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Jorge M. Fonseca

The effect of moisture conditions on yield and microbial quality of iceberg lettuce was investigated. Iceberg lettuce cvs. Honcho II and Sahara grown at the Yuma Agricultural Center were evaluated for weight, diameter, microbial population, and postharvest quality, either following different irrigation termination schedules or before/after a rainfall event. Two trials were conducted with early (24 or 18 days before harvest), middle (16 or 8 days before harvest) and late (6 or 4 days before harvest) irrigation termination. Lettuce receiving the last irrigation 6–4 days before harvest showed increased weight and diameter, higher total aerobic bacteria and shorter shelf life than plants having the early irrigation termination. The plants receiving middle termination irrigation showed similar weight at harvest, lower total aerobic count and longer shelf life than plants receiving late irrigation termination. The effect of the field's moisture prior to harvest on quality was further evaluated with lettuce harvested before and 1, 2 and 7 days after a rainfall event. After rain, mesophilic bacteria population increased 1 log10 CFU/g in outer leaves and more than 2 log10 CFU/g in head leaves. The microbial population in outer leaves declined more rapidly, possibly due to more rapid drying and higher impact of sun UV light. The results from this study suggest that managing moisture conditions at harvest is important to enhance quality of lettuce. Although the potential decrease in weight produced with an early irrigation termination is a great concern of growers, it was shown in this study that excessively late preharvest irrigation of lettuce is not necessary to obtain maximum weight at harvest.

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Jeffrey M. Hamilton and Jorge M. Fonseca

Our study evaluated changes in nutritional content of leafy vegetables at harvest and during postharvest in response to nutrient solutions of increasing salinity using a recirculating ebb and flow irrigation system within a controlled environment (CE). Two antioxidants, ascorbic acid (AsA) and total phenolics (TP), were used as proxies for nutrition as determined by the 2,6-dichloroindolphenol titrimetric and the Folin-Ciocalteu methods, respectively. Two arugulas, Diplotaxis tenuifolia (L.) DC cv. Sylvetta and Eruca sativa (P. Mill.) Thell. cv. Astro, and a garden cress, Lepidium sativum L. cv. Presto, were grown with five salinity levels ranging from 1.5 to 9.5 dS·m−1 electrical conductivity (EC) during two trials. We observed no difference in TP at harvest in response to salinity treatments in Trial 1 for all species collectively evaluated; however, during Trial 2, the TP at harvest responded to salinity treatments. In contrast, we observed a response in AsA at harvest to salinity treatments during both trials. The response of both TP and AsA to treatments was characterized by variability, both increases and decreases, when contrasting species and trials. We concluded that the variability in the nutritional content of the specific crucifers evaluated might reflect both individual species responses and the modest CE microclimate changes we recorded between trials. In addition, our research suggests that salinity levels greater that an EC of 9.5 dS·m−1 may be required to define the salinity tolerance of these specific crucifers within a CE.

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Jorge M. Fonseca, James W. Rushing and Robert F. Testin

The influence of temperature and O2 concentration on respiration and shelf life of fresh-cut watermelon was investigated. Product stored at selected temperatures from 1 to 30 °C showed increasing respiration and reduced shelf life with increasing temperature. Oxygen depletion and CO2 evolution were measured using a closed system method and rates of O2 consumption and CO2 production were computed. A mathematical model found to predict the CO2 production as function of temperature and O2 showed an elevated rate of CO2 production at about 14% O2 or lower. A modified atmosphere trial that compared product stored at 7 to 9 °C in air with product at either 14% or 8% O2 revealed increased respiration in the latter treatments, suggesting a relatively high anaerobic compensation point (ACP) at >14% O2. Our results suggest limited applicability of modified atmosphere packaging (MAP) for this product. Fresh-cut watermelon had extended shelf life and reduced respiration rate when stored at 1 to 3 °C and in >14% O2 atmospheres.

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Jorge M. Fonseca, James W. Rushing and Robert F. Testin

Fresh-cut watermelon cubes stored at selected temperatures within the range of 1.1 to 14.5 °C had decreasing quality shelf life corresponding with increasing temperature. At lower temperatures there was a random occurrence of chilling injury symptoms in some cubes that was associated with the section of watermelon from which the cubes were cut. Cubes removed from the top side of the intact watermelon fruit were more susceptible to chilling injury than cubes from other sectors of the fruit. Sanitizing cubes with chlorine (40 ul/l) or ozone (0.04 μL/L) solutions caused an initial reduction in microbial count, but, during storage, the effect diminished and became insignificant compared to controls. Overall quality was lower in cubes receiving aqueous sanitizing treatments, possibly due to mechanical injury occurring during centrifugation to remove excess solution. Overall quality of cubes exposed to UV light (≈250 nm for 1 to 5 min) was better than cubes receiving aqueous sanitizing treatment. The effectiveness of UV treatment in reducing microbial load was dependent on the amount of cube surface exposed to the light. The results emphasize the importance of preventing microbial contamination during processing of fresh-cut watermelon.

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Jorge M. Fonseca, James W. Rushing and Robert F. Testin

Fresh-cut watermelon cubes stored at selected temperatures within the range of 1.1 to 14.5 °C had decreasing quality shelf life corresponding with increasing temperature. At lower temperatures there was a random occurrence of chilling injury symptoms in some cubes that was associated with the section of watermelon from which the cubes were cut. Cubes removed from the top side of the intact watermelon fruit were more susceptible to chilling injury than cubes from other sectors of the fruit. Sanitizing cubes with chlorine (40 μL·L–1) or ozone (0.04 μL·L–1) solutions caused an initial reduction in microbial count but during storage the effect diminished and became insignificant compared to controls. Overall quality was lower in cubes receiving aqueous sanitizing treatments, possibly due to mechanical injury occurring during centrifugation to remove excess solution. Overall quality of cubes exposed to UV light (≈250 nm for 1–5 min) was better than cubes receiving aqueous sanitizing treatment. The effectiveness of UV treatment in reducing microbial load was dependent on the amount of cube surface exposed to the light. The results emphasize the importance of preventing microbial contamination during processing of fresh-cut watermelon.

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Jorge M. Fonseca, James W. Rushing, Nihal C. Rajapakse, Ronald L. Thomas and Melissa B. Riley

The purpose of this review is to promote a discussion about the potential implications of herb production in controlled environments, focusing on our recent works conducted with feverfew. Research suggests that the content of secondary metabolites in medicinal plants fluctuates with changing environmental conditions. Our studies with feverfew (Tanacetum parthenium [L.] Schultz-Bip., Asteraceae) lend support to this hypothesis. Feverfew plants exposed to different water and light conditions immediately before harvest exhibited changes in content of some secondary metabolites. The highest yield of parthenolide (PRT) was in plants that received reduced-water regimes. Phenolics concentration however, was higher in plants receiving daily watering. Light immediately before harvest enhanced accumulation of PRT, but reduced the phenolic content. Notably, PRT decreased at night whereas total phenolics decreased during the photoperiod and increased at night. PRT also increased with increased plant spacing. UV light supplementation increased PRT only in plants that had undergone water stress, whereas phenolics increased when UV was applied to continuosly watered plants. Clearly, production of medicinal plants under greenhouse conditions is a promising method for controlling levels of phytochemicals through manipulation of light and water as discussed here, and possibly other environmental factors such as temperature and daylength. However, better understanding of how the environment alter secondary metabolite levels is needed as it was revealed that manipulating the environment to favor increased accumulation of one group of phytochemicals could result in a decline of other key metabolites.

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Jorge M. Fonseca, Hyun-Jin Kim, Wesley L. Kline, Christian A. Wyenandt, Murshidul Hoque, Husein Ajwa and Ned French

The effect of preharvest application of a newly developed second-generation harpin product (2G-Harpin) on shelf life of fresh-cut lettuce (Lactuca sativa) was investigated. The lettuce plants were grown in three locations in the United States: Watsonville, CA, Cedarville, NJ, and Yuma, AZ, and treated 5 days before harvest at 140, 280, and 420 g·ha−1 (30, 60, and 90 mg·L−1). Lettuce processed and bagged were stored at 1 to 3 °C and evaluated for quality for 20 days. Lettuce from California treated with 2G-Harpin at 280 to 420 g·ha−1 consistently showed better visual quality and lower microbial population than the control. Overall results in New Jersey showed no major differences among treatments. In Arizona, microbial population was lower and visual quality was higher in lettuce treated at 280 and 420 g·ha−1 during part of the storage period. In further experimentation, we examined the phenolic content of lettuce harvested 1 and 7 days after treatment with 2G-Harpin. The results showed that phenolic content was higher in all treated lettuce than in the control lettuce after 24 h. Six days later, the levels fell back to the initial stage. Antioxidants capacity increased by 40% in head leaves when plants were treated with 280 and 420 g·ha−1 2G-Harpin, but no change was observed in outer leaves. Overall, it was revealed that a field application of 2G-Harpin can improve quality of fresh-cut lettuce under environmental conditions that need to be determined. Our results with phenolic content and antioxidant activity suggested that improvement in quality is probably the result of alteration of metabolites' composition and demonstrated that increased phenolics do not correlate with lower quality of fresh-cut products.