Optimizing water management to control respiration rate and reduce browning and microbial load of fresh-cut romaine lettuce

Abstract

We studied the influence of both deficit and excess irrigation on the quality of fresh-cut romaine lettuce, the second most important type of lettuce after iceberg. The influence of different irrigation regimes on respiration rate, browning and microbial quality of fresh-cut romaine lettuce (Lactuca sativa L.) was studied in 6 different harvests during 3 consecutive years. The total amount of water supplied to the crop was calculated taking into account the irrigation water and rainfall. Six different irrigation regimes were established based on a standard irrigation regime (SIR): −35% SIR (<221mm), −15% SIR (221–265mm), SIR (266–320mm), +15% SIR (321–370mm), +35% SIR (371–430mm) and +75% SIR (>430mm). Quality characteristics of romaine heads as raw material for fresh-cut were significantly influenced by irrigation regimes. The lowest fresh weight was obtained when cultivated with the most severe deficit and excess of water. Microbial loads were higher in whole heads subjected to the highest irrigation regime (+75% SIR). These differences were also observed in fresh-cut romaine lettuce after processing (d 0). However, after d 10 of storage, psychrotrophic aerobic bacteria increased around 3 log units without significant differences among irrigation regimes. Despite the high number of Pseudomonas bacteria in fresh-cut romaine lettuce, 6.2logcfug−1 in the deficit irrigated samples (−35% SIR) and 7.8logcfug−1 in the excess one (+75% SIR), no association with quality loss was observed at the end of storage. In fact, visual quality of the fresh-cut romaine lettuce did not differ among irrigation regimes because of the similar headspace gas composition, independently of the irrigation regime. However, when midribs were stored in air, cut edge browning was reduced in the less irrigated midribs despite the accumulation of phenolic compounds. PPO activity increased with the highest irrigation regime. Respiration rate of fresh-cut lettuce was significantly higher when lettuce was cultivated under extreme conditions, which corresponded to deficit irrigation (−35% SIR) or to excess of water (+35% SIR). In conclusion, adequate agricultural irrigation practices are needed to both guarantee the sustainability of the environment but also to assure the quality of the whole and fresh-cut produce. Because differences occur with distinct lettuce type, the optimization of water management is needed for each type of lettuce.