The use of silver nanoparticles (Ag NPs) in pesticides could lead to residual levels in food crops, thus raising both food safety and environmental concerns. The effectiveness of typical produce processing practices to remove Ag NPs from fresh produce is poorly understood. Further, little is known about the behavior of Ag NPs in wash water during commercial production of fresh-cut produce, which limits our ability to design effective mitigation strategies. The first study evaluated the behavior of Ag NPs over time when exposed to commercially applicable chlorine concentrations (2-100 mg chlorine/L) in simulated lettuce wash water. Aggregation and dissolution of Ag NPs (5 mg/L) were evaluated in the presence and absence of dissolved lettuce extract (DLE, 0.1%). Aggregate size of Ag NPs increased faster in the presence of chlorine (49 to 431 nm) compared to the control (P <0.05). Lower dissolved Ag concentrations and more negative zeta potentials were found in the presence of chlorine (0.01 to 0.03 mg/L and -39 to -95 mV) and DLE (0.01 to 0.14 mg/L and -28 to -32 mV), as compared to the control (0.54 to 0.8 mg/L and -10 to -20 mV) (P <0.05). Transmission electron microscopy with energy dispersive spectroscopy confirmed the formation of composite AgCl-Ag NPs particles in the presence of chlorine. The increased aggregate size over time likely resulted from nucleation and crystal growth of AgCl. In the presence of DLE, Ag NPs and AgCl precipitates were embedded in and bound to the DLE matrix. These observations suggest that chlorine and plant-released organic matter could substantially change the fate of Ag NPs in wash water and subsequently their environmental impact. In the second study the removal of Ag NPs from Ag NP-contaminated lettuce leaves was investigated during batch and small-scale pilot processing. First, a batch-type system (4-L carboy jar) was used to evaluate the impact of commercial produce sanitizers and simulated leafy green processing water on the removal of Ag NPs from contaminated lettuce. Peroxyacetic acid (PAA; 80 mg/L) and chlorine (100 mg/L and pH 6.5) were used with/without 2.5% (w/v) organic load as washing treatments with deionized water serving as the control treatment. Treating lettuce with the organic load alone, organic load with chlorine, chlorine alone, 0% organic load and peroxyacetic acid removed about 2.7%-6.6% of Ag from the lettuce after 5 min of washing. Thereafter, the removal of Ag NPs from contaminated lettuce was assessed using a small-scale pilot processing line during 90 seconds of flume-washing followed by centrifugal drying (a typical washing practice in commercial produce processing). The Ag removal efficiency ranged between 0.3% to 3%, probably resulting from strong binding of Ag with plant organic materials. Significantly greater Ag concentrations were found in the centrifugal water than in flume water, suggesting that centrifugation removed additional Ag from lettuce. The low Ag removal observed in both the simulated batch-type and pilot-scale produce washing systems demonstrate that typical produce processing conditions may not be effective in removing Ag NPs from contaminated produce. Thus, it is necessary to further investigate and develop effective methods for the Ag NP removal from contaminated produce.
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