Indirectly heated drum roasters and the realisation of the importance of convection as a method of heat transfer paved the way for the development of new types of roaster. The rotating drum, an essential part of directly heated roasters, is unnecessary in roasting machines that are heated entirely by hot air. In such machines the roasting chamber can be stationary; the beans are either stirred mechanically or mixed by the flow of air through the machine.
The development of recirculation systems contributed to the commercial success of hot air roasters, including indirectly heated drum roasters. Recirculation systems treat the exhaust gases, removing chaff and smoke, and then return the hot gases to the drum or combustion chamber, thereby reducing the amount of energy needed to heat the beans. In the absence of a recirculation system, a hot air–based roaster uses more fuel than a classic drum roaster. A recirculation system can reduce the energy consumption by about 25% (Nogueira and Koziorowski 2020).
Ideally, recirculation roasters use afterburners to clean the exhaust gases. Exhaust gases from a coffee-roasting machine contain chaff and particulate matter (i.e., dust and smoke) and a mixture of around 700 volatile organic compounds (VOCs), including nitrogen and sulphur compounds (De Monte et al 2003). These are major pollutants, and many jurisdictions require roasting machines to be fitted with afterburners in order to comply with pollution legislation. Roasters with recirculation systems and afterburners became common in the years following the Second World War, prompted by the need to limit energy costs and reduce pollution (Bersten 1993).
An afterburner used to treat the exhaust gases in a recirculating roaster can prevent smoky flavours in the exhaust gases from tainting the beans. The afterburner can both remove pollutants and function as the primary heat source for the roaster. A1971 patent filed by the German manufacturer Gothot describes the first roasting machine designed to use a single combustion chamber as both an afterburner and a source of heat for the roaster (Arndt and Naves 1973).