The “Lighter Method”

What is the “lighter method?”

The lighter method is a decades-old method of checking airflow through a coffee roasting machine. Essentially, you take a lighter, hold it up to the trier of a coffee roasting machine (when it is hot and full of beans), and see how strongly the air moving through the drum pulls on the lighter's flame. As the method goes, while keeping the lighter 0.25-0.5 in ( ~0.63 - 1 cm) from the trier, you should see the flame be pulled into the trier hole but not blown out.

Of course, as in professional kitchens, lighters used to be abundant in the past as most roasters and roastery staff seemed to be avid smokers. However, nowadays, this may necessitate an extra trip to the corner store to find a lighter.

Why do people use the “lighter method?”

Having airflow properly adjusted on a coffee roaster is paramount for having the correct balance between power and control when profiling your coffees. In addition, the right amount of airflow can help you roast a coffee quickly while also avoiding roasting defects like scorching, tipping, and facing. Not to be self-referential, but for more information on these defects, please jump over to my blog about them here.

Having coffee roasting machines with variable frequency drives, airflow measurements, and instrumentation is relatively new in the coffee industry. For many years, roasters got by without even having access to bean temperature gauges… let alone all of the other accouterments we have access to today. This test would allow the roaster operator to adjust the baffle or damper on the exhaust air duct to control the rate at which the air was being pulled through the roasting system to establish an ideal balance. As a testimony to how far we’ve come as an industry, it is still one of the better ways to start this conversation about airflow in coffee roasting.

What do I think about the “lighter method?”

It’s okay.

I think it’s a fine place to start, but I know it doesn’t work for everyone. On the one hand, I don’t think it goes far enough. On the other hand, it can be inaccurate in certain situations and cause people to make the wrong decision sometimes. Since it is a significant oversimplification, we can not expect that airflow moving through a small hole (often located close to the exhaust port) can tell us how airflow moves throughout the system. It is, however, better than nothing.

I doubt I’m saying anything revolutionary here or anything that others who teach roasting would disagree with. My rationale for speaking about it today is that I frequently have clients who diligently do the lighter test and still end up with the wrong airflow settings for their machines. It’s something that I am running into more and more frequently… and so I wanted to address it.

How do I dial in airflow?

This is going to require to have some equipment.

• Bean temperature probe

• Exhaust temperature probe (installed immediately after the air exists in the drum chamber)

• Inlet temperature probe (**okay, this is optional, but it’s so helpful to have, and I will advise you to add it)

When roasting, we will examine the relationship between the exhaust air temperature and the bean temperature graph for clues as to whether we should increase or decrease our airflow. In addition, we will discuss control issues that may also hint at maladjusted airflow.

1 Too low…

Please note that this curve was made with one burner setting and one airflow setting until the second crack for instructional purposes… so the dip, flick, etc., are immaterial to any conversation we’re having here.

Note the relationship between the exhaust air temperature and the bean temperature. The gap between the two is significant and stays rather significant during the roast. When the airflow is low, there will often be hotter metal and hotter inlet air to accompany the hotter exhaust air. This roast is ripe for roasting defects like scorching and tipping. So, if you notice that the exhaust air temperature doesn’t dip close to the bean temperature line, your airflow may be too restricted/slow.

2 Within “tolerance…”

Please note that this curve was made with one burner setting and one airflow setting until the second crack for instructional purposes… so the dip, flick, etc., are immaterial to any conversation we’re having here. The “too low” is featured greyed out in the background.

This airspeed is fine for sturdy coffees that are resistant to tipping. However, it can still have an inlet temperature that is too high for certain coffees and cause tipping. There are no issues with scorching when the airspeed is moving fast enough to cause this closer relationship with the exhaust air and bean temperature curves. Note that the bean temperature is turning (at the turnaround temperature) hotter, and the air temperature is turning cooler.

3 Nice and balanced

Please note that this curve was made with one burner setting and one airflow setting until the second crack for instructional purposes… so the dip, flick, etc., are immaterial to any conversation we’re having here. The “too low” is featured greyed out in the background.

This is what a safe airflow looks like. Notice how close together the curves are relative to the greyed-out background line. One may, for certain coffees, roasting styles, and batch sizes, find themselves roasting too slowly for their desires… but this can be modified by lowering batch size or going with a lower airflow setting at the beginning of the roast for a short time to accelerate roasting and hit your time and temperature goals.

4 Too slow… maybe

Please note that this curve was made with one burner setting and one airflow setting until the second crack for instructional purposes… so the dip, flick, etc., are immaterial to any conversation we’re having here. The “too low” is featured greyed out in the background.

In this final curve, you’ll notice that the exhaust air curve hugs the bean temperature significantly closer than any others. This would be an example of a roast that could be challenging to push rather quickly at this batch size. It generally works well for me to run 50% batch and use 100% airflow (as pictured) the entire roast. I can expect first crack between 8:00 and 9:00. If I want to do a larger batch size, or increase the speed at which I hit the first crack, then I need to use a combination of lower airflow at the beginning and higher airflow at the end.

Quick Comparison

Please notice, as I discuss these roasts, that an unseen world is unfolding in the inlet air temperature. For the drum roaster I did these experiments on: green = 25%, yellow = 50%, red = 75%, and blue = 100% airflow.

One must be careful when applying airflow that the inlet temperature is limited to a low enough degree that (for that roaster) the beans aren’t damaged by tipping or charring during the roast. This is why I would highly encourage everyone to also have an inlet temperature measurement. It adds some nuance to the conversation.

Control Issues…

Sometimes, control issues can also tip us off to airflow-related issues. For example, if you’ve already tried down-batching from the manufacturer-rated batch size to 75-80% capacity, and you still cannot roast reasonably fast (first crack ~ 8:30 - 9:00) then your roasting machine’s heat source may not be able to keep up with the air volume that the fan is moving through the roaster. In that case, it may be advisable to reduce your fan speed for at least part of the roast.

On the other hand, if you’re wrapping up your roast and have to lower the flame completely (or even switch it off) and still feel like you cannot sufficiently stretch your development time while keeping the roast degree light then it is totally possible that your fan speed is too low and as a result you’re having a terrible time slowing down.

Speaking in broad generalities, it is more common to use a fixed airflow or to go from lower to higher airflow. I would recommend considering making your fan speed adjustments early in the roast (before 50% complete). Also, it can be difficult to roast a medium or dark roasted coffee if you have too high of an airflow setting… not impossible, just more difficult. There are also some occasions where I can recommend going from higher to lower airflow during a roast, but that may be a story for another time.

Closing thoughts…

I’ve droned on for some time now. The reality is that this is a complex situation that requires the user to be open to and okay with nuance, uncertainty, and some trial and error.

Generally speaking, I see more people with their airflow set too low than I do people with their airflow set too high.

Also, it is really important to note that your drum speed plays a huge role in how the airflow impacts roasting… but that’s a story for a different day.

TL;DR The Lighter Test: It can tell you some helpful things, but it is also prone to error, and there may be better ways to dial in airflow in the roasting system. But maybe not.

Thanks for your time, and if you want any help understanding the heat transfer in your roasting machine, you can always schedule a zoom call with me.

Thanks,

Rob