Untargeted metabolomics investigation of volatile compounds involved in the development of apple superficial scald by PTR-ToF-MS

The superficial scald is an important physiological disorder affecting apple fruit during postharvest storage. To date, the accumulation, and further oxidation, of alpha-farnesene was considered as the most probable cause for the development of this physiopathy. In order to perform a more broad investigation, a PTR-ToF-MS (proton transfer reaction-time of flight-mass spectrometry) was employed to monitor the volatile organic compounds (VOCs) production along with the progression of this disorder in fruit of "Granny Smith", an apple variety known to be highly susceptible to scald. The untargeted metabolite investigation was performed on both skin and pulp, as well as comparing control versus treated tissues with 1-methylcyclopropene (1-MCP), an ethylene competitor widely used to prevent the development of this phenomenon. The rapid and non-destructive analysis of the VOC array carried out by PTR-ToF-MS identified three specific groups of metabolites in the skin, among which the 6-methyl-5-hepten-2-one (MHO) resulted significantly associated with the development of the superficial scald in apple. The results proposed in this work suggest the use of this novel equipment for an on-line monitoring of the VOCs released by the apple during the postharvest storage, as well as to use MHO as a possible biochemical marker for an early detection of the superficial scald symptoms.