How do the capacity and roasting volume of the roaster affect our roasting results

Understanding how roast size and roaster capacity affect roasts will give us a better understanding of how to control the different variables and produce better coffee. Next, let’s learn more about the capacity and batch size of coffee roasters.

coffee bean

Understand the recommended roast capacity for your roaster

We should not rely on the nominal capacity of the roaster as the basis for the optimal roasting batch size. According to the actual situation, the coffee with the best flavor can be roasted according to 50% to 70% of the nominal capacity of the roaster. “

But what does it really mean when it comes to coffee beans that perform best when the roaster’s total capacity is insufficient? Let’s take a look at what happens when the coffee roaster is overloaded or not.


Coffee Roaster Overload Roasting

In order to achieve a strong first crack and bring out the best flavor of the coffee beans, we need to apply enough heat at the beginning of the roast. In an overloaded roaster, there may not be enough energy to heat all the beans properly. This results in a slower ramp rate and increases the overall time required to reach the optimum stage of coffee bean flavor development. The result may be a “roasted” coffee with a substandard, bland flavor profile. This is like driving a 4-seater car with 6 people up the mountain with a 12-seater car with 6 people. The 4-seater car must spend more gasoline and take longer than the 12-seater car. In the same way: reducing the number of roasted beans or increasing the heating power of the roaster can avoid taking too long to reach the first crack node.

The roaster is too heavily loaded to turn the beans. The capacity of the drum is insufficient so the coffee beans cannot have enough space to move back and forth inside. At the same time, the volume of the coffee beans will expand during the roasting process, making the already full drum quickly become heavier. Excessive roasting in a single batch may also mean that some of the coffee beans in the drum are directly heated by excessive contact with the wall of the drum, while other parts are not heated evenly because they cannot be tumbled. The end result is uneven roasting of the beans coming out of the same oven which increases the likelihood of the beans burning. Adjusting the amount of roasting in a single batch is actually increasing or decreasing the convective flow of heat energy. The hot air in the drum can touch the beans so that the coffee beans have more surface area to receive the heat energy transferred by the hot air convection.

30KG commercial coffee roaster
coffee roaster in our factory

Imagine we are in a crowded place and everyone is smoking, so we can smell ourselves as well. The same thing happens inside the roaster: you end up with a roast full of smoky beans (even in underpowered overload roasts, we want the coffee flavor to be as clean as possible). So if we buy good coffee beans but roast them poorly, the result is meaningless. Because we must pay attention to the quality of the roasted coffee. If we want to exceed the production limit, the solution can only be to buy a roaster with a larger capacity.

Partially loaded roast

Some roasters may argue that it is best to roast in small batches to avoid the roasting problems associated with overloading. However, there are also some challenges in how to lighten the load on the coffee roaster. If we only use 1/3 of the total capacity for baking, it is not impossible, but if the load of the roaster is too small, it will be more difficult to control the heat during the roasting process. Usually, we need to adjust the lower oven temperature and the burner temperature to compensate for the variation in the batch size of different roasts.

Roasting small batches of coffee without adjusting roast parameters can lead to the following potential problems:

*Less airflow required: This is to prevent coffee beans from being sucked out of the roaster.

*Reduced drum speed: Standard drum speed may cause coffee to bounce inside the drum and out through the vent.

*The temperature probe cannot be used during the roasting process: When the amount of roasted coffee beans in the drum is too small to wrap the temperature probe, the probe cannot measure the temperature.

So small batch baking changes everything. It’s like a guessing game, and it keeps us from maintaining consistent roast quality. Of course, we should also adjust the size of the roasting batch according to the amount of coffee beans in stock. For example, for a 30kg package, we should evenly distribute the amount of roasting in a single batch according to the capacity of the roaster, roasting the same weight each time, so that there will be no leftover beans. Because variations in batch size mean differences in the amount of heat required, consistent roast batches are a must for consistent roasts.


How to Calculate the Optimal Batch Size for Roasting

Here are some useful tips to help us determine the optimal roast batch size:

1. Understand the roasting machine and its heating type

The heat likely to be generated by the roaster is a key factor in determining the optimum capacity. A roaster with a more powerful heat source can generate enough heat to heat the beans quickly at the start of roasting, which is critical for flavor development. If we use a roaster with higher thermal efficiency, we can pour more coffee at a faster speed, but the lower the thermal efficiency, the less coffee beans can be poured. “

By knowing your roaster and the heat it generates, you can determine the batch size to roast.

The material of construction of the roaster is another important consideration when determining the roast size for a single batch. If we have two different roasters, they show the same temperature on the probe, but the coffee beans may react differently due to the heat retained in the roaster barrel walls. So it cannot be assumed that the same results will be obtained using the same temperature profile on different types of roasters.

Taking different brands of roasters with a capacity of 15kg as an example, if they use drums of different materials, there must be a big difference in performance.


Therefore, it is very important to know the roaster you are using! Knowing the rated capacity of the roaster, how it is constructed and how the materials it uses retains heat, and knowing the wattage in the heat energy, we can do a good job of figuring out the ideal batch size for our machine.

2. Consider the status of green coffee beans

The variety of green coffee beans we use will also affect how much we roast in a single batch. Density, humidity and bean size all affect heat absorption, so knowing the condition of the green coffee is also very important. Only by knowing the density, water content and bean shape of green coffee beans, can we reasonably calculate the energy required for the development of these coffee beans, and then set a more prepared roasting amount through sample roasting.

Be aware that the initial stages after the beans come out of the oven require a lot of heat to heat up all the water molecules in the green coffee beans.

3. Monitor the roasting curve

Use roasting software to monitor the roasting curve. If the curve is too flat, the roasted beans will not develop flavor and will taste hollow. At this time, more power is needed to increase the temperature rise rate in the first few minutes to make the temperature curve steeper and form a large temperature difference. As the first crack of roasting occurs, the curve angle gradually slows down.

A lot of people observe the first burst happening without really thinking about why the first burst happened at that time. In this case, we run the risk of overloading the roast, and even though first crack will still occur, it will take longer to achieve first crack, and this will make the flavor of the coffee flat and appear Roasty, and flat in the mouth.

We can deduce from the ideal roasting time, if the required time node is not reached, we need to reduce the amount of roasting in a single batch. It is easy to find the correct amount of beans by experimenting with several batches.

By understanding how batch size and capacity affect the roasting process, we are able to make more informed choices when roasting. This means that samples can be roasted to gain some information and avoid wasting expensive green coffee beans.

roast finished
Checking coffee beans in roasting

Through sample roasting, roasting results are carried out and evaluated to find the ideal roasting curve, and of course we should repeat all steps carefully, because “consistency” is the key to successful roasting. If it was just the batch size, the roasting process would be very easy.

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