Temperature-Dependent Autoxidation of Conjugated Trienols from Apple Peel Yields 6-Methyl-5-hepten-2-one, a Volatile Implicated in Induction of Scald

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Abstract
Conjugated triene (CT) oxidation products of α-farnesene have long been thought to be involved in development of superficial scald in apple fruit. Early studies found that CT hydroperoxides and the volatile 6-methyl-5-hepten-2-one (MHO) are major in vitro autoxidation products of α-farnesene. However, it was recently shown that ≥99% of the oxidation products of α-farnesene that accumulate in apple peel are conjugated trienols (CTols), isomers of 2,6,10-trimethyldodeca-2,7,9,11-tetraene-6-ol. HPLC-purified CTols from fruit of two scald-susceptible cultivars, Granny Smith (GS) and Red Delicious (RD), were used to study autoxidation of these compounds in vitro. Incubation of CTols in sealed glass vials under air resulted in accumulation of MHO. Oxygen enrichment did not increase the amount of MHO produced. Regardless of which cultivar CTols were derived from, at 0 °C autoxidation yielding MHO was quite slow and linear, whereas at 20 °C MHO production was much more rapid, and after several hours the rate increased abruptly. However, CTols isolated from GS and RD fruit differed in the duration of the initial lag phase and the overall level of MHO generated at 20 °C. The sharp increase in MHO production occurred after 3 h with GS CTols and at about 12 h with RD CTols. Also, the yield of MHO from GS CTols after 6 h at 20 °C was nearly 6-fold greater than that from RD CTols after 20 h at 20 °C. The antioxidants butylated hydroxytoluene and diphenyamine reduced the yield of MHO by about 97%. Recent work has shown that MHO can induce scald-like symptoms in apple peel and that tissue sensitivity increases with time in storage. This may explain the correlation between high CTol levels and scald development, and why symptoms rapidly intensify when fruits are removed from cold storage. Keywords: α-Farnesene; conjugated triene; 6-methyl-5-hepten-2-one; autoxidation; apple fruit; superficial scald