Could it be that sweetness is the prototype of all desire?
-Michael Pollan
For some coffee diehards, adding sugar to coffee can feel sacrilegious. Yet, when it comes to talking about coffee itself, any inherent sweetness in the bean holds immense value and fascination. A pleasant cup with abundant natural sweetness signals quality.
This is why sweetness has become a key sensory category for the Specialty Coffee Association (SCA). In its new Coffee Value Assessment, an evaluation tool to help coffee tasters determine a coffee’s value, sweetness is qualitatively scored on a scaled ranking from 1-9, distinct from the old scoresheet, where sweetness was graded as a ‘yes/no’ valuation. Either you tasted sweetness, or you didn’t.
Late in 2022, SCA also announced its partnership with the Coffee Science Foundation and Ohio State University to launch a “Sweetness in Coffee” research project. “Natural sweetness makes coffee more valuable to consumers, roasters, and coffee farmers alike,” the press release states. Tellingly, the SCA’s announcement on the “Sweetness” research project doesn’t mention if they’ll study Robusta coffees or simply focus on Arabicas. For so long, we’ve equated specialty coffee—and therefore the idea of sweetness and quality—with the latter.
Regardless, it may be in our best interest to look at sweetness in Robusta. Climate change will force farmers to look beyond Arabicas and grow more climate-resilient Robustas. Despite this pressing need, it’s still hard for Robusta growers to gain an equitable price for their crops. Next to Arabica, Robusta’s higher bitterness overall still keeps it from wider acceptance among specialty consumers. While quality improvement is not a magic bullet, it can help challenge Robusta’s stigma as a commodity blend filler.
Like their colleagues in the specialty Arabica market, the fine Robusta world is no less preoccupied with exploring and maximizing sweetness. The World Coffee Research’s recent report on Robusta quality suggests that, besides wanting a clean cup, “younger generations are seeking increased sweetness and decreased bitterness, and this may be an important target for improving Robusta.”
Upping Robusta’s sweetness would be a major boost for its palatability. Time and again, high-scoring Arabicas reward drinkers with remarkable sweetness in the cup. Robustas’ potential, on the other hand, remains gravely untapped.
An Anaerobic Win for Robustas
A few critical advances in improving Robusta are already underway in the hands of producers and coffee processors. Besides growing newer cultivars with better cup profiles—like the TR varieties in Vietnam—they’re also trying experimental processes that churn out sweeter, fruitier lots. Natural anaerobic (oxygen-poor) fermentation process, in particular, has shown great results for Robustas.
Upping Robusta’s sweetness would be a major boost for its palatability. Time and again, high-scoring Arabicas reward drinkers with remarkable sweetness in the cup. Robustas’ potential, on the other hand, remains gravely untapped.
Coffee fermentation expert Lucia Solis believes that anaerobic processes such as carbonic maceration suit Robustas even better than Arabicas. She makes the case for this while reviewing a 2022 study measuring carbonic maceration’s effects on Robusta in an episode of her podcast, Making Coffee with Lucia Solis.
Carbonic maceration counts as a type of natural in-cherry anaerobic fermentation. A common red winemaking method in Beaujolais, France, carbonic maceration involves sealing whole grape clusters inside a container pumped with carbon dioxide to push away oxygen. For coffee, it means sealing whole cherries inside containers that are also flushed with carbon dioxide. In contrast, regular anaerobic processing relies on the natural production of carbon dioxide during fermentation as a by-product to keep oxygen away. Though not always, it’s becoming customary for coffee producers who embrace anaerobic processes also to experiment with long ferment time.
To Solis, a lengthy in-cherry fermentation is more detrimental than beneficial to Arabicas. Arabica cherries’ mucilage can break down so much after a long in-cherry ferment, leaving the beans with little protection and lubrication during pulping, or the process to remove the cherry skin from the bean. “The problem of fermenting whole cherries before pulping,” she cautions, “is that you’re breaking down the mucilage layer, which is good, and that’s the goal…But then when we go and pulp the [Arabica] cherries, they no longer have the mucilage layers to protect and lubricate them while they’re going through the pulper.” A coffee pulper’s “two metal plates squeezing these cherries together…can crack and damage a lot of seeds,” she says, “leading to lower yield.”
Since coffee cherries mainly comprise seeds (the beans we’re talking about when we say coffee beans), they do not hold much fermentable juice and sugars. A short ferment time for Arabica is not only sufficient but safer in Solis’s view since it reduces the risk of microbe spoilage.
Unlike Arabica, Robusta cherries’ sturdier skin and mucilage make them a better candidate for long in-cherry ferment. “Robustas tend to have a much tougher, thicker, and stubborn mucilage layer,” states Solis on the podcast. “So [a long time of 120 hours] in cherry at high heat to break down that mucilage could actually be a very helpful step, instead of one that harms the coffee.”
Another benefit of long ferment for Robusta is a decrease in bitterness. As the paper Solis discusses, called “Changes in the Chemical and Sensory Profile of Coffea canephora var. Conilon Promoted by Carbonic Maceration,” points out, “the contents of caffeine and chlorogenic acid in [Robusta] declined significantly as the fermentation time progressed.” Next to Arabica, Robusta generally tastes harsher and more bitter due to its higher caffeine and chlorogenic acid content. Arabicas could have 6-8% of their dry weight in chlorogenic acids; Robustas up to 10%. Caffeine accounts for 1-1.2% of the dry weight in Arabicas and almost double that in Robustas at 1.5-2.5%
For Solis, even without a gain in sweetness, a reduction in bitterness already signals a processing win for Robusta. “[High amount of caffeine and chlorogenic acids] can make Robusta a little bit more harsh than its Arabica counterparts,” she says on the show. “So if we can use processing to mellow out some of those sharp edges, it’s also a win.”
The Role of Microbes
Miguel Meza, co-founder of Paradise Coffee Roasters and longtime Robusta champion, concurs that long fermentations reduce Robusta’s bitterness by lowering caffeine and chlorogenic acids. As a coffee grower who experiments with various yeast inoculations, he is also interested in how different microorganisms can help the cup profile.
“There are numerous studies showing fermentations of Robusta inoculated with various microorganisms like Saccharomyces cerevisiae, Lactobacillus species, Rhizopus oryzae, and Rhizopus oligosporus can reduce the caffeine and chlorogenic acid content of the coffee as well as improve sensory quality,” writes Meza in an email.
Beyond decreasing bitterness, long ferments with different yeast and mold inoculums also seem to add fruity esters and flavorful aroma compounds to Robustas. While stipulating that there needs to be further study, Meza speculates that after fermentation, chlorogenic acids break down “into other acids like Ferulic acid. Ferulic acid could potentially be degraded in the roasting process to flavorful phenolic compounds that have spicy or vanilla-like aromas.” Here, Meza mentioned aromas, which play a more significant role than sugars in how we perceive sweetness in coffee.
There are numerous studies showing fermentations of Robusta inoculated with various microorganisms … can reduce the caffeine and chlorogenic acid content of the coffee as well as improve sensory quality. miguel meza
After fermentation and roasting, coffee holds minimal sugars—yeast eats up sugars during fermentation, and roasting degrades and caramelizes them. As one Barista Hustle blog post sums up, any sugars left after roasting sit “well below the taste threshold.” What enhances the taste of sweetness is the by-product aroma compounds like “sweet-fruity smelling esters and aldehydes [and] sweet-caramel notes of furans,” the post explains. So even if Robustas have lower sugars than Arabicas to begin with, experimental inoculation can still increase the former’s sweetness by introducing esters and other aroma compounds.
One of Paradise’s experimental Robusta releases this year seems to support Meza’s hypothesis. Collaborating with the Ecuadorian grower Denise Bustamante, they inoculated a natural anaerobic Robusta lot with “a lactic acid-producing yeast strain.”
“We asked Denise to make a special lot for us using our Lactic Natural process we developed in [our own farm in] Hawaii for Arabica coffees,” writes Meza on Paradise’s website. “The cherries were fermented anaerobically as whole cherries with a lactic acid producing yeast strain for more than a week before drying as a natural (dried in the cherry) on raised screens.”
Without oversimplifying, addressing sweetness in Robusta largely boils down to managing its high chlorogenic acids and caffeine. Extended fermentation with yeast inoculations helps break down Robustas’ tougher mucilage, cuts down bitterness, adds sweetness by decreasing caffeine, and degrades chlorogenic acids into aroma compound precursors.
The resulting coffee was sweet and mildly fruity with vanilla and toasted coconut impressions. The presence of fruit is notably gentle in acidity, distinguishing the coffee from many specialty Arabicas. Coffees like Paradise’s Lactic Natural offer a quality-focused alternative to those who enjoy a clean specialty cup but not necessarily high acidity. It shows a different facet of what natural coffee sweetness can be.
Without oversimplifying, addressing sweetness in Robusta largely boils down to managing its high chlorogenic acids and caffeine. Extended fermentation with yeast inoculations helps break down Robustas’ tougher mucilage, cuts down bitterness, adds sweetness by decreasing caffeine, and degrades chlorogenic acids into aroma compound precursors. Anaerobic natural Robustas from roasters like Paradise or Nguyen Coffee Supply already prove that Robusta can abound with sweetness.
Like for Arabica, sweetness in Robusta doesn’t happen by chance, and anaerobic fermentation typifies one proven method to improve Robusta. However, experimental processes involve more risks and investments for producers. For Robusta to gain real momentum, processing improvements need to occur on a wider spectrum, covering both the specialty and commodity ends.
As arable lands for Arabicas dwindle, centering non-Arabica species like Robusta seems most urgent. But we’ve known this for a while. The agony of procrastinating over exploring non-Arabicas is trusting that the venture doesn’t come too late. “The less prejudice we hold against non-Arabica varieties, and the more work we do now developing their potential,” says Meza in a STiR magazine interview, “the better the coffee we drink is going to be 20-50 years from now.”
