black light helped me pick them out more easily.

some green coffee with deffects detected using uv light

Flawed beans found using the black light technique. Credit: Mary Beth Stephens

The Importance of Removing Bad Beans

Why does it matter if a couple of broken or small beans make it into the roast? Because it only takes one defective bean to ruin your coffee.

Some may argue that quakers (unripened coffee beans) and other flawed beans add nuances to the cup. But a successful roast involves evenly heating each and every bean at the same time. A broken or flawed bean will not absorb heat at the same rate, causing it to be over-roasted or under-roasted. And that has an impact on the final profile of your coffee.

You may also like A Guide To Achieving Consistency In Coffee Roasting

Green beans under a black light

Green beans under a black light. Credit: Ana Valencia

I spend more time separating out questionable beans than doing anything else in the roastery. But don’t be fooled by silverskin or discolored beans, especially in natural processed coffee. They don’t negatively affect the roast, so I leave them. They will not appear as bad under UV sorting.

No matter how hard I try, a few defective beans always make it into the roaster. So I continue picking for bad beans even after roasting. It’s easy to tell both under-roasted and over-roasted beans by color.

Removing bad beans obviously means roasting less volume than I buy, but the final roasted coffee is of a higher overall quality that it would be if I didn’t take them out. Each roaster will have their own variables, but I estimate that I lose roughly 35% of my raw weight beans to moisture loss and separating out bad beans. Labor costs can also increase if we get a batch of green beans that varies greatly from the samples.

But I think it’s worth the extra cost to produce better quality coffee and our customers seem to agree.

a producer next to a roaster

The author with his roaster, Rosie. Credit: Mary Beth Stephens

It may not be possible to remove every single defective bean from a batch, but there are some ways to greatly reduce the number of flawed ones that make it into the final cup. UV lights are cheap and can make picking out bad beans faster and more accurate. So, why not try this useful quality control tool with your next batch?

Enjoyed this? Check out A Roaster’s Guide to Creating Coffee Blends

Perfect Daily Grind

Want to read more articles like this? Sign up for our newsletter!