A comparison of postharvest physiology, quality and volatile compounds of ‘Fuji’ and ‘Delicious’ apples inoculated with Penicillium expansum

Blue mold caused by Penicillium expansum is a major postharvest disease of apple fruit, which leads to fruit loss and patulin accumulation. In this study, 'Delicious' and 'Fuji' apples were inoculated with P. expansum to investigate effects of infection on physiology, quality and release of volatile compounds. The results showed that P. expansum significantly promoted ethylene production, increased respiration rate, and induced higher peaks of ethylene and respiration in fruit. The fungus also caused a lower of membrane integrity, firmness, and the content of total soluble solid and titratable acid. Moreover, P. expansum increased the content of volatile compounds in two cultivars of apple. C6 alcohol and aldehyde contents were significantly improved by inoculation in the earlier storage stage, and ester contents were improved in the middle and later storage stage. According to PLS-DA analysis, hexanoic acid and hexanal were the most important factors to distinguish the inoculated 'Delicious' and 'Fuji' from their controls. Interestingly, among specific volatile compounds detected in the two inoculated cultivars, 3 volatile compounds, including phenethyl acetate, methyl 2-methylbutanoate and ethyl benzeneacetate, were common in both of inoculated fruit. Although wounding increased ethylene production and respiration rate, decreased quality of the two cultivars and affected volatiles release, the effects were less than P. expansum. Compared with 'Fuji', the inoculated 'Delicious' had a higher of lesion diameter, ethylene production and respiration rate and content of total soluble solid, and a lower of membrane integrity, firmness and content of titratable acid. The inoculated 'Delicious' also released more volatile compounds, especially C6 volatiles. In general, P. expansum inoculation promoted ripening and quality loss, leading to change of volatile compounds during storage and release of specific volatiles. Moreover, 'Delicious' was more susceptible to P. expansum than 'Fuji'.