Effect of lyophilization on the essential oil content and composition of herbs and spices with different volatile secretory structures

In this study the influence of lyophilization on the volatiles of herbs and spices with different secretory tissues was evaluated and compared with convective drying at 40 °C and 60 °C. Oven drying at 40 °C caused only 4–7 % essential oil (EO) loss for majority of the examined species, while drying at 60 °C reduced volatiles by 26–80 %. However, the original EO composition remained significantly unchanged. Our findings revealed a relationship between the type of volatile secretory structures and the effects of freeze-drying. Lyophilization retained the EO content and composition quite similar to oven drying at 40 °C for half of the studied plant species that accumulated volatiles in external glandular hairs (Melissa officinalis L., Origanum majorana L., Salvia rosmarinus Schleid., Salvia officinalis L.), and for the other half (Mentha x piperita L., Ocimum basilicum L., Satureja hortensis L., Thymus vulgaris L.) also resulted in higher EO content than drying at 60 °C. Whereas, for species that accumulated volatiles in endogenous secretory ducts (Artemisia dracunculus L., Levisticum officinale Koch., Petroselinum crispum var. neapolitanum Danert.), freeze-drying decreased the EO content by 86–97 % and altered the composition substantially. Therefore, lyophilization is unsuitable for preserving such plants if maintaining volatiles is the goal.