Adjusting the Air:Fuel Ratio

If your burner system allows, adjust the flame so that it appears blue with orange streaks (the orange color is caused by dust in the air). If your flame is lazy and yellow, it indicates a lack of air and incomplete combustion.1 A flame starved of air will produce more soot, smoke, and carbon monoxide, and less carbon dioxide, than it should. If your flame lifts off the burner and sounds a bit like a blowtorch, it is getting too much air.1 (Ideally, allow the roaster to equilibrate for a few minutes before judging whether the mixture needs adjustment.)

This flame has an ideal air:fuel ratio.

The ideal air:fuel ratio is approximately 10:1, though it is customary to set it a little higher as a buffer against changes in air temperature or moisture.12

Charge Temperature

The charge temperature*** and initial gas setting of a batch are critical determinants of the course of a roast profile. Charging at too low a temperature can limit bean development or force the use of an excessive initial gas setting, causing the drum to overheat. Charging too hot can burn a bean or blunt some of the delicacy of its potential flavor. Knowing how to balance charge temperature and initial gas setting, as well as how to manage a roaster before charging, are essential to ensuring that every batch follows an optimal profile. To determine the charge temperature, one must consider roasting-machine design, batch size, bean density, bean size, bean-processing method, and intended roast time.

Machine Design

The first step in choosing a charge temperature is to consider the type of roasting machine one is using. A machine with direct contact between the flame and drum has a hotter drum, relative to the roasting environment, than does a machine that heats the drum indirectly. One must limit charge temperatures when using directly heated drums because such drums pose greater risk of scorching the beans.

Fluid-bed roasters, lacking drums and conductive heat transfer, can accommodate the hottest charges, at well over 550°F (288°C). Indirectly heated drum machines and machines with perforated drums can handle hot charges in the range of 450°F-525°F (232°C-274°C). Classic drum roasters require varying degrees of caution, depending on the thickness and material of the drum and whether it is single- or double-walled. Reasonable charge temperatures range from 380°F-440°F (193°C-227°C).****

As noted earlier, the drum in a classic drum roaster acts as a heat-storage device, storing tens of thousands of kilojoules of heat energy. This stored energy contributes to heat transfer early in a roast and compensates for some or all of the lower charge temperatures that directly heated drums demand. Adequate heat transfer during the first minutes of a roast is critical to inner-bean development. (See “Thou Shalt Apply Adequate Energy at the Beginning of a Roast” in Chapter 10.)

Batch Size

The larger the batch, the greater the drop in a roaster’s environmental temperature upon charging. Therefore, larger batches require hotter charge temperatures to ensure sufficient heat transfer during the first minute or two of a roast.

Bean Density

For a given bean size, it requires more energy to penetrate the core of denser beans. Charging hotter is often appropriate for exceptionally dense beans.

Bean Size

Because a larger bean has more distance from its surface to its core, penetrating a larger bean requires more energy.

Bean Processing Method

The processing of green coffee affects its density, its susceptibility to burning, and, often, its moisture content. When planning a roast, one must consider green processing on a case-by-case basis due to the myriad variables it involves. As a rule, washed-process coffees require, and can tolerate, hotter charging temperatures than natural-process ones can.

Intended Roast Time

Charge temperature and roast time must be considered together. All else being equal, one should charge hotter when roasting faster. A faster roast calls for establishing a larger AT early in the batch to ensure sufficient development. Insufficient charge temperature will impede inner-bean development. Likewise, slower roasts call for cooler charges. Charging a long roast at too hot a setting will force the machine operator to slow the roast excessively, at some point, in order to extend the total roast time. Such deceleration may create baked flavors or inhibit development.

One must consider all six of the preceding variables (machine design, batch size, bean density, bean size, bean processing method, and intended roast time) when deciding an appropriate charge temperature for a batch. For example, in a 30 kg-capacity classic drum roaster, one may charge a 12-minute, 25 kg batch of large, dense, washed Kenya AA beans at 430°F (221°C). In the same machine, the roaster may choose a 380°F (193°C) charge for a 15-minute, 20 kg batch of a small, low-density, naturally processed Brazilian. (Please ignore the unusual choice to roast the larger batch of Kenya so much faster than the Brazilian.)

In these examples, the classic drum roaster calls for a modest charge temperature for both batches. The Kenya’s larger batch and bean sizes, greater density, and washed processing each contribute to its need for a hotter charge than the Brazilian requires. Please note that these examples are hypothetical, and your beans and machine may require radically different temperatures.

Determining Roast Time

The roasting industry harbors a nearly universal misconception that slower roasting yields better development. While it’s true that roasting too fast will produce underdeveloped coffee, roasting slowly will not necessarily ensure good development. It is neither total roast time nor “development time” that determines final development. The shape of the entire roast curve influences development.

Assuming the size of a roast batch is less than or equal to a machine’s realistic capacity (see “Batch Size” in Chapter 9), a wide range of roast curves and times can create coffee with good development and flavor. I cannot tell you the exact, optimal time range for roasts in your machine, but I offer the following suggestions as rough estimates.