As a roaster, it’s important to understand the different factors at play during a roast. Not only will it help you gain more control over the outcome of each batch, it will also reduce the risk of roast defects and improve your brand’s reputation.
Although it may seem small, knowing about the different forms of heat transfer is one of the most important of these factors. Specifically, whether the roaster uses conductive or convective heat transfer – or a combination of the two.
Conductive heat transfer (or conduction) is a traditional form of roasting akin to frying: the beans come into direct contact with the hot drum and cook from the outside in.
Convective heat transfer (or convection), on the other hand, is a more modern approach to roasting. It relies on hot air continuously passing over the beans and cooking from them from the inside out. It is similar to baking.
While most roasters use both forms of heat transfer, some have started to move increasingly towards those that use convection, such as fluid bed roasters. Those who have already made the switch cite greater roast uniformity and a lower chance of “scorching” as the main benefits.
But are conduction roasters really on the way out? Or does a hybrid model still offer the best option? Read on to find out.
What forms of heat transfer occur during roasting?
Put simply, roasting coffee is the process of applying heat to raw coffee beans to develop their inherent flavours.
During a roast, the beans change colour from green to yellow to tan to increasingly darker shades of brown. As this happens, the beans lose some of their moisture, leading to a fall in weight. The size of the beans tends to increase.
Each machine used for roasting has a slightly different system for applying heat, often combining different types of heat transfer. These systems, together with the influence of the roaster themselves, has a significant impact on the development of the coffee and its characteristics.
A classic drum roaster consists of a cylindrical drum inside which green coffee rotates during a roast. Below the drum, an open flame heats the cylinder, which transfers heat to the coffee beans via contact with the hot metal surface. This form of heat transfer is called conduction.
In many roasters, pre-heated hot air is also circulated through the drum and over the beans. This air is drawn by the roasting fan after being heated in a combustion chamber. This is what’s known as convection.
It’s estimated that in a classic drum roaster, heat transfer consists of around 70% convection and 30% conduction.
The problem with conduction roasting
Conduction is a form of heat transfer in which the kinetic energy of the burner is conveyed to coffee beans through direct contact with the hot drum and other hot coffee beans.
In Ethiopian culture, roasting by conductive heat is common practice. Using a large pan (called a metat dish), the roaster will cook the raw coffee beans over hot coals, moving them around regularly to achieve an even roast. This is one of the most simple ways of roasting coffee.
Conduction plays a particularly important role at the beginning of a roast, when the room temperature beans are dropped into the hot drum.
In a classic drum roaster, rotation helps mix the beans while they absorb heat by direct contact. This is crucial, because if the beans stay in the same position, there is a risk they will be cooked unevenly or become “scorched”.
Scorching happens when only a small area of the beans are in contact with the drum, leading to dark, charred patches on the flat sections of their surface. This gives rise to unwanted characteristics, such as burnt, smoky, and bitter flavours.
With conductive heat transfer, there is also a risk that the roaster will end the roast too early judging on the colour of the beans’ surface.
Because conduction cooks from the outside in, it can mean that the inside of the beans are still partly unroasted, while the surface appears ready.
Is convection better than conduction?
Convection is generally considered to be a more efficient and consistent way of roasting coffee compared to conduction.
Although the two are often used in tandem, in recent years more and more roasters rely solely on convective heat transfer. This is predominantly due to the way in which convection heats the beans, which leads to a more even roast.
In fluid-bed convection roasters, green coffee beans enter through the funnel and circulate on a bed of hot air in the roasting chamber, before exiting through a door. Because, during this time, the air is in constant contact with the entire surface of each bean, it ensures greater roast uniformity.
However, like conduction roasting, convection has its pitfalls. Some claim that while convection is quicker than conduction roasting, it produces coffee that lacks character compared to drum roasted beans.
According to a Paradox blog post, conductive heat transfer results in coffee that has “an overall better impact on body and mouthfeel development of coffee”, compared to convective heat transfer.
There is also greater room for mistakes with convection roasters, as the roasting process is significantly quicker.
For example, the way the hot air penetrates the bean and cooks it from the inside out can rapidly mute the coffee’s acidity if the roaster does not carefully control the temperature.
It’s for this reason that convection roasting alone has been somewhat viewed with an air of caution among specialty coffee roasters. Instead, they tend to favour a hybrid of the two systems: part conduction and part convection.
No matter how you roast your coffee, it’s vital to use high-barrier coffee bags to preserve freshness between the roastery and the consumer. Any loss of flavour or aroma can lead to stale, flat, or rancid-tasting coffee, which can reduce the opportunities for repeat purchases.
At MTPak Coffee, we understand all the time and effort that goes into roasting the best coffee possible. As such, our packaging is designed to protect your roasted beans from exposure to light, oxygen, moisture, and heat.
With the help of our expert team, you can design coffee bags that not only preserve freshness but also showcase your branding. We have a range of sustainable materials, including kraft paper, LDPE, and PLA, as well as additional components such as recyclable degassing valves and resealable zippers.