Effect of Postharvest Storage Temperatures of Lemon Balm (Melissa officinalis) on Phytochemical Quality and Antibacterial Activity in Aqueous Infusions

This study investigated how postharvest storage temperatures influence the tissue physiology of lemon balm (Melissa officinalis). This herb is used in both fresh and dry forms to prepare functional infusions. Understanding proper raw material storage practices is critical for determining the phytochemical quality of the final product. Fresh bundles were stored at 2, 6, and 20 °C for 15 days, with sampling every 5 days. Tissue gas exchange, color coordinates, and pigments were evaluated alongside the tannin extractability and in vitro antibacterial activity of aqueous infusions. Results demonstrated that ambient storage (20 °C) promoted rapid quality deterioration, characterized by pronounced pigment loss and severe tissue degradation, resulting in poor visual quality by day 15. Deep refrigeration at 2 °C suppressed postharvest physiological activities, limiting cumulative moisture loss to 4.5% while preserving visual freshness and pigment retention. In contrast, mild refrigeration (6 °C) showed the highest weight loss (12.85%) and had the greatest accumulation of phenolic compounds and antioxidant capacity at the storage end. Fresh leaf storage at 2 °C showed the highest tannin extractability in the infusion. Consequently, infusions prepared from these leaves showed the strongest in vitro inhibitory activity against Staphylococcus aureus and Escherichia coli, with the lowest minimum inhibitory concentration and IC50 values. Therefore, storage at 2 °C maintained fresh leaf quality and the phytochemical content of the resulting infusions over the 15-day period tested.