**2. Circular economy**

The circular economy approach is a closed-loop system that aims to reduce the use of resource inputs, waste generation, pollution, and carbon emissions. It is a vision toward a sustainable society with great responsibility in the production of biomass and end-of-life product recovery. Reduction, repair, remanufacturing, and recycling are normally acknowledged as the representative loops of the circular economy [2]. Some authors refer to these as holistic approaches to add value to biowastes from fruit and vegetable processing [3], of which aromatic plants represent a small percentage. The goal of adding value and reducing waste to zero may invoke participation not only in physical processes related to extraction but also in chemical transformations (hydrolysis), fermentations, and bioprocessing with microorganisms. Some traditional or conventional extraction methods are not recommended due to time consumption, intensive labor demand, their use of large amounts of organic solvents, or elevated temperatures that degrade some compounds of interest. Alternative emerging technologies are faster and have reduced environmental impact. They include processes such as supercritical carbon dioxide extraction, subcritical water extraction, ultrasound mixing, microwave heating, electric pulse discharge, and enzymatic hydrolysis, which frequently lead to improved yields [4]. However, there is still a large room for improvement in scaling up their operation and changing from batch to continuous operation [5].

Plant secondary metabolites do not participate in a direct manner in basic functions such as growth and development but are very important for plant survival. As part of the plant's secondary metabolites, essential oils attract pollinators, reduce abiotic stress, and protect plants from pests and herbivores, among other direct and indirect roles. Secondary metabolites of lower volatility are not removed from plant material when essential oils are produced, which mostly happens through distillation. These other secondary metabolites include flavonoids, catechols, phenolic compounds, and other nonvolatile bioactive substances that may be recovered from the plant material to become valuable constituents of products for human well-being. A circular economy operation aims to take full advantage of essential oils, nonvolatile secondary metabolites, and secondary metabolite-depleted biomass.

**Figure 1** summarizes common processing options applied to the essential oil value chain within the circular economy approach. The first step is the retrieval of essential oil from plant material, due to its exposure to steam or mechanical compression (in the case of citrus). After a condensation step (in the case of steam distillation), the essential oil is separated by simple decantation from the aqueous phase, hydrosol,

#### **Figure 1.**

*Summary of the processing steps for the complete use of vegetal material in the essential oil agroindustry in observance of the circular economy principles.*

which retains small amounts of some essential oil components that are not completely hydrophobic. The residual biomass enters a sequence of value-addition steps that have several potential final products and no waste. A very useful first task is the use of some extraction agent (ethanol, ethanol-water, CO2) to remove waxes, pigments, flavonoids, and polyphenols from the residual biomass combined into a mixture known as the extract. This complex mixture may be further processed to obtain various fractions that are enriched in certain bioactive compounds. Composting, biochar generation, and biogas, or ethanol production from fermentation, are examples of several value-addition processes that may be applied to the lignocellulosic material that remains after solvent extraction. Thus, the circular economy version of the essential oil value chain has zero waste and several products, such as essential oil, hydrosol, extract, biofuel, biochar, and compost.
