Volatile compounds released by chocolate mint (Mentha × piperita var. chocolate) leaves after different post-harvest procedures

Background

Chocolate mint (Mentha × piperita var. chocolate) is a peppermint variety which owes its name to a chocolate-like scent and dark-red staining of the stems. It is a medicinal and ornamental plant, spread throughout the world due to nursery activities. Chocolate mint essential oil was previously characterized, while the aroma of the whole leaves has been scarcely studied. Indeed, though essential oils are commonly used in cosmetics and perfumery, whole mint leaves are broadly consumed as food products.

Goals

Fresh mint leaves were compared with hot and freeze-dried leaves with the aim of observing the differences between the volatile organic compounds (VOCs) of mint leaves subjected to different post-harvest treatments, and assessing the best method to preserve the quality of the final product.

Methodology

Chocolate mint plants (Mentha × piperita var. chocolate) were grown in 10 cm pots at the botanical garden of the University of Salento. After 4 months from planting, 3-4 apical leaves were harvested from plants and mixed in stock. Some of them were analyzed immediately (fresh sample), while others were hot-dried at 42 °C for 16 hours (hot-dried sample) or freeze-dried for 22 hours after freezing at -22 °C for two hours (freeze-dried sample). The three samples were sealed in a 20 ml vial, and a solid phase microextraction (SPME) fiber was exposed to headspace to capture the emitted VOCs. The compounds were then analyzed by gas-chromatography coupled with mass spectrometry (GC/MS).

Results

A total of 17 VOCs were detected among the three samples, with monoterpenes being the most represented category. Most of the compounds found in fresh leaves were preserved after drying, while, interestingly, some compounds occurring in dried leaves were scarcely emitted by fresh leaves. Some differences were also observed for compounds included in the menthol synthesis pathway.

Conclusions

Both hot-drying and freeze-drying preserved the characteristic aroma of chocolate peppermint leaves, proving to be effective methods for post-harvest treatments, allowing for long-term product storage. Furthermore, the data demonstrated a variable presence of chemicals part of the menthol pathway as a result of the various treatments.