Impact of Modified Atmospheres on the Vitamin C Content of Salad-Cut Romaine and other Lettuces
Modified atmosphere packaging (MAP) is a key technology for maintaining salad-cut lettuce quality, mainly by reducing cut-edge discoloration. However, the use of MAP may have detrimental consequences, and here we studied the impact of low-O2 atmospheres (0.2 to 1%) alone or in combination with high-CO2 atmospheres (3, 6, 9-m10%) on vitamin C concentrations. Iceberg and Romaine lettuces were trimmed, cut manually into salad-size pieces, rinsed in chlorinated water (50 ppm sodium hypochlorite, pH 7 for 20 s), spun in a manual spinner, packaged in perforated plastic bags, and stored in in the dark in containers with flows of humidified air or controlled atmospheres for up to 12 days at 5°C. Green leaf lettuce was prepared as whole washed leaves and also stored under the same conditions. Pieces or leaves were evaluated for appearance (overall visual quality, discoloration, and decay) and composition (ascorbic acid and dehydroascorbic acid by HPLC). Initial total vitamin C concentrations in cut iceberg, cut romaine, and green leaf lettuce leaves were 10, 25-40, and 55 mg 100 g-1 fw, respectively. The rate of vitamin C loss varied, but, in all experiments, total ascorbic acid concentrations decreased more rapidly in pieces or leaves stored in CO2-containing atmospheres than in air or low O2, alone, and before the loss of marketable quality. Of six enzymes in the ascorbate-glutathione pathway that were assayed, only ascorbate peroxidase activities were lower in a 10% CO2 atmosphere than in air-stored cut romaine lettuce. Reduced and total glutathione concentrations were substantially lower in the CO2-stored romaine lettuce.