To reveal its flavor, coffee has to be roasted. In fact, the green coffee bean contains all ingredients necessary for the later development of coffee flavor. It is now widely accepted that free amino acids and peptides are required for the generation of coffee aroma. However, the mechanisms leading to defined mixtures of free amino acids and peptides remain unknown. Information pertaining to the identification of precursor proteins is also lacking. To answer some of these questions, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) was used to follow the fate of green coffee proteins. Two conditions were considered: roasting and incubation of green coffee suspensions at 37 degrees C. Coffee beans were observed to acquire the potential to spontaneously release H(2)O(2) upon polymerization of their proteins during roasting. Fragmentation of proteins was also observed. Conversely, H(2)O(2) was found to control polymerization and fragmentation of green coffee proteins in solution at 37 degrees C. Polymerization and fragmentation patterns under the two conditions were comparable. These observations suggest that the two conditions under study triggered, at least to some extent, similar biochemical mechanisms involving autoxidation. Throughout this study, a unique fragmentation cascade involving the 11S coffee storage protein was identified. Generated fragments shared an atypical staining behavior linked to their sensitivity to redox conditions.

Coffee flavor is the product of a complex chain of chemical transformations. The green bean has only a faint odor that is not at all reminiscent of coffee aroma. It contains, however, all of the necessary precursors to generate the unmistakable coffee flavor during roasting. The levels and biochemical status of these precursors may vary in relation to genetic traits, environmental factors, maturation level, postharvest treatment, and storage. To improve our understanding of coffee flavor generation, the sensory and biochemical impact of maturation was assessed. Maturation clearly favored the development of high-quality flavor in the coffee brew. A specific subclass of green coffee beans, however, generated high-quality coffee flavor irrespective of maturation. Biochemical aspects were examined using a dynamic system: immature and mature green coffee suspensions were incubated under air or argon. On the analytical side, a specific pool of flavor precursors was monitored: chlorogenic acids, green coffee proteins, and free amino acids. A link between maturation, the redox behavior of green coffee suspensions, and their sensory scores was identified. Compared to ripe beans, unripe beans were found to be more sensitive to oxidation of chlorogenic acids. Aerobic incubation also triggered the fragmentation or digestion of the 11S seed storage protein and the release of free amino acids.

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Oxidation reactions in coffee involve redox-sensitive polyphenols and appear to control the fragmentation of coffee storage proteins both in solution and during roasting. Coffee-specific nitrogenous flavor precursors may derive from this process. Accordingly, data converge to suggest that the redox status of the green bean before roasting might control the development of subsequent redox reactions during roasting. Consequently, we decided to identify biological events that may trigger or prevent oxidation during maturation of the coffee cherry and set the final redox status of the green bean. In a previous study, we observed that the sensitivity of green coffee to oxidative processes decreased along maturation. By using the very same samples originating from open-pollinated Robusta clones, we followed the activity of three essential redox enzymes: catalase (CAT), peroxidase (POD) and polyphenoloxidase (PPO). While CAT and POD activities increased with maturation, PPO activities decreased. Thanks to the identification of an atypical immature subclass, it appeared that CAT might be an essential factor in setting the final redox status of the green bean before the roasting event.

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