Optimisation of steamer jet-injection to extend the shelflife of fresh-cut lettuce

Optimisation of short time blanching (steaming) was investigated using response surface methodology by analysing quality and microbial and nutritional markers over the shelflife of packaged fresh-cut lettuce. Steamer treatment time (5-10 s) and storage (1-10 days) were used as independent factors in order to optimise the process. Longer treatments (>10 s) were not feasible because of extensive damage caused to lettuce tissue. Significantly (p < 0.05) higher values of luminosity (L*), greenness (-a*) and sensory panel scores (fresh appearance, general acceptability and absence of browning) were obtained with samples treated for longer times with the steamer. Activity of browning-related enzymes present in photosynthetic tissue decreased in all the cases, however, longer treatment time (7.5-10 s) was needed with vascular tissues to reduce to similar activity values. These differences could be explained by the variation in tissue thickness. The longest steam treatment (10 s) reduced and maintained significantly (p < 0.05) lower mesophilic load than shorter treatments (5-8.5 s). However, significant (p < 0.05) decreases in vitamin C and carotenoids were observed in samples treated with longer treatments. Steamer treatment of 10 s could be considered the optimum time for maintaining the shelflife (mainly texture and browning) of fresh-cut lettuce for 7-10 days in optimum conditions.