Leather rot (Phytophthora cactorum) off-odour in strawberry fruit determined by sensory assessments and volatile organic compounds

Summary

Leather rot of strawberry fruit caused by the oomycete Phytophthora cactorum is detrimental to both freshly consumed and processed produce of this crop because of an off-odour flavour caused by two phenolic compounds, 4-ethylphenol (4-EP) and 4-ethyl-2-methoxyphenol (4-ethyl guaiacol, 4-EG). In this study, we have investigated differences in off-odour among strawberry genotypes and how the perception of the off-odour varies among people. Fruit of 13 strawberry genotypes, including five cultivars and eight selections, inoculated with P. cactorum were assessed by a sensory panel to determine differences in the characteristic leather rot odour. The content of aroma compounds, including the two phenolics, was analysed by headspace GC/MS and compared with sensory analysis. The amount of 4-EP and 4-EG varied greatly among the genotypes, and although the ones with the highest and lowest sensory scores also had the highest and lowest amounts of the phenolics, the correlation between their content and leather rot odour was not significant. The results clearly indicated that aroma components, of which the major ones were butanoic acid and acetic acid methyl and ethyl esters, hexanoic acid methyl ester and acetone, were important for the perception of leather rot off-odour. This suggests that natural strawberry aroma compounds can partly mask the characteristic leather rot odour independent of the level of the off-odour compounds. Consumer tests showed great variation in the ability of people to recognize the leather rot off-odour of strawberry fruit, especially at lower levels, but a few people were not able to detect the odour at all. The amount of contaminated strawberry fruit necessary for olfactory identification and to set an odour threshold of leather rot is further discussed.