Abstract
In this work, a cascade approach to obtain different valuable fractions from lemon peels waste was optimised using microwave-assisted processes. Microwave-assisted hydrodistillation (MAHD) with a Clevenger apparatus was firstly used to obtain the lemon essential oil (LEO). The remaining residue was then submitted to microwave-assisted extraction (MAE) to extract the lemon pigment (LP). A Box–Behnken design was used to evaluate the influence of ethanol concentration, temperature and time in LP extraction in terms of extraction yield and colour intensity. Optimal extraction conditions for LP were 80% (v/v) ethanol, 80 °C and 50 min, with a liquid-to-solid ratio of 1:10. The obtained yields for LEO and LP were around 2 wt.% and 6 wt.%, respectively. The composition of LEO was analysed by gas chromatography with flame ionisation detection (GC-FID), and limonene (65.082 wt.%), β-pinene (14.517 wt.%) and γ-terpinene (9.743 wt.%) were mainly identified. LP was purified by using different Amberlite adsorption resins (XAD4, XAD7HP and XAD16N), showing XAD16N the best adsorption capacity. Enrichment factors of 4.3, 4.5 and 5.0 were found for eriocitrin, diosmin and hesperidin, respectively, which were detected as the main components in LP by ultra-high-performance liquid chromatography–diode array detector–tandem mass spectrometry (UPLC-DAD-MS) analysis, with final concentrations of 4.728 wt.%, 7.368 wt.% and 2.658 wt.%, respectively. Successful antimicrobial capacity against Escherichia coli and Staphylococcus aureus was obtained for LEO. The results from this work showed the potential of applying a cascading approach based on microwave-assisted processes to valorise lemon wastes, obtaining natural pigments and antimicrobials to be applied in food, cosmetic and polymer industries.
Highlights
Agricultural and food industries produce a large amount of residues and by-products that should be valorised to accomplish the environmental challenges required by the new concepts of circular economy
The microwave-assisted extraction (MAE) process design consisted of adding the suitable solvent for pigment extraction to the extraction vessel directly after obtaining the essential oil using Microwave-assisted hydrodistillation (MAHD), without any intermediate steps
An extraction time of 10 min was set in this work to ensure most of the essential oil was extracted, it was observed that higher than 90% of the obtained yield was extracted after four min of hydrodistillation
Summary
Agricultural and food industries produce a large amount of residues and by-products that should be valorised to accomplish the environmental challenges required by the new concepts of circular economy. Citrus fruits are primarily used in juice production, generating a huge amount of residues (mainly peels and pulp) that are very often discarded or burned. These residues show noticeable amounts of potentially valuable chemicals, such as volatiles, pigments and essential oils (EOs), all of them located on the exocarp and flavedo of citrus peels [1,2]. These parts of lemon fruits contain oil glands with specific terpenoids that confer the characteristic lemon fragrance. Different terpenoids mainly present in EOs (monoterpenes such as citral, pinene, or terpinene, as well as poly(methoxyflavones) and furanocoumarins), have been used since ancient times to treat a variety of health diseases, such as spasms, fever, respiratory problems, cardiovascular diseases, gastrointestinal problems or anxiety [5]
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