THE PROCESS IN MOTION

Where do rio and the potato taste come from?

Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.

▶ Play the process01Rio begins in the rain02From chlorophenol to TCA03Potato begins on the cherry04Roasting makes the taste

Rio begins in the rain

Rio, that medicinal, phenolic or iodine-like taste, affects 20 to 25% of Brazilian production. The condition that triggers it is known: a harvest made in wet weather. Rio beans are overrun with fungi, Aspergillus, Fusarium, Penicillium, Rhizopus, and with lactic acid bacteria.

Read the stages without animation

1. Rio begins in the rainRio, that medicinal, phenolic or iodine-like taste, affects 20 to 25% of Brazilian production. The condition that triggers it is known: a harvest made in wet weather. Rio beans are overrun with fungi, Aspergillus, Fusarium, Penicillium, Rhizopus, and with lactic acid bacteria.
2. From chlorophenol to TCAThese microorganisms transform by methylation the 2,4,6-trichlorophenol present on the bean into 2,4,6-trichloroanisole, TCA, the molecule behind cork taint. It is in all rio coffees, between 1 and 100 ppb; less than half disappears during roasting, and its threshold in the brew is very low: 8 parts per trillion for the odor, 1 to 2 in the mouth.
3. Potato begins on the cherryThe “potato” defect is specific to the Great Lakes. The dominant hypothesis points to the Antestiopsis bug, present on 98.7% of Rwandan farms: its sting would open the door to bacteria. A 2025 trial shows that a simple mechanical wound is enough, without any bug.
4. Roasting makes the tasteThe marker, IPMP, forms when contaminated beans are heated between 60 and 200 °C, the range of every roast. It is sporadic, one bowl on a table, and invisible on the green coffee: a clean sample does not guarantee the lot.

To remember: A cup defect is a chain: a link in the field, a molecule formed along the way, a perception threshold in the cup.

In history: Spadone, Takeoka and Liardon identified TCA in 1990 at the Nestlé research center in Lausanne.

Read the full article and its sources ↓

The best-elucidated defect is rio, that medicinal, phenolic or iodine-like taste. It affects 20 to 25% of Brazilian production, mainly in the state of Rio, Minas Gerais, the Zona da Mata and the northwest of Espírito Santo. The condition that triggers it is known: a harvest made in wet weather.

In 1990, Spadone, Takeoka and Liardon, at the Nestlé research center in Lausanne, compared sound green beans with rioy beans. They isolated the volatiles by distillation-extraction, separated them by chromatography, then sniffed them. The culprit is 2,4,6-trichloroanisole, or TCA, the molecule behind cork taint in wine.

TCA is present in all the rio samples, between 1 and 100 ppb, together with its precursor 2,4,6-trichlorophenol. Three observations lock down the demonstration. Less than half of the TCA disappears during roasting, so the defect reaches the cup. Added to a sound coffee, it reproduces the defect. And its perception threshold in the brew is very low: 8 parts per trillion for the odor, 1 to 2 ppt in the mouth.

That leaves the origin of the molecule. The rio beans were overrun with fungi (Aspergillus, Fusarium, Penicillium, Rhizopus) and with lactic acid bacteria. This points to microbial methylation of chlorophenols, in other words to microorganisms that transform the chlorophenol present on the bean into TCA.

The “phenol” of East African coffees (Uganda, Kenya) belongs to the same family of chlorophenols. Trade pages attribute it to the chlorinated water of washing stations, phenolic pesticides, and treated pallets and container floors. No study equivalent to that of 1990 has verified these, so the causes remain trade hypotheses.

Spadone, Takeoka, Liardon, J. Agric. Food Chem. 38 (1990), p. 226-233; Green Coffee Collective, glossary “phenolic” (commercial page)

The “potato” defect (potato taste defect) is specific to the Great Lakes region: Rwanda, Burundi, Uganda, DRC. Two properties make it formidable for a buyer. It is sporadic, one bowl on a table, and it is invisible on the green coffee. In other words, it cannot be ruled out by looking at the bean, and a clean sample does not guarantee the lot.

The marker is 2-isopropyl-3-methoxypyrazine, or IPMP. It forms when contaminated beans are heated between 60 and 200 °C, that is, in the range that every roast passes through. The contamination comes earlier, but it is the roasting that makes the taste.

Ndayambaje and coauthors (Food Science & Nutrition, November 2018) isolated enterobacteria from affected beans: Pantoea, Enterobacter cloacae, Enterobacter cancerogenus, Klebsiella pneumoniae. Yet they write, in the same article, that “the source of the invasion was not identified.” Finding the bacteria does not say where they got in.

The dominant hypothesis points to the Antestiopsis thunbergii bug, present on 98.7% of Rwandan farms. Its sting would open a door to bacteria. An argument comes from the field: in Burundi, the combination of monthly insecticides, visual sorting of cherries and flotation at the station markedly reduces the proportion of bowls affected.

A 2025 experimental article (International Journal of Fruit Science) complicates this story. A mechanical wound to the cherry is enough to trigger the defect, without any bug. The cause thus shifts from the insect to the lesion, and the sting would be no more than one way among others of wounding the cherry.

Ndayambaje et al., Food Science & Nutrition (2018); Agriculture, Ecosystems & Environment (2018), Occurrence of potato taste defect in coffee and its relations with management practices in Rwanda; Mechanical Damage Causes Potato Taste Defect in Coffee Without Antestia Bug Infestations, Int. J. Fruit Science (2025), 10.1080/15538362.2025.2548292

The other defects are sorted by link. Fermented and sour come from delays between picking and pulping, from fermentation that is too long or too hot, in the tank or on the branch (overripe cherries), from storage that is too wet or from dirty water; they are the signature of excess acetic acid and ethanol. Moldy, earthy and “over-fermented” come from interminable fermentations, interrupted drying and beans brought in too wet: this is the domain where ochratoxin A appears, which justifies the storage thresholds (moisture 12.5% at most, water activity 0.7 at most).

Spadone et al. 1990; Ndayambaje et al. 2018; Int. J. Fruit Science 2025; Perfect Daily Grind 2018; Yang et al. 2026, p. 15-16

Paper, wood and the “bag” defect (baggy) are defects of time: the fat in the bean oxidizes under the effect of heat, oxygen and light, hexanal and methional are its markers, and a bean dried below 10% moisture becomes flat and stale even before aging. Paper is also the signature of quakers, those immature beans that turn pale in roasting for lack of sugars to brown. The physical sorting upstream is described on the processing plate.

Perfect Daily Grind, January 25, 2018, Coffee Defects and How to Avoid Them (trade press); Yang et al. 2026, p. 15-16; Lingle 1986; Gagné, The Physics of Filter Coffee (2021), glossary, “quakers”