Effect of Pre-harvest Periods by Replacing Nutrient Solution with Tap Water on Nitrate and Quality in Hydroponic Lettuce

Lettuce (Lactuca sativa L.) is a high-value nutritional food for human consumption, rich in minerals and vitamins. However, people are still concerned about the residues of nitrate content in hydroponic lettuce. Pre-harvesting by replacing nutrient solution with tap water is one of several factors that can help to reduce nitrate content and improve the quality of hydroponic lettuce. Therefore, this study focused on the effect of pre-harvest periods by replacing nutrient solution with tap water on nitrate and quality in hydroponic lettuce. This experiment was performed by replacing nutrient solution with tap water in a hydroponic system for 0 h, 24 h, 48 h, and 72 h before harvesting the lettuce product. Nitrate, vitamin C, phenolic, and soluble sugar contents were determined. The results showed that nitrate content was lowest under pre-harvest conditions for 72 h (19 mg/g dry weight, 63% reduction), but it was not significantly different for 48 h (20 mg/g dry weight, 61% reduction). The vitamin C content was significantly decreased after pre-harvesting for 72 h (5.3 µg/100 g, 30% reduction). The soluble sugar content was significantly increased and reached its highest content after pre-harvest for 48 h (3.3 mg/100 g, 105% increase), but there was no significant difference at 72 h (3.2 mg/100 g, 98% increase). Although the phenolic content decreased slightly, there was no significant difference among the treatments. In conclusion, nitrate contents were reduced the lowest, and soluble sugar contents were increased the highest after pre-harvest for 48 h. Vitamin C and phenolic contents were slightly decreased after pre-harvest periods by replacing the nutrient solution with tap water.