Reducing Total Plate Count of Bacteria through Dark Sealed Storage for Plant Factory Nutrient Solution Sterilization

Plant factories are efficient systems for vegetable production under a clean and controlled environment. To improve the sterilization efficiency of nutrient solutions (NS) in plant factories, this study examined the changes in the number and composition of colonies from in-use and post-use stored NS. The sterilization effect under ultraviolet (UV) light-emitting diode light was also investigated. The results indicated that the total plate count of bacteria (TPC) of newly prepared NS was 3.99 lg CFU/mL (9870 CFU/mL); within 7 days of cultivation, this number sharply increased ∼7.8 times to 4.88 lg CFU/mL (76,000 CFU/mL), then remained stable. After dark sealed storage (DSS) of post-use NS, TPC dramatically decreased by 82.8% and reached its lowest point at 4.29 lg CFU/mL (19,566 CFU/mL) on day 3, equivalent to a log survival ratio of −0.76, then rapidly rebounded by 235.6% to 4.82 lg CFU/mL (65,666 CFU/mL) on day 4. During storage, bacterial community composition was also remarkably changed, and the relative abundance of heterotrophic bacteria such as Variovorax paradoxus and Pararhizobium giardinii and photosynthetic bacteria such as Pseudo-Nitzschia multiseries were greatly decreased. The log survival ratio for in-use and post-use stored NS under UV sterilization were −1.29 and −1.30, respectively, and was not significantly different. Coupled with the dramatic decrease in TPC during DSS, the overall log survival ratio of post-use NS UV sterilization was −2.06 and was the most effectively form of sterilization in our study. In short, TPC of in-use NS remained high for a long period, and DSS may serve as a low-cost NS sterilization method in plant factories.