L-Rhamnose: Progenitor of 2,5-dimethyl-4-hydroxy-3[2H]-furanone formation by Pichia capsulata?

2,5-Dimethyl-4-hydroxy-3 [2H]-furanone (Furaneol, 1), an important aroma constituent, was detected at concentrations of up to 2 mg/l after 4 days of growth of Pichia capsulata on casein peptone culture medium containing L-(+)-rhamnose (2). Blank samples without yeast contained no 1 after the incubation period. In parallel experiments five samples of 2 exhibiting different [¹³C] abundance were given to P. capsulata. The volatile compounds formed were isolated and analysed for their [¹³C]/[ ¹²C] ratios by on-line gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). A positive correlation between the isotopes in 1 and 2 was observed; thus, 2 served as the carbon source for 1. However, 1 was formed from a postulated intermediate of 2 generated during thermal sterilization, as 1 was neither detected after sterile filtration of the culture medium nor after separate thermal sterilization of the casein peptone and 2. This observation was confirmed by an experiment investigating the time course of the formation of 1.