Sustainable Valorization of Sour Cherry (Prunus cerasus) By-Products: Extraction of Antioxidant Compounds

Abstract

Prunus cerasus, commonly known as sour cherry, is a fruit widely consumed during the summer season. Processing of sour cherries results in the generation of substantial amounts of by-products. Following the extraction of juice, the residual pomace (comprising of skin and flesh) and pits remain as by-products. This study delves into the antioxidant potential derived from the phenolic compounds present in sour cherry pomace and the oil derived from its seeds, both demonstrating significant implications for human health and longevity. The increasing demand for bioactive compounds from natural resources, to be used as ingredients in functional foods, as well as the increased global production of sour cherries, has spurred considerable interest in these by-products. The growing demand for more sustainable practices has necessitated the use of industrial fruit by-products. To this end, many articles have been published regarding sour cherry skin and pits utilization. This comprehensive review aims to elucidate advanced techniques for extracting antioxidants from sour cherries and discern optimal strategies for broad-scale industrial implementation.

1. Introduction

Nowadays, food loss and food waste, alongside the overpopulation of the earth, are severe challenges for the food supply chain worldwide [1]. It is estimated that globally, approximately 14% of food production is wasted within harvest and sale, and roughly 17% of total world food production is lost (11% in households, 5% in the food service industry, and 2% in retail) [2]. Food loss is a reduction in food availability and quality caused by the decisions and actions of food chain suppliers [3]. Food waste, on the other hand, is defined as a decline in food quality and quantity resulting from retailer, consumer, and service provider actions. Food waste and food loss are primarily caused by rapid population growth and food consumption trends in the world [4,5]. Food waste is a matter of food security, in addition to its effects on the economy due to the expenses associated with waste management and production loss. This is due to the negative effects that food waste has on the environment, such as the loss of resources used in food production, the release of greenhouse gases from discarded food in landfills and incinerators, and the deterioration of soil and water quality [6,7]. Up to 8% of human-generated greenhouse gas emissions can be attributed to inadequate management of food waste and food loss, leading to an annual discharge of approximately 3.3 billion tons of carbon dioxide equivalent, or 4.4 kilotons of carbon dioxide equivalent [8]. Due to the fact that they are among the most produced food groups and have a high degree of perishability, fruits and vegetables generate comparable quantities of food waste during the primary production and consumption stages, including edible and inedible components [9]. Leaves, rotten fruits, unused peels, pulp, or fibrous material and the inedible parts of fruits (stones, husks, kernels, seeds) are all included in the list of industrial fruit solid waste [10]. The valorization of food waste is essential for minimizing environmental and economic harm and facilitating the introduction of food waste into the circular economy [2,11]. Food waste valorization in order to produce various end-use products as well as their summary has been extensively studied; however, studies regarding their economic, environmental, and social aspects are limited [12]. Accurate measurement of the amount of food waste produced at different points in the food supply chain, in both developed and developing nations, is crucial for evaluating the environmental impact of disposing food waste in landfills and the economic benefits of converting food waste into valuable products. However, there is currently a scarcity of comprehensive data on this matter.

Phytochemicals, such as dietary fibers, phenolic compounds, and other beneficial components, are abundant in wasted fruits and vegetables [13,14]. Seeds, peels, and other parts of fruits and vegetables that are typically discarded have high concentrations of phytochemicals and essential minerals, even though most people only consume the flesh or pulp [15]. Waste produced from the food, pharmaceutical, cosmetic, and textile industries can be exploited for the isolation of bioactive compounds through extraction. While the production of such waste is inevitable, there is potential for a sustainable development initiative to be launched by making better use of the waste generated by the production of horticultural commodities. In this way, negative impact on the environment could be reduced, and the availability of foods rich in bioactive compounds like polyphenols, carotenoids, vitamins, and dietary fibers could be increased [16].

Bioactive compounds can be sorted into different categories based on their solubility, polarity, and prevalence in the natural world [17]. These chemicals can also be categorized based on their biosynthetic process, structural properties, clinical function related to pharmacological action, and botanical approach (i.e., families) [18]. Polyphenols, carotenes, tocopherols, alkaloids, and vitamins are just a few examples of the many different classes that result from this categorization [19]. Polyphenols are a class of secondary metabolites found in many plants [20]. They can be broken down into numerous subclasses, such as flavonoids, phenolic acids, stilbenes, lignans, tannins, and oxidized polyphenols [17,21], depending on the number of constitutive carbon atoms conjugated with the structure of the basic phenolic skeleton. Aromatic hydroxylated compounds are organic compounds with one or more aromatic rings and one or more hydroxyl groups; these are the end products of the shikimate and acetate pathways. Typically, they are attached to mono- and polysaccharides through one or more phenolic groups, and they can also be connected to methyl esters and esters [18,21,22]. However, they can also be either simple phenolic molecules or highly polymerized compounds. Anthocyanins comprise a group of naturally occurring, water-soluble flavonoid pigments [23,24]. They are quite common in nature and can be found not only in the orange, red, blue, and purple petals of flowers [25], but also in the roots, stems, tubers, leaves, fruits, and seeds [26,27]. Anthocyanins have potential health benefits as a result of their unique chemical structure and aroma. Extensive research indicates that anthocyanins possess a variety of biological activities, including free radical scavenging [28,29,30], antibacterial activity [31], cancer cell growth inhibition [32], and vision protection [33]. Moreover, it has been proven that anthocyanins from blueberry have various health benefits for humans [34]. Consequently, anthocyanins are utilized extensively in food, medicine, cosmetics, and other industries [35].

In recent years, there has been a significant focus on bioactive compounds due to their potential therapeutic effects on human health, including the reduction of chronic and progressive diseases such as cancer and diabetes [36,37,38]. Due to their wide range of bioactivity, natural products are attractive compounds with added value. It has been demonstrated that (poly)phenols have numerous health benefits for humans, including protection against cancer and cardiovascular disease [39,40,41]. These effects have been linked to their ability to scavenge reactive oxygen species, which are generated under oxidative stress conditions and contribute to the onset of various inflammatory and degenerative disorders [42,43,44]. Due to their advantageous effects, natural polyphenols are increasingly utilized as dietary supplements [40,45,46,47] and in other applications (such as biomedicine [48,49,50], the cosmetics industry [49,51,52,53,54], and the food industry [55,56,57,58,59]) as functionalization additives for materials. The development of improved products derived from bioactive compounds found in food waste is a novel approach to waste minimization and the creation of new economic opportunities (Figure 1). Polyphenols, vitamins, minerals, and prebiotics are all examples of bioactive compounds that improve product quality and consumer wellness [60,61,62]. Food waste can be utilized to extract these compounds, which can then be used to produce top-notch and effective food components, cosmetic products, and dietary supplements. This strategy seeks to not only lessen the amount of waste produced but also to generate fresh revenue and aid in the development of a circular economy. Future food waste valorization must take into account the long-term waste availability, its techno-economic potential, and an eco-friendly assessment of advantages and burdens based on its life cycle if it is to be environmentally and economically sustainable.

The sour or tart cherry (Prunus cerasus L.), a member of the Rosaceae family, is one of the most widely consumed fruits and is utilized in a variety of ways [63]. Juices, canned fruit, brandy, liqueurs, preserves, and other products are made with tart cherries both at home and in large-scale manufacturing facilities. The fruit has a sour flavor, juicy flesh, a reddish hue (ranging from pale to deep), and a pleasant odor. A very high amount of waste is produced from the processing of sour cherries [64,65]. After sour cherries have been juiced and quickly frozen, the pomace (skin and flesh) and seeds (pit and stone) were discarded as waste (Figure 2) [66]. The pomace and peels are known to contain polyphenols, with the main ones being catechin and neochlorogenic acid [67], as well as anthocyanins such as cyanidin 3-glucosylrutinoside [68]. Regarding the kernel of the fruit, several studies [69,70] reported the presence of C16-C20 fatty acids, with varying degrees of unsaturation. The main sterols were found to be β-sitosterol and squalene, while tocopherols have also been quantified, with δ-tocopherol being the main one. Tart cherries, and products made from them, have been shown to have promising health benefits. They have been used as preventative measures in conventional medicine for conditions like cardiovascular disease, Alzheimer, inflammation, cancer, and diabetes, all of which are characterized by elevated oxidative stress. Increased appetite, reduced blood pressure, protection from oxidative stress, lessened pain and muscle damage from exercise, better regulation of blood glucose, and reduced inflammation are just some of the benefits of consuming cherries [71,72,73]. The high concentration of antioxidant compounds in sour cherries is responsible for their health benefits to humans. Thus, this review aims to examine the extraction techniques that have been used so far for the recovery of bioactive compounds of sour cherry by-products and to highlight their industrial application. The conventional and green techniques that have been applied so far are investigated to assess which one or ones are the most effective for the greatest recovery of bioactive substances. Emphasis is placed on differentiating between conventional and green techniques in each by-product individually, pomace, peel, kernel, or seed, and on establishing the optimal technique for each of them, respectively. Moreover, future perspectives for the application of these techniques to P. cerasus by-products are discussed.

2. Extraction Techniques

2.1. Conventional Extraction Techniques

Various conventional techniques have been employed to study the extraction of bioactive compounds from food wastes [74]. Soxhlet is a conventional extraction method [75] developed by Fraiz Ritter Von Soxhlet for the extraction of lipids and fat from food matrices [76,77]. The Soxhlet extraction method is well acknowledged and commonly used as a benchmark for assessing different extraction processes [78]. For the isolation and refinement of bioactive chemicals obtained from natural sources, the employment of Soxhlet technique with liquid solvents is highly recognized as a highly efficient method. This is due to the use of heat during the extraction process, which increases the solubility of otherwise insoluble compounds at room temperature. As a result, the extraction efficiency is significantly enhanced, resulting in a more robust extraction of the desired compounds [79]. The Soxhlet extraction technique has been in use for many decades; it requires a substantial amount of time and solvents. In addition, the procedure necessitates the use of specialized equipment, known as a Soxhlet extractor [80]. Another conventional extraction technique is thermal treatment. Thermal treatment refers to any thermochemical conversion process that occurs at relatively high temperatures and alters the chemical structure of the material being processed. Thus, the analysis includes the three primary processes available for thermochemical conversion: combustion, gasification, and pyrolysis [81].

In addition, extraction procedures, such as magnetic stirring, are conducted under diffusion-controlled conditions. The diffusion process is the rate-determining step that takes place within the boundary layer separating the bulk solution and the extractant phase. The rate of this process can be accelerated by vigorously stirring the fluid. In fact, stirring can be accomplished by utilizing an external apparatus with the aid of an external power supply, or by integrating the extraction and stirring components into a unified device [82]. By maintaining a consistent low rotational speed, the compact rotating rotor can effectively blend fluids of differing viscosities. The magnetic stirrer is utilized for cell lysis due to its high mixing efficiency [83]. Utilizing magnetic stirring improves turbidity stability and accelerates mass transfer from the aqueous solution to the extraction solvent [84]. The conventional approach to disperse soil particles entails the employment of mechanical shaking, which results in substantial macroscopic mixing and turbulence. This process enables increased physical contact among particles, the extent of which is proportional to the mixing intensity. Nevertheless, employing mechanical shaking requires excess electrical energy input and is linked to a prolonged duration, potentially leading to increased expenditure [85]. Mechanical shaking is a feasible substitute for the Soxhlet method due to its simplicity, speed, cost-effectiveness, and adaptability [86].

2.2. Green Extraction Techniques

The new methods for sustainable extraction of various compounds from fruit and vegetable waste are known as green extraction techniques, also known as clean techniques, and they use organic solvents, require less time for the extraction, and consume less energy, all of which have a positive effect on the environment [87,88]. A widely used green technique is microwave-assisted extraction (MAE). Microwaves, which are electromagnetic fields with frequencies between 300 MHz and 300 GHz, cause dielectric heating and dissolve solutes. Many bioactive compounds, including phenolics, carotenoids, and flavonoids, can be extracted using this method [88]. According to studies, MAE high extraction yield and rapid acceleration are the result of a synergy between heat and mass gradients [89]. The first step in MAE is the introduction of a solvent within the plant matrix, then utilizing electromagnetic waves to disintegrate the components and transfer the dissolved components from the insoluble matrix to the bulk solution. Lastly, the liquid and remaining solid phases are separated. While a solvent is not required for MAE, it is typically performed in a mixture of ethanol and water due to the former’s high microwave energy absorption and the latter’s ability to effectively dissolve the target compound. Microwave power, extraction temperature, extraction time, and solvent volume are all variables that impact MAE [87].

In this context, cold maceration is a frequently employed extraction technique. Maceration is a common method for solid-liquid extraction in which the sample is in prolonged contact with a solvent, either at room temperature or at elevated temperatures, with or without agitation [90,91]. This procedure is repeated until all bioactive compounds in the sample have been completely dissolved in the solvent [92]. Commonly referred to as cold maceration or cold soak [93], it is the process of carrying out a maceration step at low temperatures. The application of elevated temperatures during thermo-maceration results in cellular disintegration, which can be detrimental to sensitive compounds such as anthocyanins [67]. In contrast to the Soxhlet method, cold maceration is a straightforward procedure that requires no specialized equipment. This is accomplished without compromising the integrity of the thermolabile chemicals present in the fraction by employing low extraction temperatures comparable to the cold pressing technique [80].

Ultrasound-assisted extraction (UAE), also referred to as (ultra)sonication, employs a sound wave between 20 kHz and 100 MHz; this sound wave travels through a substance and induces compression and expansion, leading to the phenomenon of cavitation [94]. The cavitation causes the generation, growth, and collapse of tiny bubbles; when the bubbles exceed a critical diameter, they burst, releasing a great deal of energy and converting the kinetic motion into heat. It is estimated that bubbles have a temperature of 4700 degrees Celsius and a pressure of 1000 atm. Cavitations occur to liquids and solids that contain liquid [95]. Hence, cavitation occurs when the pressure released on the substance decreases below the saturation pressure of the liquid-vapor, leading to the extraction of bioactive compounds [89]. UAE has a number of benefits, including a decrease in time, energy, and power consumption, less thermal degradation, and a high-quality extract [96,97].

As a pre-extraction or extraction method, enzyme-assisted extraction (EAE) can be used to obtain bioactive compounds. The plant cell wall is ripped down, and the bound bioactive compound attached to the carbohydrate and lipid chains are liberated [94]. In solvent extraction, this process is carried out under the influence of enzymes such as cellulase, pectinesterase, hemicellulase, fructosyltransferase, pectinase, α-amylase, and protease. As enzymes and water are naturally derived, as opposed to the use of hazardous solvents, EAE falls under the category of green extraction techniques [87]. EAE is employed when the plant matrix components are secured by hydrogen or hydrophobic bonds within the polysaccharide-lignin network, rendering them unextractable using a solvent in a conventional extraction method. EAE is affected by the relative humidity, particle size, chemical composition of the plant matrix, type and dosage of the enzyme, solvent quantity, time, and temperature [95].

Another novel, green, and efficient extraction technique is supercritical fluid extraction (SFE). The critical point of a substance is the state at which the densities of the vapor and liquid phases are alike, which happens when the substance is above its critical temperature and pressure. When a material is subjected to temperatures and pressures more than its critical limit, the supercritical state occurs. The fluid acquires gas/liquid characteristics of diffusion, viscosity, and density in this state. Having all of these properties enables the fast extraction of bioactive compounds [98]. In SFE, the analyte is separated into two distinct phases: the separation phase and the stationary phase. SFE is a process in which an oven contains the pumped mobile phase (typically CO₂, ethane, propane, butane, water, or pentane [87,98]), and the gas is pressurized until it reaches a high pressure, and then a vessel containing a co-solvent is similarly pumped into the extraction vessel [44,95,99]. With a critical temperature and pressure of 31 degrees Celsius and 74 bars, CO₂ is the most common solvent for SFE. It provides stable working conditions for pressures ranging from 100 to 450 bar. Due the low polarity of CO₂, extraction is primarily restricted to non-polar compounds. To surpass this restriction, it is possible to incorporate a chemical modifier, such as ethanol, methanol, water, or acetone, to increase the polarity [44,88].

High hydrostatic pressure (HHP), a non-thermal technique extensively employed to inhibit pathogenic microorganisms with minimal nutritional and organoleptic losses in foods [100,101], has become an efficient [102], easily controlled, and environmentally friendly method to trigger a physical alteration of starch with the aim of changing its functioning. HHP has the ability to disrupt or modify non-covalent interactions, such as hydrogen bonds and Van der Waals forces, within starch molecules [103] while having minimal effects on covalent bonds [104]. In general, starch processing by HHP begins with the preparation of a starch-water suspension (usually 30%) that is treated in an HHP vessel at the desired pressure (100–600 MPa) for a specific time (30 min) at ambient temperature (20–30 °C) [105,106,107]. Then, the pressure is swiftly reduced to the level of atmospheric pressure (0.1 MPa), therefore modifying the non-covalent bonds. Next, the starch suspension under pressure is subjected to either freeze-drying or air-drying, followed by milling and storage at ambient temperature [104]. There are exceptions to the aforementioned conditions, such as the use of 800 and 1000 MPa pressure to treat high amylose maize starch [108] and 650 MPa/50 °C to modify arrowroot starch [109].

3. Application and Discussion of Conventional and Green Extraction Techniques on Sour Cherry

3.1. Conventional Extraction Techniques Used for P. cerasus Pomace

A study to optimize the acquisition of phenolic components from PC pomace by conventional extraction was carried out by Yilmaz et al. [110]. The extraction was carried out in various proportions of ethanol: water (EtOH:H₂O) and specifically 0:100, 20:80, 50:50, 80:20, and 100:0 with 0.01% hydrochloric acid (HCl), at temperatures of 8, 25, 50, 75, and 92 °C. Initially, a small amount of solvent was placed in each flask, and mixing was carried out for 7–8 min. Thereafter, the following concentrations of 4, 6, 9, 12, or 14 mL/g were added for extraction in each flask separately and in various combinations with the previous factors. The extract was centrifuged at 6000 rpm for 10 min to obtain the test sample. In accordance with the results, the most suitable solvent ratio for the highest polyphenol recovery was 50:50, with the other factors not playing such an important role. Sample 2 appeared the most suitable combination of extraction factors, and the polyphenol content of PC pomace under these conditions was 14.46 mg/g. Furthermore, sample 2 turned out to have the most suitable combination of extraction factors for obtaining maximum anthocyanin yield, which reached 0.41 mg/g. However, sample 10 gathered the most optimal extraction condition for ensuring the highest amounts of the phenolic elements cyanidin-3-glucosylrutinoside, neochlorogenic acid, and catechin. Notably, sample 10 had the second-best extraction combination for obtaining high amounts of polyphenols (3.59 mg/g) and anthocyanins (0.38 mg/g).

3.2. Green Extraction Techniques Used for P. cerasus Pomace

A distinctive method was carried out by Cilek et al. [111], where they prepared microcapsules containing PC pomace extract and evaluated their antioxidant capabilities. The extraction of the microcapsules is presented in Table 1. Afterwards, condensation was performed at 40 °C in a vacuum evaporator and the final sample had a volume 13–14 times decreased compared to its initial value. The concentrated extracts were lyophilized at −52 °C for 48 h under 0.1 mPa. The dried sample was then manually ground to a fine powder. The coating materials used were maltodextrin (MD) and gum arabic (GA). Three MD concentrations with distilled water were tested (10, 12, and 16% w/v). They were left overnight at 27 °C in a 70-rpm stirring water bath. GA solutions were prepared at the percentages of 4 and 8% (v/v), 2 h before the encapsulation procedure. For the preparation of the coating solutions, the solutions were mixed using a magnetic stirrer at 1250 rpm to obtain the total solid content of 10% (w/w) with MD/GA ratios of 10:0, 8:2, and 6:4 in weight. The final step for the preparation of the different microcapsules was to mix the pomace powder with the coating solutions with pomace to coating ratios of 1:10 and 1:20 (weight). The mixtures were homogenized at high speed for 5 min and subjected to ultrasonic treatment at 160 W, 20 kHz frequency, and 50% pulse and various time periods (15, 20, and 25 min). Therefore, 15 final samples were generated and tested for their antioxidant activity and phenolic content. Following the TEAC method to determine the antioxidant activity, the highest amount was found in the sample where the MD/GA ratio was 6:4, pomace to coating ratio was 1:10, and was subjected to ultrasound for 15 min, recording a value of 181.7 mmol TEAC/g. When this sample when analyzed by the DPPH method, it exhibited antioxidant activity of 2.85 ppm DPPH/g. However, after determining the antioxidant activity by DPPH, the best value of 2.90 ppm DPPH/g was recorded to the sample with an MD/GA ratio of 10:0, pomace to coating ratio of 1:10, and ultrasonication treatment of 15 min, i.e., increased antioxidant activity by 1.75%. According to the same study, the same conditions as before but with a longer ultrasound time (20 min) should be applied to obtain the higher content of polyphenols (14 mg/g).

A green extraction method was employed by Precup et al. [112] in order to create a food industry by-product ingredient, PC pomace, to be added to classic desserts. Fresh samples of PC pomace were extracted as presented in Table 1. After extraction, the sample was centrifuged for 10 min, then the supernatant was filtered, and the filtrate was evaporated to dryness under vacuum. The polyphenol content of the PC pomace extract was 50.89 mg gallic acid equivalent (GAE)/100 g, an increase of two and a half fold from the study by Yilmaz et al. [110] using a conventional extraction method. The belief that extractions using green extraction methods lead to the export of the maximum amount of nutrients and antioxidants is therefore demonstrated [113].

The pomace consists of 24% skin and 76% kernel. Maurício et al. [114], determined the whole pomace and the skin part of pomace to study the bioactive compounds and antioxidant activity of the PC. They followed two green extraction techniques, maceration (extraction method 1), and decoction (extraction method 2) using the conditions which are referred in Table 1. As far as the pomace extract derived by extraction method 1 and the one made by extraction method 2 are concerned, the phenolic content was in the range of 40 and 173 mg/g, respectively, while as far as the PC skin extract derived by extraction method 1 and the one made by extraction method 2 are concerned, the phenolic content was in the range of 31 and 289 mg/g, respectively. Consequently, the extraction by decoction in boiling water (100 °C) for 15 min is considered to be the most appropriate of all the extraction methods tested as it ensures the maximum value of phenolics. Furthermore, the amount of anthocyanins seemed to have been enhanced by extraction method 1 with no major differences between the two methods for the two samples. In particular, the skin extracted by method 1 had 1.8 mg/g anthocyanin content while the same sample extracted by method 2 had 1.6 mg/g anthocyanin content. Moreover, although the phenolic content was benefited by extraction method 2, the antioxidant activity was favored by extraction method 1 in all cases. Specifically, the antioxidant activity measured by DPPH^● assay for PC skin that received extraction method 1 was 880 μg/mL and the one receiving extraction 2 was 330 μg/mL, i.e., reduced by 166.67% compared to that receiving extraction method 1. In addition, the PC pomace’s antioxidant activity (DPPH^● assay) using extraction method 1 was 946 μg/mL and the pomace that underwent extraction method 2 was 407 μg/mL, having a difference of 132.43%.

3.3. Combination of Conventional and Green Extraction Techniques Used for P. cerasus Pomace

Ciccoritti et al. [115] examined the pomace from two PC genotypes (Bianchi d’Offagna, BO and Bianchi Montmorency, BM) in three different ways. After the removal of the pomace from both genotype samples, 100 g of pomace formed the control sample (CS) and was studied fresh. Furthermore, 400 g of the pomace were subjected to oven drying (OP) at 60 °C, air velocity: 0.6 ms⁻¹, relative humidity < 0.5%, system power: 1.4 kW/h for 24 h, and the remaining part, i.e., 400 g, was lyophilized (FDP) at −54 °C and 0.075 mbar for 72 h. Drying of the samples, regardless of the dehydration method, continued until the samples had a moisture content of 9%. A combination of two extraction methods, one conventional (simple stirring) and an ultrasonic method, was used to obtain the analyzed samples. The extraction condition is presented in Table 1. To obtain all polyphenols and other antioxidants, the extraction procedure was repeated twice using 15 mL of fresh solvent each time and the three extracts were combined. According to the overall picture of the results, the BO genotype seemed to be richer in antioxidants (e.g., polyphenols and ascorbic acid). In addition to the genotype, the drying method has also shown to play an influential role since freeze-drying is more suitable than oven-drying, probably due to the fact that freeze-drying, apart from removing the moisture, contributes to the preservation of all the nutrients in the tested sample [116]. In order to determine the antioxidant activity, the samples were examined for their content in total polyphenols (TPC), total anthocyanins (TAC), total flavonoids (TFC), and ascorbic acid (AAC). In the BO genotype, CS displayed the highest content, reaching 45 mg/g, while the FDS sample had a content of 40 mg/g, i.e., 12.5% less. Also, regarding anthocyanin content, both CS and FDS seemed to have the same content, close to 4.5 mg/g. In contrast, the OP bale showed total flavonoid content (TFC) only 2 mg/g. Furthermore, regarding the content of the other two antioxidants in both samples, again CS seemed to be more enriched, ensuring values close to 24 mg/g of total flavones and 2.5 mg/g of ascorbic acid. Therefore, the sample that ensures the highest antioxidant content is the fresh BO, but in case of the need to use a dried sample, freeze-drying will ensure maximum results.

The evaluation of the effect of different extraction techniques on polyphenol content and antioxidant activity in PC pomace were investigated by Okur et al. [117]. As regards the conventional extraction method, the exact conditions are presented in Table 1. TPC and DPPH^● radical scavenging activity were analyzed to determine its antioxidant activity. The TPC was found to be 108.36 mg GAE/100 g fresh weight (fw) whereas the antioxidant activity reached 70%. Three green extraction methods were tested, namely UAE, MAE, and HHP. Also, the conditions of each green extraction method are presented in Table 1. Considering the green technologies, MAE at 90 s had the highest TPC, at 275.31 mg GAE/100 g fw, while between the other two methods, HHP and UAE, the highest TPC values recorded were at pressure 500 MPa and time 10 min for HHP and time 15 min for UAE, with the values being 227.51 and 239.84 mg GAE/100 g fw, respectively. In other words, the use of MAE as the extraction mode can benefit TPC compared with UAE and HHP methods by 21.01 and 12.88%, respectively. Similar results were also presented in the antioxidant capacity of the samples. Specifically, extraction with MAE at 90 s strongly benefitted the antioxidant activity where 89.90% was obtained. Meanwhile, in the extractions with HHP (at 500 MPa pressure and 10 min time) and UAE (at 15 min time), the antioxidant activity was 84.33 and 85.77%, respectively, i.e., reduced by 6.60 and 4.81%, respectively. In conclusion, by comparing the results of green extraction methods with the conventional extraction method, it is evident that irrespective of the green extraction method used, both the TPC and antioxidant activity of the PC pomace sample were enhanced. This fact was expected as many researchers have reported that green extraction methods promote the extraction of bioactive compounds compared to conventional extraction methods [118].

Sezer et al. [119] carried out one of the latest studies on the PC pomace in order to examine all extraction methods, conventional and green, in order to find the most suitable one for obtaining the maximum antioxidant amount (TPC and antioxidant activity). In addition to extraction by conventional or green extraction alone, a combination of conventional and green methods together was also performed. Further, an emerging green method that is also used is the use of enzymes, which allows the release of the analyzed compounds through the degradation of plant cell walls. More specifically, the destruction of the cell wall aims increase the isolation of the bioactive components of the plant and, therefore, enhance the efficiency of the extraction [120]. Prior to the application of any extraction method, PC pomace was dissolved in citric acid buffer (50 mM) at an acidic pH of 5.0 and left for 24 h in a refrigerator for hydration. After hydration, the samples were subjected to digestion either before or after each green extraction method (thermal, high-pressure, microwave heating). However, in the enzymatic hydrolysis procedure, the samples after hydration were subjected to digestion using 0.1% (w/w) of the enzyme cellulase, with continuous stirring at 50 °C for 1 h. Following digestion, boiling was carried out for 5 min in order to inactivate the enzyme. The control sample was prepared receiving only hydration as treatment. The conventional extraction method employed was thermal treatment of the hydrated sample at 60 °C for 1 h and then heating at 100 °C for 5 min. The result of TPC was 3.12 mg/g and the antioxidant activity was 5.48 mmol DPPH/100 g. Compared to the control sample, heat treatment is perceived to promote the antioxidant activity of PC pomace, since the control sample exhibited 2.72 mg/g TPC (14.71% reduced content) and 2.83 mmol DPPH/100 g antioxidant capacity (93.64% reduced antioxidant capacity). As far as the green extraction modes are concerned, four different methods were used such as enzymatic hydrolysis at 60 °C for 1 h followed by heating at 100 °C to deactivate the enzyme (sample 1), high pressure at 300 MPa (sample 2) and 600 MPa (sample 3) for 15 min both. A microwave technique at 850 W for 60 s (sample 4) was also implemented. Regarding the results of TPC of the samples, the following results were presented, 3.22, 3.92, 4.04, and 5.18 mg/g for samples 1, 2, 3, and 4, respectively. According to the results the most suitable extraction method was microwave extraction where a higher TPC was exhibited by, 60.87, 32.14, and 28.22% with respect to samples 1, 2, and 3, respectively. Moreover, concerning the control sample, its TPC was reduced by 90.44% compared to the microwave-extracted sample. Finally, the antioxidant activity recorded was 3.89, 4.51, 6.76, and 9.51% for samples 1, 2, 3, and 4, respectively. Over 10 different combinations of extraction methods were performed and analyzed. In particular, sample 5 was subjected to a thermal treatment at 60 °C for 1 h followed by heating for 5 min at 100 °C after hydration, which was followed by a high pressure of 300 MPa for 15 min. Thermal treatment was also applied to sample 6 where the same conditions were maintained followed by a high-pressure method at 600 MPa for 15 min. Enzymatic hydrolysis was also applied to samples 7 and 8 with the pressure of 300 MPa and 600 MPa for 15 min, respectively. In addition, enzymatic hydrolysis was applied at 60 °C followed by heating for 5 min at 100 °C and then subjected to high pressure at 300 MPa and 600 MPa for 15 min in samples 9 and 10, respectively. However, most combinations of methods were performed by microwave heating. Sample 11 was prepared after microwave extraction followed by high pressure at 300 MPa while sample 12 was prepared by microwave and high pressure at 600 MPa. Similar treatment was given to samples 13 and 14, where sample 13 was the same as sample 11 with the addition of enzyme and sample 14 was subjected to the same extraction as sample 12 with the addition of enzyme, as well. The last two samples were extracted by microwave heating followed by enzyme hydrolysis for 1 h at 60 °C and then heating at 100 °C for 5 min for enzyme suppression (sample 15) while sample 16 was subjected to the same methods as 15 with the additional treatment of high pressure of 600 MPa for 15 min after enzyme inhibition. Regarding the yields, the highest TPC (5.39 mg/g) and antioxidant activity (9.94 mmol DPPH/100 g) was recorded for sample 12. It is easily demonstrated that the use of a microwave is the most beneficial method for the extraction of PC pomace mainly in combination with a high pressure of 600 MPa as this extraction ensures the highest content of antioxidants.

3.4. Green Extraction Techniques Used for P. cerasus Pomace and Peel

Şahin et al. [121] examined the effect of a green deep eutectic solvent (DES) on the phenolic content of PC peel using a green extraction method (MAE). The definition of DES indicates liquids that belong close to the eutectic composition of the mixtures [122]; in this case, the solvent was prepared with a 1:4 molar ratio, citric acid/ethylene glycol, following a previous study [123]. The extraction of PC peels was carried out with a laboratory-scale microwave system according to Table 1. Specifically, sample 1—35% water: 0.5 g mass, sample 2—50% water: 0.5 g mass, sample 3—20% water: 0.5 g mass, sample 4—35% water: 0.3 g mass, sample 5—50% water: 0.3 g mass, sample 6—20% water: 0.3 g mass, sample 7—50% water: 0.1 g mass, sample 8—35% water: 0.1 g mass, sample 9—20% water: 0.1 g mass. The highest TPC was found in the samples with the lowest mass (0.1 g). In detail, in sample 7 polyphenols reached 16.62 mg/g, in sample 8 polyphenols reached 17.70 mg/g, and in sample 9 polyphenols reached 18.14 mg/g (maximum value). The anthocyanin content of the samples reached up to 3.31 mg/g, amount recorded in sample 7, whereas the highest amount of cyanidin-3-glucoside (5.49 mg/g) was also recorded to the same sample. In conclusion, it might be said that for PC peel the ratio of 50% water/50% DES: 0.1 g mass for UAE system can ensure high amounts of antioxidants. The above-mentioned studies are shown in Table 1.

Table 1. Comparison of conventional and “green” techniques in the extraction of bioactive compounds from tart cherry pomace and peel.

Technique	Extraction Type	Extraction Conditions	Antioxidant Activity Due to Bioactive Compounds	Ref.
Conventional	Orbital shaking	Ethanol:water 50% v/v with 70 rpm for 24 h at 30 °C	DPPH: 290 ppm DPPH/g	[111]
	Extraction	9:1 solid-to-liquid PC pomace with ethanol:water 50:50, 0.01% hydrochloric acid at 50 °C	TPC: 14.46 mg/g, TPC: 0.41 mg/g	[110]
	Extraction in water bath	30 min at 50 °C, 10 g, 100 mL 80% methanol with 1% HCl	TPC: 108.36 mg GAE/100 g fw	[117]
	Thermal treatment	60 °C for 1 h and then heating at 100 °C for 5 min	TPC measured 3.12 mg/g	[119]
“Green”	US	20 mL solvent (hydrochloric acid:methanol:water 1:80:19), 30 min, 40 °C, triple	TPC at 50.89 mg GAE/100 g	[112]
		20 mL of extraction solvent with a hydrochloric acid/methanol/water ratio of 0.1:80:19 in an ultrasonication bath for 30 min at 40 °C	BO CS 45 mg/g FDS 40 mg/g, total anthocyanin content both 4.5 mg/g OP 2 mg/g TAC, TFC 2 mg/g and 2.5 mg/g ascorbic acid in CS	[115]
	UAE	Frequency of 24 kHz, power of 400 W, and amplitude of 20–100%	TPC: 227.51 mg GAE/100 g fw	[117]
	MAE	900 W, 1900 MHz, 60 °C, 90 s	TPC: 275.31 mg GAE/100 g fw
	HHP	Solvent:solute 10:1, 20 °C, 500 MPa, 10 min	TPC: 239.84 mg GAE/100 g fw
	Maceration	1:10 w/v solid:liquid ratio, 900 rpm, reduced light at 25 °C for 24 h	TAC: 1.8 mg/g	[114]
	Decoction	1:10 w/v solid:liquid ratio, 900 rpm, reduced light at 100 °C for 15 min	TAC: 1.6 mg/g
	Enzymatic hydrolysis	60 °C for 1 h, then heat at 100 °C	TPC: 1. 3.22 mg/g, 2: 3.92, 3: 4.04, and 4: 5.18 mg/g	[119]
	HHP	300 MPa for 15 min	TPC: 3.92 mg/g
		600 MPa for 15 min	TPC: 4.04 mg/g
	MAE	850 W for 60 s	TPC: 5.18 mg/g
		500 W, extraction time of 3 min, and 50 mL of solvent with 9 different ratios of solvent (DES/water)	TPC: 18.14 mg/g, anthocyanin content up to 3.31 mg/g	[121]

Table 1. Comparison of conventional and “green” techniques in the extraction of bioactive compounds from tart cherry pomace and peel.

Technique	Extraction Type	Extraction Conditions	Antioxidant Activity Due to Bioactive Compounds	Ref.
Conventional	Orbital shaking	Ethanol:water 50% v/v with 70 rpm for 24 h at 30 °C	DPPH: 290 ppm DPPH/g	[111]
	Extraction	9:1 solid-to-liquid PC pomace with ethanol:water 50:50, 0.01% hydrochloric acid at 50 °C	TPC: 14.46 mg/g, TPC: 0.41 mg/g	[110]
	Extraction in water bath	30 min at 50 °C, 10 g, 100 mL 80% methanol with 1% HCl	TPC: 108.36 mg GAE/100 g fw	[117]
	Thermal treatment	60 °C for 1 h and then heating at 100 °C for 5 min	TPC measured 3.12 mg/g	[119]
“Green”	US	20 mL solvent (hydrochloric acid:methanol:water 1:80:19), 30 min, 40 °C, triple	TPC at 50.89 mg GAE/100 g	[112]
		20 mL of extraction solvent with a hydrochloric acid/methanol/water ratio of 0.1:80:19 in an ultrasonication bath for 30 min at 40 °C	BO CS 45 mg/g FDS 40 mg/g, total anthocyanin content both 4.5 mg/g OP 2 mg/g TAC, TFC 2 mg/g and 2.5 mg/g ascorbic acid in CS	[115]
	UAE	Frequency of 24 kHz, power of 400 W, and amplitude of 20–100%	TPC: 227.51 mg GAE/100 g fw	[117]
	MAE	900 W, 1900 MHz, 60 °C, 90 s	TPC: 275.31 mg GAE/100 g fw
	HHP	Solvent:solute 10:1, 20 °C, 500 MPa, 10 min	TPC: 239.84 mg GAE/100 g fw
	Maceration	1:10 w/v solid:liquid ratio, 900 rpm, reduced light at 25 °C for 24 h	TAC: 1.8 mg/g	[114]
	Decoction	1:10 w/v solid:liquid ratio, 900 rpm, reduced light at 100 °C for 15 min	TAC: 1.6 mg/g
	Enzymatic hydrolysis	60 °C for 1 h, then heat at 100 °C	TPC: 1. 3.22 mg/g, 2: 3.92, 3: 4.04, and 4: 5.18 mg/g	[119]
	HHP	300 MPa for 15 min	TPC: 3.92 mg/g
		600 MPa for 15 min	TPC: 4.04 mg/g
	MAE	850 W for 60 s	TPC: 5.18 mg/g
		500 W, extraction time of 3 min, and 50 mL of solvent with 9 different ratios of solvent (DES/water)	TPC: 18.14 mg/g, anthocyanin content up to 3.31 mg/g	[121]

3.5. Conventional Extraction Techniques Used for P. cerasus Kernel

The study by Górnas et al. [70] investigated the lipophilic bioactive compounds (essential fatty acids, carotenoids, tocopherols, sterols, and squalene) present in the kernel oils derived from six different cultivars from Baltic countries and Russia (“Haritonovskaya”, “Bulatnikovskaya”, “Tamaris”, “Shokoladnica”, “Latvijas Zemais”, and “Zentenes”) of sour cherry. The oil was extracted with a sole conventional technique with hexane. The mixture was centrifuged, the supernatant was collected, and the procedure was repeated, with the final combined mixture having hexane evaporated through vacuum rotary evaporator. In terms of the overall content of monounsaturated fatty acids, the concentration exhibited a range of 26.0–46.1%, with the cultivar Latvijas Zemais having the highest proportion. The total polyunsaturated fatty acids also showed a similar trend, ranging from 44.0–62.3%, with the highest value achieved by the same cultivar. In comparison with tocopherols, the observed values ranged from 118.2–163.6 mg/100 g oil, with the Zentenes cultivar presenting the highest concentration. The Tamaris cultivar revealed a statistically significant difference (p < 0.05) in both total carotenoid and total sterol content. Total carotenoids ranged from 0.54–1.18 mg/100 g, with Tamaris cultivar achieving 1.75 mg/100 g of oil. A greater amount of 1041.3 mg/100 g of oil was measured in total sterol content, compared to the other cultivars which ranged from 313.6–416.24 mg/100 g of oil. Finally, Bulatnikovskaya cultivar had the highest content in squalene, which ranged from 65.8–102.8 mg/100 g of oil. Squalene, the primary component of skin surface polyunsaturated lipids, has moisturizing properties and protects skin cells from free radical oxidative damage [124]. The results indicate that the composition of bioactive compounds in sour cherry kernel oils is significantly influenced by the cultivar. In addition, it was observed that oils extracted from cultivars with a high oil content may contain lower concentrations of sterols and carotenoids.

The primary focus of the study by Korlesky et al. [125] was the comprehensive analysis of sour cherry kernel oil, containing its inherent properties and chemical composition. Fine powder of rinsed sour cherry kernel was extracted with hexane through the Soxhlet apparatus. The results showed that the sour cherry kernel oil yielded high concentrations of both monounsaturated (48.7%) and polyunsaturated fatty acids (39.3%). The authors of the study observed the presence of a specific fatty acid, α-eleostearic acid, which displayed noteworthy antioxidant properties. This fatty acid was found at a concentration of 5.72%. High total tocopherol content (TTC) was also measured at 525.2 ppm, with γ-tocopherols yielding the highest amount from other tocopherols (400 ppm). Consequently, cherry kernel oil has the potential to be beneficial in various dietary applications and incorporated into cosmetic products.

The production of sour-cherry jam and juice results in several by-products, one of which is the sour-cherry seed. Both the sour-cherry kernels and the seed shells are useful byproducts of sour-cherry production. To that end, both parts have been examined and evaluated in a study conducted by Farhadi et al. [66]. Powdered sour-cherry seeds were analyzed for their fatty acid content, TPC, and DPPH^• scavenging activity. Oven-dried ground kernels were extracted through Soxhlet extraction. The results showed that total polyphenols were present in high concentrations in sour-cherry seed (27.02 mg GAE/g). Chlorogenic acid (1887.50 μg/mg), 3,4-dihydroxybenzoic acid (262.30 μg/mg), quercetin (13.5 μg/mg), and rutin (58.45 μg/mg) were all found through chromatographic polyphenol analysis. Regarding sour cherry kernel oil, TPC was measured at 6.41 mg GAE/g, whereas the oil analysis revealed a high concentration of unsaturated fatty acids with oleic acid (45.03%), linoleic acid (40.61%), and linolenic acid (3.87%). In sum, sour-cherry seeds have promising physical properties that could make them useful in a variety of applications, such as functional beverages and dietary supplements. Initial research on the sour cherry seed properties has revealed promising potential for its use in nutritional supplements and healthful food products.

There is ongoing research into discovering new plant oils that may provide a natural source of nutraceutical compounds. Eight Polish sour cherry cultivars (‘Koral,’ ‘Naumburger’, ‘Lucyna,’ ‘Montmorency’, ‘Wanda’, ‘Wigor’, ‘Wolynska’, and ‘Wróble’ varieties) were analyzed for their crude fat, protein, and oil content in a study by Stryjecka et al. [69]. The chemical properties of the oils were evaluated after they were extracted through a Soxhlet extractor. Crude oil ranged from 24.6–35.4%. Fatty acid composition ranged significantly (p < 0.05) in oleic acid (25.4–41.0%), linoleic acid (39.1–46.2%), and α-linolenic acid (0.09–0.43%). Non-significant differences (p > 0.05) were recorded to the measurement of α-eleostearic acid in seven cultivars (~8.0–8.5%) but was recorded considerably higher in cv. ‘Naumburger’ (15.62%). The data presented above showed that the dominant fatty acid content in cherry oil varies depending on the cherry cultivar. In addition, α-eleostearic acid was discovered to be third, after oleic and linoleic acids, and its content was found to be cultivar-specific. Due to the large amount of beneficial unsaturated fatty acids and relatively low proportion of unhealthy saturated fatty acids, sour cherry kernel oil has many uses in human nutrition and medicine. Total sterol content (averaged 495 mg/100 g oil), total tocochromanols (averaged 134 mg/100 g oil), TPC (averaged 22.8 μg GAE/g), and total carotenoids (averaged 0.84 mg/100 g oil) indicated the strong presence of bioactive compounds in the sour cherry kernel oil. It could be also indicated by the vast antioxidant activity, as in DPPH^• assay averaged 60.4 mg Trolox/100 g oil and ABTS^•+ 40.4 mg Trolox/100 g oil. These compounds are antioxidants that help prevent cell death and tissue damage throughout the body and possess medicinal effects against cancer, inflammation, bacteria, fungi, viruses, and allergies [126].

3.6. Green Extraction Techniques Used for P. cerasus Kernel

In recent years, cold-pressed seed oils have become increasingly popular in the West as a healthy alternative to conventional cooking oils. The study conducted by Başyiğit et al. [127] highlighted that the natural bioactive compounds, especially unsaturated fatty acids present in sour cherry kernel oil, have the potential to enhance the nutritional value of food and pharmaceutical materials. A “green” technique including a laboratory-scale cold pressing machine with a capacity of 12 kg kernels/h was employed to extract oil from sour cherry kernels. The results showed that sour cherry kernel oil had high monounsaturated (39.74%) and polyunsaturated (43.34%) fatty acids composition, yielding a sum of 83.08% of unsaturated fatty acids. The authors also quantified specific individual carotenoids in the oil samples. Zeaxanthin (13.15 mg/kg oil), β-carotene (1.75 mg/kg oil), and cryptoxanthin (0.08 mg/kg oil) were the bioactive compounds of interest that were quantified. The antioxidant activity of the oil is also enhanced by known individual polyphenols determined chromatographically. For instance, compounds such as resveratrol (66.14 μg/kg), p-coumaric acid (101.61 μg/kg), ellagic acid (612.93 μg/kg), kaempferol (135.00 μg/kg), and naringenin (1678 μg/kg) were identified in the study samples. Compared to other oils, this kernel oil could be tested as an enrichment ingredient in a wide range of formulations.

Kazempour-Samak et al. [128] investigated, for the first time, the extraction of oil from the kernel of Iranian sour cherry (Cerasus vulgaris Miller) using “green” technique. Indeed, a cold-press extraction technique was employed to extract oil from sour cherry kernel samples with the temperature rising at least 35 °C. The authors evaluated the antioxidant properties of sour cherry kernel oil. The content of kernel oil in total polyphenols (33.44 mg GAE/g dm), TFC (46.37 mg quercetin/g dm), total tannins (1.21 mg GAE/g dm), total anthocyanin content (177.84 mg cyanidin 3-glucoside/mL), and TTC (832.5 mg/kg oil) indicated its considerable antioxidant activity. The same research team conducted a study [129] that involved the extraction of sour cherry (C. vulgaris Miller) kernel oil, again using a cold-press extraction technique, followed by an assessment of its chemical composition. The authors presented comprehensive results regarding the bioactive properties of the oil dry matter. The study explored the TPC (33.0 mg GAE/g), total tannin content (1.50 mg GAE/g), TFC (45.87 mg quercetin/g), total anthocyanin content (177.34 mg cyanidin 3-glucoside/mL), and TTC (562.5 mg/kg). This study demonstrated that sour cherry by-products possess noteworthy bioactive properties, particularly in terms of their polyphenol content and antioxidant capacity. Nevertheless, it is imperative to conduct additional research in their potential antioxidant preservatives for the food industry, and to assess their impact on human health.

The purpose of the study from Atik et al. [65] was to identify the bioactive properties of oils extracted from sour cherry kernels using the cold press method. Oils were extracted using a cold press machine that could process 6 kg of kernels/h. To protect the unique qualities of oil, the process did not exceed 50 °C. It has been established that the fruit’s kernels, which are commonly discarded as waste in the fruit juice industry, can be refined into vegetable oil and used as an edible component. The authors identified linolenic acid as the major fatty acid in sour cherry kernel oil and was quantified at ~42%, summing an average of ~54% in polyunsaturated fatty acids and ~39% in monounsaturated fatty acids. The oil exhibited considerable antioxidant activity (1.86 mmol TE/g), which was attributed to the high TPC at 33.65 mg GAE/kg, with benzoic acid being the major polyphenol measured at 79.7 ppm. TTC was also measured at 224.43 ppm. The sterol composition analysis revealed that β-sitosterols (6018.27 ppm) showed high concentration in the kernel oil. In the same study, the authors also investigated wild plum (Prunus spinosa) kernel oil. It was evident that sour cherry kernel oil had higher values of TPC and sterols from wild plum kernel oil (28.32 mg GAE/g and 2509.93 ppm of β-sitosterols). The antioxidant activity also found reduced in wild plum kernel oil (1.44 mmol TE/g).

3.7. Combination of Conventional and Green Extraction Techniques Used for P. cerasus Kernel

The objective of the study conducted by Yilmaz et al. [130] was a comprehensive analysis of the chemical composition of kernels from sour cherry, with the intention of exploring their potential applications as a viable source of oil and antioxidants. Conventional extraction required successive extraction of the grounded kernel sample with hexane, as represented in

Table 2. Comparison of conventional and “green” techniques for the extraction of bioactive compounds from tart cherry kernel.

Technique	Extraction Type	Extraction Conditions	Bioactive Compounds	Ref.
Conventional	Orbital shaking	5:2 liquid-to-solid ratio with hexane, 15 min extraction	Oleic acid ranged from 45–48%, β-carotene: 8.47 mg/L without 3% v/v ethanol and 10.03 mg/L with 3% v/v ethanol	[130]
	Vortex shaking	5:1 liquid-to-solid ratio with hexane for 1 min	Linoleic acid (35.5–46.06%), oleic acid (25.25–45.30%), α-eleostearic acid (7.43–15.76%) and palmitic acid (5.06–7.38%), total tocochromanols (118.2–163.6 mg/100 g oil), total carotenoid content (0.51–1.75 mg/100 g oil), squalene content (65.8–102.8 mg/100 g), sterols content (313.6–1041.3 mg/100 g oil)	[70]
	Soxhlet	Hexane for 24 h	High concentrations of both monounsaturated (48.7%) and polyunsaturated fatty acids (39.3%), α-eleostearic acid content (5.72%), TTC (525.2 ppm)	[125]
		24 h of extraction at 60 °C with methanol	Sour cherry seed content: TPC (27.02 mg GAE/g), chlorogenic acid (1887.5 g/mg), 3,4-dihydroxybenzoic acid (262.3 g/mg), quercetin (13.5 g/mg), and rutin (58.45 g/mg), Sour cherry kernel content: TPC (6.41 mg GAE/g), oleic acid (45.03%), linoleic acid (40.61%), and linolenic acid (3.87%)	[66]
		Extraction with diethyl ether for 6 h	Oleic acid (25–41%), linoleic acid (39.1–46.2%), α-linolenic acid (0.09–0.43%), α-eleostearic acid (~8–15%), total sterol content (495 mg/100 g oil), total tocochromanol content (134 mg/100 g oil), TPC (22.8 μg GAE/g), total carotenoid content (0.84 mg/100 g oil)	[69]
		1:4 solid-to-liquid ratio with n-hexane or methylene chloride for 6 h	Oleic acid (41.44–41.46 g/100 g seed oil), linoleic acid (47.00–47.17 g/100 g seed oil), α-linolenic acid (0.32–0.33 g/100 g seed oil), TTC (37.37–46.36 mg/100 g seed oil)	[131]
“Green”	SC-CO2	Extraction at 60 °C and 300 bar pressure, flow rate 25 g/min	β-carotene: 5.65 mg/L without 3% v/v ethanol and 6.00 mg/L with 3% v/v ethanol, TPC: 27.86 mg GAE/L	[130]
	Cold-press	12 kg kernels/h	High monounsaturated (39.74%) and polyunsaturated fatty acids (43.34%), zeaxanthin (13.15 mg/kg oil), β-carotene (1.75 mg/kg oil), cryptoxanthin (0.08 mg/kg oil), resveratrol (66.14 μg/kg), p-coumaric acid (101.61 μg/kg), ellagic acid (612.93 μg/kg), kaempferol (135 μg/kg), and naringenin (1678 μg/kg)	[127]
		<35 °C	TPC (33.44 mg GAE/g dm), TFC (46.37 mg quercetin/g dm), total tannin content (1.21 mg GAE/g dm), total anthocyanin content (177.84 mg cyanidin 3-glucoside/mL), and TTC (832.5 mg/kg oil)	[128]
		<35 °C	TPC (33.0 mg GAE/g), total tannin content (1.50 mg GAE/g), TFC (45.87 mg quercetin/g), total anthocyanin content (177.34 mg cyanidin 3-glucoside/mL), and TTC (562.5 mg/kg)	[129]
		6 kg kernels/h, <50 °C	Polyunsaturated fatty acids (~54%), monounsaturated fatty acids (~39%), linolenic acid (42.42%), TPC (33.65 mg GAE/kg), benzoic acid (79.7 ppm), TTC (224.43 ppm), β-sitosterol (6018.27 ppm)	[65]
	SC-CO2	4 h extraction with pressure (200, 275, and 350 bar), temperature (40, 55, and 70 °C), and CO2 flow rate (0.2, 0.3, and 0.4 kg/h)	Oleic acid (40.8–41.65 g/100 g seed oil), linoleic acid (46.32–47.37 g/100 g seed oil), α-linolenic acid (0.32–0.33 g/100 g seed oil), TTC (10.06–60.61 mg/100 g seed oil)	[131]
	Cold-press	70 °C, 26 Hz frequency	Oleic acid (41.92 g/100 g seed oil), linoleic acid (46.82 g/100 g seed oil), α-linolenic acid (0.33 g/100 g seed oil), TTC (38.09 mg/100 g seed oil)

. The results showed the impact of ethanol presence in the extraction process on the fatty acid composition of kernel oil, as it was found to be statistically insignificant (p > 0.05), as the most abundant acid (C18:1, oleic acid) ranged from ~45–48%. However, total tocopherol concentration (TTC) was measured at 428.62 mg/L in the conventional extraction without 3% v/v ethanol. A similar pattern was seen in the quantification of β-carotene. Conventional extraction was found to have high efficiency and it seems that the presence of ethanol has positive effects on the extraction of this compound. It was measured 8.47 and 10.03 mg/L with the conventional technique, respectively, verifying the vast impact of ethanol. In the same study, the authors also investigated a “green” SC-CO₂ technique. This method yielded the highest TPC at 27.86 mg GAE/L, significantly higher than any other technique by ~29–76%. It is of high importance to highlight that the application of ethanol in both techniques (conventional and “green”) resulted in a considerable increase of the overall TPC and β-carotene of the kernel oil samples. However, the “green” technique was found to be the preferable technique as it yielded higher TPC and better polyphenol recovery, despite the higher tocopherol recovery provided by conventional extraction. The implementation of an environmentally and human-friendly technique to extract the oil renders it suitable for consumption in food products.

The objective of the conducted study by Dimic et al. [131] was to assess the efficacy of Soxhlet extraction to isolate oil from cherry seeds. The investigation focused on the analysis of oils with regard to their extraction yield, fatty acid composition, TTC, and antioxidant capacity. The raw material underwent fractionation, resulting in fractions with sizes less than 800 μm and greater than 800 μm. In the Soxhlet extraction procedure, milled cherry seeds were subjected to extraction using an appropriate non-polar solvent (n-hexane or methylene chloride) in a ratio of 1:4 solid-to-liquid. Oleic acid content did not differ significantly between the different solvents (~42 g/100 g of sour cherry seed oil). Linoleic acid averaged ~47 g/100 g seed oil) and α-linolenic acid was measured at ~0.32 g/100 g seed oil. Regarding TTC, it ranged from 37–46 mg/100 g cherry seed oil. In the same study, “green” techniques were also used for the isolation of kernel oil, i.e., supercritical carbon dioxide (SC-CO₂) and cold-pressing extraction. The laboratory-scale high-pressure extraction plant was utilized to conduct the SC-CO₂ process. Further information about extraction conditions is shown in Table 2. The maximum content was achieved using SC-CO₂ at 70 °C, 0.4 kg/h at 200 bar. Oil yield ranged from 2.50–13.02%, with the highest yield obtained from SC-CO₂ method with 350 bar, 70 °C, 0.4 kg/h, in which the particle size (<800 μm) had a significant role. Linolenic acid content ranged with non-significant differences (p > 0.05) from 46.68–47.37 g/100 g of sour cherry seed oil. The highest content was achieved with a “green” technique, cold-press extraction. Finally, antioxidant activity was measured in order to evaluate the healing effect of sour cherry seed oil. DPPH^• assay (2.18–6.22 μM TE/g oil) highlighted the pressure effect in SC-CO₂ method with 70 °C, 0.4 kg/h at 275 bar. Subsequently, it was proved that bioactive compounds yield was increased further with “green” techniques. More specifically, SC-CO₂ extraction parameters like pressure, temperature, and flow rate were tuned. tocopherol yield in oil was increased when these factors were kept to a minimum during extraction. Particle size also had a crucial impact on SC-CO₂ because it can affect the amount of the sample extracted and the amount of the tocopherol produced. Extracts potentially rich in bioactive compounds may be obtained through the SC-CO₂ method without the need for further purification. In the case of optimizing the SC-CO₂ process for selectively recovering desired bioactive compounds and obtaining high-value-added products, this can be considered. When compared to other extraction methods, the SC-CO₂ process allows for the recovery of chemical-free, “green” extracts. The results of the above studies are demonstrated in Table 2.

4. Conclusions and Future Perspectives

Food loss is a huge problem nowadays. Huge quantities of food, especially fruit and vegetables, are discarded every day, resulting in a burden on both the economy and the environment. It is now imperative to utilize food by-products and convert them into value-added products. The utilization of industrial by-products, such as those resulting from the processing of sour cherries, is a sustainable method that can meet the needs that have arisen. The by-products of sour cherry, mainly peels, pulp, and pits, are rich in polyphenols, anthocyanins, and other antioxidants. All these bioactive compounds can be isolated by various extraction techniques, conventional and non-conventional. Among the conventional ones are Soxhlet, maceration, and simple extraction. Among the green techniques are microwave-assisted extraction, ultrasound-assisted extraction, supercritical fluid extraction, high hydrostatic pressure, and enzyme-assisted extraction. Green techniques, the most recent trends in extraction, have an equivalent or even higher-quality extract, higher extraction yields, less extraction time, less solvent use, and reproducible and repeatable efficiencies compared to conventional methods. In addition, the use of green techniques is a sustainable option for the extraction of bioactive compounds, as it is possible to work with green solvents such as water, and in some cases without any solvent at all. This was confirmed above, as the studies conducted on the extraction of bioactive compounds from the by-products of P. cerasus revealed that green techniques were superior to conventional ones. More specifically, regarding the peel and pomace of P. cerasus, UAE appeared to be the most effective technique, while for the kernel, SC-CO₂ was the most effective technique. Both techniques are efficient, and fast, and lead to high yields. The combination of multiple green extraction techniques yields superior performance in terms of extraction yield and extract purity.

Other green techniques could be applied to the by-products of P. cerasus. These include pulsed electric field and ionic liquid extraction. The pulsed electric field technique is a fast, efficient technique that applies electricity to break the cell membrane, thus helping the extraction of bioactive compounds. The effect of the electric field on the by-products of P. cerasus could be investigated. Furthermore, ionic liquid extraction is a novel type of solvent extraction that utilizes ionic liquid instead of volatile organic compounds as diluents, extractants, or both. Thereby, the effect of ionic liquid on P. cerasus peel, pomace, and kernel could be studied leading to potentially higher recoveries of bioactive compounds from them. In addition, another extraction technique called Cloud point extraction could be applied or combined with one of the other green or conventional techniques. The advantage of this method is that it uses non-toxic, edible surfactants that encapsulate the bioactive compounds so that these can then be easily employed as food additives. All these techniques could also be easily applied to the by-products of all fruits belonging to the Prunus genus.

Significantly, the potential uses of Prunus peels, pomace, seeds, and kernels in food manufacturing appear to have been ignored. In the future, it is important to consider using Prunus seeds in the development of new juices containing bioactive compounds, bread products, veggie-meat based products, and industrial fermentation for the production of various food additives like enzymes, proteins, and flavorings. The optimization of the optimum amount of each waste component to ensure the quality of the enriched products should also be taken into account and may be customized according to consumer preferences.