Green Extraction of Bioactive Compounds from Plant Biomass and Their Application in Meat as Natural Antioxidant

Plant extracts are rich in various bioactive compounds exerting antioxidants effects, such as phenolics, catechins, flavonoids, quercetin, anthocyanin, tocopherol, rutin, chlorogenic acid, lycopene, caffeic acid, ferulic acid, p-coumaric acid, vitamin C, protocatechuic acid, vitamin E, carotenoids, β-carotene, myricetin, kaempferol, carnosine, zeaxanthin, sesamol, rosmarinic acid, carnosic acid, and carnosol. The extraction processing protocols such as solvent, time, temperature, and plant powder should be optimized to obtain the optimum yield with the maximum concentration of active ingredients. The application of novel green extraction technologies has improved extraction yields with a high concentration of active compounds, heat-labile compounds at a lower environmental cost, in a short duration, and with efficient utilization of the solvent. The application of various combinations of extraction technologies has proved to exert a synergistic effect or to act as an adjunct. There is a need for proper identification, segregation, and purification of the active ingredients in plant extracts for their efficient utilization in the meat industry, as natural antioxidants. The present review has critically analyzed the conventional and green extraction technologies in extracting bioactive compounds from plant biomass and their utilization in meat as natural antioxidants.


Introduction
The meat industry's focus is shifting towards antioxidants that are novel, efficient, economical, green, or natural alternatives to potentially harmful synthetic preservatives. There has been an ever-increasing effort by the meat industry to explore novel, effective and edible antioxidants and antimicrobials compounds obtained from natural sources. Recently, there have been renewed interests in applying natural compounds (plant extracts, herbs, spices) in food preservation. Since ancient times, these additives have been added to foods as flavorings/seasonings, folk medicine, and food preservatives owing to antioxidant, bacteriostatic or bactericidal properties of polyphenolic compounds, and essential oils. The consumption of these compounds is known to exert beneficial effects on consumer's health in addition to their preservative role. These herbs, spices, essential oils, or plant extracts exhibit various protective actions against the occurrence of diseases, and thus are widely used as potential green alternatives to food additives, preservatives, and dietary supplements.
Natural antioxidants are safe to consume, available in large quantities at a relatively lower price than their synthetic counterparts, and can be extracted and applied in the meat

Plant Extracts as Natural Antioxidants
At present, various plant extracts are increasingly being explored in meat products as potential natural antioxidants for improving oxidative stability due to the potent antioxidant potential of these compounds in the meat matrix. The antioxidant attributes of these plant extracts are due to flavonoids (flavanol, flavones, anthocyanins), phenolic acid (hydroxybenzoic, hydroxycinnamic acids), diterpenes tannins (hydrolyzable and condensed tannins), stilbenes, coumarins, lignans, quinones, curcuminoids, and others including phenolic alkaloids, phenolic glycosides, phenolic terpenoids, and essential oil) [4,5]. Tea phenolics, such as epigallocatechin gallate (EGCG), have antioxidant potential in terms of their having electron reduction (550 mV) potential comparable to alpha-tocopherols or vitamin E (480 mV) [6].

Extraction Protocols
The concentration of recovered bioactive molecules in extracts and their yield largely depend upon the various processing parameters. It is desirable to have a higher yield along with a significant concentration of active compounds. A proper extraction protocol is a primary step in ensuring the proper availability and utilization of these natural extracts, with higher preservative potential, and at an economical rate to industry [9].The overall composition of the extract obtained varies with the extraction protocols, plant species, part of the plant used (root, stem, leaves, root), geographical origin, harvesting time, type of storage, drying methods in case of use of powder and developmental stage of plant [10][11][12]. The use of extraction solvents, method of sample preparation, extraction duration, ratio of sample to extraction solvent, and temperature affect the composition and concentration of active compounds in an extract, thereby affecting its antioxidant efficacy [13][14][15][16]. Figure 2 depicts the various technical aspects of using plant extracts as a natural antioxidant in meat.

Green Solvents for Extraction
Vegetable matter, such as specific parts of plants or as such a whole plant, when properly harvested for maximum yield, is cleaned and mostly dried at a low temperature to preserve the maximum activity of heat-labile active compounds. This dried material is ground into powder to increase its surface area for better segregation of bioactive compounds from the plant matrix. The finely ground powder is dissolved in either a suitable solvent or a combination of solvents (depending upon the suitability and efficiency of these solvents) and dissolves bioactive compounds from plant powders. The yield and concentration of active principles increased in the extracts upon increasing the total surface or contact area between the powders and solvents. In the plant kingdom, a huge variety of bioactive active ingredients showing antioxidant potential exist. These compounds have different chemical and physiological attributes that affect their solubility. Hence, these bioactive compounds are soluble in different solvents. It is practically not feasible to recommend a single universal extraction solvent to extract active ingredients from all plant biomass.
The commonly utilized solvents for extraction are ethanol, methanol, ethyl acetate, acetone, heptane, dimethyl sulfoxide (DMSO), and water. The food industry prefers to use solvents that are non-toxic and easy to handle. DMSO and methanol solvents for extraction have food safety issues due to their toxic nature; hence, mostly preferred as solvent of choice for various non-food-related uses. DMSO can be used for dissolving both polar and non-polar compounds, as well as its ability to mix in organic solutions, including water, completely. Methanol was also reported suitable for lower molecular weight polyphenols extraction whereas water has been recommended to extract polyphenols with larger flavanols [17]. Thus, a combination of these two has been recommended.
Methanol retards polyphenol oxidase, and hence is recommended for phenolic-compounds extraction from the raw plant matrix [18]. Water is very efficient in extracting non-polar compounds, whereas the use of organic solvents proves efficient in extracting organic compounds from plant biomass. The hydrophilic compounds present in plants are the most active compounds that can be efficiently extracted by using water as a solvent. Water was also reported as a suitable extraction medium for tea catechins, as compared with combinations of methanol (80%) and ethanol (70%) [19]. Do et al. [20] reported significantly higher antioxidant activity in Limnopholia aromatic extracts obtained by absolute ethanol with maximum yield by the combination of 50% acetone and 50% water. They recommended using combinations of aqueous and organic solvents in suitable concentrations for the efficient extraction of active principles that are soluble in water and the organic solvent.
Organic solvents are commonly used to efficiently extract phenolic compounds, but their left-over residues may be potentially harmful to consumers' health even in traces. Thus, while using organic solvents as an extraction medium, it is imperative to take utmost care to remove all extracting solvents from the filtrate. The application of aqueous media in extraction has several inherent advantages. Water is considered a cheap and the safest solvent but is not as efficient as organic solvents, especially in extracting active compounds possessing antioxidant potential [21]. Turkmen et al. [22] utilized water, acetone, N, N dimethylformamide, ethanol/methanol as extraction solvents for obtaining green tea and mate tea compounds and reported the use of 50% aqueous ethanol and 50% aqueous acetone to get the maximum yield and so antioxidant potential.
The food industry increasingly prefers green solvents for extraction due to their nontoxic nature, food safety, and recycling, such as water, ethanol, deep eutectic solvents, synthetic ionic liquids, and carbon dioxide, and water again, as a supercritical fluid in extraction [23][24][25]. The ionic liquids are salt mixtures in the liquid phase at room temperature, comprising ionic components that bind ionically [25]. These solvents have very low vapor pressure, high conductivity, are very stable at high temperature, and have a wide range of polarity, but their food-safety risk, high cost, and poor environmental degradability hampers their proper utilization in the extraction sector [26].

Extraction Methodology
Extraction conditions have a critical role in determining the overall efficacy of extracts. Generally, it is recommended to perform extraction at lower temperature and avoid prolonged exposure to high temperature, as the latter could lead to significant loss of active principle of the extract. Based on the type of plant biomass used, suitable solvent and extraction techniques must be employed to get the maximum concentration and yield of the active compounds in the extract. After extraction in solvents for a sufficient time, the solution is filtered. The resultant filtrate is dried at low temperature using advanced technologies such as vacuum (vacuum oven/rotator evaporator/rotavapor) or freeze-drying (for aqueous solvent mostly). Extracts should be stored under frozen conditions, in suitable, inert packaging material, such as glass bottles/trays, to prevent reaction between the active ingredients and packaging material. A solvent used in the extraction of bioactive compounds or essential oils should have low toxicity, a high mass transfer rate, a low boiling points, preserve the active ingredients, and should not interact with the organic compounds in the plant biomass or food [27].

Traditional Extraction Methods
Water or alcohol-based extraction methods, steam distillation, or Soxhlet extraction are widely used methods for extracting bioactive compounds from plant biomass. These methods are simple, low cost, and fast, but have some practical problems, such as difficult and complex operations, lower yield/inefficient methods, energy-intensiveness, and extract at high temperatures, leading to the degradation of heat-sensitive active compounds, problems in the purification of extracts, and removal of solvents-especially, poor recovery of solvents and the generation of organic wastes, a requirement of specific organic and inorganic solvents [28]. Under solvent extraction or portioning, active ingredients from plant biomass (solid or liquid) are separated by dissolving them into a suitable solvent. This is one of the most commonly used traditional methods by the food industry. The active ingredients (solutes/extracts) are extracted from one solid or liquid phase to another liquid i.e., solvent. This extraction process is generally completed at a higher temperature for long exposure, resulting in the possible degradation of the active principle in the extract and high energy consumption.
Under Soxhlet extraction, a small, dried sample is placed in an extractor and placed in direct contact with a suitable solvent (water, petroleum ether, and hexane) repeatedly until the extraction is complete. Its higher solvent consumption, energy requirement, and long extraction time are important issues with the Soxhlet extraction technology [29].
Maceration is the process of breaking/subdividing or softening matter into pieces of plant biomass in a suitable solvent. This method is recommended to preserve the typical essence of extracts of some valuable herbs that contain very delicate, heat sensitive, and volatile compounds. It is used for the preparation of traditional food products with specific flavors and organoleptic attributes.
Hydrodistillation is applied to extract volatile compounds from food/plants by using distilled water. This process, of extracting volatile organic compounds by azeotropic distillation and non-volatile organic compounds by boiling water, takes 6-8 h. It comprises three processes viz. penetration of water into solutes (hydrodiffusion), hydrolysis, and some level of degradation of heat-labile compounds due to high temperature [30]. Figure 3 provides a comparative description of conventional and green extraction technologies.

Greener/Advanced Extraction Methods
The high energy and solvent consumption and lower solvent recovery in traditional extraction methods has forced various researchers and biochemists to explore a more efficient method utilizing environmentally friendly and less harmful solvents. These technologies, such as supercritical fluid extraction (SFE), ultrasound-assisted extraction (UAE), subcrit-ical water extraction (SWE), microwave-assisted extraction, instant controlled-pressure drop extraction (French-DIC détente instantanée contrôlée) are widely referred to as green extraction technologies [23,31]. These technologies require a lower amount of organic solvent or utilizeg less harmful or environmentally friendly solvents, show better reuse of solvents, use solvents from sustainable sources with reduced or no toxicity, consume less energy, have a lower environmental impact/footprint, better worker safety; that is, they are highly efficient extraction methods with higher yields, containing the maximum percentage of desirable bioactive ingredients in active form [23,32].
Various operational aspects of various green extraction technologies are described in Table 1.

Supercritical Fluid Extraction
A supercritical fluid is a dense gas or compressed liquid lacking any molecular interactions. These gases, in a supercritical fluid state, are compressed, exhibiting properties of both gas-like (mass transfer, diffusion, lower viscosity, higher penetration power) and liquid-like (high density, solvent power, reduced surface tension, and viscosity) attributes (mesophase) at temperatures and pressures above critical points, with even minor change/adjustment in these parameters resulting in a significant change in density (tunable). Active principle or bioactive compounds are separated from the plant matrix (solid or liquid) by utilizing supercritical fluids to extract solvents. Carbon dioxide and water are the two commonly used supercritical fluids in the industry on a large scale.
Recently, there is an increasing focus on extracting bioactive compounds from the plant by using various solvents such as carbon dioxide, propane, ethylene, and butane in the supercritical fluid stage (SCF), due to their use in efficient and environmentally friendly technology. This supercritical fluid technology has resulted in the development of several other subsidiary technologies, used in the pharmaceutical and food sectors, such as supercritical fluid extraction (SFE), gas antisolvent process (GAS), supercritical solutions (RESS), supercritical antisolvent process (SAS), and their suitable modifications, such as solvent-extraction systems (ASES), supercritical solvent impregnation (SSI), and supercritical assisted atomization (SAA) [34]. Supercritical fluid extraction (SCFE) is a fast, efficient, economical, selective, practically solvent-free method, having a simple procedure for sample preparations' extraction of bioactive compounds from plant biomass [35]. Carbon dioxide is the most commonly applied gas in SCFE. Supercritical fluid extraction with carbon dioxide is performed at 31 • C and 74-bar pressures. These processing conditions viz. temperature and pressure depend on the nature and type of the extracts to be obtained; for example, low pressure (100 bar) is used to extract volatile oils/essential oils, polyphenols, and unsaturated fatty acids [36]. SC CO 2 is the most common (about more than 90% of total SCFs used) used supercritical fluid (SC CO 2 ) used in the extraction of flavor and fragrance compounds. The SC CO 2 has the following operational advantages: Various parameters that make carbon dioxide as most common gas used for SCFE are presented in Figure 4.
The carbon dioxide gas in the supercritical fluid stage exhibits properties of a lipophilic solvent, with greater and easier manipulation of its solvent between its use as gas solvent versus a liquid one. SC CO 2 has a low polarity index, and thus is useful in dissolving lipophilic compounds such as lipids and essential oils, but unsuitable for polar compounds. To overcome this challenge, SC CO 2 is used with other co-solvents to increase its polarity index [38,39], or an anti-solvent with plant material is injected into SC CO 2 . For the extraction of polar compounds, supercritical nitrous oxide (SC N 2 O) fluid (7.1 MPa Pc and 36.7 • C Tc) is utilized. Still, it has safety issues regarding its high concentration in organic compounds [40]. Water exhibits a supercritical fluid state at a high critical pressure (Pc 22.1 MPa) and critical temperature (374 • C Tc). Still, its corrosive nature has limited its use in the food and pharmaceutical industry [41]. The pre-treatment of plant biomass with water before extraction with SC CO 2 or used as a co-solvent with carbon dioxide, in a subcritical or supercritical state, for the extraction of polar compounds from aromatic plants significantly improves the composition of its extracts [42,43]. Carbon dioxide can segregate flavor and aroma compounds from complex mixtures in various herbs and spices, such as ginger, garlic, and pepper.
Besides CO 2 , ethane (4.8 MPa Pc and 32.4 • C Tc), dimethyl ether, and propane (4.2 MPa Pc and 96.6 • C Tc) are also used in extracting bioactive compounds from plant biomass, due to their critical temperature and pressure being close to CO 2 and better extraction efficiency in extracting polar compounds due to their higher polarizability as compared with CO 2 [44]. There is a range of solvents that have the potential to be used in supercritical fluid extraction, such as n-butane, nitrous oxide, freon, propane, methoxymethane and water, but various food-safety matters and operational hazards/harsh handling should be given due consideration before their, application such as the very high critical temperature of water (400 • C), which poses serious handling challenges during its application in SFE.

Propane as a Supercritical Fluid
Propane's application in supercritical fluid extraction processes has certain inherent merits over carbon dioxide, such as higher yield, an improved solubility of non-polar compounds, and a fast and efficient process requiring less time with better specificity, and less solvent consumption [45]. Liquified petroleum gas (LPG) is a mixture of propane and butane used in domestic food preparation and heating. In extraction under high pressure (245 g/kg), it improves the operation speed. It improves the catalytic power of enzymes, such as increasing enzymes' activity, leading to improved enzymatic hydrolysis of lignocellulosic material of sugarcane bagasse, for example [46]. The uses of propane and LPG as supercritical fluids in the extraction of bioactive compounds are presented in Table 2. Extract quality same with both solvents, and temperature and pressure have an important role. [52] Sunflower seed Propane and CO 2 High concentration of tocopherol in the oil [53] LPG as supercritical fluid

Rice bran
Compressed LPG and SC CO 2 LPG decreased extraction time and save energy of re-compression [54] Elaeis guineensis Compressed LPG Advantages in terpene extraction with improve speed and reducing cost [55] SC CO 2 -supercritical carbon dioxide, LPG-liquified petroleum gas, SFE-supercritical fluid extraction.

SCF Extraction Process
The SFE unit has the following essential components viz. a carbon dioxide compressor or pump, along with a modifier pump in a co-solvent, such as water or an organic solvent, an extraction reactor, and a collection or fractionation vessels. To improve the selectivity and sequential separation of active compounds from plant biomass in SFE operations, a series of reactors are applied, or a various combination of temperature, pressure, and solvent flow rate is applied [56]. Other processing variables that affect extraction in SFE are flow rate, extractor diameter, particle size (larger particles require more time), diameter-tolength ratio, the application of extractors, bed porosity, and stirring diameter [57]. Samples are pre-treated by various processes, such as microwaves, ultrasound, irradiation and enzymatic treatment, to increase the efficiency of the process and improve the recovery of bioactive compounds. The preparation of raw samples has an impact on the recovery of bioactive compounds, such as how air-drying may lead to a loss of heat-labile compounds, whereas freeze-drying helps in retaining the activity of these heat-labile compounds.
The optimization of processing conditions is critical for obtaining the desired outcome in supercritical fluid extraction. In general, an increase in pressure above certain points results in increasing fluid density and improving solute solubility, leading to higher yield. In some cases, pressure increase also proves disadvantageous, by decreasing the diffusivity of the supercritical solvents, thereby decreasing contact/interactions and dissolving solutes or even in some cases, cause the formation of void fractions and undesirable outcomes due to a compact solid matrix [58,59]. Temperature exhibits a crossover effect in SFE, where high-temperatures decreases yield by reducing density, increasing diffusivity, and imparting high energy to the system, whereas low temperature causes increasing extract yield by increasing density and reducing vapor-pressure solutes [33]. However, in essential oils, the diffusion has a greater impact than density, thus higher temperature result in higher yields [60].
Water is soluble in SC CO 2 at 0.3% v/v [35], and this was observed to affect the extraction process, depending upon the nature of plant biomass. Paprika, for example, with 85% moisture recorded a marginal increase in yield [61], while no effect was observed on extract yield from Nannochloropsis sps containing 5-20% moisture [62], though extract yield increased (40%) and CO 2 consumption was reduced by 25% by soaking the flowers of Helichrysum italicum prior to processing [63].
Overall, the SFE is completed in five basic steps: (e) Precipitation of compounds/solutes by suitable pressure and temperature modifications [34,40].
The various components of an SFE unit are depicted in Figure 5.

Other Extraction Methods as SFE-Adjunct
The application of various other extraction methods in combination with SFE has been reported to improve the efficiency and recovery of active compounds in the SFE process. Porto et al. [64] observed an 11%-higher yield (up to 34%) than controls (23%) upon microwave pre-treatment followed by CO 2 SFE at 40 • C and 300 bar with Moringa oleifera seeds at 100 W for 30 s.

Enzyme-Assisted SFE
The enzymatic treatment of plant materials, such as cellulase, alpha-amylase, hemicellulose, and pectinase, also improves the extraction efficiency and yield by enzymatic degradation of the structural integrity of plant's cell wall, thereby increasing the interactions between solvent and solute. Black pepper, treated with alpha-amylase obtained from Bacillus licheniformis and followed by CO 2 supercritical fluid extraction (60 • C, 300 bar at 2 L/min flow) increased black pepper oleoresin yield by 53%, with a 46% more piperine-enriched extract and higher enzymatic activity, noticed in continuous flow (2.13%), than the batch process (1.5%) [65].
Pre-treating pomegranate peel with recombinant enzyme mixtures containing cellulase, pectinase, and protease (2:1:1) followed by SFE (using SC CO 2 and ethanol as co-solvent) resulted in a two-fold increasing crude extract yield with higher levels of total phenolics as compared with extracts obtained by only the SFE process. Vanillic acid (108.36 µg/g), ferulic acid (75.19 µg/g), and syringic acid (88.24 µg/g) were reported as principal phenolic compounds in the pomegranate peel extract [66]. The authors recommended using enzyme-assisted supercritical fluid extraction (EASFE) as a state-ofthe-art green technology for extracting bioactive compounds from plant biomass. The pre-treatment of black tea leftovers with 2.9% kemzyme (2.8% w/w at 45 • C, pH 5.4 for 98 min) followed by SFE using SC CO 2 , along with ethanol as a co-solvent, increased extract yield by five-fold, displaying a higher amount of caffeic and para-coumaric acid as compared with controls. The enzymatic treatment hydrolyzed the cellulose and hemicellulose of the black tea leftovers, thus releasing the non-extractable polyphenols [67]. The application of plant cell-wall glycosidase to freeze-dried tomato matrix, prior to SC CO 2 (500 bar, 86 • C, 4 mL/min, SC CO 2 flow) extraction, was observed to increase lycopene content three fold [68].

Ultrasound-Assisted SFE
Ultrasound application at 10-20 kHz creates a mechanical breakdown of plant biomass and improves the extraction yield with less solvent and energy consumption. A longer ultrasound treatment, above permissible limits, causes undesirable outcomes due to nonenzymatic browning reactions and oxidative degradation, for example, a 200 W and 10 min ultrasonic treatment followed by SC CO 2 extraction process increased oil yields by 3.3%, whereas further increasing the duration of ultrasonic treatment lead to an undesirable outcome [69]. Ultrasonic pre-treatment of ginger rhizome (40 kHz, 40 • C, 1 h), followed by SC CO 2 extraction (250 bar, 40 • C and 15 g/min flow rate), resulted in a 110% improvement in extract yield as compared with controls [70]. The application of various extraction processes as SFE-adjuncts are presented in Table 3.

SFE of Bioactive Compounds
Various essential oils present in these aromatic herbs and spices are the principal compounds affecting the flavor and fragrance of food and pharmaceutical products, owing to the presence of oxygenated compounds such as phenols, ketones, aldehydes, acids, alcohols, ethers, acids, and esters, along with mono and sesquiterpenes hydrocarbons [34]. The flavor and aroma of food products has significant impact on its sensory attributes and acceptance by consumers. Various aromatic herbs or spices are added during their preparation to improve their flavor and aroma. These compounds also form a significant part of the global market of natural products. SFE is increasingly being applied in the food, pharmaceutical and cosmetic sectors as suitable alternatives to traditional solvent or steam distillation [35], the latter possessing lower selectivity, is energy-intensive/expansive, and exhibits a loss of volatile compounds during the extraction process [78]. The extraction of various bioactive compounds by using SFE technology are presented in Table 4.

Pressurized Liquid Extraction (PLE)
Under pressurized liquid extraction (PLE) or accelerated solvent extraction (ASE), solvents at high temperature (25-200 • C) and pressure (up to 200 bar or 20 MPa) are used for the extraction of bioactive compounds. The high temperature and pressure decrease the solvent's surface tension, which facilitates penetration into the pores of the plant matrix, thereby improving the mass transfer of the active compounds to the solvent [127]. The solvent under pressure remains in a liquid state, even at its boiling point, and facilitates extraction at a higher temperature. Under these conditions, solvents that are not efficient in extracting analytes such as phenolic compounds or anthocyanins under normal conditions may be used for the same extraction. Pressurized solvents have improved, featuring desirable physicochemical properties, such as increased diffusivity, solubility, viscosity and dielectric constant, and these can be further modified by changing temperature and pressure. This is a rapid (completed within 30 min) and efficient process with reduced solvent consumption, but higher temperature-induced damage to heat-labile active compounds [128]. The requirement of large and sophisticated equipment and extraction at higher temperatures are drawbacks to this method.
The use of pressurized extraction technology for extracting bioactive compounds from plant biomass is presented Table 5.

Ultrasound-Assisted Extraction (UAE)
Plant biomass, exposed to high-intensity ultrasonic waves (20 kHz-100 MHz), results in tiny cavitations/bubbles around the cells. These bubbles suddenly collapse during the procedure, producing shockwaves that disintegrate the cell wall and release its intracellular contents, thereby improving the release of target compounds. This technology is simple to operate, has relatively inexpensive extraction systems, increases mass transfer, increases kinetics, yields bioactive compounds, and is suitable for a wide range of solvents for industrial applications [132]. There are two main physical phenomena in the UAE viz. diffusion of solvent through the cell wall and dissolving cell content after breaking the cell wall.
Ultrasound-treated plant biomass increases the extraction yield by fragmentation or reduction of particle size; erosion and improved accessibility to the solvent; sonocapillary effects, by increasing the velocity and depth of penetration of the solvent through pores and canals; sonoporation, by increasing cell membrane pore size; local shear stress by friction between the liquid molecules; detexturization; and combinations of these methods [133]. UAE efficiency is affected by the temperature of solvent, pressure, the frequency of ultrasound waves/energy, and the sonication time [134]. This method is relatively easy to use and can be suitable/applied for/to a range of plant matrices, features tunability, and requires low capital cost compared with other methods.
Ultrasound can be applied directly to the extraction medium by a probe, which amplifies the intensity of the ultrasound waves (up to 100 times), whereas indirect ultrasound treatment is given by using an ultrasound water bath, wherein the water acts as a transport medium for the ultrasound waves [135]. Ultrasound-assisted extractions of bioactive compounds from plant biomass are presented in Table 6.

Microwave-Assisted Extraction (MAE)
Microwaves (300 MHz-300 GHz frequency, 1 mm-1 m wavelength) penetrates food materials, agitating water molecules and charging ions, leading to non-contact heating. It enables faster heat transfer by converting electromagnetic energy to thermal energy through ionic conduction and dipole rotation [147]. This technique is commonly used to extract active ingredients, by combining microwave heating and traditional solventextraction techniques. This combination of microwave heating and conventional solvent extraction has several merits over conventional extraction methods, such as less use of solvents with better yields; it is afast process; it's economics are better as it is a fast and efficient process with less energy consumption and carbon dioxide emissions [148]. Due to these procedural and technological advantages, this technology has become a popular method for extracting substances from plant materials.
At present, several advanced technological innovations have been incorporated in this method, such as pressurized microwave-assisted extraction (PMAE), and solventfree microwave-assisted extraction (SFMAE). The microwave-assisted extraction resulted in a significant increase in apple-pomace-extract yield with active ingredients (55%), as compared with other extraction methods, such as ultrasound-assisted extraction (33%), pressurized liquid extraction (33%), and maceration (43%) [149]. Microwave-assisted extraction of bioactive compounds from plant biomass are presented in Table 7.

Pulsed Electric Field Assisted Extraction
Pulse electric field (PEF) is a novel, environment friendly, non-thermal and green technology. Its application increases the cell membrane permeability and mass-transfer rate. It has wide application in the meat industry, such as accelerated curing, drying and freezing, meat tenderization, novel product development, and restructured meat products. The application of PEF is performed by placing the food items between or passing it through two electrodes while applying a very high-voltage electric field for a very short duration [157]. To achieve a visible PEF effect on preservation and quality of food-the purpose here underconsideration-electric pulses of 20-1000 µs (several nanoseconds to several milliseconds) are required in an electric field several orders of magnitude strong (0.1-80 kV/cm) [158]. The overall efficiency of PEF depends upon the applied field strength, temperature, and energy-delivery efficiency. During the application of PEF, irreversible structural changes occur in cells' membranes, leading to a marked increase in cell permeability and an enhanced mass-transfer rate across the membrane. These two factors ultimately result in the breakdown of cellular tissue [159].
This technology could be very useful in recovering bioactive compounds of more specificity from plant matrices more economically and with a low environmental impact, by softening and disrupting the cell membrane, resulting in the release of intracellular compounds [160]. Pulse electric field-assisted extraction can utilize renewable, alternative, efficient, and less toxic solvents obtained from plant resources, such as water and agri-solvents, which exhibit lower energy consumption, and produce higher quality and purity yields. Parniakov et al. [161] applied pulse field-assisted extraction of bioactive compounds from the Agaricus bisporus mushroom and obtained mushroom extracts with higher polysaccharide content and high purity with less energy. Boussetta et al. [162] also noted increased yield and improved extraction efficiency, even at low temperatures and with less solvent, in this method. It facilitates easier removal of bioactive compounds from the plant matrix without damaging it and facilitates segregation and purification later. Table 8 depicts the application of pulse electric field extraction of bioactive compounds from plant biomass. Polyphenol and antioxidant potential increased between 1.3 and 6.6-fold and from 2.0 to 13.7 fold, respectively as compared with conventional methods [166] Apple juice 3 µs, 3 kV/cm, electric conductivity-2.3 mS/cm Higher polyphenols content and reduced processing time as compared with conventional methods [167] Although PEF systems have low maintenance costs, high efficiency, and are fast, their exorbitant initial capital cost is still prohibitory. With the introduction of new technologies and the scaling-up of production, its cost is decreasing steadily and finding wider acceptance by the food industry.

Miscellaneous
The other extraction methods are high-voltage electric discharges and high hydrostatic pressure. The high-voltage electric current is applied through an aqueous solution through electrodes, inducing an avalanche of electrons, bubble cavitation, and pressure shock waves, causing cell damage and resulting in the release of bioactive compounds in a process similar to ultrasound [134,160]. The electric energy leads to chemical electrolysis and free radical formation, which may react with bioactive compounds, leading to reduced antioxidant activity of the extracts [161].
The high-hydrostatic-pressure method is seen as an alternative to thermal processes in improving the microbiological quality of food products. It is a green technology, involving thermal treatment and increased mass transfer rates and metabolites movement [168]. High-pressure treatment leads to protein denaturation by deprotonation and breakage of hydrophobic linkages and salt bonds, improving permeability and diffusion of the solvent, resulting in a higher yield of bioactive compounds [169].
Thus, the selectivity and yield of traditional and novel green extraction techniques depend on the proper selection of extraction protocols, such as critical input parameters, the nature and state of the plant biomass, the structure and heat sensitivity of its bioactive compounds, the yield of its extracts, and the equipment needed. However, these technologies should be commercialized and developed on a large scale, with suitable technological advancements and better, safer, and greener solvents to maximize benefits for the food, pharmaceutical, and nutraceutical sectors. Higher capital costs or larger initial investments are still prohibitive and must be reduced to facilitate their large-scale adoption.

Plant Extracts as Natural Antioxidants in Meat
The most common method of applying these extracts in meat processing is mixing them with water (mostly for water-soluble extracts; for organic solvent-assisted extracts, it is advised to mix in vegetable oil or fat) during preparation. It ensures the homogenous distribution of extracts in the product. The overall oxidative stability of products depends upon the concentration of the extracts; the higher the extract concentration, the greater the antioxidant effect. Phenolics, catechins, flavonoids, quercetin, anthocyanin, tocopherol, rutin, chlorogenic acid, lycopene, caffeic acid, ferulic acid, p-coumaric acid, vitamin C, protocatechuic acid, vitamin E, carotenoids, β-carotene, myricetin, kaempferol, chrysin, carnosine, zeaxanthin, sesamol, rosmarinic acid, chlorophyll, carnosic acid, carnosol, and gallic acid are compounds, present in plants, possessing antioxidant potential [170].
The important bioactive compounds present in various plant biomass and their meat application as natural antioxidants are described next.
Moringa oleifera a common vegetable in South Asian and African countries, is widely explored for its use as natural preservatives, owing to its various bioactive compounds viz. rhamnetin, isoquercitrin, kaempferol, kaempferitrin, saponins, triterpenoids, tannins, anthraquinones, alkaloids, and terpenoids [171], with concentration varying with the maturity of the plant and climatic and geographical conditions. M. oleifera is a rich source of protein, provitamins, vitamin C, A and E, zinc, calcium, iron, and potassium along with anti-cancerous agents such as glycerol-1-9-octadecanoate, glucosinolates, isothiocyanates, and glycoside compounds.
Rosemary contains a high amount of rosmarinic acid, carnosol, and carnosic acid in extract, and eucalyptol, α-pinene-bornyl acetate and camphor in rosemary essential oil [172,173]. Rosemary leaves were reported as a rich source of vitamin C, as much as as 18.51 g/100 g raw materials and extracts (0.26 mg/100 mL aqueous, 0.34 mg/100 mL alcoholic, and 0.36 mg/100 mL acetonic) have been reportedly obtained [10]. Monoterpenes hydrocarbons, esters, oxygenated sesquiterpenes, phenol, sesquiterpene hydrocarbons, oxygenated monoterpenes, ketones, and alcohol are the primary flavoring compounds of rosemary [173]. Generally, camphor, caryophyllene, borneol, bornyl acetate, and verbenone are chief compounds present in rosemary extracts. Peng et al. [174] advocated the application of supercritical fluids for the extraction of rosemary extracts, as it is associated with saving time, maximizing yield, and inhibiting the conversion of carnosic acid into carnosol. For rosemary stems, flowers, and leaves extracts, a suitable extraction methodology must be adopted, as lipophillic solvents and water (in the case of fresh samples were reported to result in a significant loss of phenols due to the action of phenoloxidase enzyme [175,176]. Arjuna or Arjun tree (Terminalia arjuna) bark, stem, leaves, roots, and fruits are a rich source of various beneficial bioactive compounds (polyphenols, flavonoids, triterpenoids, tannins, glycosides, sitosterol) and minerals. The Arjuna tree bark was reported to have the highest concentration of flavonoids such as arjunolone, quercetin, flavones, kaempferol, baicalein, and pelargonidin. Ethanolic and aqueous arjuna fruit extracts have been reported to contain a good amount of total phenolics (11.04-16.53 mg gallic acid equivalents/g), exerting significant scavenging activity (50.02-58.62%) [177].
Cinnamon bark is considered a promising antioxidant source, even exhibiting antioxidant potential comparable to synthetic antioxidants [178,179]. Most of the bark available in the market is Ceylon bark; dried bark (cork and cortex) of shoots of tree Cinnamomum zeylanicum F. lauraceae, containing approximately 0.5-1.0 % of volatile oil made up of 50.5% cinnamaldehyde, 8.7% cinnamic acid, methoxycinnamaldehyde and cinnamyl acetate, and 4.7% eugenol [178]. Chan et al. [180] reported ahigher antioxidant efficacy of deodorized cinnamon in meatballs, and noted significantly reduced lipid oxidation without causing any detrimental effects to its sensory attributes. Spices in the Lamiaceae family contain high rosmarinic acid, the major phenol (ranging from 1086-2563 mg/100 g of dry-weight) [181]. Rosmarinic acid possesses strong antioxidant activity due to two ortho-dihydroxy groups in its structure. This compound can act as an antioxidant and control the oxidation of low-density lipoproteins (LDLs). Rosmarinic acid also possesses anti-inflammatory activity by stimulating interleukin-10 (IL-10) secretion [182].
Several studies have confirmed the anti-inflammatory activity of oregano extracts, which are aqueous extracts observed to inhibit cyclooxygenase-2 (COX-2) secretion in epithelial carcinoma cells [183], and to exhibit anti-inflammatory properties by controlling stress-induced gastritis and hypersensitivity [184]. Oxidative stress leads to incorrect protein folding in the endoplasmic reticulum. Kaempferol, an aglycone flavonoid widely present in aloe vera, ivy gourd, saffron coccus and Peking spurge, is known to prevent hepatocellular carcinoma by controlling oxidative stress caused by reactive oxygen species [185]. Five bioflavonoids obtained from moss fern (Selaginella doederleinii) were observed to inhibit non-small cell lung cancer cells by suppressing XIAP and survivin expression, increasing the upregulation of caspase-3/cleaved-caspase-3, inducing cell apoptosis in A549 cells with low toxicity to non-cancer cells MRC-5 cells [186]. The Kalmia angustifolia extract exerted antioxidant, anti-inflammatory and anti-aging effects, at concentrations up to 200 µg/mL, by enhancing the expression of elastin and collagen-1 [187]. Flavonoids, such as apigenin, myricetin, and luteolin, were observed to exert anti-cancer effects against a range of human epithelial cancers by selectively reducing the viability of cancer cells, the alteration of ROS signaling, and the arrest of cell multiplication [188]. The extract of Polyalthia spp. is known to exert antioxidant, anti-ulcer, anti-plasmodial, anti-cancer, anti-microbial and anti-inflammatory effects due to the inhibition of COX-2 activity, inhibiting downstream prostaglandin E2 (PGE2) production, and inhibiting focal adhesion kinase, phosphoinositide 3-kinase, 3-hydroxy-3-methylglutaryl co-enzyme A reductase, and dipeptidyl peptidase 4 [189].
Clove exerts the most potent antioxidant activity among all spices and condiments commonly used in the food industry. Carnosic acid and carnosol, tocopherols, carotenoids, and sterols are important active ingredients present in herbs. Clove extracts mainly contain eugenol, caryophyllene, and eugenyl acetate. These two compounds have strong antioxidant potential and are considered the main bioactive compounds responsible for the increased antioxidant activity of clove, equivalent to vitamin E [190]. While processing, n-hexane solvent was recorded to produce a better yield, with high antioxidant activity (total flavonoid content-15.54 mg GAE/g, total polyphenol content-54.05 mg GAE/g, FRAP = 0.69 mg/mL, DPPH = 0.29 mg/mL), and antimicrobial activity as compared with extracts obtained with other solvents (alcohol, water, and petroleum ether) [191]. Clove extract, at 0.25%, reported exerting antimicrobial activity with an 8.8 mm-to-9.27 mm minimum inhibitory concentration, as seen in S. typhimurium and E. coli, respectively [191].
Citrus byproducts/coproducts viz. peel and pulp are a rich source of various active ingredients, such as dietary fiber, minerals, vitamins, organic acids, flavonoids (ferulic, sinapic acids, and chlorogenic), phenolics (hesperetin, hesperidium, diosmin, and narirutin), carotenoids (carotene, zeaxanthin, lutein) [192][193][194][195]. These compounds are applied in the meat industry due to their associated health effects, such as antimicrobial, antioxidant, anticancer, anti-allergic, and antihypertensive effects [196]. The antioxidant activity of different citrus extracts varies with the methodology of their extraction, their fruit type, their environmental conditions, such as soil type and climate, their fruit-ripening stage, and their harvesting time [197,198]. Nayak et al. [199] reported the high antioxidant potential of Citrus sinensis peel extracts obtained by using microwave and ultrasoundassisted accelerated extraction technology at 337. .09 mL/L DPPH (50% inhibition). Aqueous and methanolic extracts of lemon pomace had high antioxidant potential, wherein the methanolic extract exhibited higher antioxidant potential as compared with the aqueous extract, as measured in terms of DPPH assay (0.17, 0.13 mg Trolox equivalents/g, respectively) and ABTS (0.403, 0.458 mg Trolox equivalents/g, respectively) [200].
Catechin, present in green tea, is a group of flavonoids (flavan-3-ols), especially epicatechin, epicatechin-3-gallate, epigallocatechin (EGC), and epigallocatechin-3-gallate (EGCG), with EGC and EGCG regarded as the main active principle in green tea, and which is studied for its various antioxidant effects in food processing. The incorporation of tea catechins at 300 ppm in beef, duck, ostrich, pork, and chicken markedly reduced their TBARS values during refrigerated storage and was noted to exert 2-4 times higher antioxidant potential as compared with vitamin E [201]. Catechins reduced the production of putrescine and tyramine in dry, fermented pork sausage. Green tea catechins (GTC) and green coffee antioxidants (GCA) in linseed oil and fish oil, added in pork sausage at 200 mg/kg, decreased the lipid peroxidation of the sausages during seven days refridgerated storage. A significantly decreased lipid oxidation rate and higher organoleptic attributes were recorded in the fish-oil-substituted sausages [202].
Grape seed extract has a larege amount of polyphenols (such as epicatechin, gallic acid, resveratrol, and procyanidin dimers) and was reported to exert high antioxidant efficacy [203]. The antioxidant activity of grape seed is reported to be 20-50 times greater vitamins E and C [204], due to its possessing proanthocyanidins and oligomers of flavan-3-ol units. Grape seed extract (GSE) is the richest known source of polyphenolic compounds (catechins, flavanols, phenolic acids, anthocyanins, and proanthocyanidins) and has 20-50 times more antioxidant potential as compared with vitamins E and C, respectively [205,206]. Libera et al. [207] prepared grape seed extract by heating a grape seed slurry in water at 50-60 • C under high pressure (100-175 hPa or 10000-17500 bar). The authors evaluated the antioxidant potential of grape seed extract by incorporating it into pork neck containing Lactobacillus rhamnosus LOCK900 and reported that the lipid oxidation (PV-1% oleic acid and TBARS-0.46 mg MDA/kg) in the extract-incorporated samples was comparable to samples containing sodium ascorbate (PV-0.9% oleic acid and TBARS-0.53 mg MDA/kg).
The aqueous extract of Cantharellus cibarius hads high antioxidant potential due to its high concentrations of polyphenols, beta-carotene, and ascorbic acid, but has lower ABTS radical scavenging potency than vitamin C in cases where the extract is prepared by the water-decoction method i.e., first smashing and grinding, followed by boiling in water to obtain the extract [208,209]. The incorporation of Cantharellus cibarius water decoctions during the preparation of frankfurters, at 0.75-and 1.5% levels, resulted in significantly reducing the total plate counts with potent inhibitory action against Candida albicans, and improved the sensory attributes of the frankfurters during 60 days of storage under refrigerated conditions [209].
The ethanolic extract of mesquite leaf of Proposis was reported to exert high antioxidant potential due to its total phenolic content (278.5 mg GAE/g) and total flavonoid content (226.8 mg RE/g). The incorporation of the extract (0.05-0.10%) during the preparation of pork patties resulted in significantly improved oxidative stability of the treated patties as measured by theie marked reduction in TBARS value (90%) and conjugated dienes (40%) as compared with the positive control, i.e., patties with BHT [210]. The Carob (Ceratonia siliqua L.) tree is an underutilized tree of the Mediterranean region. Carob has high dietary fiber content and has 1.2-7.0% polyphenolic compounds, especially catechins, myricetin, rutin, and gallic acid [211].

Current Scenario
The studied plant extracts showed strong antioxidant activity in meat products, owing to strong H • -donating activity, high radical-scavenging capacity or the ability to sequester metal catalysts. Various studies had been undertaken to standardize the extraction protocols and incorporation of extracts in meat products, and to document the preservative effect of plant extracts, as indicated, by reduced oxidation (in terms of lower TBARS value, PV, FFA value, carbonyl content, and free thiols) and microbial growth inhibition (in term of total plate count, coliform count, psychrophilic count, and food pathogens), inhibiting various carcinogens, reducing levels of heterocyclic aromatic amines, and better preservation/maintenance of sensory attributes (such as freshness, color, juiciness, flavor, and overall acceptability). At present, advanced technologies are being applied (such as supercritical water extraction, microwave-assisted extraction, ultrasonication, vacuum drying, and freeze-drying) to produce high-quality extracts with higher concentrations of active principle, on an industrial scale, for food application.
Various commercial preparations of some commonly used plant extracts are now readily available for application in food processing. The active ingredients or bioactive compounds from plant extracts are concentrated or purified using various ion-exchange adsorptions, followed by elution or chromatography technology. These are prepared using these improved and advanced technologies to obtain the maximum concentration of active ingredients in the extracts and protect any loss or degradation of active ingredients due to undesirable liquids, enzymes, and fermentation leading to alcohol production due to vinification. Angeletti and Sparapani [239] patented a process for the preparation of grape seed extract, containing a lower concentration of monomeric polyphenols and a higher concentration of procyanidin oligomers, by using a combination of acetone and methyl alcohol as a primary solvent in addition to ethyl acetate, methylene chloride, and dichloroethane.
The purified preparations of these plant extracts are now readily available in the market as premier products/dietary supplements/nutraceuticals. These are recommended for various health benefits, such as reducing oxidative stress, maintaining cardiovascular health, lowering LDLs and proper blood pressure, improving memory, controlling atherosclerosis, and anticancer effect. Natural Products Insiders Inc. California, USA had commercial preparation of grape seed extracts with the trade name ORAC-15 M (80% polyphenols and 15000 oxygen radical absorbing capacity) and recommended its use in alleviating the oxidative stress of the body. Other commercial preparations of Natural Products Insiders Inc. are MegaNatural, Enovita, and Leucoselect. Activin, Gravinol-S, Activin, Gravinol Super are some commercial preparations of grape seed extract produced and marketed by Inter Health Nutraceuticals Inc. (Broadfield Park, Crawley RH11 9RT, United Kingdom). Pycnogenol, an extract obtained from pine bark is commercialized by Natural health Science Inc. New York, USA and Horphag Research, Cointrin, Switzerland. It has a higher concentration of procyanidins, phenolic acids, and bioflavonoids, and is recommended for cardiovascular health, skin and eye care, proper brain functioning, and improving respiratory and women's health. Applied Food Science Inc. has a wide range of commercial preparations of plant extracts, such as green coffee extract (GCE-50% chlorogenic acids from raw coffee beans), green coffee caffeine extract (Java G with 45% chlorogenic acid, 60% polyphenols, and 40% caffeine), organic ginger powder (PureGinger with 2% gingerol), cascara fruit extract (CoffeeNectar from coffee cherry), caffeine from green tea (Purtea), etc. Herbalox Seasoning HT-25 and Herbalox HT-25 are trade names of rosemary extracts manufactured by Kalsec Inc. Kalamazoo, Michigan, USA. Kemin Americas Inc., Iowa, USA, manufactures, who markets the rosemary extract as Fortium R 20.
The consumer preference towards healthier and greener alternatives to synthetic preservatives has been the main driving force of the rapid growth of plant extracts. The global market value of plant extracts was reported to be 43.32 billion USD in 2019 and is forecasted to grow with a compound annual growth rate (CAGR) of 6.0%. The market value of plant extract is expected to reach 66.81 billion USD by 2027 [240]. With the increasing preference of consumers towards green alternatives and minimally processed or natural foods, this sector is expected to maintain high growth in the near future.

Prospects and Challenges
As plant extracts are very potent antioxidants and rich in phenolic compounds, there is a high probability that, at higher concentrations or incorporations, these will lead to bitterness or aftertaste and darkened color of the developed product. It is always desirable to explore and use plant extracts, owing to their higher antioxidant ability with minimum effect on the sensory attributes of meat products. Alternatively, combinations of various plant extracts are also used to achieve desired effects without modifying/altering sensory attributes. As flavor and taste are crucial in determining the overall acceptability and acceptance of food products by consumers, these extracts should be added to meat products, if well within the recommended limit/optimum level. Another challenge before the meat industry is the differing outcomes of plant phenolics in in-vitro systems and in the food matrix. It has been noticed that these compounds exert a potent antioxidant effect, even at small concentrations, in in-vitro conditions, but need comparatively higher concentrations in meat model systems, which may result in bitterness and off-flavor. This could be due to interactions between these phenolic compounds with other meat components, such as muscle protein, lipid and minerals [241,242].
The application of plant extracts in the meat industry shows promising results in extending storage life and preserving product quality by significantly inhibiting the oxidation of meat lipids and protein and microbial growth. A proper extraction protocol is critical for obtaining the desired yield percentages and activities of plant extracts that determine their overall effectiveness and success. The high energy and solvent consumption, associated food-safety hazards due to organic solvent residue and waste, are major concerns among food technologists, to the extent that even brand extracts are being prepared from such green alternative methods [241]. The remnants of toxic solvent in plant extracts make them unsuitable for use in the food industry, so to for the loss of active ingredients due to improper high temperature and storage, change in color and flavor, bitterness, increasing cost of production are some important considerations and various food technologists, and researchers are studying these aspects. Further, the current focus of the food industry is on the proper segregation and identification of active principles and assessing their safety levels, toxicity, if any, and optimum levels of incorporation of these active principles in food.
Mbah et al. [243] and Porkorny [244] noted the economical cost, better safety, and easier application of synthetic antioxidants, compared with natural antioxidants, as major factors leading to the higher utilization of these synthetic antioxidants in the food industry. Consumer perceptions about stored products as stale, their preferences towards the use of meat products with shorter shelf lives, without preservatives and minimally processed are some factors limiting the use of these plant extracts in the meat industry. There is a need for exploring a better suitable, efficient, readily available, safer, easy-to-use application; proper labeling; and cost-effective materials for application in the meat industry. Proper and uniform legislation is required for extracts, as some extracts have been classified as safe for consumption since ancient times but nonetheless produce allergic reactions in some consumers upon consumption.

Conclusions
Natural extracts are extracted from various plant sources and are rich in several bioactive compounds possessing potent antioxidants. Some of these plant extracts exhibit comparable or even better antioxidant properties than commonly used synthetic antioxidants in the meat industry. Plant extracts should be utilized by following proper extraction protocols to obtain higher yields with higher concentrations of bioactive compounds exerting desired antioxidant effects. Green extraction technologies are proving to be a better and more efficient method for extracting bioactive compounds from plant biomass. However, the high initial cost and lack of scaling-up have proven to be a significant challenge for food technologists and biochemists. The application of combinations of extraction technologies is recommended, such as using conventional means with supercritical fluid extraction, and Soxhlet extraction paired with microwave-assisted extraction due to their synergistic effects. There is also a need for further identification and separation of the active ingredients in plant extracts and their potential toxicological effects and permissible limits in meat products.