Effect of ethylene and 1-methylcyclopropene on bitter compounds in carrots (Daucus carota L.)

Abstract

Sensory appearance of carrots is determined by a multiplicity of compounds, while a specific bitter taste depends on the content of phenolics including isocoumarins, polyacetylenes, and terpenes, being modulated by sugar content. Since exposure of carrots to ethylene may cause a bitter taste, the compounds potentially contributing to ethylene-induced bitterness were investigated. Storage roots of 3 carrot cultivars were stored in the dark for 7d at 17°C under aeration with and without added 7–10μLL−1 ethylene, and an aliquot was pre-treated with 1-methylcyclopropene (1-MCP) to inhibit the effect of ethylene. Accumulation of 6-methoxymellein (6-MM), total phenolics, polyacetylenes, terpenoid volatiles, and sugars was monitored, and sensory analysis was performed by a trained panel. Ethylene enhanced the accumulation of 6-MM and other phenolics, but partially inhibited the accumulation of terpenoid volatiles. However, the increase of polyacetylene levels during storage was found to be independent of ethylene treatment. In general, accumulation of 6-MM was inhibited after pre-treatment with 1-MCP. While the respiration rate of carrots was stimulated by ethylene, total sugar content during storage remained unchanged. Sensory profiling and principal component analysis revealed a close relationship between 6-MM content and other phenolics with ethylene-induced bitterness of carrots, whereas contents of the predominant polyacetylene falcarindiol and terpenoids were related to the characteristic carrot, but not bitter, taste. Pre-treatment of carrots with 1-MCP resulted in significantly lower sensory scores for bitterness of ethylene-exposed carrots compared to carrots without 1-MCP treatment. In conclusion, the results strongly suggest that ethylene-induced bitterness of carrots primarily results from the accumulation of 6-MM accompanied by further phenolics, while terpenoid and polyacetylene levels did not substantially contribute.