Roasting Science

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Heat Transfer

RS 4.09 Batch Size

Any coffee roaster has a maximum and a minimum amount of coffee that it can roast while maintaining an acceptable level of consistency and quality. The upper and lower limits of a roaster’s capacity depend not only on its size but also on its heat-transfer characteristics.

The nominal capacity of a roasting machine, as given by the manufacturer, is often a theoretical maximum. Most professional roasters choose a batch size smaller than the maximum capacity given by the manufacturer, to allow shorter roast times and give them more control over the roast profile. Depending on the manufacturer, the maximum batch size for optimal roasting may be as low as 50% of the stated capacity of the roaster (Rao 2014).

The minimum quantity of coffees that can be roasted, meanwhile, depends on the position of the bean-temperature probe and the amount of control the roaster has over the gas settings. When roasting a smaller batch, the bean-temperature probe has more contact with the roasting gases. This makes the bean-temperature reading more sensitive to changes in airflow and gas settings. With very small batches, the probe may not be fully immersed in the bean pile and therefore will not provide meaningful data on the bean temperature.


Burner Capacity

The most important factor in determining the maximum capacity of a roaster is the overall amount of heat available to roast the beans. If the burner output is too small for a given amount of coffee, the time needed to complete a roast will become excessively long, leading to flat, baked flavours.

For single-pass roasters (machines that do not recirculate hot air), Scott Rao suggests that the minimum heat output from the burner should be 11,500 kJ/hr per kilogram of green coffee (5,000 btu/hr per lb) (Rao 2020). Recirculating roasters are more energy efficient (Nogueira and Koziorowski 2020), so they need less overall heat output to roast the same amount of coffee.

In some cases,