In 2008, ionizing irradiation was approved by the U.S. FDA for use on fresh Iceberg lettuce and spinach at doses not exceeding 4 kGy to enhance microbial safety and to extend shelf life (FDA, 2008). Both whole head lettuce and fresh-cut lettuce are allowed to be treated with radiation. There have been studies on quality of irradiated fresh-cut lettuce (Fan and Sokorai, 2002, 2008, 2011; Fan et al., 2012; Hagenmaier and Baker, 1997) showing that cut lettuce in MA packaging can tolerate up to 1 kGy radiation. Earlier studies on whole head Iceberg lettuce conducted in the 1950s and 1960s showed that doses of 0.5 to 2.0 kGy reduced decay of lettuce but caused severe radiation injury as manifested by spotting, browning, and pink rib, thus precluding the use of irradiation (Bramlage and Lipton, 1965; Thomas, 1988). Kader (1986) put lettuce and leafy vegetables in the category of low tolerance to irradiation.
Our preliminary results showed that irradiation of whole head lettuce caused injury with a symptom similar to russet spotting, a physiological disorder characterized by the appearance of numerous small brown spots along both sides of the midrib (Ke and Saltveit, 1989). It is known that russet spotting can occur in lettuce that is exposed to ethylene (Ke and Saltveit, 1988), although biotic and abiotic stresses during growth, harvest, and storage can also lead to its development even without exposure to ethylene (Peiser et al., 1998). The development of russet spotting without ethylene exposure is probably the result of endogenous production of stress ethylene. It is also known that irradiation, as a stress, can induce ethylene production in some fruits and vegetables (Abdel-Kader et al., 1968; Hagenmaier and Baker, 1998). Ethylene exerts its effect through binding to its receptors. The ethylene action inhibitor 1-methylcyclopropene (1-MCP) (Sisler and Blankenship, 1996; Sisler and Serek, 1997) blocks ethylene binding to its receptor. Earlier studies (Fan et al., 1999; Fan and Mattheis, 2000) demonstrated that exposing whole Iceberg lettuce to 1-MCP decreased or eliminated tissue discoloration induced by ethylene. However, it is unknown whether 1-MCP would reduce injury caused by irradiation.
Development of russet spotting and other tissue discolorations generally involves accumulation and oxidation of phenolic compounds catalyzed by polyphenol oxidase (PPO), peroxidase (POD), and other enzymes such as phenylalanine ammonia-lyase (PAL) and indole-3-acetic acid oxidase (IAA). It is known that ethylene induces PAL and POD activities and increases the synthesis of phenolic compounds, which are converted to brown pigments by the action of PPO (Ke and Saltveit, 1988). Low oxygen and high CO2 can inhibit ethylene action (Burg and Burg, 1969), and the inhibition of ethylene action prevents the induction of PAL and IAA oxidase and inhibits activities of PPO and POD, thus inhibiting tissue discoloration (Ke and Saltveit, 1988, 1989). Indeed, low oxygen (0.5% to 8%) effectively reduced development of russet spotting (Lipton, 1967; Zagory and Kader, 1988). Our earlier study (Fan and Sokorai, 2011) on cut Iceberg lettuce showed that irradiation induced tissue browning, and MA packaging (MAP) reduced the development of tissue browning. However, it is unknown whether low oxygen and/high CO2 would reduce irradiation-induced injury in whole head lettuce. The disorders developed on whole Iceberg lettuce were different from those on cut lettuce.
In our preliminary study, whole head lettuce was irradiated at 0.5 and 1.0 kGy. After 7 and 14 d of storage in air at 4 °C, irradiated lettuce had lower scores of overall appearance than the non-irradiated samples. In addition, individual leaves of heat lettuce also developed russet spotting, rusty brown, vein browning (vein stain; Fig. 1) and pink rib. The present study focused on the effects of 1-MCP, nitrogen flushing, and MAP on irradiation-induced injury.
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