In recent years, growing consumer demand for natural-based products in the personal care and well-being sectors has sparked renewed interest in medicinal and aromatic plants as sources of bioactive and functional components, especially in the wake of the recent pandemic events. Globally, the phytochemicals market is experiencing a significant positive trend lately, with a Compound Annual Growth Rate (CARG) of about 8.5%, corresponding to a trade value of about USD 7 million in 2023 [1]. Phytochemicals (term generically referring to the secondary metabolites of plants such as polyphenols) are non-nutrient bioactive components of plant origin recognized to be of great interest for their beneficial effects on human health and beyond. They include a plethora of different chemical structures that can be grouped into polyphenols, terpenoids, alkaloids, glucosinolates, etc. [2].

Yet, the choice of which extraction method needs to be applied to the plant matrix for their recovery is the first critical issue in maintaining their quality and biological potential unaltered. Extraction can be described as a critical transport phenomenon that transfers matrix components to the solvent ^[3][4][5][6][4,5,6,7], which can be carefully controlled to preserve odor, flavor, and biologically active compounds ^{[7][8][9][10]}[8,9,10,11]. The biological potential of natural extracts is influenced by both the technology used to obtain them and the quality of the plant matrix used ^{[6][9][11][12]}[7,10,12,13]. Conventional extraction strategies to isolate bioactive compounds from plants are based on the use of organic solvents, most of which have a negative environmental impact ^{[13][14][15][16][17]}[14,15,16,17,18]. These extraction methods, including steam distillation, hydro distillation, and liquid–solvent extraction, require high temperatures, long processing times, and at times many extraction steps, limited extraction efficiency, safety issues, and environmental concerns regarding the use of toxic solvents ^{[18][19][20][21][22][23]}[19,20,21,22,23,24]. Recently, green, and non-thermal alternatives such as ultrasounds, pressurized liquids, supercritical fluid extraction, pulsed electric fields, and cold plasma (CP) have been introduced to partially overcome these problems ^{[6][24][25][26][27]}[7,25,26,27,28]. These new methods can reduce energy consumption, process time, use of organic solvents, and loss of nutrient/nutraceutical compounds, enabling high-quality functional plant extracts ^{[13][28][29][30]}[14,29,30,31].

CP is an emerging and relatively unexplored non-thermal technology with promising applications in various areas of food processing ^[31][32][33][72,73,74]. It is regarded as an innovative method, which uses highly reactive charged molecules and gaseous species to inactivate contaminating microorganisms in food. From a chemical-physical perspective, plasma represents the fourth state of matter ^[32][34][73,75]. When the energy of molecules in a system increases, solids turn into liquids and liquids into gases. The intermolecular configuration is altered. A further increase in the energy of gases causes all interactions to vanish, releasing positive and negative ions and causing some molecules and atoms to ionize, giving rise to plasma, which can then be defined as an ionized gas ^[31][32][33][72,73,74]. Currently, CP is used to improve the shelf life of food ^[34][35][75,76], reduce food contamination ^[36][77], and improve the functional properties of proteins ^[37][78]. It is also used for structural modification ^[38][79], enzyme inactivation ^[39][80], removal of toxins ^[40][81], change of food packaging characteristics ^[41][82], wastewater treatment ^[7][42][43][8,83,84], control of biofilm ^[44][85], and food waste valorization ^[23][45][24,86] (Figure 1). CP can also reduce the unpleasant effects of thermal treatments, such as discoloration and loss of nutrients ^[31][46][72,87].

In recent years, a growing number of scientific studies have highlighted the potential of using CP to increase the extraction yield of phytochemicals due to its non-thermal properties and energy efficiency ^{[18][20][29][36][47][48]}[19,21,30,77,88,89], ensuring faster times, reducing the degradation of heat-sensitive substances, and achieving sophisticated extraction without impacting the environment. However, to date, most studies have focused on the microbial decontamination properties of CP, while there are few studies on the effect of CP treatment on the production of plant extracts and their phytochemical profile and relative biological potential. Thus, rethisearchers aim work aims to review up-to-date information on the application of CP technology for the extraction of valuable plant compounds, considering its effect on (a) extraction yield, (b) total phenolic content and phytochemical profile, (c) antioxidant capacity, and (d) antimicrobial properties of the obtained extracts.

2. Principles, Types, and Sources of Cold Plasma Production

Plasma is considered the fourth state of the matter. It is a partially or fully ionized gas, macroscopically neutral, in which the species that can be identified are the molecules of the gas, fragments of the same (atoms, positive and negative ions, and radicals), and reaction products between all the species present ^[49][90] (Figure 2); these may be in different states of excitation depending on how the energy is distributed within the system. Any gas at a temperature above 0 °K contains a certain concentration of charged species (electrons and ions), but it is only considered a plasma if the concentration of charged species is such as to affect its motion. Any form of energy, including UV and gamma radiations, electrical energy, and electromagnetic radiation, has the potential to generate plasma. Plasma can be classified based on temperature (hot plasma or cold plasma), pressure (low, atmospheric, or high pressure), working gas (air, oxygen, argon, or helium), and mode of production. Non-thermal atmospheric pressure plasma (non-equilibrium plasma) is commonly described as CP, in which the temperature of the ions and neutral atoms is significantly lower than that of the electrons. Thus, it is characterized by a non-uniform distribution of energy among the constituent particles. Since the ions and neutral atoms remain relatively cold, CP can be successfully used with heat-sensitive compounds. CP emits light with wavelengths in both the visible and ultraviolet spectral regions. In addition to the emission of UV radiation (wavelength range: 100–380 nm), an important property of low-temperature plasma is the presence of high-energy, highly reactive electrons, which cause several chemical and physical processes such as oxidation, the excitation of atoms and molecules, the production of free radicals, and UV photons that can decompose covalent bonds and produce many chemical reactions, such as surface etching (creation of pores/tissue damage), depolymerization (formation of new compounds through the breakdown of cell wall polysaccharides), and cross-linking (cleavage of C-OH polymeric chains for adding new C-O-C bond by eliminating water) ^[31][46][72,87]. In addition, UV photons can increase the activity of specific enzymes such as phenylalanine ammonia-lyase, thereby increasing the amount of total phenols extracted from the plant matrix ^[50][91]. Plasma can be generated artificially by supplying a gas with sufficiently high energy employing lasers, shock waves, electric arcs, and electric and magnetic fields, i.e., by applying energy to a gas in such a way as to reorganize the electronic structure of species (atoms, molecules) and produce excited species and ions. One of the most common ways to artificially create and maintain plasma is by using an electric discharge in a gas. Other ways include the use of jet plasma (a jet plasma system is connected to a power source, which induces ionization in the surrounding gas) and microwave plasma (plasma torches with two dielectric tubes that let the gas pass through). In the case of CP, so-called non-thermal discharges are used ^[29][31][51][30,72,92]. The two main types of non-thermal discharge at atmospheric pressure are corona discharge and dielectric barrier discharge (DBD) ^[52][93]. DBD is the commonly used system for the extraction of bioactive compounds from plant matrices ^{[19][31][47][53]}[20,72,88,94]. In a DBD system, an electrical discharge is generated by two electrodes that are separated by a dielectric layer (a material with high electrical resistance) (Figure 3). It operates at approximately atmospheric pressure (0.1–1 atm), at frequencies up to 104 Hz, and alternating voltages up to 100 kV ^[31][72]. The most important parameters influencing the CP process are the plasma generation voltage or power (higher power increases the electron density, generating more reactive species capable of interacting with the matrix to be treated), the amount of material to be treated (efficiency decreases as the amount increases), the duration of exposure (time), the gas pressure (affects the rate of plasma volatilization), the gas flow rate (higher gas flow rate can improve treatment efficiency), and the type of feed gas used ^[29][51][30,92]. The degree to which reactive species are produced depends crucially on the gas used to generate the plasma (working gas). Plasma gases, such as oxygen, nitrogen, and dry air, are commonly employed in the food processing industry ^[31][46][48][72,87,89]. However, gas mixtures such as He/N₂, He/O₂, N₂/N₂O, Ar/O₂, etc., can also be used. Another important parameter influencing the effectiveness of plasma treatment in extracting phytochemicals is the frequency of plasma generation, due to the inverse relationship between the frequency and the activity of certain crucial enzymes such as peroxidase and polyphenol oxidase ^[54][95].

3. Influence of Cold Plasma on the Phytochemical Profile and Biological Activity of Plant Extracts

3.1. Phytochemical Profile

From a chemical point of view, the term ‘phenolic’ or ‘polyphenol’ can be defined as a substance that possesses one or more aromatic ring and various functional derivatives such as esters, methyl ethers, glycosides, etc. ^[55][132] (Figure 49). In addition, most phenolic compounds possess two or more hydroxyl groups, which contribute to their potential hydrogen binding with other compounds ^[55][132]. Phenolic compounds are positively correlated with the nutraceutical and sensory quality of fresh and processed plant foods ^[56][133]. They possess a broad spectrum of biological properties such as antioxidant, antiproliferative, and anti-inflammatory (Table 1), making them excellent health-promoting compounds for the food industry ^[57][58][59][134,135,136]. Several studies have shown how CP treatment can affect the extraction of these compounds, by increasing the total polyphenol content (TPC) in the obtained plant extracts. Ahmadian et al. (2023) demonstrated that ultrasounds coupled with cold atmospheric plasma (CAP), as a pre-treatment, improved the extraction of phenolic components from Hyssopus officinalis L. (Hyssop) by about 22% compared with ultrasound alone ^[60][96]. Phenolic components increased in connection with cell membrane destruction by active chemical species, UV photons, and charged particles generated from CP, simplifying the extraction of phenolic compounds from the plant matrix. Also, CP supplied enough energy to break phenolic covalent bonds within polysaccharides (plant cell walls). Moreover, nitrogen plasma-pretreated samples exhibited a higher TPC than the other samples (air plasma and conventional solvent extraction). This was probably due to the breakdown of the aromatic rings of phenolic compounds by molecular ozone, which caused their destruction. However, a longer pre-treatment time with CP can create more reactive components, providing sufficient energy to separate and release the bound phenols ^{[47][53][60][61]}[88,94,96,99]. Although CP-generated ozone can affect the properties of plant extracts, it is very unstable to remain in the final product ^[62][63][64][137,138,139]. Bao et al. (2020) tested a high-voltage atmospheric cold plasma (HVACP) pretreatment using different working gases (air, Ar, He, and N₂) to evaluate the extraction of phenolics from tomato pomace. According to the main results, samples treated with He and N₂ plasma showed significantly higher extraction yields than untreated samples. In contrast, argon and air plasma treatments showed no significant differences compared with the control. Eight phenolic compounds (such as phenolic acids and flavonoids) were also identified and confirmed by liquid chromatography mass-spectroscopy (LC-MS). CP treatments showed no effect on the concentrations of trans-ferulic acid, gallic acid, rutin, and isoquercetin, while concentrations of caffeic acid, chlorogenic acid, quercetin, and naringenin increased depending on the working gas. Additionally, the extraction rate of flavonoids was more significant than that of phenolic acids, since the release of bound flavonoids required less energy ^[47][88]. In a similar study, Bao et al. (2020) investigated the effect of HVACP on the extraction of phenolics and anthocyanins from grape pomace. Treatment with CP (5 and 15 min) significantly increased the extraction of phenols, while the sample treated for 10 min showed no significant differences compared with the control. Similarly to the phenol content, the anthocyanin content increased after treatment with CP (processing time: 5 and 15 min) compared with the untreated sample; however, the authors noted no significant difference from the control for samples treated for 10 min with CP. In the analyzed samples, anthocyanins, quercetin, and phenolic acids (such as protocatechuic and gallic acids) were identified by ultra-performance liquid chromatography (UPLC). About these compounds specifically, after treatment with CP for 5, 10, and 15 min, the results showed higher quercetin concentrations than in the control, while CP treatments lasting 5 and 15 min were able to produce increased anthocyanidin concentrations in the respective extracts. The authors also reported that, under the experimental conditions tested, flavonoids were more extractable than phenolic acids ^[53][94]. Umair et al. (2022) observed an increase in the TPC of carrot juice due to the combined effect of HVACP and ultra-high hydrostatic pressure (UHHP). According to the authors, the increase in TPC by plasma-induced active species was related to the disruption of cell membrane bonds and the biosynthesis of phenolic compounds through the metabolism of phenylpropanoid enzymes ^[65][111]. An investigation by Kashfi et al. (2020) indicated that treatment of peppermint with low-pressure cold plasma (LPCP) (20 and 50 W, for 20 min) increased the TPC in the extract compared with the control sample ^[66][109]. Keshavarzi et al. (2020) evaluated the efficacy of DBD cold plasma variables (gas, time, and power) on the TPC extraction rate from green tea leaves. They reported that the TPC of the samples decreased and increased after air and nitrogen DBD plasma treatment, respectively. In the first case, oxygen radicals formed during treatment with air CP led to the degradation of phenolic compounds. In contrast, in nitrogen CP treatment, ion bombardment led to erosion of the superficial epidermal layer of the leaves, facilitating solvent penetration and the extraction of more phenolic compounds. The study also showed that the interaction between time and power required an increase in both parameters to achieve the best TPC. Finally, the analysis of the individual phenolic compounds under optimal nitrogen CP conditions (15 W and 15 min) showed an increase in the concentration of gallic acid and catechin, with a slight decrease in epicatechin and epigallocatechin gallate (EGCG) ^[20][21]. Pogorzelska-Nowicka et al. (2021) evaluated the effect of CP pretreatment on the TPC of 12 types of grounded and water-suspended herbs. This approach has not been found in previous studies. In 11 extracts from 12 herbs, they observed a significant increase in total phenols (approximately 10%) after the CP treatment (8 min, 20 kHz). This was probably due to the destruction of the cell membrane by reactive plasma species. In addition, the hydration of ground herbs can facilitate the extraction process because of the equal penetration of radicals in the whole sample surface. They also stated that the breakdown of larger polyphenols into minor compounds can increase the TPC of the extract. Anthocyanin and flavonoid content also increased but only in four herb extracts. However, in 8 out of the 12 samples, no significant difference was observed in individual phenolic compounds ^[18][19]. Rodríguez et al. (2017) reported that nitrogen-CP treatment significantly increased the TPC of cashew apple juice. However, in the case of treatment performed for 5, 10, and 15 min, no significant differences between the samples were observed ^[67][97]. Lee et al. (2023) studied the effect of surface dielectric barrier discharge (SDBD) on oat (Avena sativa L.) sprout extracts ^[68][117]. In Othis study, oat seeds, after hydration for 12 h, were exposed to plasma for 6 min per day for 3 days after sowing, with no significant effect on sprout growth, but with a significant increase in the content of bioactive metabolites. They stated that the phenolic content of oat sprouts increased through the stimulation of the antioxidant system, the release of phenolic compounds, and their decomposition into minor compounds, similar to the study conducted by Kim et al. (2014) ^[69][108]. According to the results reported by the authors, the single plasma treatment (1 day) produced a significant increase in the content of free amino acids (39.4%), γ-aminobutyric acid (53%), and avenacoside B (23%) compared with the control, while the increase in the number of CP treatments (3-days) increased the content of hexacosanol, the most plentiful polycosanol found in oat sprouts ^[68][117]. Herceg et al. (2016) demonstrated that processing time and gas flow rate were able to improve the TPC of pomegranate juice ^[70][98]. Several researchers have also reported that CP treatment/pretreatment may reduce TPC. Faria et al. (2020) investigated the effect of glow discharge plasma (GDP) pretreatment on the ultrasound-assisted extraction of total phenolic compounds from sea asparagus. CP pretreatment for 60 min resulted in a significant decrease (83%) in TPC ^[19][20]. Afshar et al. (2021) investigated the effect of oxygen and nitrogen CP treatment on the physicochemical properties of oil extracted from sunflower and sesame seeds. They stated that the TPC in both extracted oils decreased significantly as exposure time increased, especially when an oxygen atmosphere was applied. Indeed, ROS in an oxygen-CP atmosphere can interact with phenolic compounds, leading to structural degradation that results in a decrease in total phenolic compounds ^[71][114]. Hemmati et al. (2021) reported that the total phenolic content of green tea powder decreased with the increase in exposure voltage (20, 22, and 25 kV) and time (2, 4, 6, and 8 min). Similarly, the TPC of apple juice was also reduced with increasing cold plasma treatment time ^[61][72][99,140]. Zielinska et al. (2022) investigated the modification of okra pod cell wall polysaccharides and phytochemicals using cold plasma. CP treatment (5, 15, and 30 s) slightly reduced the TPC of okra pods by 5, 13, and 20%, respectively. The results indicated that treatment with CP destroyed the basic structures of phenolic compounds and that oxidation of these compounds, such as phenolic acids and their derivatives, reduced TPC ^[73][74][100,106]. Almeida et al. (2015) reported the effect of CAP and ozone on prebiotic orange juice. Orange juice can be affected by direct and indirect DBD CP treatment exposure. They observed that the TPC decreased with 70 kV plasma treatment applied for 15–60 s ^[75][101]. Liu et al. (2021) and Leite et al. (2021) found a similar effect of DBD CP on the reduction in TPC in kiwi turbid juice and cashew apple juice extraction ^[76][77][102,103]. In other studies, Abedelmaksoud et al. (2022) stated that the TPC of mango pulp increased with the application of dielectric barrier discharge plasma (DBDP) for up to 6 min and then decreased at 8 and 10 min. According to the authors, the increase in phenolic compounds was caused by plasma polymerization and phenylalanine ammonia-lyase activity, which caused the decomposition of cell wall polysaccharides and the release of conjugated phenolic compounds ^[78][104]. Furthermore, Yodpitak et al. (2019) reported that the phenolic content of DBD-treated brown rice (PGBR) reached its maximum value and increased during germination within 0.5 days, whereas in the control sample (GBR), germination peaked after 1.5 days and then decreased rapidly ^[79][80][81][105,107,122]. Noteworthily, Seelarat et al. (2023) reported that the TPC of white Cordyceps militaris blended by Cordyceps militaris through cold plasma jet (CPJ), increased as time did too, from 30 s to 90 s, and decreased when the treatment time reached 120 s ^[82][110]. These studies have shown that the quantity and quality of the phytochemical profiles in the extracts are mainly related to the interaction of CP with the treated plant matrix. Among the operating parameters of the CP process, the processing time can play a crucial role in the reaction between plasma reactive species and phenolics, e.g., by increasing the oxidation of flavanols ^[49][90]. The impact of processing time also depends very much on the chemical structure of the individual bioactive compound being considered. For example, in the case of anthocyanins, short CP treatment facilitates their extraction by disrupting the cell vacuoles where these substances are enclosed. On the other hand, longer treatment times (>20 min) can lead to a loss of anthocyanins due to photodegradation by UV photons generated in the plasma, which transform these compounds into chalcones and thus into benzaldehyde and benzoic acid ^[83][141]. These reactions are also affected by the type of plasma, and more operating variables, such as the power or voltage applied, the type of feed gas, the target phenolic compound, and the food matrix. Generally, DBD CP using high voltages (>60 kV) decreases the phenolic content of plant extracts due to oxidation side reactions because of the generation of a high concentration of ROS and ozone in the plasma. However, when DBD CP is used at high voltages but low frequencies (50 Hz), an increase in the hydroxybenzoic acid content can be observed. It is important to note that plasma-side reactions are not just limited to oxidation ones; they also include hydrogenation, dehydrogenation, hydrolysis, and dehydration reactions, which may boost or reduce the phenolic content of the plant extract ^[70][98]. For a more in-depth look at the effect of CP on the chemical structure of individual phenols, rwesearchers recommend reading the review by Kumar et al. (2023) ^[29][30]. Considering these considerations, it seems clear that after treatment with CP, the phytochemical profile of a plant extract may differ due to the different degrees of structural decomposition of the cells or different side reactions that may occur during the extraction process. Further studies are therefore needed to determine the optimal CP processing conditions to maximize the phenolic compound yield of the extract and optimize its phytochemical profile.  Table 3 summarizes the changes in phytochemicals induced by CP application.

3.2. Antioxidant Capacity

Antioxidants are key substances that can control free radicals, bind oxygen, and block oxidation, maintaining the nutritional and nutraceutical value of food ^[84][142]. Among the bioactive substances present in food, polyphenols are considered to contribute most to the antioxidant potential of foods and extracts of plant origin. Any chemical changes in the profile of these compounds, such as oxidative degradation and the cleavage of double bonds by reactive oxygen species generated through CP treatment, such as 𝑂𝐻^.��., O³�3, and O₂⁻�2−, may increase or decrease the antioxidant capacity of the extract ^[36][51][85][77,92,143]. Several studies have therefore analyzed the effect of cold plasma on the antioxidant capacity of extracts and essential oils (EO) ^[86][144]. For example, Bao et al. (2020) reported an improvement in the antioxidant potential of the phenolic extract from tomato pomace when samples were subjected to HVACP pretreatment produced with different gases. Samples treated with nitrogen CP showed the highest improvement in antioxidant capacity (30%) compared with samples treated with argon, helium, and air plasma. They stated that this observed general improvement was probably related to the higher levels of more potent phenolic antioxidants and hydrophilic phenols present in the final extract due to the increase in surface hydrophilicity following the treatment ^[47][88]. Similarly, Poomanee et al. (2021) indicated that Luem Pua (LP) pre-treated with CP showed the highest increase in antioxidant capacity compared with the other tested species and untreated LP. The IC₅₀ value recorded for the pretreated LP sample was 0.080 ± 0.002 mg/mL, while the value for the untreated LP was 0.187 ± 0.025 mg/mL. According to the authors, this improvement may result from the higher concentration of cyanidin-3-O-glucoside (CNG) and phenolic compounds in the pretreated LP extract, as no other new compounds were observed in the HPLC analysis to which this improvement could be attributed ^[81][122]. However, Bao et al. (2020) believed that another reason may be responsible for the improved antioxidant potential in plasma-treated samples. Indeed, they showed that when exposing the grape pomace to HVACP pretreatment, the phenolic extract obtained had a higher antioxidant potential regardless of the treatment duration (5, 10, and 15 min) than the untreated sample, due to the release of the intercellular compounds caused by the strong breakdown of the plant tissue following the experimental treatment. However, regardless of the overall increase in antioxidant capacity, it was observed a slight, though not significant, reduction in it from 5 to 10 min ^[53][94]. Interestingly, some researchers observed that although CP treatment reduces TPC, total flavan content (TFC), or vitamin C, the antioxidant potential remains unaffected. The possible reason for this stability is that the unbounded antioxidant compounds, which are released from carbohydrate and protein complexes during CP treatment, can be replaced by part of the antioxidant compounds that are decomposed by the plasma reactive species generated during CP treatment ^[60][73][96,100]. For instance, Faria et al. (2020) reported that despite the drastic loss of phenolic content in sea asparagus (Salicornia neei) extract, when the pre-treatment time with CP was extended to 60 min, the antioxidant capacity did not change significantly. One possible reason for this stability, according to the authors, could be the presence of more active antioxidants formed by highly reactive species by the CP pretreatment employed before ultrasound-assisted extraction (UAE). Notably, with a shorter CP exposure time (5 min), an improvement in the antioxidant capacity of the extract was observed, probably due to a more severe cell wall cleavage that led to greater solvent penetration ^[19][20]. Hou et al. (2019) also mentioned the importance of CP treatment exposure time since the antioxidant potential of blueberry juice (assessed by FRAP, ABTS, and DPPH techniques) declined when the operation time was extended from 2 to 6 min ^[87][145]. Similar results were reported by Abedelmaksoud et al. (2022) for fresh mango pulp treated with DBDP for four minutes ^[78][104]. It is worth noting that CP treatment does not always positively affect antioxidant capacity. Keshavarzi et al. (2020) indicated that air-working gas in CP pretreatment before the extraction of phenols from green tea leaves led to a significant reduction in the antioxidant capacity in all treatments evaluated ^[20][21]. According to the authors, ROS formed through air-CP might be responsible for the cleavage of phenolic compounds, thus reducing the antioxidant potential of the extracts. In contrast, cold nitrogen plasma increased the antioxidant potential. This increase can be due to the presence of specific phenolic compounds, usually restricted to the membrane and cell wall, which are now rapidly released as sufficient energy is supplied under CP conditions. Another potential reason can be the accelerated solvent permeability due to the severe tissue breakdown caused by ion bombardment during CP pretreatment. This investigation continued by optimizing nitrogen CP pretreatment conditions (exposure time of 15 min and generation power of 15 W) to reach the highest antioxidant capacity of the final extract (152.114 μM Trolox/g) by central composite design ^[20][21]. Kashfi et al. (2020) reported that plasma power values (20, 50, and 60 W for 20 min) during radiofrequency LPCP treatment affected the antioxidant potential of plant extracts. The authors showed that when peppermint (Mentha piperita L.) was subjected to LPCP immediately before the drying process, an increase in the antioxidant capacity of the extract was observed at a plasma power value of 50 W (IC₅₀: 0.240 mg/mL), probably resulting from an increase in the concentration of bioactive compounds in the peppermint extract pretreated with plasma compared with the control sample. However, by increasing the power to 60 W, a significant decrease in the antioxidant capacity was observed. Furthermore, all the LPCP-treated samples demonstrated higher ferric-reducing ability compared with the untreated control sample ^[66][109]. In agreement with this study, Kim et al. (2019) pointed out that the intensity of active species generated during CP treatment was highly dependent on different CP parameters such as exposure time, generating power, plasma-working gas, and plasma source ^[88][120]. Pogorzelska-Nowicka et al. (2021) reported that, when subjecting various herbs (12 species) to cold plasma pre-treatment, the extract obtained from nine species showed a significant increase in their antioxidant potential, while Sanguisorba officinalis showed a slight reduction, and the extracts of Andrographis paniculata and Polygonum aviculare showed no significant changes following CP treatment ^[18][19]. As far as the effect of CP on the biological properties of essential oils (EOs) is concerned, Buonopane et al. (2019) reported that the EOs from CP-treated sweet basil plants showed a higher antioxidant capacity than untreated samples due to the higher eugenol extraction (48–94.82%) ^[89][121]. In contrast, Afshar et al. (2022) reported that for sunflower and sesame seeds pre-treated with CP, there was a significant decrease in the antioxidant capacity of the extracted oils, attributable to the degradation of phytochemicals during treatment, especially in cold air plasma, caused by the presence of ROS in the plasma. Furthermore, sunflower oil samples showed a higher scavenging capacity than sesame oil samples in the DPPH assay ^[71][114]. In this regard, it is worth noting that employing different methods provides a more accurate view of the potential antioxidant alteration in food products ^[67][90][97,146]. Several studies have shown different trends in antioxidant potential values about the type of in vitro test used to measure it, emphasizing the need to use more than one analytical test to more reliably determine the antioxidant potential of an extract or food, also depending on the fact that the antioxidant compounds most reactive to a specific type of method may be affected differently during plasma processing ^[60][87][96,145]. In agreement with the above statements, Ahmadian et al. (2023) reported inconsistent results when examining the antioxidant potential of extracts from hyssop (Hyssopus officinalis L.) pretreated with CAP using FRAP and DPPH techniques, respectively. According to the FRAP assay, pretreated samples with N₂ and air plasma showed an increasing trend in antioxidant properties compared with control samples. In contrast, the DPPH assay showed that CAP treatment led to lower antioxidant capacity. However, N₂ plasma-treated samples exhibited a higher radical scavenging rate than the untreated ones ^[60][96]. It is worth emphasizing that the different process parameters of the CP treatment, influencing the phytochemical profile of the extract, can similarly modulate its antioxidant potential. Rodríguez et al. (2017) reported that among different process parameters during the CP treatment of cashew apple juice, N₂ plasma flow rate and exposure time played a determinant role in the FRAP assay. The most significant changes were observed at the 15th min of treatment and flow rates of 30 mL/min (164% relative antioxidant activity) and 50 mL/min (163% relative antioxidant activity). In addition, the results showed that the DPPH and ABTS assays were significantly dependent on the period of plasma treatment of the sample, where a longer exposure time, regardless of the rate of N₂ flow, resulted in a continuous decrease in the antioxidant capacity (the highest antioxidant potential was recorded at the 5th min of CP treatment) ^[67][97]. Almeida et al. (2015) considered the ABTS assay a more accurate method in comparison with the DPPH one to reflect the antioxidant capacity alteration, since the CAP-treated prebiotic orange juice showed lower antioxidant activity in ABTS technique, while no significant difference was observed among treated and untreated samples through DPPH assay ^[75][101]. Ramazzina et al. (2016) evaluated some functional properties of minimally processed Pink Lady apples treated with DBD CP in comparison with untreated samples, through different in vitro and ex vivo tests, pointing out that plasma treatment caused only a slight reduction in antioxidant content and antioxidant capacity (up to 10%), mainly limited to the amphiphilic fraction ^[91][147]. It is worth noting that CP treatment may reduce antioxidant properties or even have no significant effect, due to the oxygen in the air plasma, which plays a role in suppressing antioxidant capacity by creating highly destructive ROS. Regardless of different types of working gases, the CP process parameters such as exposure time, power, and storage period have an important role in the antioxidant capacity changes. However, further investigation is recommended for a more accurate determination of different action mechanisms of the CP treatment on the antioxidant, through simultaneous assays such as ABTS, FRAP, and ORAC, to gain more reliable information.  An overview of the effects of CP on antioxidant capacity is shown in Table 3.

3.3. Antimicrobial Properties

The use of plant extracts for the control of microbial infections and food spoilage is becoming increasingly popular in recent years. The ability to eliminate or control an exhaustive range of harmful organisms by complex plant extracts or essential oils, rich in polyphenols, terpenoids, and alkaloids, has led to the use of these bioactive compounds as food preservatives ^[92][93][148,149]. In this regard, the quality of the extraction process is one of the essential factors affecting the antimicrobial properties of plant extracts. In some studies, CP has been introduced as a pioneering technique for obtaining a high-quality extraction process with minimal loss of valuable compounds, thus preserving their antimicrobial properties. In one of these studies by Pogorzelska-Nowicka and coworkers (2021), the antibacterial potential of various dried herb extracts obtained by pretreatment with CP was evaluated against aerobic bacteria. Overall, the CP samples reduced total aerobic bacteria and most of them showed germicidal properties. Depending on herb species, the number of aerobic bacteria decreased by 1.27 up to 3.48 logCFU/g ^[18][19]. Kashfi et al. (2020) evaluated in vivo the efficacy of dried peppermint (Mentha piperita L.) extracts obtained by LPCP at three different powers (20, 50, and 60 W) for 20 min against the pathogen E. coli ^[66][109]. According to experimental observations, the bacterial growth was completely controlled with peppermint extract using high CP powers (50 and 60 W), whereas this process was unable to inhibit E. coli at 20 W. Faria et al. (2020) investigated the effect of ultrasound-assisted CP pretreatment on in vitro antimicrobial activity of the S. neei extract against S. aureus and E. coli by the disc diffusion method ^[19][20]. The S. neei extract (0.1 g/mL) showed an inhibition halo (4 mm) against E. coli and no inhibition halo against S. aureus. In general, the decomposition of the extract caused by CP treatment produces bioactive compounds such as ferulic, caffeic, and p-coumaric acids, which have shown synergistic antibacterial properties. Despite the few studies on the indirect antimicrobial effects of CP through the use of the plant extract obtained by this technique, many results confirm the direct application of CP in food disinfection. Most studies stated that the microbial count decreased while increasing plasma treatment time and voltage ^[61][69][99,108]. In general, it has been found that Gram-negative bacteria are more sensitive to plasma treatment than Gram-positive ones. CP efficacy seems to be directly correlated to bacterial cell wall thickness. Gram-negative bacteria are characterized by a thin (<10 nm) inner cell wall layer and an additional outer membrane, consisting of phospholipids and lipopolysaccharides that are highly sensitive to plasma-generated ROS. Moreover, due to the presence of porins on the outer membrane of these bacteria, plasma penetration is certainly more effective than in the case of Gram-positive bacteria, which are characterized by a thicker peptidoglycan-based cell wall (20–80 nm) that cannot easily break down by reactive plasma species ^[94][95][96][150,151,152]. However, Hemmati et al. (2021) noticed that CAP was more effective in inhibiting Gram-positive bacteria (E. faecalis) than Gram-negative ones (E. coli) ^[97][153]. A summary of the effects of CP on the antimicrobial properties of the plant extracts obtained is given in Table 3. To date, studies on the antibacterial properties of these extracts are lacking, making it difficult to assess their efficacy. Therefore, it is necessary to extend research studies in this area with the help of statistical strategies to model and optimize the extraction conditions.

3.4. Other Relevant Biological Properties

The use of cold plasma to produce phytoextracts is a very recent technique, and most studies have focused on the effect of the extraction technique on the antioxidant and antimicrobial activity of the extracts obtained. However, some studies have evaluated other biological properties of phytoextracts obtained using plasma. Poomanee et al. (2021) optimized the CP extraction of some purple rice varieties, obtaining up to a five-fold higher amount of Cyanidin-3-O-glucoside in plasma-treated samples compared with the untreated one, with a significantly more substantial anti-ageing potential ^[81][122]. Mehta et al. (2022) optimized the extraction process of polyphenols by atmospheric and vacuum CP from rice and maize bran. The resulting phytoextracts showed not only higher antioxidant activity, but also better in vitro digestion, higher anti-inflammatory responses, and better overall quality than a conventional extraction ^[98][129]. In another study, they extracted xylooligosaccharides (XOS) from rice and corn bran dietary fibers by CP. Similar to polyphenols in their previous study, the extracted XOS showed better gastric digestion and anti-inflammatory responses with no cytotoxicity against RAW 264.7 and HepG2 cell lines ^[99][154]. Jeong et al. (2019) found that atmospheric DBD cold plasma treatment resulted in the dimerization of trans-resveratrol in a methanol solution. The generated trans-resveratrol dimers showed higher inhibitory activities against α-glucosidase and α-amylase than parent trans-resveratrol. Inhibiting the activity of these enzymes can contribute to the improvement of anti-diabetic properties through controlling postprandial hyperglycemia, and reducing the risk of developing diabetes ^[100][155]. The lack of studies on the biological properties, such as anticarcinogenic or anti-mutagenic, of plasma-derived phytoextracts, opens exciting new research prospects for scholars in the field to implement this technique commercially.