How Do Roaster Capacity & Batch Size Affect Your Coffee Roast?


When planning to roast a batch of coffee, do you consider your roaster’s capacity? How much thought do you give to the size of your batch of beans? Knowing how batch size and capacity can affect your roast will give you more insight into how to control different variables and produce better coffee.

Let’s find out more about coffee roaster capacity and batch size.

Lee este artículo en español Tueste de Café: ¿Cuál es el Tamaño de Lote Adecuado?

Freshly roasted coffee beans in cooling tray. Credit: Neil Soque

Every coffee roaster comes with a recommended capacity size, as determined by the manufacturer. But this is the maximum recommended amount of beans you can roast, not the optimal amount for the best results. Manufacturers may be inclined to state the largest amount possible.

Carola Mapp is Head Roaster at Pharmacie Coffee Roasters in Brighton, UK. She tells me that “to my knowledge, it is common in specialty coffee to roast only up to about 80% of the roaster’s total capacity in order to be better able to control the roast.”

In The Coffee Roasters Companion, Scott Rao writes, “One should not assume that a machines stated capacity is its optimal batch size; I have found that many, if not most, machines produce the best coffee at 50 to 70% of their nominal capacity.”

But what does it mean to say a roaster performs best at less than its total capacity? Let’s look at what happens when you overload or underload a coffee roaster.

You may also like Understanding Roaster Drum Speed & Its Affect on Your Coffee

Large batch size of roasted coffee

Coffee beans cool after roasting.

Overloading Your Coffee Roaster

To get to a strong, rolling first crack and stimulate optimal bean development, you need to apply sufficient heat at the beginning of a roast. In an overloaded roaster, there may not be enough energy to heat all of the beans properly. This will result in a slow rate of rise and increase the total amount of time needed to reach optimal bean development. The result can be a “baked” coffee with subpar, flat flavor.

Steve Cuevas is Head Roaster at Black Oak Coffee Roasters in Ukiah, California. He says, “if you’re driving a V4 car and you have six people in the car, it’s going to take way more gas and a lot longer to go up that hill than if you’ve got a V12 and you have those same six people. You’re going to travel a lot faster.” The point is clear: run fewer beans or increase roaster power to avoid roasts taking too long.

High capacity roaster

A coffee roaster and bags of green beans at a roastery. 

Overloaded roasters also poorly agitate coffee beans. The drum is simply not large enough to allow the beans to move around. Coffee beans also expand in size during roasting, so a full drum can quickly become even more overloaded. 

Using too big of a batch size can mean that some coffee beans are over-exposed to the direct heat of the drum walls, while others are under-exposed. This means an uneven roast and increases the potential for scorching.

Steve says, “when you change your batch size, you’re actually increasing or decreasing the amount of convection, the air that touches the beans, so you’re actually creating more surface area for the beans to transfer the heat from.”

Alexandru Niculae, co-founder of Bob Coffee Lab in Bucharest, says, “Imagine yourself in a crowded place where everyone smokes…you will smell like smoke very fast. Same thing happens inside the roaster, you will get tons of smoky beans [in an underpowered, extended roast]… and we want the coffee to be as clean as possible.

“There is no point in buying high quality beans and roasting them badly. Focus on quality. If you are going over capacity, it is time to invest in a bigger machine,” he says.

Freshly roasted coffee beans.

Underloading Your Batch Size

You may think that to avoid the problems associated with overloading, it’s better to use a small batch size. But there are some challenges with underloading your coffee roaster too.

Carola says that “roasting much smaller batches is still possible, in my experience down to about a third of total capacity, but the smaller the batch size, the more difficult it is to control the heat during roasting… Generally, you will need a different charge temperature and burner in order to compensate for the change in batch size.”

In The Coffee Roasters Companion, Scott Rao warns that using a small batch of coffee without adjusting variables can cause some issues. He lists the following potential problems:

  • Less airflow is needed: this keeps beans from being sucked out of the roaster.
  • Slower drum speed: standard drum RPM can cause beans to bounce inside the drum and exit through the exhaust.
  • Roasting without the bean probe: the probe can become useless when the bean pile is too small to keep the probe immersed.

Alexandru says that ”[with small batches] everything changes and it’s more of a guessing game rather than you being consistent.”

Carola acknowledges that “sometimes it makes sense to adjust batch sizes according to the amount of coffee you have, for example doing a 7.5 kg batch for 30 kg vac packs, just because it means you’ll not have any left over.” But she also says that “as a change in batch size always means a difference in the heat needed, a consistent batch size is a requirement for consistent roasting.”

A Probat roaster at a roastery. 

How to Calculate Optimal Batch Size

Here are some practical tips to work out your optimal batch size:

Know Your Roaster & Type of Heat

The amount of heat a roaster can generate is a critical factor in determining optimal capacity. A roaster with a more powerful heat source will generate enough thermal energy for beans to rise quickly in temperature at the beginning of the roast, which is crucial for bean development. Steve says, “If you have more BTUs in a roaster, you’re able to push more coffee at a faster pace, but the lower your BTUs are, the less coffee you’re able to push through.”

By knowing your roaster and understanding the heat it can produce, and you can start to identify the size of batch you want to roast.

A small roaster ready for use. Credit: Neil Soque

The material construction of the roaster is another important consideration when determining batch size. You may have two different roasters that show the same temperature on a probe, but due to the heat retained in the roaster walls, the beans may respond differently. Get to know your own machine and don’t assume that you’ll get the same results with another model just because you use the same temperature.

Steve says, “let’s say you have four different roasters and they’re all 15 kg, but they are all different materials. Now, they’re all meant for 15 kilos, but they perform differently.”

Alexandru says, “Know your machine! Very important.” If you know your roaster’s stated capacity, understand its construction and how that material holds heat, and know your power source in BTUs, you are well on your way to calculating your ideal batch size. 

Freshly roasted coffee beans drop into the cooling tray. 

Consider The Green Coffee Beans

The type of green coffee beans you are using also has an impact on establishing batch size. Bean density, humidity, and size will affect the absorption of heat, so it’s important to understand the green beans you are roasting. If you know the density, water content, and size of the green coffee, you can reasonably calculate the energy required to develop those beans. Then make sure to do sample roasts. 

Steve says “It takes a lot of energy at the very beginning to heat up all the water molecules [in green beans].”

Learn more in A Roaster’s Guide to Green Bean Samples

A coffee roaster and bags of coffee beans at a roastery.

Monitor Your Roast Curve

Keep an eye on roast curve using roast profile software. If it is too flat, the roast will taste hollow and underdeveloped. More power is needed to increase the rate of rise in the first few minutes and steepen the curve, with it tapering off as the roast passes through first crack.

Steve says that “a lot of people observe the first crack but they’re not really thinking why they got there at those times.” With this roasting philosophy, you run the risk of overloading – the first crack will still come, but it will take much longer to get there with a larger load and that can result in flat, baked, and bland coffee.

Work backward from an ideal roast time. If you don’t hit that mark, decrease the batch size. With some trial batches, it will be easy to find the correct load weight.

A roaster checks the roast. 

By understanding how batch size and capacity can impact the roasting process, you will be able to make more informed choices. This means you can do start sample roasts with some insight and avoiding costly green coffee waste.

From there, cup your results to assess the results. And once you find the ideal roast profile, repeat everything meticulously. As Alexandru says, “Consistency is key to a successful roaster. It would be very easy if it were just the batch size.”

Enjoyed this? You may also like A Guide to Achieving Consistency in Coffee Roasting

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