How many times have you circled back on a coffee that you’ve roasted before, excited for what you know is possible for that coffee, and been stunned that it doesn’t taste the same? Welcome to the club. Mastering consistency throughout the entire journey from green bean to cup is one of the greatest challenges for anyone roasting coffee.
If you’re a professional, or own a café or online coffee business, you know that customer satisfaction hinges on your consistency. If you’re new to roasting, you can’t learn what needs tweaking until you minimize the variables.
Most roasters understand this general challenge and the importance of replicating processes like a roast profile, but many overlook inconsistent operating temperatures of the roaster itself. Ensuring your equipment is presenting the same roasting environment from batch to batch to batch to batch is critical to predictable results.
Preheating. Roast consistency begins by properly (and thoroughly!) preheating your roaster before charging your first batch, and then reheating your roaster in a consistent manner between batches.
It’s obvious that the larger your roaster is, the longer it takes to preheat. But if you’re watching a temperature readout during preheat, remember that the probes are only feeding data about the environmental air temperature inside the chamber, not necessarily the temperature of the roasting equipment or drum. The ambient air temperature can reach your target charge (drop) temperature much quicker than the roast chamber can, so don’t get fooled by your temperature readouts.
We like to think of it like preheating a cast iron pan. If you blast it with high heat for a few minutes and focus a digital thermometer at the center of it, you might see your desired steak-searing temperature, but the pan won’t be heated evenly and your steak will quite literally fizzle out. Take the slow and steady approach (this is free cooking advice). The steel walls of a roasting chamber work in the same way.
Depending on your equipment, there are several ways to accomplish a full preheat. One option is taking the cast iron approach: a slow, steady preheat until you reach your target charge temperature. The other is to overshoot your target charge temperature, then back off until you return to that target. In either case, once you reach that temperature, let it “idle” for 10 minutes or so to ensure your roaster is fully heated before charging your first batch (this timing assumes you’re operating a drum roaster with modest thermal mass). For small, home coffee roasters, there isn’t that much mass you need to preheat, so it may only take minutes to properly heat.
Reheating. Reheating between batches is about making sure that you precisely replicate your process between each roast. Achieve this by letting your equipment return to temperature, holding it steady for a minute or two to make sure everything is fully (re)energized with heat, then charge your next batch. Every single time.
Because there is no hard and fast formula for this, it’s important to cup your batches to check your results and adjust accordingly.
Cheat code (until you master the above). If you always roast 5 batches of your Guatemala Cimarron every Monday morning for example, blend them together. At least your combined Guat “batch” will be consistent from week to week.