## 1 Introduction
Among these novel technologies, microwave and ultrasound energy, as well as subcritical water and enzymatic amplification of the extraction process, stand out [12]. In microwave-assisted extraction (MAE), microwave power provides rapid local heating and can lead to cellular rupture. In this way, pectin extraction becomes easier, the processing time is significantly shortened, and extraction yields are enhanced. Furthermore, microwave power can prevent pectinase enzyme activities that cause pectin degradation. Microwave-assisted heating systems are considered a promising technique for processes such as extraction, synthesis, etc. However, the restricted penetration depth of microwave irradiation into absorbing materials is the main limitation of microwave scale-up processes. At present, this limitation is overcome by developing continuous or stop-flow microwave reactors. There are some studies in the literature that have demonstrated a successfully completed scale-up process of microwave technology in HMF production [13], polymerization [14], and flavonoid extraction [15]. Temperature or microwave power, time, and liquid/solid ratio are process parameters that have crucial impacts on the yield and quality of target compounds in microwave-assisted extraction. As the temperature increases, the diffusion rate of the solvent increases and the compound passes into the solvent, increasing the yield of the extract. However, prolonged processing at extreme temperatures can cause the degradation of heat-sensitive compounds, reducing the yield and/or purity of the target compounds [16]. In general, with increased extraction time comes increased yield. But sometimes, extending the time can reduce the yield and quality due to some undesirable reactions, such as enzymatic degradation, oxidation, burning, etc. The liquid/solid ratio also affects the extraction efficiency in MAE. [PERSON] et al. [17] reported that pectin yield increased when increasing the liquid/solid ratio; however, it must be kept at an optimum level because if the solvent volume is too high, microwave energy may be insufficient to heat the solvent. Additionally, microwave radiations are nonionizing rays, do not cause changes on the chemical bonds of the compound, and therefore do not change the molecular structure of the compound. Hence, the compound in the cell can be obtained in high purity [18]. The use of water and CO\({}_{2}\) as solvents is nontoxic and nonflammable, and safe for the sustainable environment. When CO\({}_{2}\) dissolves in water, carbonic acid is produced and acidifies the water. This facilitates transfer of pectin to the solvent. Additionally, the polarity of water decreases at temperatures above the boiling point of water in a pressurized environment (such as 180 \({}^{\circ}\)C), subcritical conditions occur, and water acts as an organic solvent [16]. Many extractions have been combined and tested with microwave. For example, [PERSON] et al. [19] obtained pectin from dragon fruit by microwave-ultrasound-assisted extraction. [PERSON] et al. [20] obtained pectin from jackfruit peel by ultrasonic microwave-assisted extraction with citric acid. [PERSON] et al. [21] combined microwave-assisted extraction with pulsed electric field for pectin extraction from jackfruit waste. Pectin is a unique ingredient with its versatile role and has benefits not only for the food industry, but also for the pharmaceutical and cosmetic industries. Pectin is currently produced by extraction methods that require harsh chemicals. There are limited numbers of studies on alternative environmentally friendly pectin extraction methods. Transformation of the traditional methods into safe methods is essential to achieve sustainable production systems. In this respect, the novelty of this study is that MW-HPCO\({}_{2}\), an environmentally friendly method for pectin extraction, is discussed for the first time. The aim of this study was to extract pectin from graprefruit peel using a new environmentally friendly extraction method, MW-HPCO\({}_{2}\) extraction.