Evaluation of Postharvest Washing on Removal of Silver Nanoparticles (AgNPs) from Spinach Leaves

Persistence of Silver Nanoparticles Sorbed on Fresh-Cut Lettuce during Flume Washing and Centrifugal Drying

Increased agricultural use of silver nanoparticles (Ag NPs) may potentially lead to residual levels on fresh produce, raising food safety and public health concerns. However, the ability of typical washing practices to remove Ag NPs from fresh produce is poorly understood. This study investigated the removal of Ag NPs from Ag NP-contaminated lettuce during bench-top and pilot-scale washing and drying. Ag NP removal was first assessed by washing lettuce leaves in a 4-L carboy batch system using water containing chlorine (100 mg/L) or peroxyacetic acid (80 mg/L) with and without a 2.5% organic load and water alone as the control. Overall, these treatments removed only 3-7% of the sorbed Ag from the lettuce. Thereafter, Ag NP-contaminated lettuce leaves were flume-washed for 90 s in a pilot-scale processing line using ∼600 L of recirculating water with or without a chlorine-based sanitizer (100 mg/L) and then centrifugally dried. After processing, only 0.3-3% of the sorbed Ag was removed, probably due to the strong binding of Ag with plant organic materials. Centrifugation only removed a minor amount of Ag as compared to flume washing. However, the Ag concentration in the ∼750 mL of centrifugation water was much higher as compared to the flume water, suggesting that the centrifugation water would be preferred when assessing fresh-cut leafy greens for Ag contamination. These findings indicate that Ag NPs may persist on contaminated leafy greens with commercial flume washing systems unable to substantially reduce Ag NP levels.

Behavior of Silver Nanoparticles in Chlorinated Lettuce Wash Water

ABSTRACT

Use of silver nanoparticles (Ag NPs) in pesticides may lead to residual levels in food crops, thus raising food safety and environmental concerns. Because little is known about Ag NP behavior in wash water during typical commercial washing of fresh produce, this study assessed the temporal changes in Ag NP behavior when exposed to 2 to 100 mg/L free chlorine (Cl2) in simulated lettuce wash water for up to 10 days. Aggregate size and zeta potential of Ag NPs (5 mg/L) were evaluated in the presence and absence of dissolved lettuce extract (DLE, 0.1%), with Ag NPs in deionized water serving as the control treatment. In the presence of chlorine, greater aggregation of Ag NPs occurred over time (49 to 431 nm) compared with the control treatment (P < 0.05). Lower zeta potentials (-39 to -95 mV) were observed in the chlorine-only treatments, likely due to the formation of AgCl particles. Larger aggregates and lower zeta potentials were also observed in DLE (84 to 273 nm and -28 to -32 mV, respectively), as compared with the control treatment. After 7 to 10 days, larger aggregates were seen in the chlorine-only treatments as compared with the DLE treatments, despite lower zeta potentials, probably facilitated by nucleation and crystal growth of AgCl. Transmission electron microscopy with energy dispersive spectroscopy confirmed the formation of AgCl-Ag NP composite particles with chlorine and the embedding of AgCl and Ag NPs in the DLE matrix. Thus, DLE might stabilize and protect Ag NPs from chlorine. These findings indicate that chlorine and plant-released organic material can substantially change the behavior of Ag NPs, which may, in turn, impact both removal from fresh-cut produce during washing and their environmental fate.

HIGHLIGHTS

Production of Plant-Based Seafood: Scallop Analogs Formed by Enzymatic Gelation of Pea Protein-Pectin Mixtures

This study investigated the possibility of using a phase separation, mixing, and enzymatic gelation approach to construct seafood analogs from plant protein-polysaccharide mixtures with properties mimicking real seafood. Heat-denatured pea protein (10%, w/w) and pectin (0-1%, w/w) were mixed to produce phase separated biopolymer blends. These blends were then subjected to mild shearing (350 rpm) to obtain fiber-like structures, which were then placed in molds and set by gelling the pea proteins using transglutaminase (2%, w/w). The appearance, texture, and cooking properties of the resulting scallop analogs were characterized and compared to those of real scallop. The presence of the pectin promoted the formation of a honeycomb structure in the scallop analogs, and microscopic orientation of the proteins was observed in the plane parallel to the applied shear flow. Lower pectin concentrations (0.5%, w/w) led to stronger gels with better water holding capacity than higher ones (1.0%, w/w). The appearance and texture of the plant-based scallop analogs were like those of real scallop after grilling, indicating the potential of using this soft matter physics approach to create plant-based seafood analogs. One of the main advantages of this method is that it does not require any expensive dedicated equipment, such as an extruder or shear cell technology, which may increase its commercial viability.

Evaluating green silver nanoparticles as prospective biopesticides: An environmental standpoint

The current method of agriculture entails the usage of excessive amounts of pesticides and fertilizers. The blatant use of conventional pesticides and fertilizers over several decades has led to their bioaccumulation with adverse effects on soil biodiversity and the development of resistance by pests. With the decline in clinically useful antibiotics and increase in multi drug resistant microbes, it is imperative to develop new and effective antimicrobial therapies. Growing awareness and demand for efficacious biorational pesticides are on the rise. Silver nanoparticles are widely known antimicrobials and have been in use for several purposes for a long time. This work reviews the implications of applying silver nanoparticles in agriculture and their possible consequences. The physiological and biochemical changes in plants due to the uptake of silver nanoparticles as a consequence of its morphology, capping biomolecules and method of application are comprehensively discussed in this review article. Studies on tolerance levels or stress due to silver nanoparticles by variation in concentration/doses on diverse flora and fauna are also analyzed here. Further, phytotoxicity and genotoxicity due to the metal as well as its transformation in soil, water and sludge are taken into account. We also gauge the potential of biogenic silver nanoparticles-viable antimicrobial agents for enhanced applications in agriculture as biopesticides.

In situ and real time investigation of foliarly applied silver nanoparticles on and in spinach leaves by surface enhanced Raman spectroscopic mapping

Understanding the behavior and biological fate of silver nanoparticles (AgNPs) applied on plant surfaces is significant for their risk assessment. Our study's objective is to investigate the interactions between AgNPs and plant biomolecules as well as to monitor and quantify the penetration of AgNPs in spinach by an in situ and real-time surface enhanced Raman spectroscopic (SERS) mapping technique. AgNPs (2 μg per leaf) of different surface coatings (citrate, CIT, and polyvinylpyrrolidone, PVP) and sizes (40 and 100 nm) were foliarly applied onto spinach leaves with different exposure times (1-48 h). Cysteine is the major biomolecule that interacts with AgNPs in spinach based on the in situ and in vitro SERS pattern recognition. The interaction between CIT-AgNPs and cysteine happened in as early as 1 h after AgNP foliar deposition, which is faster than the interaction between PVP-AgNPs and cysteine. Also, the SERS depth mapping shows that particle size rather than surface coating determines the penetration capability of AgNPs in spinach, in which 40 nm AgNPs show a deeper penetration than the 100 nm ones. Last but not least, based on the results of SERS mapping, we detected significantly higher amounts of 40 nm CIT-/PVP-AgNPs than 100 nm CIT-AgNPs internalized in the leaf tissues after 1 h exposure. The estimated percentage of internalized AgNPs (0.2-0.8%) was significantly smaller than that of the total residual Ag (9-12%), indicating the potential transformation of the AgNPs into other Ag species inside the plant tissues. This study facilitates a better understanding of the behavior and biological fate of AgNPs in plant tissues.

Sonication-Assisted Production of Fosetyl-Al Nanocrystals: Investigation of Human Toxicity and In Vitro Antibacterial Efficacy against Xylella Fastidiosa

Recently, there is a growing demand in sustainable phytopathogens control research. Nanotechnology provides several tools such as new pesticides formulations, antibacterial nanomaterials and smart delivery systems. Metal nano-oxides and different biopolymers have been exploited in order to develop nanopesticides which can offer a targeted solution minimizing side effects on environment and human health. This work proposed a nanotechnological approach to obtain a new formulation of systemic fungicide fosetyl-Al employing ultrasonication assisted production of water dispersible nanocrystals. Moreover, chitosan was applicated as a coating agent aiming a synergistic antimicrobial effect between biopolymer and fungicide. Fosetyl-Al nanocrystals have been characterized by morphological and physical-chemical analysis. Nanotoxicological investigation was carried out on human keratinocytes cells through cells viability test and ultrastructural analysis. In vitro planktonic growth, biofilm production and agar dilution assays have been conducted on two Xylella fastidiosa subspecies. Fosetyl-Al nanocrystals resulted very stable over time and less toxic respect to conventional formulation. Finally, chitosan-based fosetyl-Al nanocrystals showed an interesting antibacterial activity against Xylella fastidiosa subsp. pauca and Xylella fastidiosa subsp. fastidiosa.

Controlled Tuning of the Size of Ag-Hydrosol Nanoparticles by Nonstabilized THF and Detection of Peroxides in THF

Surface plasmon extinction (SPE) spectra of plasmonic nanoparticles (NPs) are sensitive indicators of their composition, size, shape, interparticle interactions, and of the dielectric constant of their ambient. In this study, rapid changes in SPE spectra of Ag NPs suggesting variations in NP size and concentration were detected after addition of aged tetrahydrofuran (THF). Using time-dependent UV/vis spectroscopy combined with factor analysis, transmission electron microscopy imaging, selected-area electron diffraction, and energy-dispersive X-ray analysis, we observed that an over-limit amount of aged THF fully dissolved Ag NPs with no plasmon recovery. By contrast, an under-limit amount led to incomplete dissolution of Ag NPs and, after reaching the turnover point, to spontaneous recrystallization on residual Ag nuclei, as demonstrated by the SPE band intensity recovery to the original or even higher values. The newly formed Ag NPs were isometric, and their diameter was dependent on the added amount of THF. Furthermore, both Ag NP dissolution and recrystallization were caused by THF peroxides and their reduction products. Therefore, the dissolution of Ag NPs and the resulting hydrosol bleaching may be used as an indicator of the presence of peroxides in THF. Moreover, the reaction of aged THF with Ag NPs can be employed as a tool for tuning the size of Ag NPs in hydrosols.