The 2026 review makes it possible to say, link by link, what is established. Origin first: of 176 arabicas analyzed by metabolomics, it is geographic origin that structures the profiles most, ahead of processing, altitude and variety. The same article reports, however, a few pages earlier, that post-harvest would account for about 60% of the cup profile; the two figures are not reconciled, and the most defensible reading is that they do not measure the same thing, one being the variance among three processings of the same lot, the other the variance among coffees from around the world.

Processing moves a coffee; the origin decides where it starts from. The variety fixes the stock: more caffeine and CGAs and less sugar in canephora, hence more bitterness and less matter to caramelize; and the introgression of canephora into resistant arabicas comes with a measurable “quality penalty” in the genome, the only place in the chain where a loss in the cup can be tied to an identifiable piece of DNA. The same variety does not have the same cup everywhere: a Geisha sometimes disappoints, a Caturra at 1,400 m surprises.

Yang et al. 2026, p. 3, 12-13; LC ch. 17

Altitude is a real indicator and an insufficient predictor. Coffees from 1,950 to 2,100 m beat coffees from 1,600 to 1,680 m in appearance, aroma and overall quality, because each 100 m slice above 900 m adds about ten days to ripening; but caffeine and CGAs fall with altitude in Ethiopia and rise in Costa Rica, depending on the cultivar, the local climate and the aspect of the slope; and above 2,000 m cold stress triggers lipid peroxidation that weakens the bean in storage.

The real parameter is temperature: heat speeds up ripening and depletes precursors, raises butane-2,3-diol and butane-1,3-diol along with earthy and green notes, whereas cold favors ethanol, acetone, acidity and fruit. Shade is a thermal correction, not a virtue: 30 to 50% improves the cup in hot, low zones, more than 60% makes flowering, yield and CGAs plunge, and at cool altitude (Oporapa, Colombia) shade degrades fragrance, acidity, body and preference.

Fertilizer can be tasted: ammoniacal forms of nitrogen inhibit taste enzymes, and potassium chloride degrades the cup through the accumulation of chloride, whereas sulfate and nitrate are neutral.

Yang et al. 2026, p. 7-12; LC ch. 17

Processing makes three chemistries, not three degrees on one scale. The dry process leaves more CGA analogues, hence more bitterness and astringency after roasting, alongside the fruit it is sold for; the washed process gives more caffeine, more bitterness and more body, contrary to its reputation for lightness; honey produces phenolamides of its own, including N-caffeoyltryptophan.

Fermentation, a cleaning operation for a century and a half, reshapes the stock of precursors through the enzymes of yeasts and lactic acid bacteria, cellulases, pectinases, amylases, proteases, lipases; pectinase brings a fermentation of 36-72 hours down to 24 hours; and the authors call it a “black box,” for lack of knowing how temperature, pH and oxygen steer the microbial succession. Roasting, finally, is covered in no review of the chain: the published science of taste stops at the green bean, just where the roaster's trade begins.

Water, which decides the extraction of acids and the rendering of acidity through its hardness and its alkalinity, has its own plate (water), and the grind, which sets the surface offered and the uniformity of extraction, has that of the preparations.

Yang et al. 2026, p. 12-14; LC ch. 16b and 17