The recent outbreaks of foodborne illness associated with leafy greens highlights the limitations of chemical sanitizers that are currently used by the produce industry. A “kill” step is needed to ensure the safety of such produce. Ionizing radiation is known to effectively eliminate human pathogens such as Escherichia coli O157:H7 on fresh produce (Niemira and Fan, 2006). In 2008, ionizing irradiation was approved by the U.S. Food and Drug Administration for use on fresh Iceberg lettuce and spinach to enhance microbial safety at doses not exceeding 4 kGy. Either whole lettuce or fresh-cut lettuce is allowed to be treated with radiation. Irradiation is ideally applied after packaging to prevent post-process contamination. However, the commercial application of irradiation to fresh produce is still limited partially as a result of concerns about possible damage to sensory and nutritional quality.
Fresh-cut Iceberg lettuce for food service consists predominantly of shredded lettuce. In the retail market, fresh-cut Iceberg lettuce is often mixed with other fresh-cut vegetables for use as pre-packaged salads. Processing of fresh-cut lettuce typically involves removal of outer leaves, shredding, washing, sanitizing, rinsing, and dewatering before MAP using film bags (Barth et al., 2009). Fresh-cut lettuce has a relatively short shelf life even when stored in MAP, mostly as a result of the development of tissue browning and development of off-flavor/off-odor (Hodges and Toivonen, 2008).
Hagenmaier and Baker (1997) studied the effect of irradiation on cut Iceberg lettuce under MAP and found irradiation at 0.15 to 0.5 kGy in combination with chlorine washing reduced the microbial population by 3 to 4 log colony-forming units/g. Massey et al. (2006), using the bending of lettuce discs under application of a given weight as an index of radiation damage, suggested that lettuce can tolerate 0.6 to 1 kGy radiation. Increased O2 consumption and production of CO2 by the irradiated lettuce were also observed. Foley et al. (2002) found that 0.55 kGy radiation in combination with chlorination caused a 5.4 log reduction of E. coli O157:H7 on fresh-cut Iceberg lettuce. Irradiation at this dose did not cause any change in texture or other sensory attributes of lettuce packaged in film bags, indicating that low-dose irradiation can improve both safety and shelf life. Kim et al. (2005) found irradiation at 1.0 kGy reduced Listeria ivanovii to below the limit of detection and did not adversely affect visual quality of lettuce during cold storage in air. In our previous study (Fan and Sokorai, 2002), fresh-cut Iceberg lettuce was irradiated at doses up to 4 kGy in MAP, and it was found that overall visual appearance of lettuce irradiated at 1 or 2 kGy was the best. The better quality is related to high CO2 and low O2 levels observed in packages of the irradiated samples. Warm water treatment (47 °C, 3 min) in combination with low-dose (0.5 or 1 kGy) irradiation helped maintain the sensory quality of fresh-cut lettuce (Fan et al., 2003). Although there have been studies of the quality of irradiated Iceberg lettuce, packaged either in air or with MAP, showing that fresh-cut lettuce can tolerate low-dose irradiation, there have been no quality studies of irradiated fresh-cut Iceberg lettuce in which MAP is directly compared with air storage.
Many factors have to be considered for commercial application of irradiation on lettuce. The availability of irradiation facilities is one of the factors. Ideally, an irradiation facility is located on the site of the fresh-cut processing plant so that fresh-cut lettuce can be irradiated immediately after being packaged. However, currently, there are only a limited number of radiation facilities in the country, and most are located away from major fresh produce packing and processing facilities. For fresh-cut produce to be irradiated, it might have to be shipped to an off-site irradiation facility, which could take 1 to 2 d, depending on the distance, and thereby delaying the time of irradiation. Therefore, the objectives of this study were to study the effects of MAP and delay of irradiation on quality of irradiated fresh-cut lettuce.
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