Effects of Fertigation Practices on Postharvest Quality and Microbial Safety of Microgreens

Abstract

Microgreens are young edible plants harvested at the cotyledon or first true leaf stage. They are valued for their high nutritional content, vibrant color, unique texture, and rapid growth cycle. As demand increases, optimizing fertilization practices to enhance crop quality while maintaining food safety is critical, particularly given that organic and synthetic fertilizers differ in nutrient release, microbial inputs, and potential impacts on microbial persistence. This thesis evaluated the effects of fertilizer type (organic vs. synthetic) on microgreen growth, postharvest quality, and nutritional attributes. Extended storage (up to 21 days) was included to better capture changes in quality and nutritional composition over time. Emphasis was also placed on Salmonella survival as a key food safety concern, assessing pathogen persistence under conditions relevant to commercial handling. In the first experiment, four microgreen species (amaranth, beet, cilantro, and kale) were grown under controlled greenhouse conditions. Two commercially-available liquid fertilization treatments were used: a synthetic fertilizer (Miracle-Gro) and an organic liquid fish emulsion. Growth parameters including fresh yield and hypocotyl length were evaluated at harvest. Postharvest storage was evaluated for up to 21 days at 4 °C, with data collected at 0, 7, 14, and 21 days. Quality and nutritional attributes were assessed by measuring weight loss, color, anthocyanins, phenolics, antioxidant activity, and chlorophyll and carotenoid content. Both microgreen species and fertilizer treatment significantly influenced microgreen growth, yield and quality. Synthetic fertilization generally increased fresh yield and hypocotyl length, although responses differed among species. Kale had the highest fresh yield (1658.92 g·m-2), while cilantro had the lowest (322.5 g·m-2). Weight loss increased throughout storage, with kale exhibiting the lowest loss (0.18%). Organic fertilization increased anthocyanin, phenolic, and antioxidant activity levels by 26%, 45%, and 20%, respectively, compared to synthetic fertilization. In beet microgreens, anthocyanins, phenolics, and antioxidant activity increased by 8%, 14%, and 2%, respectively, during the first 14 days of storage before declining by day 21. Chlorophyll content generally decreased during storage, while carotenoid content remained stable or increased depending on the species. In the second experiment, the role of organic Earth Juice (plant-based), organic Fish Emulsion (animal-based) and synthetic fertilizer (Miracle-Gro) as potential sources of contamination were evaluated. Salmonella (S.) enterica survival was assessed in kale microgreens grown in controlled conditions under full spectrum light emitting diodes (LED). Fertilizer solutions were inoculated with a two-strain Salmonella enterica cocktail and were applied through bottom irrigation after seed germination. The S. enterica populations were monitored in grow mats and edible kale microgreen tissues during production. Similarly, populations in kale microgreen tissues were evaluated during postharvest storage at 4 °C on days 0 and 7 (d). Salmonella populations were significantly influenced by fertilizer type during production, with higher populations observed in organic fertilizers (7.2 log CFU·g⁻¹) compared to synthetic fertilizers (6.17 log CFU·g⁻¹). Salmonella decreased over time by 0.85 log CFU·g⁻¹ in grow mats and 1.89 log CFU·g⁻¹. However, the pathogen persisted throughout production and postharvest storage regardless of fertilizer treatment, indicating that contaminated fertilizer solutions can serve as a potential source of pathogen contamination in microgreen production systems. Overall, these results demonstrate fertilizer type plays an important role in influencing microgreen quality, growth, nutritional value and food safety.