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Contaminated coring tools may transfer bacteria to iceberg lettuce. The efficiency of coring tool design modifications in reducing bacterial transfer to lettuce heads was evaluated under simulated field operations. The standard coring tool consists of a stainless steel cylindrical tube welded to a tab that is inserted into a plastic handle. Design modifications included removal of the welded portion, incorporation of a shorter front straight bottom edge, or an angled bottom edge toward the front. In the first study, coring tools of four different designs were inoculated by dipping in a tryptic soy broth (TSB) suspension that contained 8.85 Log CFU/mL of Escherichia coli K-12 and then were used to core 100 lettuce heads, consecutively. Use of the standard tool resulted in 91% ± 9% positive lettuce heads. Removing the welded surface from the standard tool resulted in the highest reduction of E. coli transfer (44% ± 11.9% positive lettuce heads, P < 0.05), whereas incorporation of a short front straight edge with no welding resulted in 65.6% ± 5.6% of the cored lettuce heads being positive for E. coli. Removal of the welded surface resulted in a 40% decrease in E. coli contamination among the last 20 cored lettuce heads (81 to 100), which indicates that coring tool design modifications resulted in reduced cross-contamination. In the second study, the transfer of Salmonella to coring tools after their immersion in rinsing solutions was evaluated using imaging. The tools were dip inoculated for 2 min in water, water with lettuce extract, or TSB containing 7 Log CFU/mL bioluminescent Salmonella Newport; they were then imaged to observe spatial distribution of bacteria. There was greater retention and spatial distribution of Salmonella on the surface of tools immersed in water containing lettuce extract than in TSB and water. The results of the second study indicate that rinsing solutions that contain lettuce particulate and organic load could facilitate cross-contamination of Salmonella Newport to tool surfaces.

The influence of salinity stress on the growth, appearance, and nutritional compounds, especially phenolic compounds and carotenoids, of romaine lettuce (Lactuca sativa L.), a low salt tolerant plant, was studied. The dry weight, height, and color of the lettuce plants were significantly changed by long-term irrigation (15 days) with higher NaCl concentration (i.e., >100 mM). However, no significant differences were observed in the growth and appearance among the control, all short-term treatments (2 days; 50, 100, 500, and 1000 mM), and long-term irrigation with low salt concentration. Moreover, in romaine lettuce treated with long-term irrigation with 5 mM NaCl, the total carotenoid content increased without color change, and the contents of major carotenoids in romaine lettuce, lutein and beta-carotene, increased 37 and 80%, respectively. No differences were observed in lutein and beta-carotene contents in short-term-treated lettuce. The phenolic content of the romaine lettuce declined with short-term salt irrigation, whereas there were no significant differences among treatments exposed to long-term irrigation. This research indicates that long-term irrigation with relatively low salt concentration, rather than short-term irrigation with high salt concentration, can increase carotenoid content in romaine lettuce without causing a tradeoff in yield or visual quality.

The effect of exogenous methyl jasmonate (MeJA) on antioxidative compounds of romaine lettuce ( Lactuca sativa L.) was investigated. Lettuces were treated with various MeJA solutions (0, 0.05, 0.1, 0.25, and 0.5 mM) before harvest. Total phenolic compounds content and antioxidant capacity of romaine lettuce significantly increased after MeJA treatments (0.1, 0.25, and 0.5 mM). The total content of phenolic compounds of the romaine lettuce treated with 0.5 mM MeJA (31.6 microg of gallic acid equivalents/mg of dry weight) was 35% higher than that of the control. The increase in phenolic compound content was attributed to a caffeic acid derivative and an unknown phenolic compound, which also contributed to increased antioxidant capacity. The induction of phenylalanine ammonia-lyase (PAL) activity by the MeJA treatment indicated that phenolic compounds were altered due to the activation of the phenylpropandoid pathway. Total content of carotenoids, including lutein and beta-carotene, of the MeJA-treated lettuce did not change after 8 days of treatment, whereas the content of the control without MeJA decreased after 8 days. This research indicated that preharvest application of MeJA could increase the nutritional value of romaine lettuce under determined conditions discussed in this work.

Iceberg lettuce was evaluated for yield, microbial population, and postharvest quality either following different irrigation termination (IT) schedules or before and after a rainfall event. Lettuce receiving late (4 d before harvest) IT showed increased weight and diameter, higher aerobic bacteria counts, and lower quality than plants subjected to early (16 d before harvest) IT. Middle (8 d before harvest) IT produced similar yields, and lower microbial population in lettuce than late IT. After rain, microbial population increased by 1.5 and 3.0 log colony-forming units (CFU) /g in outer and head leaves respectively. The results revealed the importance of managing moisture conditions at harvest to enhance overall quality of lettuce. [ABSTRACT FROM AUTHOR]