Planning a Roast Batch (roasting)

Vietnamese Coffee Exporter
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Planning a Roast Batch – Before beginning a roast, the roaster must make several decisions. Before deciding on the beginning temperature (charge stamp), gas setting, and airflow, he must consider batch size, machine design, and varied grain characteristics.

Coffee roaster

Coffee beans are matured seeds from the coffee cherry. After that, they are processed and dried into coffee beans. Coffee beans are green before roasting and have a beany and grassy aroma. Green coffee beans, in fact, do not smell like coffee at all. We produce 800 to 1000 different aroma compounds when we roast coffee. These compounds contribute to the flavor of the coffee. Roast profiling allows us to influence the presence of these aroma compounds in coffee while also determining the flavor of the coffee.

Size of a Batch

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The first step is to determine the best range of machine batch sizes when preparing a roast. To determine which batch size will taste the best, roasters must consider drum size, airflow space, and rated burner power. You should not assume that the manufacturer’s batch design is the best batch size; I’ve discovered that many, if not most, roasters produce the best results when run at 50-70 percent of nominal capacity.

Roaster makers frequently exaggerate their devices’ capabilities since most consumers, exceptionally tiny specialty roasters, are swayed by the headline number. By first looking at the manufacturer’s offer, one can determine the maximum batch size. That figure primarily denotes the giant batch a roaster should try to fit inside the drum. Overfilling the drum can result in improper seed mixing during roasting or the exhaust fan pulling the cf beans out.

Next, and most crucially, the maker recommends that you think about a machine’s power output. According to the researcher, 1kg of 20dgC cf kernels needs 1000-1500 kJ to reach medium roast. On the other hand, a standard single-pass roaster is inefficient, transmitting just a portion of the heat from the burning energy to the beans. The chimney loses most of the heat, while the roasting room and adjacent sections lose more. Single-pass roasters are only 1-0-15 percent energy-efficient, based on my experience with dozens of various roasters. To put it another way, a single-pass roaster consumes 7-10 times the power required to convey heat to the beans.

So, if a 12kg roaster has a 105,506 kJ rating, your maximum batch size should be around 9kg. You may find that the best batch size is somewhat greater or smaller after a few attempts, and it may vary with different beans, but I recommend starting with the 9kg batch size for this experiment. Because fluid-bed roasters are less efficient than single-pass roasters and recirculation roasters are more efficient than sing-pass roasters, batch sizes vary depending on their burner ratings. Furthermore, the recirculation roaster’s efficiency improves as the heat is recirculated.

Most machines, presuming they can “rest” with a continuous gas setting as low as 204 dg C, do not have the most petite batch sizes. However, there are several practical considerations when roasting in batches. Small (less than 25% of the machine’s capacity) is extremely challenging.

Tiny batches necessitate, among other things:

There is less airflow. Too much airflow, especially if the cage RPM is high, can pull the beans out of the roasting cage.

Drums played at a slower tempo. The grain can ricochet around the drum at ordinary drum RPMs, causing uneven roasting and grain to escape into the exhaust stream.

The Bean probe may or may not be used by the roaster. The search becomes less dependable or ineffective when the batch is so tiny that the investigation does not sink in the grain.

Airflow Control

I’ve been tweaking my airflow setup for years based on a simple test with a cigarette lighter. Remove the trowel from the machine while it is roasting, and the fire is on to do this. Place the lighter over the trowel hole and see if the flame is directed into the hole, away from the spot, or upright.

Changing the Air-to-Fuel Ratio

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Change the color of the burner to a blue fire with an orange streak (the orange color is due to dust in the air). If the fire is weak and yellow, it indicates a lack of oxygen and incomplete combustion, which will result in more soot, smoke, and CO while producing less CO2. There’s too much air if the fire is on the burner, and it sounds burning. (It’s best to let the machine run for a few minutes before deciding whether or not to make a change.)

The ideal air-fuel ratio is 10:1, but most people set it slightly higher to account for air temperature and humidity variations.

Temperature of Charging

The initial gas setting and the charge temperature of a roast are the decisive variables for the entire roast (course of a roast profile). A low charge temperature can limit bean development or require the use of an initial excess gas setting, resulting in drum overheating. A high charge temperature might burn the grain or mask some subtle nuances. To ensure that each batch follows an appropriate profile, you must know how to balance charge temperature and starting gas settings and control the machine before roasting. The design of the roaster, batch size, grain thickness, grain size, grain processing method, and prolonged roasting duration must all be considered while determining charge temperature.

Designing Machines

Consider the type of roaster you’re using before deciding on a charging temperature. Compared to the roasting medium, a machine with direct fire and drum contact has a hotter drum than one that indirectly heats the drum. When using a directly heated drum, he must keep the charge temperature low to avoid scorching the grain.

The hottest charge temperature, reaching 288 dogs, may be accommodated in a fluidized-bed roaster without a drum or conductive heat transfer. Charge temperatures of 232-274 degrees Celsius can be handled by machines with directly heated drums and drilled drums. Traditional drum roasters demand varying levels of attention based on thickness and drum material and whether the drum is single or double-walled. The charge temperature should be between 193- 227 dg.

A roaster’s drum contains a traditional drum that serves as a heat-storage mechanism, storing tens of thousands of KJ of heat energy. This heat aids heat transfer early in the roast and compensates for some or all of the lower charge temperatures required to heat the drum directly. Proper heat transfer during the initial few minutes of roasting is critical for inner-bean development. (See also “You should expend all of your energy at the start of the roast”).

Size of a Batch

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The loss/decrease in the initial roaster ambient temperature is proportional to the size of the batch. As a result, larger sets necessitate a higher charge temperature to ensure enough heat transfer during the first minute or two of roasting.

The density of Beans Roast

More energy is required to enter the center of a particle with a denser (or higher specific gravity) particle for the same particle size. A higher Charge Temp is often more appropriate for particles with a specified specific gravity.

Bean Dimensions

Heat entering larger particles requires more energy than heat entering smaller particles because larger particles have higher specific gravity than smaller particles. A higher charge temperature aids in the penetration of bigger particles.

Bean Processing Techniques

The specific gravity, flammability, and moisture content of the grain are all affected by how the kernels are processed. Because of the numerous factors involved, you must examine the processing procedure case-by-case basis while planning your roast. Wet processing often necessitates and tolerates a hotter charging temperature than natural processing.

Roast Time (roasting)

Both the charge temperature and the roasting time must be taken into account. They should be balanced; if you want to roast faster, you should use a hotter charging temperature. Faster roasting necessitates a higher delta setting early in the batch to provide proper growth. The low charging temperature will stifle seed growth. Slower roasting necessitates a lower charging temperature. In a long roast, a high charge temp will require the operator to abruptly slow down the roast to increase the entire roast time. The deceleration can produce a toasted flavor or stifle growth.

When deciding on the right charging temperature for your batch, you must evaluate six criteria (machine design, power, grain specific gravity, grain size, processing method, and prolonged roasting time). For example, you can charge wet-processed Kenya AA seeds with a high specific gravity at 221 dg C in 12m, 25kg each batch with the standard 30-kg drum roaster. For a naturally processed low specific gravity Brazilian model, you can choose a charge temperature of 193 dg C for a 15m batch, 20kg of grain, and the same machine design. (Please disregard the odd decision to roast a sizeable Kenyan batch significantly faster than a Brazilian batch.)

The typical drum roaster requires a low charging temperature for both batches in these examples. The larger the Kenya batch, the greater grain size and specific gravity, and wet processing contribute to a higher charge temperature than the Brazilian batch. Please keep in mind that this is a hypothetical example, and your cf and machine may require an entirely different charge temperature.

Choosing a Roasting Time

The coffee industry is based on the belief that slow roasts result in superior development. While roasting too soon will result in underdeveloped beans, roasting too slowly may not always yield optimal growth. The final product is not determined by the entire roasting or development time. The overall contour of the roasting curve is influenced.

Assuming that the batch size is less than or equal to the actual machine capacity (see Chapter 9), a wide range of roasting and time curves can be used to make cf with good flavor development and development.

RPM of the drums

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The drum’s revolutions per minute (RPM) should primarily be determined by the drum’s inner diameter and batch size. The RPM setting should allow optimal bean mixing, resulting in a consistent roast with no risk of surface burn. I offer the following guidelines for drum RPM when roasting in batches of roughly 60-80 percent of initial capacity, based on my experience and informal polls of various roasting companies.

These are only preliminary estimates based on standard drum sizes. The actual test of precise drum RPM is uniform bean mixing and roasting with little chance of surface burn.

When determining RPMs, roasters should consider the following factors:

Higher RPMs enhance airflow and convection heat transfer by a small amount.

Smaller batches necessitate a lower RPM.

If the machine’s drum speed is easily adjustable, consider gradually increasing it by a few RPMs. As the beans expand, this will keep the beans mixed and rotated.

Moisture, density, and size of beans

Most roasters shape each new kernel cf batch by trial and error. Before the roaster starts roasting for the fresh sample, this procedure can take a few days to a few weeks. Throughout the testing process, the customer received inconsistent and frequently unsatisfactory cf.

While any new batch may necessitate some unique treatment, roasters can avoid the trial-and-error process by analyzing specific gravity, grain size, and moisture content. The roaster can assess how much heat is delivered to the beans during roasting if they know these three factors. The specifics of turning these data into decisions are beyond the purview of this book. Still, I recommend that you keep track of these three variables (specific gravity, bean size, and humidity) for each batch. Multiply the cf batch by the roasting results to see the correlation between these metrics and the roasting results.

[1] Charge Temperature is a shaky phrase. While it always refers to the temperature in an empty drum (without seeds) just before the seeds are inserted, there is no agreement on what charge Temp means: some roasters determine the charge temp depending on transducer data. Others rely on data from air probes. Not only are these two measures non-switching between machines, but two typical successive batches of roasts charged at the same roaster in Temp, for example, 204 dg C, can present significantly differently. The issue is that Charge Temp is only a partial and often non-uniform representation of the thermal energy of the roaster. For instance, in the preceding case, the drum’s Surface Temp on the first batch charge might be 260 dogs, but 271 dogs on the second batch charge (air temp 204 dogs). Because of the tiny change in drum temperature, the roast profile will be varied. Every roaster has encountered this phenomenon when trying coffee to get the first batch to function like later roasts. Most roasters accept that single, two, or three sets per day will roast differently.

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1 comment

  1. June 11, 2022 at 9:52 am
    cdsclxqlrv

    That’s an interesting way of looking at it.

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