Roast Approach: Robusta

In today's Roast Approach, we are going to be discussing how to approach roasting Robusta.  Robusta is chemically unique to the Arabica species of coffee, creating a unique cup character.  

Robusta Basics

Robusta is a species of coffee (Coffee canephora) that originates in central and western sub-Saharan Africa.  Robusta accounts for 40-44% of the global coffee production, with substantial production in Vietnam and Brazil.  Brazil is known for production of a variety of Robusta known as "conilon".  Robusta is highly regarded for its disease resistance, ease of cultivation and high yield.  In an age of advancing climate change, Robusta cultivation is an important diversification.  Farms no longer able to grow Arabica due to rising temperatures can become productive Robusta farms. 

Robusta is traded on the London futures market under the symbol RC.  Robusta is priced per metric ton and traded on the market in units of 10 metric tons per lot.  Robusta is a key component in the classic Italian espresso, where it's heavy body and thick viscosity create a rich crema.

Comparing Robusta to Arabica

Robusta and Arabica differ chemically in the coffee seed.  These differences don't make a huge impact in the way in coffee roasts, however it does make a big impact in how the coffee shows up in the cup.  Below is a chart showing the chemical differences between Arabica and Robusta.  With each coffee species, the exact chemical makeup will be influenced by the soil, cultivation, and post harvest processing. 

The main differences between Robusta and Arabica that will impact the flavor profile of the coffee are the differences in caffeine content, chlorogenic acids and soluble carbohydrates (sugars).  

Robusta can have more than 4x the caffeine content of Arabica coffee.  Aside from giving you a major buzz, caffeine has a very bitter flavor.  Robusta also nearly double the amount of chlorogenic acids (CGA's) as Arabica.  On a positive note, chlorogenic acids are responsible for the health benefits of coffee, but they are also responsible for flavors of bitterness, sourness, and astringency.  During the roasting process, chlorogenic acids break down into quinic acid, which is very bitter.  Robusta coffee also has less soluble carbohydrates, which include sucrose, monosaccharides, and polysaccharides.  These are the chemicals that are responsible for perceived sweetness. 

With these chemical differences, we know going into a roast that we will be trying to minimize the impact of the higher amount of chlorogenic acids and trying to maximize whatever sweetness we can find.  These coffees are naturally going to have the cup characteristics of a darker roasted Arabica coffee, with the higher level of chlorogenic acids and more bitter flavors.  Roasting these coffees into a dark roast will yield an even higher intensity of these flavors.

Roast Approach: El Salvador Finca Colomba Oro Tinto Robusta Especial Honey Process

The Roaster:

For these roasts, I'll be using a Proaster 1.5 kilo drum roaster hooked up to natural gas.  Batch size is 400g, approximately 35% of roaster capacity.  I've used a small batch size on this roaster to get closer to mimicking the gas power that you might find on a larger production roaster.  It must always be mentioned that each roaster is different.  The type of probe, placement of probe, and batch size will all influence the way in which bean probe temperatures are registered in a roaster.  With that said, don't read too much into the exact temperatures, but look more closely at the theory behind the approach, time spent in different development periods, and rate of rise to get a better idea of how to translate this profile over to your own machine. 

The Goal:

For this coffee, my goal was to balance the acidity and maximize the sweetness.  I didn't want to emphasize the acidity with a hot short roast.  Instead, I was looking for a roast that provided sugar caramelization, heavy body, while not having too strong of bitterness develop.  

Hopefully this will provide you with the background you need to confidently approach this coffee in your own roasting adventures.

If you would like to learn more about my four phase approach to roast profiling, check out my “Roast Approach: Roast Strategy” blog.

The Approach: 

For this El Salvador coffee, I did five versions of a roast with an extended caramelization phase.  In these roasts, I adjusted the charge temperature as well as the end temperature.  Overall, I found that extending the post crack development emphasized flavors of clove spice, cocoa powder and bitterness.  At an ending temperature around 405, with 100 seconds of post crack development, I found the coffee to have a maximized sweetness and notes of fruit leather were present.  Despite efforts at trying to pull back the heat application ahead of entering the caramelization phase, the coffee carried substantial momentum through the finish of the roast.  The coffee hit first crack around 381 degrees and had a post crack development time of 1:39.  

This roast accentuated the fruity character of the coffee, both in the aroma and in the flavor.  The aroma was sweet with notes of raspberry jam and malt.  The flavor could be described as boozy fruitcake.  There were notes of sweet cherry cordial and dried fruits, combined with a cereal sweetness.  

Roast Approach: India Kerehaklu Estate Robusta Congusta 62hr Anoxic Natural

A few quick notes about this coffee.  Congusta is a Robusta varietal that is a hybrid of species Coffea congensis and Coffea canephora.  Congensis comes from the Congo and has a sweeter cup profile than the Robusta species.  The coffee was crossed with Robusta to improve yield and disease resistance.  The hybrid was developed in Dutch East Indies colonial coffee farms in Indonesia.

The Goal:

In a similar way with the El Salvador, I was looking to maximize the sweetness and minimize any harsh acidity or bitterness.  Going in to these roasts, I had the idea of trying to "bake out" some of the harsh acidity in the coffee.  It had been a bit difficult to pull the El Salvador Robusta back enough to get all the time in caramelization that I had hoped for. So in these roasts on the India Robusta, I played around with keeping the gas application at 50% and making adjustments to the charge temperature.  Robusta is a very dense coffee.  I had a theory that with just the right charge temperature, I may be able to generate enough momentum to push the coffee through the roast. 

The Approach: 

These roasts started out as just a research experiment, but I found the results to be really nice in the cup.  I did three different roasts on this coffee.  The first was a repetition of the El Salvador roast above.  In this roast, the coffee had notes of bitter chocolate, fruit punch, molasses and fruit leathers.  It still had a harshness to it that I was hoping to remove.  I next did two roasts that had a consistent 50% gas application (from beginning to end).  The first one I started low, at 375.  I thought that this could be beneficial to the coffee having such a high density.  This roast definitely hit the "baked" goal that I was looking for.  At a charge temp of 375, the coffee finished with a roast time of 10:13.  In this roast, the cup came out with notes of bitters, orange acidity, raisin and a "cherry cough syrup" finish.  In my experience roasting high density coffees, I found that changing the charge temperature created a cascading momentum throughout the rest of the roast.  For the last roast (featured below), I increased the charge temperature to 400 and kept the heat application at 50%.  This higher charge temperature ran the risk of burning or scorching on the exterior of the bean, however I did not observe any of this on the final roast.  This coffee has a fairly high first crack point of 387.  This coffee is a good example of why I don't rely too heavily on first crack.  Despite having a post crack development of 1:29, the coffee spent 3:21 in a period where caramelization was occurring.  If I just looked at the post crack development time, this roast profile would appear to have a fairly short development time.  This coffee finished at 408.6°F.  

Unsurprisingly, the quicker roast had a more complexity and fruit forward acidity than the more "baked" versions of the roast that I tried.  The coffee had a sweet, fruity aroma.  In the cup, notes of orange citric acidity were present with flavors of cherry fruit leather, bakers chocolate, molasses sweetness and fruit snacks.  There were definitely bitter notes in the cup, but they weren't overwhelming.  With the chemical makeup of Robusta, I don't think a person can avoid the natural bitters that will be found in the roast.  Accentuating the sweetness of the coffee in order to balance against the bitters is a better strategy with Robusta roasting.  I also found this coffee to have a sweet finish that was very pleasant.

The Takeaway:

Today, people throw around the word "sustainability" casually.  To me, true sustainability means economic opportunity for all coffee producers.  It means taking care of the people who have devoted their lives to growing coffee.  I have always thought that incorporating Robusta coffee into a coffee program is meaningful sustainability.  Many coffee producers find themselves relegated to commercial coffee production because of the elevation or climate conditions of their farm.  By finding processing methods that accentuate the sweetness of the coffee and then finding usage of these coffees into specialty coffee programs, we create meaningful economic opportunity for ALL coffee producers.  Both of these Robusta coffees would fit perfectly into a specialty coffee program.  Blending 10-25% of either Robusta into an espresso roast would add heavy body and an excellent crema to your espresso extraction.  The body and roasty flavors of these coffees would fit perfectly into any dark roast blend as well.  When blending the Robusta in, my only recommendation would be to do a post roast blend, as the flavors that will complement a dark roast occur earlier in the roast than they would with an Arabica.  In a pre-roast blend, you may find that the Robusta flavors come off as "burnt" or "over developed" due to this difference.

Happy roasting!