The FR does not require the presence of Ca 2+ or a strong base, nor does it depend on the presence of contaminating carbohydrates like glycolaldehyde (GA) . The origin of a complex mixture of carbohydrates from the FR in the absence of a priming aldol catalyst and the generation of a "priming sugar" by a non-obvious conversión of HCHO to glycolaldehyde via an unknown and slow mechanism remains a problem in understanding the FR . Caramelization in the FR remains an enduring phenomenon which also requires explanation.