INTRODUCTION
1.4. Extraction of bioactive compounds
The effectiveness of bioactive compounds are dependents on the quality and quantity of the compounds. The qualitative and quantitative studies of bioactive compounds from plant materials mostly rely on the selection of proper extraction method (Azmir et al., 2013). Extraction is the first step of any bioactive compound study, which plays a crucial role in the result and outcome.
The extraction of bioactive compounds depends on several factors, such as the extraction technique, raw materials, extraction condition, and the extraction solvent. The extraction techniques can be classified into the conventional and non-conventional process (Meireles, 2008). Conventional techniques require organic solvents, temperature, and agitation for the extraction such as soxhlet, maceration, and hydrodistillation. Modern techniques or non-conventional techniques such as enzyme-assisted extraction, microwave-assisted extraction, supercritical fluid extraction, and ultrasound-assisted
extraction, are green or clean techniques due to less use of energy and organic solvent, which are beneficial to the environment (Soquetta et al., 2018).
1.4.1. Conventional extraction
The conventional extraction techniques are widely used for the extraction of bioactive compounds from plant materials. These techniques are based on the extracting efficiency of different solvents and the application of heat and/or mixing. To obtain bioactive compounds from plants, the existing classical techniques are (1) Soxhlet extraction, (2) Maceration, and (3) Hydrodistillation.
1.4.1.1. Soxhlet extraction
The Soxhlet extraction has widely been used for the extraction of valuable bioactive compounds from various natural sources. It is used as a model for the comparison of new extraction techniques. In this process, a wide range of solvent is used as extractant, which carries extracted solutes. Few researchers have applied the soxhlet extraction techniques to extract bioactive compounds, especially anthocyanin from various plant sources (Lapornik et al., 2005; Veggi et al., 2011). The rate and degree of extraction of anthocyanins from plant materials through soxhlet depends upon a number of factors such as types of solvent (ethanol, methanol, water), temperature, time and acid (HCl, citric, tartaric, formic, acetic, propionic) (Ameer et al., 2017). Though this process is widely used for the extraction, however, this technique requires long extraction time and large amounts of solvent (Heleno et al., 2016) which enhance the degradation of the bioactive compounds.
1.4.1.2. Maceration
In the maceration process, the sample is ground into smaller particles to increase the surface area for an efficient mixing with the solvent. During maceration, the continuous agitation makes the extraction process easier in two ways: by increasing the diffusion and removing the concentrated solution from the surface of the sample. However, this process requires a long extraction time to obtain bioactive compounds from plant sources (Azmir et al., 2013). Romero-Cascales et al. (2005) reported that the maceration process increases the rate of anthocyanin extraction from skins of Monastrell grapes. However, in this process the exhaustive bulk extractions can be time-consuming, taking from a few hours to several weeks, and consume large volumes of solvent. Thereby decreases the extraction efficiency of bioactive compounds.
1.4.1.3. Hydrodistillation
Hydrodistillation is performed with distilled water and is generally used to extract the volatile fraction from foods. Hydrodistillation extraction process usually takes 6–8 h without organic solvents. This technique involves three main physicochemical processes:
hydro-diffusion, hydrolysis, and decomposition by heat. Due to the presence of heat, this process generally is not used for the extraction of the heat-sensitive compounds. Though Santana-Méridas et al. (2014) extracted various polyphenol, and antioxidant from the solid residue of Rosmarinusofficinalis L. using hydrodistillation process. Santana-Méridas et al.
(2014) have identified various phenolic compounds such as carnosol, carnosic acid, cirsimaritin in extract having antioxidant and bioplaguicide activities. Hydrodistillation consumes high levels of energy and is time-consuming (Okoh et al., 2010). Moreover, it also involves a heating process, which restricts its application for the extraction of heat- sensitive bioactive compounds from plant material.
1.4.2. Non-conventional extraction techniques
The major challenges of conventional extraction are longer extraction time, high purity solvent, evaporation of the huge amount of solvent, low extraction selectivity and thermal decomposition of thermolabile compounds (Wang and Weller, 2006). To overcome these limitations, non-conventional extraction techniques are introduced. Some of the most promising non-conventional extraction techniques are enzyme-assisted extraction, microwave-assisted extraction, supercritical fluid extraction, pressurized liquid extraction, and ultrasound-assisted extraction. These include less hazardous chemical, energy efficiency, reduce derivatives, catalysis, less degradation, and less time (Vieira da Silva et al., 2016). Thereby, the non-conventional extraction techniques nowadays are commonly used for the extraction of bioactive compounds.
1.4.2.1. Enzyme-assisted extraction
Enzyme-assisted extraction methods are gaining more attention because of the eco- friendly feature of the extraction process (Puri et al., 2012). Enzymes have been used particularly for the treatment of plant material before conventional methods for extraction.
Various enzymes such as cellulases, pectinases, and hemicellulase are often used to disrupt the structural integrity of the plant cell wall, thereby enhancing the extraction of bioactive compounds from plants. Xu et al. (2016) and Landbo and Meyer (2001) extracted anthocyanins from blueberry and black currant residue using enzyme-assisted extraction process. Xu et al. (2016) proposed that the enzyme-assisted extraction is more efficient than
existing extraction process and easy to scale‐up for industrial application. Enzyme-assisted extraction is a non-thermal method which can increase oxidative stability and antioxidant activity of bioactive components (Nadar et al., 2018). However, it has numbers of disadvantage, such as separation of enzyme and cost-effective (Puri et al., 2012).
1.4.2.2. Microwave-assisted extraction
In the microwave-assisted extraction process, microwave energy is used to heat the solvent and reduce the size of the sample to increase the mass transfer rate of the solutes from the food matrix into the solvent (Delazar et al., 2012). The microwave energy causes molecular motion by the migration of ions and rotation of dipoles (Gujar et al., 2010). This process is generally influenced by solvent nature and volume, extraction time, microwave power, matrix characteristics, and temperature. Microwave-assisted extraction used to extract bioactive compounds more rapidly, and recovery efficiency is higher than conventional extraction processes (Chemat and Cravotto, 2012). Pap et al. (2013) extracted anthocyanin from black currant marc by microwave-assisted extraction and reported that there was a 20 % (v/v) increase in anthocyanin yield as compared to the conventional extraction process. Yang and Zhai (2010) optimized the microwave-assisted extraction of anthocyanins from purple corncob. They also suggested that microwave-assisted extraction was highly efficient and rapid in comparison with conventional solvent extraction.
Nevertheless, microwave irradiation can accelerate the chemical reactions and can modify the chemical structures of the target compounds. Compared with ultrasonic-assisted and conventional extraction, apparatuses, and equipment of microwave-assisted extraction are more expensive, and their operation is more difficult in many cases (Zhang et al., 2011).
1.4.2.3. Supercritical fluid extraction
Supercritical fluids extraction has several advantages over the conventional extraction process, such as high dissolving power, higher diffusion coefficient, and easy to separate. Due to their low viscosity and relatively high diffusivity, supercritical fluids have high transport properties than liquids; thereby, it can diffuse easily through solid materials and give faster extraction rates. Moreover, the solvents generally used for the extraction are recognized as safe (da Silva et al., 2016). Few researchers were used supercritical fluids extraction process to extract anthocyanin from the various plant sources (Garcia-Mendoza et al., 2017; Maran et al., 2014; Paes et al., 2014). Garcia-Mendoza et al. (2017) and Paes et al. (2014) used supercritical fluids extraction process to extract anthocyanin from juçara
process had higher efficiency in extracting anthocyanin than pressurized liquids and conventional extraction process. Paula et al. (2014) extracted anthocyanins and luteolin from Arrabidaeachica using supercritical CO2, ethanol, and water as solvents and demonstrated that pure supercritical CO2 showed lowest extraction yield and with ethanol as co-solvent showed the highest yield. However, this process has a few disadvantages like the requirement of co-solvent, high operating pressure, higher capital, and operating costs (Wang and Weller, 2006).
1.4.2.4. Ultrasound-assisted extraction
Ultrasound-assisted extraction has been revealed to be an economically feasible technique for the extraction of heat-sensitive bioactive compounds (Chemat et al., 2017).
According to Pingret et al. (2013) ultrasound has several advantages over conventional extraction processes such as higher diffusion, higher mass transfer, breakdown of plant cells, more solvent penetration, and capillary effects of ultrasound (Sono capillary Effect).
The ultrasound-assisted extraction does not act with one mechanism but through different independent or combined mechanisms between fragmentation, erosion, capillarity, denaturation, and sonoporation. The efficiency of ultrasound-assisted extraction depends upon sonochemical effects, frequency, wavelength, amplitude, size of ultrasonic reactors, and medium parameters such as solvent type, temperature (Chemat et al., 2017). Few researchers extracted anthocyanin from various plant sources such as haskap berries (Jang and Xu, 2009), purple waxy corn (McGhie and Walton, 2007) mulberry (Jang and Xu, 2009) and red rose petals (McGhie and Walton, 2007) using ultrasound-assisted extraction.
They also suggested that ultrasound-assisted extraction showed higher extraction yield than conventional extraction process. Moreover, the ultrasound-assisted extraction used as a continuous process in the industry for extraction (Pan et al., 2011). Thereby, ultrasound- assisted extraction is now gaining interest to extract the bioactive compound.
However, to further enhance the activity of extracted bioactive compounds, its need to separate specifically from undesirable components and concentrate using non-thermal process.