3.2.3. Pressurized liquid extraction (PLE)

The pressurized liquid extraction technique can also be called “accelerated solvent extraction (ASE)” (Fig. 7). It is one of the current techniques developed for extracting phytochemicals. It requires the use of high temperature and pressure (Nieto et al., 2010). High pressure ranging from 3.3 to 20.3 ​MPa is applied in combination with high temperature; ranging from 40 to 200 ​°C to facilitate the desorption and solubility molecules into solvents (Zhang et al., 2018). Nieto et al. approved that PLE is an advanced extraction method which ensures rapid extraction process using a few volume of solvents as compared to the conventional methods of extraction (Nieto et al., 2010). Besides, it encourages a better usage of water as extraction solvent such as the so-called subcritical water extraction due to the involvement of high temperature (Teo et al., 2010).

Pictorial representation of PLE technique.

At higher temperatures of around 200 ​°C, there will be a change in the dielectric properties of water, making it act as a standard organic solvent, thereby improving the capacity of the extraction (Plaza and Turner, 2015). Among the reported positives of the PLE as per various researchers include purity of the extracts compared with extracts from the classical methods. This eliminates the need for any purification step especially liquid chromatography mass spectrometry analysis (Sosa-Ferrera et al., 2013). However, the major limitations of PLE are low analytes selectivity during extraction, presence of interferents during the extraction process, high level of extracts dilution especially when using several numbers of cycles, and need for advanced instrumentation which is a costly process. Despite these limitations, pressurized liquid extraction is a common extraction method which had been utilized severally for extracting polyphenols via various sources (Erdogan et al., 2011; Liazid et al., 2014).

In order to improve the efficiency of a typical PLE process, some parameters must be optimized and the most important of these parameters is solvent selection. Even though the properties of any solvent can be modified under the elevated temperature used in PLE, the success of the process still depends on the solvent employed. Several solvents and their mixtures had been utilized for extracting phenolic component through several sources, but the most frequently used solvents are methanol, ethanol and their combination with water in various ratios. The decision on the combination of these solvents must be systematically taken as it has been proven that adequate selection of solvent may affect the level of extraction of several phenolics from the same sample because of the large distinct in the chemical composition of such components.

For instance, phenolic compounds extraction from parsley had been proven to be impossible due to the difficulty of finding a common solvent that will extract all the phenolic compounds in parsley (Maqsood et al., 2014). In fact, the water content of any extraction solvent has an impact on the compounds to be effectively extracted using such a solvent. Water acidification can improve the efficiency of phenolic compounds extraction if the chemical nature of such compounds is considered. This technique has been reportedly used to extract anthocyanin from the grape skin. Having carefully optimized the solvent selection by considering different percentages of acidified water, the complex solvent mixtures consisting of HCL, acetone, methanol, and water at the ratio of 0.1:40:40:20 was found to achieve the total acylated anthocyanins and maximum phenolic compounds recovery (Ju and Howard, 2003).

Similarly, the other parameters including time, temperature, sample packaging inside the extraction cell sample size, and solvent flow rate need to be optimized as they could influence the process. Although pressure as a parameter is believed to facilitate the rupture of the sample matrix and allow a better extraction experience; nevertheless, studies have proven that pressure has no significant influence on the extraction result if its value is highly sufficient in sustaining the solvent at liquid phase throughout the process of extraction.

The pressurized liquid extraction is usually combined with solid-phase extraction in an off-line or in-line mode to achieve better isolation of the target phenolic compounds. Regarding on-line coupling, it implies the positioning of solid phase within the extraction cells together with the targeted extracted sample but demarcated through a dispersing agent. This step ensures a better sample purity prior to chemical analysis. Several studies have employed the off-line and in-line PLE-SPE technique for extracting phenolic compounds (Plaza and Turner, 2015). Other on-line commercial platforms had been reported to be used in quantifying concentration of proanthocyanins in malt (Pritor and Liwei, 2005). This case requires the use of an automated device which will mediate the transfer of extracts from PLE collection bottle to SPE device.