In situ microwave assisted extraction of clove buds to isolate essential oil, polyphenols, and lignocellulosic compounds

Clove buds is a spice of relevance in food, traditional medicine, pharmaceutics and cosmetics and, among the spices, they have the highest content of total polyphenols with exceptional antiviral and antimicrobial properties. Various approaches have been reported for the isolation of essential oil from clove buds. Nonetheless, the qualitative and quantitative analysis of hydrosoluble polyphenols and solid residues simultaneously yielded during the extraction process has not been explored yet. This work is focused on the analysis of some variables effect on yield and composition of the clove buds essential oils on a green microwave assisted extraction, and the characterization and quantification of the different compounds obtained from the extraction process. A versatile coaxial dipole antenna, to directly apply the electromagnetic energy inside the extraction medium, was used to thermally activate the hydrodistillation. The composition profiles of clove buds essential oil and hydrosoluble polyphenols obtained during in-situ microwave assisted extraction (IMWAE) were analysed and quantified by head space gas chromatography mass spectrometry (HS-GC–MS) and liquid chromatography with UV/visible diode array/fluorescence detector (HPLC-DAD-FD). The solid residue was characterized by Fourier Transform Infrared (FTIR) spectroscopy and its composition in term of lignin, cellulose and hemicellulose was predicted. The green IMWAE process was compared with the conventional hydrodistillation (CH) in terms of yield and quality of isolated products. Thermogravimetry coupled to FTIR analyses of the evolved gases from the solid residue evidenced that the solid residue obtained from IMWAE of clove buds is richer in cellulose-hemicellulose than the residue obtained from CH. This can be because of microwaves that allow to remove a higher amount of phenolic compounds/lignin oligomers. The enthalpy of combustion values (ΔcH) (kJ/g) of IMWAE and CH residues were determined by calorimetric combustion and were compared with the –ΔcH (kJ/g) values calculated using the hemicellulose, cellulose and lignin compositions predicted by partial least square chemometrics. The ΔcH highlighted the energetic features of solid residues from IMWAE and CH for their potential uses as alternative biomass for fuel production and here firstly reported for this kind of biomass. The extraction approach here presented is environmentally friendly, highly flexible, easily controllable, time saving, and enables to break the scale-up barrier in microwave assisted industrial processes aimed to valorise aromatic herbs and eventually to exploit vegetable wasting materials. This leads to a lowering of production costs and, therefore, of the market price of isolated extracts from aromatic herbs.