Extraction methods to obtain essential oils are usually influenced by many parameters. The optimization of the process is crucial because the extraction yield is low, i.e., less than 5 %. Fortunately, there are different ways to obtain essential oils and therefore to optimize the process. This article reviews recent studies regarding the use of Response Surface Methodology (RSM) in the optimization of essential oils extraction. It attempts to detail the important parameters of both innovative methods and conventional methods. Even when the essential oil extraction yield is the most common optimized response in these publications, lately the concentration of any chemical compound of interest and/or the antioxidant activity of the essential oils are also candidates for optimization. In addition, not only a high yield of essential oil is desirable but also a high quality due to its final application. In this review, we cover a comprehensive overview of the studies made about the optimization of essential oils extracted from plants that use the RSM. This document was elaborated through a comprehensive and systematic review of the literature published between 2010 to 2020.

The essential oils (EOs) extracted by hydrodistillation (HYDRO) and steam distillation (SD) from Mexican Salvia officinalis L were analyzed for yield, chemical composition (GC-MS), particle morphology (SEM), antioxidant activity (ABTS), and antibacterial activity against Enterobacter agglomerans, Citrobacter freundii, Salmonella sp, E. coli, and Pseudomonas aeruginosa. The influence of the factors (method, quantity, and sample) was evaluated using a 2³ full factorial design, Pareto chart, normal probability plot, main effects, and interaction plots in variance analysis on yield and antioxidant activity. The quantity, methods, sample, and the methods×sample and methods×quantity interactions were the most significant factors on yield (). The sample, methods, and quantity×sample interaction were significant for antioxidant activity. EO yields were between 0.35 and 1.27 ( w/w), and the highest value was obtained by the HYDRO method using 50g of whole sage leaves. The antioxidant activity values were in the range of 2.35 to 3.44mg Trolox equivalent/g. Camphor, limonene, camphene, and caryophyllene were the main compounds identified. Micrographs of sage leaves showed relevant changes in the structure after extraction. The antibacterial activity was confirmed with the inhibition diameter and inhibition percentage of all bacteria, and P. aeruginosa was the most resistant bacteria. Finally, S. officinalis EO potentials can be considered an alternative natural preservative for the food and pharmaceutical industries.

Clinopodium mexicanum (CM) has been used by the population as a home remedy for inducing sleep, and as a sedative and analgesic. This study presents the first evaluation of the antioxidant activity of the essential oil (EO) obtained through hydrodistillation of the aerial parts of CM. NaCl, CaCl₂, and Tween 20 were used as additives and the yield, chemical composition, and toxicity of the EO were evaluated. The findings revealed that the highest yield of EO was obtained through hydrodistillation without additives, and the additives significantly influenced the antioxidant activity of the EO. The main components of the EO were found to be pulegone and menthone. The toxicity of the EO was determined using the brine shrimp assay, with an LD₅₀ of 32 mg/L. Based on these results, the authors suggest that the Clinopodium mexicanum EO has the potential to serve as a natural antioxidant.

Bioactive compounds from medicinal plants have applications in the development of functional foods. However, since they are unstable, encapsulation is used as a conservation alternative. This work aimed to assess the bioactive properties (antioxidant and hypoglycemic) of different extracts, including the infusion, as well as their spray-dried microencapsulates from Tecoma stans leaves. A factorial design was proposed to determine the best extraction conditions, based on ABTS and DPPH inhibition. Maltodextrin (MD), arabic gum (AG), and a 1:1 blend (MD:AG) were used as encapsulating agents. Moreover, characterization through physicochemical properties, gas chromatography/mass spectrometry (GC-MS) and scanning electron microscopy (SEM) of the best two powders based on the bioactive properties were analyzed. The results showed that the combination of stirring, water, and 5 min provided the highest inhibition to ABTS and DPPH (35.64 ± 1.25 mg Trolox/g d.s. and 2.77 ± 0.01 g Trolox/g d.s., respectively). Spray drying decreased the antioxidant activity of the extract while preserving it in the infusion. The encapsulated infusion with MD:AG had the highest hypoglycemic activity as it presented the lowest glycemic index (GI = 47). According to the results, the microencapsulates could potentially be added in foods to enhance nutritional quality and prevent/treat ailments.

Mexican giant hyssop ( Agastache mexicana subsp. xolocotziana ) is an endemic Mexican plant traditionally used as a sleeping inducer. The aim of this study was to evaluate the effect of selected processing variables on the extraction of essential oil (EO) using surfactant-assisted hydrodistillation (SAH). Response surface methodology (RSM), coupled with a face-centered central composite design, was used to optimize the oil extraction process. The factors studied included the concentration of Tween 20 (0.001 – 0.007 g/mL), the solid–liquid ratio (15.8 – 45.8 g/L), and the extraction time (0.5 – 3.0 h). The experimental data obtained were fitted to a second-order polynomial model. The results showed that the concentration of Tween 20 and the solid–liquid ratio had a significant effect ( P < 0.005) on the essential oil yield. Additionally, all three factors studied significantly affected ( P < 0.005) the antioxidant activity, as measured by the DPPH radical scavenging activity. The optimal conditions for extracting oil from Agastache mexicana subsp. xolocotziana ( AMX ) were determined to be 0.001 g/mL of Tween 20, a solid–liquid ratio of 45.8 g/L, and an extraction time of 2.86 h. The GC-MS analysis of the essential oil showed that nerol was the main component, whose presence was confirmed by infrared spectroscopy. The antibacterial activity of EO was evaluated by determining the inhibition diameter, with results indicating that the antibacterial activity was higher when using hydrodistillation compared to SAH. The study concluded that the AMX essential oil showed significant DPPH radical scavenging activity and antibacterial activities. Additionally, surfactant-assisted hydrodistillation (SAH) was identified as a sustainable method for essential oil extraction, due to its reduced environmental impact. Graphical abstract