Handling the Sugar: Preventing Thermal Defects in Natural Process Coffees

When you roast natural process coffees, you are not just roasting a seed; you are roasting a seed coated in dried fruit sugars. This changes the thermodynamics completely.

The processing method, where the fruit dries on the bean, means these coffees have a higher concentration of simple sugars (fructose and glucose) on the exterior and within the outer layers of the bean. Simple sugars caramelise and burn at much lower temperatures than the complex carbohydrates found in washed coffees.

Tipping (burns on the bean’s embryo or tips) and Scorching (burn patches on the flat face of the bean) are conductive heat defects. They happen when the bean touches hot metal for too long or at too high a temperature. Because natural coffees are essentially "glazed" with sugar, they are hypersensitive to these defects.

Here is how you must adjust your physics to prevent them.

1. Lower Your Charge Temperature

The most common cause of scorching on naturals is a charge temperature that is too high. If you drop sugar-coated beans into a drum sitting at 200°C or higher, the sugars on the surface will carbonise instantly upon contact.

You must reduce your charge temperature compared to your washed coffee profiles. By starting cooler, you reduce the difference in temperature (Delta T) between the hot metal of the drum and the cold bean. This prevents that initial "searing" effect.

2. The "Soak" is Mandatory

For naturals, the "soak" is your safety net. After charging, turn your gas off or to its lowest pilot setting. Allow the beans to turn in the drum for the first 60 to 90 seconds using only the stored energy in the metal.

This allows the beans to release their surface moisture gently. This steam creates a microscopic protective barrier around the bean, shielding the sugars from direct contact burns. Only once the beans have warmed up and the drum temperature has stabilised should you apply your primary gas input.

3. Increase Drum Speed (RPM)

Scorching is a function of time and temperature. Specifically, it is the amount of time a bean spends in direct contact with the hot drum wall.

To mitigate this, you should increase your drum speed (RPM).

• If the drum spins too slowly, the beans slide along the hot metal rather than tumbling. This prolonged contact burns the flat face (scorching).

• By increasing the RPM, you increase the agitation. The beans spend more time in the air (convection) and less time touching the steel (conduction). This ensures the heat is distributed evenly across the surface rather than concentrated on one spot.

4. Rely on Air, Not Steel

Once the roast is underway, you want to shift your heat transfer method from conduction to convection.

• Conduction (heating via the drum surface) is risky for naturals because of the sugar.

• Convection (heating via the air) is gentler and more uniform.

To achieve this, avoid pushing your burner to 100%. Instead of using a high flame with low airflow, try using a moderate flame with higher airflow. This pulls hot air through the bean pile, cooking the centre of the bean without charring the sugary exterior.

5. Check Your Batch Size

Finally, ensure your batch size is appropriate for your machine.

• Under-loading: If you roast a very small batch (e.g., 2kg in a 12kg roaster), the beans will have too much contact with the hot metal because there isn't enough coffee to absorb the heat energy. This almost guarantees scorching on naturals.

• Over-loading: If you overfill the drum, the beans cannot tumble properly. They press against the hot walls, leading to tipping.

Make it Count

When roasting naturals, you must think like a pastry chef. You are working with volatile sugars. If you apply high heat directly, you make caramel; apply a little more, and you make carbon.

You must be gentle. Lower your charge, soak the beans, keep them moving fast in the drum, and wash them in hot air rather than searing them on hot steel. This is how you preserve the fruitiness without the ash.

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