Excitation–emission matrix fluorescence spectroscopy for monitoring coffee fermentation: contribution of chlorogenic acid to the emission band and discrimination of the fermentation stage

Chlorogenic acid (CGA) is widely proposed as a fluorescent indicator of coffee fermentation, but whether the fluorescence of a coffee extract reports its CGA content has not been tested. Excitation–emission matrices (EEMs) were acquired for water extracts and ground powder from Indonesian Robusta beans sampled after 0–4 d of fermentation, with CGA determined by HPLC. A CGA standard fluoresced near 370 nm excitation and 450 nm emission. In a standard-addition experiment the amplitude of the added standard, fitted freely, matched the volumetrically known amount (α/f = 0.8–1.1), so CGA fluoresces additively and roughly quantitatively in the extract. It nevertheless supplies only one-third to two-thirds of the band, whose intensity does not increase with CGA content, so the band is not a CGA read-out. The reference was also poorly defined for an individual extract: beans within one batch differed in CGA more than the fermentation stages did, only 33 % of the variation being between stages. We therefore do not claim quantitative CGA determination from the EEM. Linear discriminant analysis, by contrast, assigned the fermentation stage of extracts from unseen beans with 71 % accuracy for water versus 57 % for powder (chance 20 %), 73 % among the 1–4 d stages alone, and unfermented beans without error. One fermentation run was sampled, so these accuracies bound the performance expected on independent runs. EEM of a water extract is thus a promising indicator of the fermentation stage, whereas CGA quantification would require a reference measured on each extract.