Causes of irradiation-induced softening in peaches

Highlights: • Irradiation induced immediate softening in 'Mid Pride' peaches. • Irradiation caused an increase in electrolyte leakage. • Irradiation increased PME activity. • Loss of firmness in irradiated peaches was related to pectin hydrolysis. - Abstract: Acceleration of ripening has been offered as a cause of irradiation-induced loss of firmness in peaches (Prunus persica). This work aimed to correlate softening with biochemical and physiological responses induced by irradiation in fresh peaches during subsequent storage. Freshly harvested and cooled 'Mid Pride'peaches were treated with gamma irradiation at 1030 Gy, then stored at 1 °C for 7 days, and at 22 °C for another 6 days, during which time ripening and biochemical parameters were measured. The changes in respiration rate, ethylene, firmness, total soluble solids (TSS), titratable acidity (TA), water-soluble pectin (WSP) and activity of polygalacturonase (PG) and pectin methylesterase (PME) due to treatment and storage were evaluated. Firmness was significantly reduced immediately following irradiation treatment, whereas total pectin remained unaltered. WSP, however, increased upon irradiation and also during ripening. Despite an increase in respiration rate, ethylene evolution, TSS and TA were unaffected by irradiation. PG activity was not affected by irradiation, but, both PME and electrolyte leakage increased following irradiation treatment. For early ripe irradiated 'Mid Pride' peaches, softening during storage appears to be related to pectin hydrolysis, PME activity and electrolyte leakage, and not associated with ethylene evolution or exo-PG activity.