Vacuum and convection drying effects on volatile compounds profile and physicochemical properties of selected herbs from Lamiaceae family

During drying herbs, the aim is to preserve their bioactive properties and characteristic aroma, which requires choosing a drying method and parameters that minimize degradation. In this study, the effects of convection and vacuum drying methods on peppermint ( Mentha piperita L.), sweet basil ( Ocimum basilicum L.), and lemon balm ( Melissa officinalis L.) were compared. Fresh herb leaves were dried at 35 °C (using both methods) with an airflow of 0.5 m·s −1 for convection (CONV) and with reduced pressure (300 mbar) for the vacuum method (VAC). Physical and chemical properties were measured (color measurement in the L*a*b* system, weight loss, profile of volatile compounds, total phenolic content (TFC), total flavonoid content (TFC), and total antioxidant activity (TAA)). After drying, a reduction in the brightness of the leaf color and the proportion of red color was observed in most replicates (e.g., for peppermint—control sample L* = 43.11, CONV sample L* = 40.21, VAC sample L* = 37.30). Water evaporated by varying degrees depending on the herb species and the drying method as follows:. The TPC of peppermint was 332.44 mg gallic acid equivalent (GAE)/100 g of sample for CONV and 451.38 for VAC; for sweet basil sequentially: 80.59 and 105.63 GAE/100 g; for lemon balm 248.31 and 314.41 GAE/ 100 g. TFC of peppermint was 244.12 mg quercetin/100 g of sample for CONV and 261.92 for VAC; sweet basil sequentially: 76.91 and 83.31 mg quercetin/100 g; lemon balm 186.50 and 203.87 mg quercetin/100 g. The TAA of peppermint was 84.28% for convection drying, 87.49% for vacuum drying, and sweet basil, it was 20.35% and 47.35% sequentially, and for lemon balm, it was 87.67% and 89.69%. The vacuum method was statistically more effective in preserving the polyphenols and flavonoids in the herbs. Different drying methods also affected the volatile compounds recognized in dried herbs using the Electronic Nose.