Exhaust temp vs bean temp of roasters

 Exhaust temperature indicates the energy within the roaster and is used to control the roast, while the gap between the exhaust and bean temperature is crucial for even roasting. A small gap (10-30°C) is ideal, with the exhaust temperature slightly above the bean temperature to ensure energy transfers efficiently without scorching or baking the beans. Monitoring this gap helps a roaster adjust burner power and airflow to achieve desired outcomes like the "sweet spot" of development. 

How exhaust temperature guides the roast

Controls energy and momentum: The exhaust temperature reflects the total energy in the roasting chamber and its momentum, helping the roaster control the rate of rise (ROR) of the bean temperature.

Helps manage airflow: A roaster can adjust airflow to vent moisture and smoke, which can otherwise cause a dip in the ROR around stages like the Maillard reaction.

Guides heat adjustments: As the roast progresses, roasters will make adjustments to the burner and airflow. They use the exhaust and bean temperature readings to make proactive decisions. 

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The importance of the gap between exhaust and bean temperatures

Ideal gap: A gap of about \(10-30\degree C\) (20-50°F) between the exhaust and bean temperature is a reliable benchmark for a quality roast.

Too large a gap: A gap exceeding \(50\degree C\) mid-roast can signal too much burner power or insufficient venting, potentially scorching the coffee.

Too small a gap: An exhaust temperature too close to or below the bean temperature can indicate inadequate heat or poor circulation, leading to a stalled development and flat flavor.

Toward the end of the roast: The goal is often for the bean and exhaust temperature lines to approach each other near the end of the roast, indicating that the beans have absorbed most of the energy available to them. 

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Using exhaust temperature in practice

Data-driven decisions: Monitoring exhaust temperature, along with bean temperature and airflow, provides crucial data for consistently replicating roasts and dialing in new ones.

Making adjustments: If the gap between temperatures widens unsustainably, a roaster might reduce burner power or increase airflow to correct the roast. Conversely, if the gap is too small, they might increase heat or adjust airflow to increase the ROR.

Proactive roasting: Instead of reacting, roasters can use temperature data to anticipate changes and make adjustments proactively to guide the roast along a specific profile.