Isotope labeling studies on the formation of 5-(hydroxymethyl)-2-furaldehyde (HMF) from sucrose by pyrolysis-GC/MS

Although it is generally assumed that the reactivity of sucrose, a nonreducing sugar, in the Maillard reaction is due to its hydrolysis into free glucose and fructose, however, no direct evidence has been provided for this pathway, especially in dry and high temperature systems. Using specifically C-13-labeled sucrose at C-1 of the fructose moiety, HMF formation was studied at different temperatures. Under dry pyrolytic conditions and at temperatures above 250 degrees C, 90% of HMF originated from fructose moiety and only 10% originated from glucose. Alternatively, when sucrose was refluxed in acidic methanol at 65 degrees C, 100% of HMF was generated from the glucose moiety. Moreover, the relative efficiency of the known HMF precursor 3-deoxyglucosone to generate HMF was compared to that of glucose, fructose and sucrose. Glucose exhibited a much lower conversion rate than 3-deoxyglucosone, however, both fructose and sucrose showed much higher conversion rates than 3-deoxyglucosone thus precluding it as a major precursor of HMF in fructose and sucrose solutions. Based on the data generated, a mechanism of HMF formation from sucrose is proposed. According to this proposal sucrose degrades into glucose and a very reactive fructofuranosyl cation. In dry systems this cation can be effectively converted directly into HMF.