Effects of vacuum level and microwave power on rosemary volatile composition during vacuum–microwave drying

The influence of the vacuum level and the microwave power on aroma compounds and sensory quality of Rosmarinus officinalis dehydrated by vacuum–microwave (VM) method was evaluated. VM drying kinetics consisted of two periods: linear until a critical point and exponential beyond that point. The time needed to dry rosemary was shorter for high values of microwave power and vacuum intensity. Volatile compounds of rosemary samples were extracted by steam–hydrodistillation and analyzed by GC–MS. Thirty-one compounds were identified, with α-pinene, verbenone and 1,8-cineole being the major components; monoterpenoids were the predominant chemical family. The total quantity of volatiles of fresh rosemary (27.2 g kg−1) decreased during drying, independently of treatments, to a mean of 14.8 g kg−1. Soft conditions of VM drying (low vacuum level and microwave power) are recommended to get the highest concentrations of volatiles and the best sensory quality (e.g. 72–74 kPa and 360 W).