Microwave extraction of essential oils (from black pepper and coriander) at 2.46 GHz

Essential oils are composed of a wide range of bioactive chemical compounds. They traditionally found application as flavour, fragrances and medicinal aroma. Today, the essential oils are sought-after for innumerable applications starting from markers for plant identifications to base for semi-synthesis of highly complex molecules. The extraction of highly delicate essential oils from plants remains a crucial step in all these applications. By using microwaves to mediate the extraction, it is possible to maintain mild conditions and effect superior extraction. However, apart from laboratory trials, essential oil extraction using microwave energy is largely an unexplored area. In the current work, an integrated procedure for microwave extraction followed by volatiles sampling and analysis from selected botanical raw materials (viz. black pepper, Piper nigrum and coriander Coriandrum sativum) was developed. There are two problems to overcome in the extraction from solid plant materials: that of releasing the essential oil from solid matrix and letting it diffuse out successfully in a manner that can be scaled-up to industrial volumes. Towards this end, an innovative volatiles extraction unit was conceived, designed and developed that used thin layer, for microwave exposure and rotational mixing, to mitigate the effects of thermal gradient and non-uniform exposure of bulk matter. The effect of varying the microwave field on the essential oils extracted was studied. The microwave field that coupled in the region of extraction was estimated from temperature rise measurement using the microwave power equation (with water as reference dielectric). The essential oil extracted under different microwave fields were compared using gas chromatography-mass spectrometry (GC-MS) and data analysis with SAS statistical software. The microwave field at the site of extraction was sensed by symmetrical placement of biomaterial sample and a reference, in a rotational extractor, such that they both couple the same field during tumbling motion. By measuring the temperature rise in the reference accurately, it is possible to estimate the microwave field present at that position. The rotational extractor has a second degree of freedom, in that it can slide along the axel taking discrete positions. Each position leads to a different microwave exposure of the sample. It is possible to measure the relative variation of microwave field using temperature rise data at each position. It was found that, at position labeled R4, located at 65.6 mm from the right extreme of the microwave cavity had the highest effective microwave field strength of value 92.7 V/m. The volatiles released from the biomaterials, black pepper and coriander, were sampled using solid phase micro extraction and analyzed using gas chromatography-mass spectrometry. The highest peaks representing beta-caryophyllene in black pepper and linalool in coriander were identified using mass spectrometric peak matching using NIST library.The extract (in terms of ion count) for each microwave parameter (such as field, water content level and solvent type) was plotted as a trend graph. The current experiment successfully tested the procedure for following the microwave process in the extraction of sensitive spice volatiles (from black pepper and coriander). With the microwave field measured at the region of extraction, it was possible to plot pepper extraction versus the microwave field to which the pepper sample was exposed. The extraction was represented in terms of cumulative value of ion counts obtained in GC-MS analysis. This unique procedure developed in the current research allows for the graphical comparison of the microwave extractions. It was found that black pepper has a better response to microwave extraction than coriander. The pepper extraction was found to increase proportionately with increase in microwave field strength. The extraction was also enhanced proportionately by the incremental addition of water content at constant microwave field