Abstract
Lettuce is a cool-seasoned vegetable, which is very fragile and should therefore be handled with great care. Processed lettuce (loose leaves and shredded lettuce), is more prone to handling damage than intact lettuce crisp heads. Lettuce produces very little ethylene (less than 0,1 ul/kg-1/h-1), but is extremely sensitive to damage from ethylene. In lettuce, ethylene induces a postharvest physiological disorder known as russet spotting. Due to its climacteric nature, lettuce is very sensitive to ethylene. Ethylene sets in motion a programmed series of events that accelerates senescence in lettuce. Besides causing russet spotting, ethylene also stimulates cell membrane and cell wall damage. In wounded (lightly processed) tissue ethylene production increases, resulting in increased russet spotting and accelerated senescence. Reactive oxygen species are likely role-players involved in membrane damage. Hydrogen peroxide is the primary mediator in membrane degradation; hydrogen peroxide levels remained constantly high during storage. Plant free radical scavengers, such as ascorbate peroxidase, catalase and superoxide dismutase protect membranes from damage by free oxygen radicals. The activities of ascorbate peroxidase and catalase was determined during storage and was found not to increase during senescence. Applying different postharvest treatments such as controlled atmospheric storage, 1-MCP, and short heat-shocks to iceberg lettuce significantly suppressed the negative effects of ethylene. These treatments were effective in suppressing russet spotting and senescence as indicated by reduction in moisture loss and ion leakage. Treatment with electro-activated water, did not affect ethylene sensitivity, but did limit moisture loss by lowering the rate of transpiration by increasing stomatal resistance, thus contributing to the retention of crispness and improved longevity.
Prof. C.S. Whitehead