How to Roast Coffee

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Prologue — Getting Set Up

HTR 0.05 Choosing the Right Size of Machine

Roasting machines vary hugely in size, from industrial machines such as the Probat PX120 (left), with a capacity of 120 kilograms, to tabletop roasters such as this 1-kilogram model (right), designed for use in cupping labs.

When deciding what size of roasting machine to purchase, the first thing to work out is how much coffee you need to roast per week. For a comfortable roasting program, we recommend that you purchase a machine that can roast this amount of coffee in no more than 25 hours per week. 

Almost invariably, a roasting machine’s real capacity will be less than its stated capacity, thanks to a number of factors: 


Weight Loss

Typical weight losses for roasted coffee range from 14–20%, the lesser amount for light roasts and closer to 20% or more for very dark roasts. The weight loss for beans from certain prominent coffee chains can be as much as 22–25%. 


The Burner Capacity

A machine’s burner capacity is the real limiting factor on how much coffee a machine can roast per hour.

Roasting 1 kg of green coffee in a reasonable time requires a burner capacity of approximately 11,500 kJ/hour.

(Alternatively, roasting 1 lb. of coffee requires about 5,000 BTU/hour.) A machine that recirculates hot air back into the roasting chamber, such as a Loring, will have a slightly higher roast capacity.



Whilst a roaster stuffed full of beans will accommodate well beyond its stated capacity, this also means the coffee will take longer to roast. A salesperson may claim that you can roast 15 kilograms per batch in a 15-kg machine. But to make sure your drum roaster is never underpowered, use the following rough formula to estimate your effective roasting capacity:  

When planning your roasting program, assume you will roast 3–3.5 batches of green coffee per hour at 50–70% of a machine’s stated capacity. Then deduct the 14–20% weight lost per batch to calculate how much roasted coffee per hour a machine can produce.