Top Reasons Your Coffee is Unevenly Roasted

Uneven roasted coffee beans
Beware the uneven roast
5 reasons your roasts are coming out uneven. Check out these quick fixes for a more even roast every time.

Are all of your roasts coming out uneven? Some beans are barely yellow while others are perfectly brown? Is it that some of your beans are burning in spots, or scorching along the flat side?

I want to take a moment to look at the top reasons why your coffee is unevenly roasting, and give a few tips for what you can do to solve your problem.

#1 Top Reason: Bad Beans

If you’re consistently getting uneven roasts, but it’s only with one particular bean, you may want to reexamine that bean. If you’re buying green from a home roasting specialty source, you may want to reexamine that as well. Buying top quality green coffee goes a long way in ensuring that your roasts will come out evenly.

Green coffee may roast unevenly due to:

  • Producers picked unripe or underdeveloped coffee cherries
  • Coffee beans were under or over fermented, or not turned enough during fermentation
  • Drying process was too rapid or uneven

In general, if green coffee wasn’t meticulously prepared and cared for during the fermentation and drying process, it will affect your roast.

Troubleshoot #1

Stick with green bean sources you trust. I recommend Sweet Maria’s. Quality sources will be sure to weed out the producers that aren’t as careful with their green coffee, so you’re basically guaranteed to get great quality coffee.

#2 Top Reason: Roasting Too Fast

If your roasts are coming out in less than 7 minutes, you’re roasting too fast. This can happen sometimes with popcorn poppers and the like.

Some beans, even in top quality green coffee, are more dense than others. If you roast super fast, you’re not giving your beans time to equally dehydrate in the roaster. Only after a significant portion of the water has been roasted from the bean will the sugars begin to caramelize: that’s the yellowing and browning process in action.

By roasting fast, you’re allowing some of the beans to begin caramelization before others have exhaled their steam.

Troubleshoot #2

If you’re using an electric roaster and you’re roasts are coming out too fast, try using an extension cord. This slows the roast a good deal and will help you get an even roast.

#3 Top Reason: Slow Drum Speed

If you’re using a drum roaster and you’ve been seeing the tips, edges, or faces of your coffee beans develop carbonized spots, you’re a victim of tipping, facing, and general scorched beans.

If these carbonized marks are only on a few of your beans, it won’t cause much of a difference in the final cup of coffee. However, if you’re noticing that many of your beans have scorch marks, you may want to take a look at your roasting process.

If your drum is running slowly, it may cause some of the beans to sit on the hot drum for too long. Your beans should mainly roast through convection; not through conduction. If they sit too long on a hot surface, just like everything else, they burn.

Troubleshoot #3

Speed up your drum. In some machines, like the Behmor 1600, this is as easy as pressing a button. Other models may be more difficult or require extra tools.

#4 Top Reason: Drum is too Hot

Another reason that you may be getting uneven roasts with your coffee is that your drum is too hot. The reasoning here is the same as in reason #3. You want to be roasting mainly through convection. Some research says that the average roast is 75% convection and 25% conduction.

This usually only happens if you start with too high of a charge temperature or if you do multiple batches back to back.

Troubleshoot #4

Lower your charge temperature. You can do this by preheating less, or programming in a lower temp. If you’re roasting multiple batches, give your roasts ten minutes between each other. This allows the drum to adjust back to its surrounding temperature.

#5 Top Reason: Too Crowded

If you’re getting an uneven roast, you may want to look at how much coffee you’re roasting at once. If your roast chamber is too crowded, you won’t be getting the necessary agitation to keep your beans moving. They will either stick to the surface of the drum or at the bottom of your air roaster. In either of these cases, all of the beans in your chamber wont receive the same heat, and that will cause an uneven roast.

Troubleshoot #5

Use half as many beans as you currently are. Measure the amount of beans you use in every roast. Keep a consistent charge weight. This will help you ensure that you’re not overloading your roast chamber.

Anything I missed?

For more information about roasting coffee at home, check out my roasting resources.

If you have any other suggestions for why a roast might be uneven, let us know in the comments section below!

  1. This happens to me only on specific beans I get from a supplier and not others. Would you think the issue is definitely supplier or could the issue still be one of the other pints you mentioned in the post?

    1. The beans might be an older crop. It may also just be the type of bean– for instance, some dry process ethiopians can have a wide range of colors, but still taste great. Back when I roasted commercially, I noticed that Brazil roasted really unevenly, but that’s because we bought really inexpensive stuff, so the density was all over the place (and thus it roasted to different levels in the same batch). The main thing to consider is the taste. If the taste is sour, bitter, or just not satisfying, then switch up the beans. I’ve also seen some suppliers keep their stuff in hot dirty warehouses, so it may be worth looking into that. For the record: All major US suppliers do a great job of temp regulating their beans. But if you buy your beans from a larger local roastery, they may be the middle man you want to cut out.

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