Rwandan Coffee arabica has caffeine increasing with altitude, but undetectable phytomelatonin

• Rwandan C. arabica at high altitude had more caffeine than at lower altitudes. • TLC, GC–MS, and HP-LC detected no melatonin in Rwandan C. arabica . • Melatonin products from Belgium failed the quality assessment test. Altitude affects plants’ constituents. The presence of caffeine and melatonin in many plants, including C. arabica, has been widely reported. However, a possible variation of their quantity in Rwandan C. arabica samples across different altitudinal locations is not yet known. This study sought to provide this information. C. arabica samples from Kayonza, Ruhango, Gakenke, Gatsibo, Nyagatare, and Karongi districts, covering different altitude ranges in Rwanda, were obtained from the National Agriculture Export Development Board (NAEB). Analytical techniques like Thin-Layer Chromatography (TLC), High-Performance Liquid Chromatography (HPLC), and Gas Chromatography-Mass Spectrometry (GC–MS) were used. C. arabica samples from higher altitudes yielded higher caffeine content, while instant coffee samples showed a broad caffeine range. However, all the techniques detected no phytomelatonin in the C. arabica samples collected from different altitudinal locations in Rwanda following two extraction methods, even though melatonin supplements from pharmaceutical outlets manufactured in the USA (110 %), Canada (106 %), Denmark (102 %), and Belgium (2.8 %) had detectable melatonin. Rwandan C. arabica has no phytomelatonin but shows increased caffeine content with increasing locational altitude. Further studies to analyze the precursors and enzymes involved in the phytomelatonin biosynthetic pathways in plants are needed to unravel this mystery.