Caffeine Crystal Growth on Roasted Coffee Beans Misidentified as Biological Spoilage ^†

Abstract

The spoilage of roasted coffee products is largely suppressed through industry standards regarding storage and packaging. Conversely, biological contamination can be attributed to cross-contamination. Meanwhile, the increasing trend at the upper end of the specialty coffee industry consisting of more complex (e.g., yeast inoculation and fruit infusion) and longer post-harvest processing (e.g., extended anaerobic fermentation) methods might be considered at risk for potential spoilage. Here, we report the first case of a mistakenly confused accumulation of mold mycelium on high-end roasted Coffea arabica beans (Gesha variety from Barú, Panama; natural anaerobic process and greenhouse-dried), ultimately identified through UV/Vis spectroscopy (against a caffeine solution) as a conglomerate of needle-like caffeine crystals growing over the beans’ surface. Biological spoilage was unambiguously discarded due to negligible microbial activity, as shown through colony counting and a mycotoxin analysis. Furthermore, we demonstrated that the roast degree (development time after the first crack) had a significant effect on the growth of the crystals upon storage (RT and moderate light exposure). Darker roasts (70 s after the first crack) showed a higher accumulation of caffeine crystals than lighter roasts (20 s after the first crack), most likely promoted by coffee oil secretion. However, micro-crystal growth had already been detected in lighter roasts. While uncommon in roasted beans and, so far, poorly documented, crystal formation relates to increased caffeine availability at the surface through initial seed germination. Through the migration towards the endosperm surface, the xanthophyll antibacterial function is activated. A similar confusion of spoilage can be observed in soluble coffee granules. Yet, here, crystals growth was solely related to the high caffeine concentration. In addition, apparent spoilage in soluble coffee does not have the same economic burden as in high-end roasted coffee, and it also remains unrelated to any physiological process.