Valorization of Pistachio Green Hull: Advances in Extraction and Characterization of Phenolic Compounds

Abstract

Substantial agro-industrial waste is generated by the food industry, including pistachio green hulls (PGH), which can constitute 40% to 60% of the fresh fruit weight. This by-product contains bioactive functional components, especially phenolic compounds (PCs). An overview of research focused on PCs extracted from PGH is presented, highlighting their chemical composition, extraction methods, compound identification, and antioxidant and antibacterial activities. Extraction techniques such as ultrasound, microwave-assisted extraction, and solid-state fermentation are utilized, with mild organic solvents like water, ethanol, methanol, or their mixtures employed. The quantification of PCs is commonly performed using the Folin–Ciocalteu assay, HCl-Butanol technique, and aluminum chloride colorimetric assays. Furthermore, identification of compounds is generally accomplished through high-performance liquid chromatography (HPLC) or gas chromatography (GC), often coupled with mass spectrometry or photodiode-array detectors to enhance accuracy and reliability. Gallic acid, kaempferol, quercetin, cyanidin, and catechin are the main PCs identified, with their antioxidant activity validated by ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), DPPH (2,2-diphenyl-1-picrylhydrazyl), and FRAP (ferric-reducing antioxidant power) assays. Antibacterial effectiveness has been demonstrated against bacteria using disk diffusion and minimum inhibitory concentration methods. These findings indicate potential uses of PGH by-products in the food, cosmetic, and pharmaceutical industries, contributing to a circular economy and enhancing agro-industrial waste management.

1. Introduction

The food industry has been expanding to meet the needs of 8 billion people, resulting in increased agro-industrial waste [1]. Although the use of these wastes is still limited, there is increasing interest among researchers because of the many bioactive compounds they contain that could offer health benefits to humans [2,3].

Pistachio green hull (PGH) is a by-product rich in phenolic compounds (PCs) that has attracted significant interest. This interest is mainly due to the presence of bioactive compounds, such as gallic acid, quercetin, eriodictyol, luteolin, and kaempferol. These compounds exhibit a range of biological properties, such as anticancer, antioxidant, anti-diabetic, anti-inflammatory, antiangiogenic, antihypertensive, and antimicrobial activities [2,4,5]. In 2024, about 1.2 million tons of pistachios were produced worldwide [6], with PGH accounting for 40–60% of the total fresh fruit weight, resulting in approximately 600,000 tons of waste [5,7,8]. Therefore, PGH utilization offers a valuable option for managing agro-industrial waste, supporting the circular economy, and reducing environmental impact by decreasing methane emissions from its breakdown [9].

The PCs extracted from PGH have exhibited effects similar to those of commonly used synthetic antioxidants, such as butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) [10]. As a result, valorizing PGH could reduce dependence on synthetic antioxidants. This review aims to compile research on PCs from PGH, including their chemical composition, extraction methods, quantification, identification, and antioxidant and antibacterial activities.

2. Review Methodology

This study conducted a comprehensive review of scientific literature across international databases, including Scopus, ScienceDirect, Wiley, and Springer. The search used combined keywords such as “pistachio green hull” and “phenolic compounds extraction,” restricted to Title, Keywords, and Abstract. Inclusion criteria for articles in this review were based on the extraction, quantification, or identification of PCs, as well as the analysis of proximal composition and antioxidant or antibacterial activities derived from PGH. The literature search extended until 14 August 2025. An initial selection yielded 332 scientific papers; after further screening, 66 articles were ultimately included. These selected articles were carefully analyzed regarding the extraction methods used, the techniques used for PC identification, and the reported biological activities. Additionally, we excluded articles that did not meet the inclusion criteria or were derived from encyclopedias, book chapters, or conference abstracts.

3. Pistachio Fruit

Pistachio (Pistacia vera L.) in Iran is recognized as “green gold” owing to its substantial economic and nutritional significance as a traditional dried fruit [11]. This tree species belongs to the family Anacardiaceae and is one of the most commercially important species within the genus Pistacia, alongside P. lentiscus, owing to the diverse food, cosmetic, and health products derived from its fruits [12]. It is originally native to Middle Eastern nations such as Iran, Syria, Türkiye, and Afghanistan. The cultivation of pistachios has since expanded to various countries in the Mediterranean region, including Greece, Spain, and Italy, as well as to North America, particularly the United States and Mexico (see Figure 1) [13,14].

In Mexico, the pistachio crop was introduced in the 1990s because the environmental conditions in Chihuahua State meet the requirements for establishing productive pistachio orchards [13,15]. However, despite its economic potential in Mexico, the country has low annual production rates, classifying it as an emerging producer [16].

This crop is the only species within the genus Pistacia that produces fruits with edible kernels, widely consumed for their health benefits, due to the presence of valuable phytochemicals [14,17]. Its nutritional profile includes vitamins (α-tocopherol, γ-tocopherol, A, B1, B2), minerals (Fe, Se, Zn, and P), sterols, carotenoids, chlorophyll, fatty acids, and PCs [11,14,18]. Furthermore, it serves as a source of protein, accounting for 21.7–23% of the total protein content [19].

Pistachio kernels are used in different forms, including salty and roasted snacks. They are also added to liqueurs, cakes, sweets, ice cream, condiments, wafers, and chocolate-based products [14,20].

Pistachio is classified as a dry fruit in the form of a semi-drupe, where the kernel is enclosed by testa, endocarp, mesocarp, and epicarp. It is surrounded by a thin, hard shell of a light yellowish color, and the PGH appears as a thick, fleshy husk with colors ranging from green-yellow to reddish [2,21].

Figure 1. Countries producing pistachios worldwide [22].

The production and processing of pistachios generate by-products that are often discarded. While pistachio kernels, with or without their hard shells, are marketed, other parts, such as stems, leaves, and PGH, are usually discarded. This disposal creates considerable waste and environmental challenges, with approximately 660,000 tons of fruit waste discarded worldwide each year [23,24]. However, PGH has recently gained research interest due to its compounds, which have potential significance for human health [25].

4. Pistachio Green Hull By-Products

The PGH (Figure 2) is the main by-product of the pistachio fruit [5,7,8]. Without proper management, such as polyphenol extraction, PGH can cause environmental problems due to its high content of PCs, leading to soil changes. Also, PCs can interfere with livestock digestion by interacting with organic molecules, thereby preventing the absorption of proteins and sugars, reducing their bioavailability, and hindering their absorption [26,27]. Furthermore, PGH has a moisture content exceeding 70%, which encourages the growth of microorganisms that may represent health risks [2]. It also creates a pest hotspot, leading to the buildup of various nematodes and arthropods and accelerating material decay, resulting in widespread contamination. Additionally, the decomposition process generates methane emissions into the environment [9].

Figure 2. Parts of a pistachio fruit. (a) Fresh pistachio; (b) PGH with in-shell pistachio; (c) Shell and kernel; (d) Kernel.

Presently, following the hulling process, PGH aggregates are in substantial quantities. Nevertheless, initiatives are underway to mitigate environmental impact by blending PGH with soil and subsequently applying the blend to agricultural fields and orchards. Furthermore, PGH has been utilized as an herbal medicinal remedy and in the production of jam [4,28,29].

Composition of Pistachio Green Hull

The PGH chemical composition (Table 1) has excellent potential for developing new products, considering its nutritional content, including protein, ash, crude fiber, carbohydrates, fat, and moisture. The differences in results could be linked to geographical location, harvest season, variety, maturity stage, or the analytical techniques used.

The PGH contains over 15% of neutral and acid-detergent fiber and carbohydrates, making it a valuable source of fermentable sugars. Bakhshizadeh et al. [30] reported the analysis of PGH from Iran, where the contents of acid- and neutral detergent fiber were 18.7 and 26.2%, respectively. Additionally, Bohluli et al. [31] reported values of 20 and 25% for acid- and neutral detergent fiber, respectively, for PGH from Iran. For their part, Özbek et al. [29] studied PGH from Türkiye, and reported values of 15.7 and 17.08% for acid-detergent fiber and neutral-detergent fiber, respectively. Given these values for detergents and crude fiber, PGH could have a prebiotic effect [32], as its polysaccharide and monosaccharide content can promote the growth of intestinal flora and aid digestion [14].

Additionally, PGH contains several fatty acids, including palmitic, stearic, myristic, linoleic, and oleic acids. Among these, linoleic and oleic acids, which are unsaturated fatty acids, are the most common [2,33]. Monounsaturated fatty acids have a vasodilatory effect that helps lower blood pressure and reduce the risk of heart disease. Their consumption also enhances the fatty acids β-oxidation [13].

Table 1. Nutritional content of PGH.

Country	Carbohydrates (%)	Fat (%)	Protein (%)	Ash (%)	Crude Fiber (%)	Moisture (%)	Reference
Tunisia	39.70	20.41	11.23	14.74	ND	10.46 (DB)	Hamed et al. [34]
Iran	ND	ND	0.24*	ND	ND	97.33 *	Azhdari et al. [35]
Iran	ND	9.67	13.1	13.1	ND	ND	Mohammadi-Moghaddam et al. [36]
Iran	ND	ND	11.30	15.08	ND	DB	Bakhshizadeh et al. [30]
Iran	ND	5.8	12.15	11.98	15.17	DB	Noruzi et al. [37]
Iran	40	5.7	16.6	12.7	25	DB	Bohluli et al. [31]
Mexico	63.37–67.70	3.67–4.89	8.78–10.22	10.79–11.98	ND	DB	Martínez-Ruíz et al. [13]
Türkiye	13.8	9.5	8.26	12.56	ND	71.05 (WB)	Özbek et al. [29]
Türkiye	ND	ND	7.27–14.99	8.50–19.86	18.25–22.49	DB	Boğa et al. [38]

DB = dry basis; WB = wet basis; ND = not determined; * = PGH extract.

Table 1. Nutritional content of PGH.

Country	Carbohydrates (%)	Fat (%)	Protein (%)	Ash (%)	Crude Fiber (%)	Moisture (%)	Reference
Tunisia	39.70	20.41	11.23	14.74	ND	10.46 (DB)	Hamed et al. [34]
Iran	ND	ND	0.24*	ND	ND	97.33 *	Azhdari et al. [35]
Iran	ND	9.67	13.1	13.1	ND	ND	Mohammadi-Moghaddam et al. [36]
Iran	ND	ND	11.30	15.08	ND	DB	Bakhshizadeh et al. [30]
Iran	ND	5.8	12.15	11.98	15.17	DB	Noruzi et al. [37]
Iran	40	5.7	16.6	12.7	25	DB	Bohluli et al. [31]
Mexico	63.37–67.70	3.67–4.89	8.78–10.22	10.79–11.98	ND	DB	Martínez-Ruíz et al. [13]
Türkiye	13.8	9.5	8.26	12.56	ND	71.05 (WB)	Özbek et al. [29]
Türkiye	ND	ND	7.27–14.99	8.50–19.86	18.25–22.49	DB	Boğa et al. [38]

DB = dry basis; WB = wet basis; ND = not determined; * = PGH extract.

Another essential product found in PGH is pectin, a valuable resource for the food industry, used as a gelling or emulsifying product [14]. Kazemi et al. [3] reported a pectin content of 18.13%, while Azhdari et al. [35] noted 4.53%. The first value aligns closely with other sources, such as orange and apple peels, indicating that PGH is a potential source of this compound [2].

Essential oils, products of plants’ secondary metabolism, are found in PGH. Shahdadi et al. [39] and Smeriglio et al. [40] identified essential oils from the PGH using gas chromatography coupled with mass spectrometry (GC-MS), revealing the presence of hydrocarbon monoterpenes, oxygenated monoterpenes, and sesquiterpenes, with α-pinene, limonene, 4-carene, α-terpinolene, δ-3-carene, L-bornyl acetate, camphene, and β-pinene as the most prominent compounds.

Despite previous discussion of PGH, the most notable component to draw researchers’ attention is the content of PCs [2,25].

5. Phenolic Compounds

The PCs are secondary metabolites produced by plants as defense mechanisms against environmental threats; however, they have gained attention for their benefits in human health [10]. These compounds usually contain one or more aromatic rings with hydroxyl groups attached and are synthesized via the shikimic acid pathway [41,42].

The PGH contains many PCs, ranging from small molecules such as gallic acid to complex molecules such as gallotannins (GLT). The PCs present in PGH belong to phenolic acids (PA), flavonoids (FLA), and tannins (TAN) groups [2].

The PAs are the simplest group of PCs, derived from hydroxycinnamic acids and hydroxybenzoic acids, respectively. Notable compounds in the hydroxybenzoic group are gallic acid, ellagic acid, and protocatechuic acid. The hydroxycinnamic acid group includes coumaric acid, caffeic acid, chlorogenic acid, and ferulic acid (Figure 3A) [43,44].

Conversely, TAN are divided into two types: hydrolyzable tannins (HT), which consist of glucose esters with gallic acids; and condensed tannins (CT), also called proanthocyanidins (PRAN), which are polymers of FLA (notably flavanols) linked by C-C bonds (see Figure 3C) [43,44]. Structurally, FLA has two aromatic rings and a single oxygenated cyclic ring. Their classification includes flavones, flavonols, flavanones, isoflavones, and anthocyanins (ANT) [43].

Flavonols, including myricetin, kaempferol, and quercetin, are among the most common FLA in various foods and are often attached to sugars such as rhamnose or glucose. Flavanols, represented by catechin and epicatechin, undergo polymerization to CT. Flavones, which are present in lower amounts, typically exist as glycosides of apigenin or luteolin. Conversely, isoflavones like genistein and daidzein act as phytoestrogens. Flavanones, including naringenin, eriodictyol, and hesperidin, mostly appear in citrus fruits. Additionally, ANT such as cyanidin, delphinidin, pelargonidin, and malvidin are among the most studied FLA because of their essential role in food coloration (Figure 3B) [43,44,45].

Figure 3. Classification of PCs from PGH [46]. (A) Phenolic acids; (B) Flavonoids; and (C) Tannins.

Many of these PCs are in PGH and can be recovered through various extraction methods, giving this agro-industrial by-product a useful purpose.

5.1. Extraction of Phenolic Compounds from Pistachio Green Hull

Recently, PGH has become more important as an agro-industrial by-product in the Mediterranean and parts of Asia, prompting research into its phytochemical composition. The standard extraction methods for obtaining PCs from PGH include chemical maceration with ethanol, methanol, acetonitrile, acetone, butanol, and distilled water, as well as alternative techniques like ultrasound, microwave, solid-state fermentation, and enzyme-assisted fermentation (Table 2).

The extraction method used can significantly influence the yield or presence of different PC groups. Therefore, selecting the appropriate extraction method is crucial. This includes the collection of raw materials, pretreatment, PCs extraction using different solvents, and subsequent characterization or identification.

The PCs in PGH are important due to their proven biological activities, including antioxidant and antibacterial effects. As a result, these PCs could be used in various sectors of the consumer goods industry (Figure 4).

Figure 4. Process flow diagram for the extraction of PCs from PGH.

Maceration is the most conventional method in PC studies from PGH; this process involves soaking the plant material in the selected solvent at room temperature for a designed period, sometimes with agitation [43,47]. Additionally, ultrasound-assisted extraction is another technique used in PGH for PCs extraction, producing small bubbles that explode and break down the plant matrix, thereby enhancing the release of PCs and their solubility in the solvent [43]. Microwave-assisted extraction has also been used in PGH research; this method heats the plant matrix and solvent without creating a thermal gradient, thus facilitating the breaking of the bonds and release of PCs [47].

Table 2. Extraction techniques employed in the recovery of PCs from PGH.

Extraction Technique	Yield	Cost/ Effectiveness	Environmental Impact	Scaling-Up	References
Decoction	24.54% (TPC)	Low cost/ Moderate efficiency	Moderate-high energy consumption	Highly scalable with low maintenance and instrumentation cost	Shahdadi et al. [39]
Maceration	0.0001–81% (TPC) 0.03–32.66% (FLA) 2.31–7.91% (TAN) 0.001–0.71% (CT) 1.82–11.86% (ANT) 0.99% (NTAN) 0.009–0.018% (PRAN)	Low cost/ Low-Very high efficiency	Large volume of solvent required	Scale-up easily controlled	Farrokhi et al. [48] Roudbari et al. [5] Karaogul and Ugurtay [49] Shakerardekani et al. [50] Noruzi et al. [37] Moreno-Rojas et al. [51] Elhadef et al. [52] Ozay et al. [53] Pakdaman et al. [54] Noorolahi et al. [55] Rafiee et al. [56] Barreca et al. [21] Grace et al. [33] Tabaraki and Ghadiri [57] Rajaei et al. [58] Goli et al. [59]
Soxhlet	0.024% (TPC)	Moderate cost/ Low efficiency	High energy consumption	Scale-up involves a high solvent consumption and large waste production	Kepekci et al. [60]
Microwave-Assisted Extraction	1.47–20% (TPC) 0.29–7.5% (FLA)	High maintenance cost/ Moderate-high Efficiency	Reduced energy consumption	Highly scalable by reducing energy consumption and solvent use	Seker and Akbas [61] Özbek et al. [29] Özbek et al. [10] Garavand et al. [4] Tabaraki and Ghadiri [57]
Ultrasound-Assisted Extraction	0.5–22% (TPC) 0.34–11.98% (FLA) 1.06% (GLT) 6.75% (ANA)	Moderate-high maintenance cost/ Moderate-high Efficiency	Reduced energy consumption	Highly scalable by reducing energy consumption and solvent use	Elakremi et al. [62] Erşan et al. [63] Garavand et al. [4] Tabaraki and Ghadiri [57] Goli et al. [59]
Atmospheric Cold-Plasma-Assisted	1.8% (TPC) 39.09% (FLA)	High initial setup and operating cost/ High efficiency	Low energy consumption	Expensive scale-up by initial investment and maintenance cost	Farrokhi et al. [48]
Ohmic Heating- Assisted	2.05% (TPC) 48.41% (FLA)	Low maintenance cost/ High efficiency	Environmentally friendly process	Scale-up involves adaptations for efficient energy Utilization	Farrokhi et al. [48]
Subcritical fluids	0.66–3.95% (TPC) 0.07–0.57% (FLA) 2.04–3.31% (GLT) 0.11–0.28% (ANA)	High cost/ Low efficiency	Environmental and safety issues caused by high pressure	Expensive scale-up by equipment and conditions	Erşan et al. [63] Goli et al. [59]
Enzymatic- Assisted Extraction	3.21–10.1% (TPC) 3.66–3.69% (FLA) 2.23–2.91% (TAN) 5.51–6.24% (HT) 0.47–12.11% (GA) 2.88–3.57% (PHG) 0.22% (NRG)	High cost/ Moderate Efficiency	Environmentally friendly process	Expensive scale-up by cost and availability of enzymes	Ghandehari-Yazdi et al. [64] Ghandahari-Yazdi et al. [65] Azhdari et al. [35]
Solid-State Fermentation- Assisted Extraction	0.054–6.3% (TPC) 0.019–0.18% (FLA) 3.47% (HT) 0.68% (CT)	Low cost/ Low-moderate Efficiency	Environmentally friendly process	Scale-up involves adaptations for reactors and operating conditions	Ordoñez-Cano et al. [66] Karimi et al. [67] Abbasi et al. [68]

For detailed information by study, see Table S1. TPC = total phenolic compounds; FLA = flavonoids; GA = gallic acid; PHG = phloroglucinol; NRG = naringin; TAN = tannins; HT = hydrolyzable tannins; CT = condensed tannins; ANT = anthocyanins; NTAN = no tannins; PRAN = proanthocyanidins; GLT = gallotannins; ANA = anacardic acids. Yield (%) = (mg PCs/g of sample) × 100.

Table 2. Extraction techniques employed in the recovery of PCs from PGH.

Extraction Technique	Yield	Cost/ Effectiveness	Environmental Impact	Scaling-Up	References
Decoction	24.54% (TPC)	Low cost/ Moderate efficiency	Moderate-high energy consumption	Highly scalable with low maintenance and instrumentation cost	Shahdadi et al. [39]
Maceration	0.0001–81% (TPC) 0.03–32.66% (FLA) 2.31–7.91% (TAN) 0.001–0.71% (CT) 1.82–11.86% (ANT) 0.99% (NTAN) 0.009–0.018% (PRAN)	Low cost/ Low-Very high efficiency	Large volume of solvent required	Scale-up easily controlled	Farrokhi et al. [48] Roudbari et al. [5] Karaogul and Ugurtay [49] Shakerardekani et al. [50] Noruzi et al. [37] Moreno-Rojas et al. [51] Elhadef et al. [52] Ozay et al. [53] Pakdaman et al. [54] Noorolahi et al. [55] Rafiee et al. [56] Barreca et al. [21] Grace et al. [33] Tabaraki and Ghadiri [57] Rajaei et al. [58] Goli et al. [59]
Soxhlet	0.024% (TPC)	Moderate cost/ Low efficiency	High energy consumption	Scale-up involves a high solvent consumption and large waste production	Kepekci et al. [60]
Microwave-Assisted Extraction	1.47–20% (TPC) 0.29–7.5% (FLA)	High maintenance cost/ Moderate-high Efficiency	Reduced energy consumption	Highly scalable by reducing energy consumption and solvent use	Seker and Akbas [61] Özbek et al. [29] Özbek et al. [10] Garavand et al. [4] Tabaraki and Ghadiri [57]
Ultrasound-Assisted Extraction	0.5–22% (TPC) 0.34–11.98% (FLA) 1.06% (GLT) 6.75% (ANA)	Moderate-high maintenance cost/ Moderate-high Efficiency	Reduced energy consumption	Highly scalable by reducing energy consumption and solvent use	Elakremi et al. [62] Erşan et al. [63] Garavand et al. [4] Tabaraki and Ghadiri [57] Goli et al. [59]
Atmospheric Cold-Plasma-Assisted	1.8% (TPC) 39.09% (FLA)	High initial setup and operating cost/ High efficiency	Low energy consumption	Expensive scale-up by initial investment and maintenance cost	Farrokhi et al. [48]
Ohmic Heating- Assisted	2.05% (TPC) 48.41% (FLA)	Low maintenance cost/ High efficiency	Environmentally friendly process	Scale-up involves adaptations for efficient energy Utilization	Farrokhi et al. [48]
Subcritical fluids	0.66–3.95% (TPC) 0.07–0.57% (FLA) 2.04–3.31% (GLT) 0.11–0.28% (ANA)	High cost/ Low efficiency	Environmental and safety issues caused by high pressure	Expensive scale-up by equipment and conditions	Erşan et al. [63] Goli et al. [59]
Enzymatic- Assisted Extraction	3.21–10.1% (TPC) 3.66–3.69% (FLA) 2.23–2.91% (TAN) 5.51–6.24% (HT) 0.47–12.11% (GA) 2.88–3.57% (PHG) 0.22% (NRG)	High cost/ Moderate Efficiency	Environmentally friendly process	Expensive scale-up by cost and availability of enzymes	Ghandehari-Yazdi et al. [64] Ghandahari-Yazdi et al. [65] Azhdari et al. [35]
Solid-State Fermentation- Assisted Extraction	0.054–6.3% (TPC) 0.019–0.18% (FLA) 3.47% (HT) 0.68% (CT)	Low cost/ Low-moderate Efficiency	Environmentally friendly process	Scale-up involves adaptations for reactors and operating conditions	Ordoñez-Cano et al. [66] Karimi et al. [67] Abbasi et al. [68]

For detailed information by study, see Table S1. TPC = total phenolic compounds; FLA = flavonoids; GA = gallic acid; PHG = phloroglucinol; NRG = naringin; TAN = tannins; HT = hydrolyzable tannins; CT = condensed tannins; ANT = anthocyanins; NTAN = no tannins; PRAN = proanthocyanidins; GLT = gallotannins; ANA = anacardic acids. Yield (%) = (mg PCs/g of sample) × 100.

In this context, Tabaraki and Ghadiri [57] compared the extraction of PCs using maceration, ultrasound, and microwave methods under optimal conditions. Furthermore, they reported yields of 4.59%, 5.81%, and 6.26%, respectively, with the microwave extraction providing the highest recovery of compounds. Garavand et al. [4] found that both ultrasound and microwaves increased the content of PCs compared to the control. The optimal conditions were ultrasound at 35 kHz and microwaves at 60% and 80% of potency. They also reported that distilled water yielded a higher PCs content due to its high polarity and the compounds’ good water solubility in PGH. Additionally, Elhadef et al. [52] evaluated the effectiveness of hexane, acetone, acetonitrile, and distilled water as solvents, reporting total phenolic compound (TPC) yields of 14.12%, 16.77%, 17.94%, and 19.73%, respectively. These findings suggest that distilled water is the most effective solvent for extracting PCs.

It is noteworthy that substantial efforts have been undertaken to recover PCs employing environmentally sustainable technologies, notably through solid-state fermentation and enzyme-assisted extraction, as documented in studies on PGH. These technologies help mitigate environmental impact by reducing the use of low process temperatures (30 °C), lowering energy consumption, and using solvents in smaller quantities, such as distilled water, thereby reducing the water footprint. Solid-state fermentation offers the potential to lower costs by enabling the broad spectrum of enzymes synthesized during the bioprocesses, which represents an advantage for heterogeneous substrates. Compared to SSF, enzyme-assisted extraction can be expensive due to the use of specific and purified enzymes [69].

Research by Ordoñez-Cano et al. [66], Karimi et al. [67], and Abbasi et al. [68] shows that using fungi in solid-state fermentation can increase the release of PCs; however, Karimi et al. [67] observed a decrease compared with the unfermented control. Additionally, Azhdari et al. [35], Ghandehari-Yazdi et al. [64], and Ghandehari-Yazdi [65] investigated the use of commercial enzymes such as tannase, pectinase, and cellulase for PCs extraction, highlighting the effectiveness of these methods in recovering the target compounds.

Solid-state fermentation involves growing microorganisms in a limited water environment, often using fungi as enzyme producers [70]. Both solid-state fermentation and enzyme-assisted extraction aim to break down plant material components to release bound phytochemicals [43,70]. The main difference is that enzyme-assisted extraction uses isolated enzymes, while solid-state fermentation relies on enzymes produced by microorganisms during the culture.

Farrokhi et al. [48] compared emerging extraction techniques, including atmospheric cold-plasma-assisted and ohmic heating-assisted methods, with the maceration technique. They observed that both methods produced more than twice the extraction yield, with ohmic heating showing the best results (2.05% for TPC and 48.41% for FLA). Meanwhile, Erşan et al. [63] noted that although the subcritical water method recovered a greater amount of phenolic compounds than ultrasound with methanol as the solvent, ultrasound extraction resulted in a higher yield of phenolic lipids.

All studies indicate that PGH can serve as a potential source of PCs, which can be extracted using different techniques and solvents. The choice of extraction method is crucial and should be tailored to the specific target (PA, FLA, or TAN). To maximize compound recovery, it is essential to employ appropriate techniques and select the correct solvent. For example, regarding solvents, it has been observed that TPC from PGH shows affinity for distilled water, while FLA shows affinity for organic solvents (ethanol and methanol). Additionally, accurately quantifying the extracted compounds is crucial to evaluating the effectiveness of the extraction technique.

For industrial applications, ultrasound-assisted extraction has become feasible through a cost-effective system capable of managing large-scale solvent volumes. Similarly, subcritical water extraction is applicable in industrial settings, as it is environmentally friendly due to its exclusive use of water and the absence of toxic solvents [71]. Although maceration is a conventional technique with highly variable extraction efficiency in PGH, it is one of the most widely used due to its easy scalability and the diversity of solvents available for PC recovery.

5.2. Quantification of Phenolic Compounds in Pistachio Green Hull

Research on PGH shows that this by-product constitutes a valuable source of PCs due to its high yield, favorable quantity, and potential to develop diverse products. Table 3 presents the methods used to quantify the concentration of PCs in PGH.

Quantifying PCs is essential for evaluating extraction methods and the importance of these compounds in research and product development. The Folin–Ciocalteu assay is the most commonly used technique for measuring TPC. It involves reducing the Folin–Ciocalteu reagent in an alkaline environment, producing blue chromophores from molybdenum ions, which can be detected at 765 nm [72]. Two primary methods are used to measure CT or PRAN. The first is the Butanol-HCl method, which depolymerizes PRAN into monomers using butanol-HCl, a ferric agent, and heat. The second method involves the vanillin assay, where a complex forms between vanillin and CT at low pH (caused by hydrochloric or sulfuric acid, and vanillin in methanol), producing red chromophores that can be detected at 500 nm [71,73]. On the other hand, FLA are usually measured using the aluminum chloride method, which uses their ability to chelate metals, especially aluminum, forming complexes with yellow chromophores detectable between 410 and 440 nm [73].

Although spectrophotometry helped identify various groups of PCs, such as FLA and CT, current research is concentrating on the quantitative analysis of specific compounds using alternative analytical methods [41]. For example, Ghandahari-Yazdi et al. [65] measured gallic acid, phloroglucinol, naringin, protocatechuic acid, catechin, and vanillic acid with high-pressure liquid chromatography coupled with a photodiode array detector (HPLC-DAD) in PGH samples.

Table 3. Studies reporting quantification of PCs in PGH.

Component	Content	Unit	References
TPC	0.0002–810	mg GAE g dm−1	Karaogul and Ugurtay [49]; Noruzi et al. [37]; Roudbari et al. [5]; Seker and Akbas [61]; Ghandehari-Yazdi et al. [64]; Pakdaman et al. [54]; Noorolahi et al. [55]; Özbek et al. [29]; Garavand et al. [4]; Tabaraki and Ghadiri [57]; Karimi et al. [67]; Rajaei et al. [58]; Kepekci et al. [60]; Farrokhi et al. [48]; Azhdari et al. [35]; Elhadef et al. [52]; Özbek et al. [10]; Grace et al. [33]
	0.1–41.48	mg PCs g dm−1	Ordoñez-Cano et al. [66]; Shakerardekani et al. [50]; Ozay et al. [53]
	245.43	mg GAE mL of extract−1	Shahdadi et al. [39]
	1.46–5.92	mmol GAE 100 g fm−1	Moreno-Rojas et al. [51]
	218.97	mg GAE g de−1	Elakremi et al. [62]
	22.2–81.8	g PCs kg dm−1	Erşan et al. [63]
	163.3–614.9	mg GAE g fe−1	Rafiee et al. [56]
	6.74–11.7	µM GAE g fw−1	Barreca et al. [21]
	~49–63	mg CAE g dm−1	Abbasi et al. [68]
	5.02–34.7	mg TAE g dw−1	Goli et al. [59]
FLA	2.22–484.1	mg QE g dm−1	Karaogul and Ugurtay [49]; Seker and Akbas [61]; Farrokhi et al. [48]; Elhadef et al. [52]; Grace et al. [33]
	30.46–85	mg CE g dm−1	Noruzi et al. [37]; Noorolahi et al. [55]; Garavand et al. [4]; Azhdari et al. [35]
	0.34–0.688	mg QE g fm−1	Shakerardekani et al. [50]; Barreca et al. [21]
	119.75	mg CE g de−1	Elakremi et al. [62]
	0.7–5.65	g FLA kg dm−1	Erşan et al. [63]
	27.4–73.3	mg CE g fe−1	Rafiee et al. [56]
	0.186–1.855	mg RE g dm−1	Karimi et al. [67]
CT	7.07	mg CT g dm−1	Karaogul and Ugurtay [49]
	0.32–6.77	mg CE g dm−1	Ordoñez-Cano et al. [66]; Elhadef et al. [52]
	0.013–0.071	mg CE g fm−1	Barreca et al. [21]
HT	34.71–62.35	mg GAE g dm−1	Azhdari et al. [35]; Ordoñez-Cano et al. [66]
ANT	18.21–40.98	µg C-3-O-glu g dm−1	Elhadef et al. [52]
	35.5–118.6	µg Cy-3-g g fe−1	Rafiee et al. [56]
PRAN	0.088–0.177	mg CE g fm−1	Barreca et al. [21]
TAN	23.14–32.03	mg GAE g dm−1	Noruzi et al. [37]; Noorolahi et al. [55]
	22.29–29.09	mg TAE g dm−1	Azhdari et al. [35]
NTAN	9.93	mg GAE g dm−1	Noorolahi et al. [55]
ANA	1.13–67.5	g ANA kg dm−1	Erşan et al. [63]
GLT	10.6–33.1	g GLT kg dm−1	Erşan et al. [63]
Gallic Acid	4.69–121.10	mg g de−1	Ghandahari-Yazdi et al. [65]
Phloroglucinol	28.82–35.71	mg g de−1	Ghandahari-Yazdi et al. [65]
Naringin	2.21	mg g de−1	Ghandahari-Yazdi et al. [65]

For detailed information by study, see Table S2. TPC = total phenolic compounds; TAN = tannins; FLA = flavonoids; NTAN = no tannins; HT = hydrolyzable tannins; CT = condensed tannins; ANT = anthocyanins; PRAN = proanthocyanidins; ANA = anacardic acids; GLT = gallotannins; mg GAE = milligrams of gallic acid equivalents; mg QE = milligrams of quercetin equivalents; mg CE = milligrams of catechin equivalents; mg PCs = milligrams of phenolic compounds; mg TAE = milligrams of tannic acid equivalents; µg C-3-O-glu = micrograms of cyanidin-3-O-glucoside; µM GAE = micromoles of gallic acid equivalents; µg Cy-3-g = micrograms of cyanidin-3-glucoside; mg RE = milligrams of rutin equivalents; mg CAE = milligrams of caffeic acid equivalents; g dm = gram of dry mass; mg dw = milligram of dry weight; g fm = gram of fresh mass; g de = gram of dry extract; g de = gram of ground dry extract; kg dm = kilogram of dry mass; g fe = gram of fresh extract; g dw = gram of dry weight.

Table 3. Studies reporting quantification of PCs in PGH.

Component	Content	Unit	References
TPC	0.0002–810	mg GAE g dm−1	Karaogul and Ugurtay [49]; Noruzi et al. [37]; Roudbari et al. [5]; Seker and Akbas [61]; Ghandehari-Yazdi et al. [64]; Pakdaman et al. [54]; Noorolahi et al. [55]; Özbek et al. [29]; Garavand et al. [4]; Tabaraki and Ghadiri [57]; Karimi et al. [67]; Rajaei et al. [58]; Kepekci et al. [60]; Farrokhi et al. [48]; Azhdari et al. [35]; Elhadef et al. [52]; Özbek et al. [10]; Grace et al. [33]
	0.1–41.48	mg PCs g dm−1	Ordoñez-Cano et al. [66]; Shakerardekani et al. [50]; Ozay et al. [53]
	245.43	mg GAE mL of extract−1	Shahdadi et al. [39]
	1.46–5.92	mmol GAE 100 g fm−1	Moreno-Rojas et al. [51]
	218.97	mg GAE g de−1	Elakremi et al. [62]
	22.2–81.8	g PCs kg dm−1	Erşan et al. [63]
	163.3–614.9	mg GAE g fe−1	Rafiee et al. [56]
	6.74–11.7	µM GAE g fw−1	Barreca et al. [21]
	~49–63	mg CAE g dm−1	Abbasi et al. [68]
	5.02–34.7	mg TAE g dw−1	Goli et al. [59]
FLA	2.22–484.1	mg QE g dm−1	Karaogul and Ugurtay [49]; Seker and Akbas [61]; Farrokhi et al. [48]; Elhadef et al. [52]; Grace et al. [33]
	30.46–85	mg CE g dm−1	Noruzi et al. [37]; Noorolahi et al. [55]; Garavand et al. [4]; Azhdari et al. [35]
	0.34–0.688	mg QE g fm−1	Shakerardekani et al. [50]; Barreca et al. [21]
	119.75	mg CE g de−1	Elakremi et al. [62]
	0.7–5.65	g FLA kg dm−1	Erşan et al. [63]
	27.4–73.3	mg CE g fe−1	Rafiee et al. [56]
	0.186–1.855	mg RE g dm−1	Karimi et al. [67]
CT	7.07	mg CT g dm−1	Karaogul and Ugurtay [49]
	0.32–6.77	mg CE g dm−1	Ordoñez-Cano et al. [66]; Elhadef et al. [52]
	0.013–0.071	mg CE g fm−1	Barreca et al. [21]
HT	34.71–62.35	mg GAE g dm−1	Azhdari et al. [35]; Ordoñez-Cano et al. [66]
ANT	18.21–40.98	µg C-3-O-glu g dm−1	Elhadef et al. [52]
	35.5–118.6	µg Cy-3-g g fe−1	Rafiee et al. [56]
PRAN	0.088–0.177	mg CE g fm−1	Barreca et al. [21]
TAN	23.14–32.03	mg GAE g dm−1	Noruzi et al. [37]; Noorolahi et al. [55]
	22.29–29.09	mg TAE g dm−1	Azhdari et al. [35]
NTAN	9.93	mg GAE g dm−1	Noorolahi et al. [55]
ANA	1.13–67.5	g ANA kg dm−1	Erşan et al. [63]
GLT	10.6–33.1	g GLT kg dm−1	Erşan et al. [63]
Gallic Acid	4.69–121.10	mg g de−1	Ghandahari-Yazdi et al. [65]
Phloroglucinol	28.82–35.71	mg g de−1	Ghandahari-Yazdi et al. [65]
Naringin	2.21	mg g de−1	Ghandahari-Yazdi et al. [65]

For detailed information by study, see Table S2. TPC = total phenolic compounds; TAN = tannins; FLA = flavonoids; NTAN = no tannins; HT = hydrolyzable tannins; CT = condensed tannins; ANT = anthocyanins; PRAN = proanthocyanidins; ANA = anacardic acids; GLT = gallotannins; mg GAE = milligrams of gallic acid equivalents; mg QE = milligrams of quercetin equivalents; mg CE = milligrams of catechin equivalents; mg PCs = milligrams of phenolic compounds; mg TAE = milligrams of tannic acid equivalents; µg C-3-O-glu = micrograms of cyanidin-3-O-glucoside; µM GAE = micromoles of gallic acid equivalents; µg Cy-3-g = micrograms of cyanidin-3-glucoside; mg RE = milligrams of rutin equivalents; mg CAE = milligrams of caffeic acid equivalents; g dm = gram of dry mass; mg dw = milligram of dry weight; g fm = gram of fresh mass; g de = gram of dry extract; g de = gram of ground dry extract; kg dm = kilogram of dry mass; g fe = gram of fresh extract; g dw = gram of dry weight.

To quantify the PCs in PGH, understanding their composition is an essential key for selecting the right appropriate analytical technique. The choice of solvents is also crucial since some compounds dissolve better in water while others in organic solvents, depending on their polarity. Ozay et al. [53] evaluated five solvents for the extraction of PCs from PGH using the Folin–Ciocalteu method. They found distilled water was most effective, yielding 17.6 mg of PCs g⁻¹, followed by methanol (~8 mg g⁻¹), ethanol (~3.5 mg g⁻¹), acetone (~3 mg g⁻¹), and hexane (~0.1 mg g⁻¹). Similarly, Elhadef et al. [52] evaluated PCs extracts using acetonitrile, hexane, distilled water, and acetone. Once again, distilled water proved most effective for TPC, demonstrating significant efficacy for TPC (132.14–197.25 mg GAE g⁻¹), FLA (18.07–23.29 mg QE g⁻¹), TAN (46.24–79.06 mg CE g⁻¹), and ANT (27.21–40.98 µg C-3-O-glu g⁻¹).

Significant variations in reported values were attributed to the evaluation of three different PGH varieties: Sfax, Sidi Bouzid, and Gafsa. In another study, Rajaei et al. [58] found that distilled water was the most effective solvent for TPC, yielding 49.32 mg GAE g⁻¹, whereas solvent mixtures such as ethanol/water/acetic acid, ethanol/water, and methanol/water/acetic acid produced values near 40 mg GAE g⁻¹. In contrast, Erşan et al. [74] reported that a methanol/water (80/20 v/v) mixture increased the concentration of compounds by enhancing their solubility across a range of polarities, including the low-polarity anacardic acids (ANA). They noted that using only organic solvents, such as hexane, acetone, and diethyl ether, would mainly extract low-polarity compounds, whereas adding water would allow extraction of higher-polarity compounds, such as gallic acid and quercetin. Additionally, a mixture of solvents, such as ethanol, methanol, and acetone, with water can extract a wider range of compounds, including FLA, GLT, and ANA.

Because of the long-standing methods used to quantify compounds by groups, accurately identifying specific compounds in the plant matrix has become essential, along with linking their benefits to their presence.

5.3. Identification of Phenolic Compounds in Pistachio Green Hull

The PCs are valuable because of their biological properties, including antioxidant, antimicrobial, antimutagenic, anticancer, and antitumor activities [44]. However, the specific compounds responsible for these effects are still being studied. Table 4 presents studies on PGH that identified PCs.

Table 4. Identification studies of PCs in PGH.

Technique	Main Compounds	Reference
LC-DAD-ESI-MS/MS	Gallic acid, protocatechuic acid, catechin, rutin, eriodictol-7-O-glucoside, quercetin and naringenin	Farrokhi et al. [48]
RP-HPLC-ESI-MS	Gallic acid 4-O-glucoside and geraniin	Ordoñez-Cano et al. [66]
LC-MS/MS	Tannic acid, gallic acid, protocatechuic acid, isoquercetin, miquelianin, ellagic acid, p-coumaric acid, rutin, hesperidin, quercetin, naringenin, luteolin, kaempferol and apigenin	Kepekci et al. [60]
GC	Gallic acid, protocatechuic acid, vanillic acid, chlorogenic acid, naringenin, naringin, quercetin, quercetin-3-galacatoside, luteolin, apigenin, catechin and epicatechin	Shakerardekani et al. [50]
HPLC	Catechin and gallic acid	Shirzadi-Ahodashti et al. [75]
LC-MS/MS	Gallic acid, caffeic acid, cinnamic acid, ferulic acid, gentisic acid, coumaric acid, vanillic acid, syringic acid and succinic acid	Ozay et al. [53]
HPLC/DAD	Phloroglucinol, gallic acid, protocatechuic acid, catechin, vanillic acid and naringin	Ghandahari-Yazdi et al. [65]
HPLC-DAD-ESI/MSn	Gallic acid, digallic acid, protocatechuic acid, galloyl- shikimic acid, penta-O-galloyl-β-D-glucose, quercetin 3-O-glucoside, anacardic acids and myricetin hexoxide	Erşan et al. [63]
RP-HPLC-DAD	Phloroglucinol, gallic acid, naringenin, vanillic acid, catechin and protocatechuic acid	Fattahifar et al. [76]
HPLC-DAD-ESI/MSn and UHPLC-DAD-ELSD	Gallic acid, galloyl-shikimic acid, protocatechuic acid, digallic acid, cyanidin-3-O-β-D-galactopyranoside, luteic acid, myricetin 3-O-galactoside, quercetin 3-O-glucoside, kaempferol hexoxide and anacardic acids.	Erşan et al. [74]
HPLC-DAD-FLD-MS	Gallic acid, phloroglucinol, vanillic acid, p-coumaric acid, sinapic acid, naringenin and catechin	Garavand et al. [4]
RP-HPLC-DAD-FLU	Gallic acid, protocatechuic acid, vanillic acid, chlorogenic acid, eriodictyol-7-O-glucoside, naringenin, quercetin, kaempferol, luteolin, apigenin, catechin and epicatechin	Barreca et al. [21]
HPLC-DAD-ESI-MSn and HPLC-ESI-HR-MS	Gallic acid, protocatechuic acid, cyanidin 3-O-β-D- galactopyranoside, luteolin, quercetin 3-O-glucuronide, kaempferol hexoside, anacardic acids, methyl digallate and myricetin galloyl hexoxide	Erşan et al. [77]
GC–MS and HPLC-MS-IT-TOF	Gallic acid, luteolin, quercetin, catechin gallate, myricetin gallate, digallic acid, myricetin-3-glucoside, galloylshikimic acid, quercetin-3-glucoronide, tetragalloyl hexoxide and anacardic acids.	Grace et al. [33]

LC-DAD-ESI-MS/MS = Liquid chromatography with electrospray ionization coupled to tandem mass spectrometry and photodiode array detector; RP-HPLC-ESI-MS = Reverse phase high-pressure liquid chromatography with electrospray ionization coupled to a mass detector; LC-MS/MS = Liquid chromatography coupled to a tandem mass detector; GC = Gas chromatography; HPLC = High-pressure liquid chromatography; HPLC/DAD = High-pressure liquid chromatography coupled to photodiode array detector; HPLC-DAD-ESI/MSⁿ = High-pressure liquid chromatography with electrospray ionization, photodiode array detector and coupled to mass spectrometry; RP-HPLC-DAD = High-pressure liquid chromatography in reversed phase with photodiode array detector; UHPLC-DAD-ELSD = Ultra-pressure liquid chromatography coupled to a photodiode detector and evaporative light scattering detector; HPLC-DAD-FLD-MS = High-pressure liquid chromatography with photodiode array and fluorescence detector coupled to mass spectrometry; RP-HPLC-DAD-FLU = High-pressure liquid chromatography in reversed phase with photodiode array and fluorescence detector; HPLC-ESI-HR-MS = High-pressure liquid chromatography with electrospray ionization coupled to high-resolution mass spectrometry; GC–MS = Gas chromatography coupled to mass spectroscopy; HPLC-MS-IT-TOF = High-pressure liquid chromatography with electrospray ionization coupled to ion trap mass spectrometry and time-of-flight mass spectrometry.

The studies on polyphenols from PGH by-products mainly used HPLC techniques coupled with different array detectors and mass spectrometry. However, Shakerardekani et al. [50] used GC to identify compounds in PGH. Similarly, Grace et al. [33] used both GC and HPLC.

Table 5 lists the techniques used to provide information on PCs identification in PGH, but the analysis’s objective and the nature of the sample must be established for correct identification.

Table 5. Identification techniques comparation.

Identification Technique	Sensitivity	Cost	Suitability
LC	High with MS detector	Low-moderate by solvents used	Phenolic compounds
HPLC	High-very high with detectors	Moderate-high by solvents used and maintenance cost	Large, volatile and non-volatile compounds
UHPLC	Very high with detectors	High initial and maintenance cost	Highly complex samples
GC	High with detectors	Low cost by inert gases and small volumes	Volatile compounds

LC = Liquid chromatography; HPLC = High-pressure liquid chromatography; UHPLC = Ultra-pressure liquid chromatography; GC = Gas chromatography.

All studies reported a similar profile of compounds, highlighting gallic acid, quercetin, luteolin, apigenin, catechin, vanillic acid, naringenin, and phloroglucinol. In the study by Ordoñez-Cano et al. [66], only gallic acid 4-O-glucoside and geraniin were identified, likely due to the use of XAD-16 Amberlite for sample purification, which may have limited the detectable compounds.

Similarly, Shirzadi-Ahodashti et al. [75] reported the detection of catechin and gallic acid, but did not specify the purification methods used. This observation is noteworthy, as Kepekci et al. [60], Shakerardekani et al. [50], Erşan et al. [63,74,77], Barreca et al. [21], and Grace et al. [33] utilized methanolic extracts, which revealed a wider range of compounds, including gallic acid, catechin, quercetin, luteolin, kaempferol, and apigenin. The authors observed that PGH compounds have a strong tendency to bind with organic solvents, especially FLA. Additionally, some studies have found compounds in water-based extracts; for example, Ozay et al. [53] identified ten phenolic compounds in PGH’s water extract. Notably, all these compounds were PA, such as gallic acid, caffeic acid, cinnamic acid, coumaric acid, and vanillic acid.

Conversely, Erşan et al. [63,74,77] and Grace et al. [33] documented the presence of ANA, which are phenolic lipids serving as taxonomic markers within the Anacardiaceae family [2]. These compounds were identified in the fruit husk of this family, particularly in species of the genus Pistacia. ANA are known to confer health benefits, including the prevention of cardiovascular diseases, and exhibit antioxidant, antibacterial, and anticancer activities [78]. Variations in PCs are associated with factors such as pistachio variety, climate, extraction methodologies and conditions, as well as geographical location, as previously discussed [76].

Furthermore, PGH contains HT such as gallic acid-4-O-glucoside or pentagalloyl glucoside, and ellagitannins including geraniin, which are known for their anticancer, antioxidant, antitumor, antibacterial, and antidiabetic properties [79]. Additionally, CT and polymers of FLA, such as catechin, exhibit similar biological activities [80].

5.4. Biological Activities of Pistachio Green Hull

5.4.1. Antioxidant Activity

The PA accounts for one-third of all PCs and has attracted attention for its antioxidant activity [44]. This activity results from phenolic rings that capture free radicals via their hydroxyl groups [44,81]. Notably, the antioxidant properties of PGH have been extensively studied (Table 6).

Table 6. Studies on the antioxidant capacity of extracts derived from PGH.

Type of Assay	Antioxidant Activity	Unit	Reference
ABTS	~72–92	%	Farrokhi et al. [48]
	132.75–466.73	mg TE g dm−1	Ordoñez-Cano et al. [66]; Noorolahi et al. [55]
	427.06	mg TE g de−1	Elakremi et al. [62]
	3.32	mg AAE mg PCs−1	Rajaei et al. [58]
	~37–50	% free radical inhibition 80 mg/L−1	Ghandahari-Yazdi et al. [65]
	0.47–1.18	mmol TE g dm−1	Erşan et al. [61]
	1.28–3.53	mmol TE 100 g fm−1	Moreno-Rojas et al. [51]
ABTS (IC50)	1.2278	mg mL−1	Karaogul and Ugurtay [49]
ABTS (EC50)	0.09–0.91	mg mL−1	Elhadef et al. [52]
DPPH	~50–90	%	Farrokhi et al. [48]; Pakdaman et al. [54]; Tabaraki and Ghadiri [57]; Abbasi et al. [68]
	131.68–411.98	mg TE g dm−1	Ordoñez-Cano et al. [66]; Noorolahi et al. [55]
	332.92	mg TE g de−1	Elakremi et al. [62]
	~87–95	% free radical inhibition 80 mg/L−1	Ghandahari-Yazdi et al. [65]
	31.2–50.4	% free radical inhibition 300 µg dm−1	Karimi et al. [67]
	0.51–0.84	mmol TE g dm−1	Erşan et al. [63]
	1.28–3.53	mmol TE 100 g fm−1	Moreno-Rojas et al. [51]
DPPH (IC50)	0.003–4.49	mg mL−1	Karaogul and Ugurtay [49]; Kepekci et al. [60] Roudbari et al. [5]; Azhdari et al. [35]; Özbek et al. [29]; Özbek et al. [10]
	206.32	µL L−1	Shahdadi et al. [39]
DPPH (SC50)	0.022	mg mL−1	Benli and Bahtiyari [82]
DPPH (EC50)	2.53	µg PCs mL DPPH−1	Rajaei et al. [58]
	0.025–1.5	mg mL−1	Elhadef et al. [52]; Ghandehari-Yazdi et al. [64]; Garavand et al. [4]
FRAP	504.59–2230.8	mg Fe+2 g dm−1	Ordoñez-Cano et al. [66]; Noorolahi et al. [55]
	517.96	mg TE g de−1	Elakremi et al. [62]
	~130–350	µM Fe+2 80 mg/L−1	Ghandahari-Yazdi et al. [65]
	0.49–1.20	mmol TE g dm−1	Erşan et al. [63]
	1.111	µmol TE mg ext−1	Benli and Bahtiyari [82]
	380.4–508.8	µmol Fe+2 g dm−1	Tabaraki and Ghadiri [57]
	2.36–6.04	µM TE 100 g fm−1	Barreca et al. [21]
ORAC	3.5–15.8	mmol TE 100 g fm−1	Moreno-Rojas et al. [51]
	~50–250	µmol TE g dm−1	Özbek et al. [10]
	1.79–3.48	µmol TE 100 g fm−1	Barreca et al. [21]

For detailed information by study, see Table S3. ORAC = oxygen radical absorbance capacity; FRAP = ferric-reducing antioxidant power; DPPH = 2,2-diphenyl-1-picrylhydrazyl; ABTS = 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid); IC₅₀ = 50% inhibitory concentration; SC₅₀ = 50% scavenging capacity; EC₅₀ = 50% mean effective concentration; mg TE = milligrams of Trolox equivalents; mg Fe⁺² = milligrams of ferric ion; µmol TE = micromole of Trolox equivalents; ppm = parts per million; mmol TE = millimoles of Trolox equivalents; µM Fe⁺² = micromoles of ferric ion equivalents; µM TE = micromoles of Trolox equivalents; mg AAE = milligrams of ascorbic acid equivalents; g dm = gram of dry mass; mg ext = milligram of extract; g de = gram of dry extract; g fm = gram of fresh mass; µg PCs = microgram of phenolic compound; mg PCs = milligram of phenolic compound; µg dm = microgram of dry matter.

The synthetic antioxidants currently employed, including BHT and BHA, are effective at low concentrations; however, concerns have been raised about their potential mutagenic, carcinogenic, and teratogenic effects, as well as the risk of hepatic damage. Consequently, owing to these potential health hazards, it is imperative to substitute them with compounds derived from natural sources, such as polyphenols found in PGH, which are biocompatible and economical to produce [54].

The importance of PCs as antioxidants comes from their ability to neutralize free radicals. These compounds neutralize free radicals because of their aromatic rings, which contain hydroxyl groups capable of donating hydrogen atoms. In this way, different mechanisms allow PCs to exhibit their antioxidant activity (Figure 5).

The first mechanism is hydrogen atom transfer (HAT), where the compound donates hydrogen to stabilize free radicals, as seen in the ORAC assay (oxygen radical absorbance capacity). The second mechanism is single-electron transfer (SET), where the compound donates electrons to stabilize free radicals, used in assays such as FRAP (ferric-reducing antioxidant power) and CUPRAC (cupric-reducing antioxidant capacity). A third mechanism combines single-electron transfer and hydrogen atom transfer (SET/HAT), which eliminates stable chromophores in reagents such as DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)). In the DPPH assay, a color change occurs from deep purple to light yellow, while the ABTS assay shows a discoloration from blue-green [72,83,84].

Figure 5. Mechanisms of antioxidant activity facilitated by phenolic compounds [85]. HAT = hydrogen atom transfer; SET = single electron transfer.

Evidence indicates that PCs are linked to antioxidant activity, with studies showing a positive correlation between PCs content and antioxidant capacity. Ordoñez-Cano et al. [66] observed a positive correlation for HT but a negative one for CT using DPPH, ABTS, and FRAP assays. Ghandahari-Yazdi et al. [65] also reported a positive correlation between PCs and antioxidant capacity, measured by the ABTS, DPPH, and FRAP assays. Conversely, Elhadef et al. [52] identified an inverse relationship between TPC and antioxidant capacity, as evaluated through DPPH and ABTS assays. Nonetheless, they reported a positive correlation between TAN and ANT content in purple grapes. Additionally, when the authors examined the correlation between the TAN and ANT content of PGH and its antioxidant activity, they observed a positive association.

Studies on PGH show differences in the antioxidant capacity values analyzed, ranging from 132.75 to 466.73 mg TE g⁻¹, 131.68–411.98 mg TE g⁻¹, and 504.59–2230.8 mg Fe⁺² g⁻¹ for ABTS, DPPH, and FRAP, respectively.

These findings show that this by-product is a promising source of PCs and demonstrates antioxidant activity. This development presents opportunities for research focused on creating products that utilize this biological property.

Efficiency of Extraction of Phenolic Compounds on Antioxidant Activity

The antioxidant activity exhibited by various natural extracts is extensively correlated with their PCs content. Different extraction methodologies were employed to recover PCs and subsequently assess their antioxidant efficacy. Noorolahi et al. [55] reported macerating PGH in water with eight hours of stirring, yielding 3.31, 6, 0.99, and 2.31% for TPC, FLA, no tannins (NTAN), and TAN, respectively. The extraction yields of PCs reported antioxidant capacity values of 466.73 mg TE g⁻¹, 411.98 mg TE g⁻¹, and 2230.8 mg Fe²⁺ g⁻¹ for ABTS, DPPH, and FRAP assays, respectively. Elakremi et al. [62] conducted ultrasound-assisted extraction of PCs using 50% ethanol as the solvent, reporting an extraction yield of 21.9% for TPC and 11.98% for FLA. When evaluating the antioxidant capacity of the extract, the authors obtained values of 427.06 mg TE g⁻¹, 332.92 mg TE g⁻¹, and 517.96 mg TE g⁻¹ for ABTS, DPPH, and FRAP assays, respectively. Ordoñez-Cano et al. [66] reported the recovery of PCs through solid-state fermentation, followed by extraction using an ethanol/water/lactic acid (80/19/1, v/v/v) solvent. The study indicated extraction yields of 3.47%, 0.68%, and 4.15% for HT, CT, and TPC, respectively. Concerning antioxidant capacity, values of 132.75 mg TE g⁻¹ dry matter (dm), 131.68 mg TE g g⁻¹ dm, and 504.59 mg Fe²⁺ g⁻¹ dm were obtained for ABTS, DPPH, and FRAP assays, respectively. The results of these studies were expressed as Trolox equivalents and iron (II) equivalents.

In the context of free radical scavenging, Farrokhi et al. [48] documented the recovery of TPC utilizing ohmic heating-assisted, atmospheric cold plasma-assisted, and maceration techniques, with extraction yields of 2.05%, 1.8%, and 0.76%, respectively. The values obtained for FLA indicate extraction yields of 48.41%, 39.09%, and 36.66%, respectively. Concerning antioxidant capacity, they demonstrated free radical scavenging values of approximately 92%, 87%, and 72% for ABTS, and approximately 89%, 84%, and 74% for DPPH, respectively, for ohmic heating-assisted, atmospheric cold plasma-assisted, and maceration extracts. Tabaraki and Ghadiri [57] reported the recovery of PCs through maceration, microwave, and ultrasound techniques, with TPC yields of 4.59%, 6.26%, and 5.81%, respectively. However, the highest antioxidant activity was observed in the ultrasound extract, followed by maceration and microwave, with free radical scavenging values of 84.2%, 80.5%, and 74.6%, respectively. In this study, the extract exhibiting the highest extraction yield did not correspond to the highest antioxidant activity. This discrepancy may be attributed to the presence of other antioxidant phytochemicals within the extract.

The aforementioned studies do not establish a correlation between high compound extraction yields and increased antioxidant activity. Nevertheless, the presence of PCs does positively influence antioxidant activity. Various extraction methodologies have been examined, including maceration, ultrasound-assisted extraction, microwave-assisted extraction, solid-state fermentation, and ohmic heating-assisted extraction; however, none of these techniques demonstrates a definitive advantage over the others. Instead, external factors, such as the PGH cultivar used in the study or the season in which the sample was obtained, may significantly impact the phenolic compound content and their associated antioxidant capacity.

5.4.2. Antibacterial Activity

The PCs exhibit biological activities beyond their antioxidant effects. One important function is their antimicrobial activity. For example, PA exhibits antiviral, antifungal, antibacterial, and even insecticidal properties [45]. The TAN are known in plants as defense mechanisms against both biotic and abiotic stresses in their environment, showing capabilities as antibacterial, antifungal, and antiviral agents [45].

Table 7 presents studies on PGH extracts that analyze their effectiveness against both Gram-positive and Gram-negative bacteria. The evaluation methods include determining the minimum inhibitory concentration (MIC) and performing disk diffusion testing.

Table 7. Antibacterial activity of extracts from PGH against bacteria.

Assay	Bacterial Strains	Result	Reference
MIC	Escherichia coli Salmonella subsp. enterica serovar Typhimurium Staphylococcus aureus Pseudomonas aeruginosa Bacillus subtilis Bacillus cereus Enterococcus faecalis Enterococcus hirae Enterobacter aerogenes Klebsiella pneumonia Acinetobacter baumannii Proteus mirabilis Legionella pneumophila Streptococcus faecalis	0.42–857.12 mg mL−1 0.26 mg mL−1 0.16–807.5 mg mL−1 0.31–1615 mg mL−1 0.25–121.25 mg mL−1 0.21–1 mg mL−1 4–100.9 mg mL−1 50.7–428.56 mg mL−1 485.00 mg mL−1 4–14.55 mg mL−1 4 mg mL−1 4 mg mL−1 202.95–857.12 mg mL−1 0.21 mg mL−1	Ordoñez-Cano [86] Karaogul and Ugurtay [49] Shahdadi et al. [39] Shirzadi-Ahodashti et al. [75] Ozay et al. [53] Hasheminya and Dehghannya [87] Smeriglio et al. [40] Rajaei et al. [58]
Disk diffusion method	Escherichia coli Salmonella subsp. enterica serovar Typhimurium Salmonella enterica Staphylococcus aureus Pseudomonas aeruginosa Bacillus subtilis Bacillus cereus Enterococcus faecalis Enterobacter aerogenes Klebsiella pneumonia Streptococcus faecalis Clostridioides difficile Listeria monocytogenes	11.3–20.8 mm 21.5 mm 13.83–16 mm 7.36–21.8 mm 9.31–21.8 mm 12.96 mm 12–22.5 mm 3.30–9 mm 15.18 mm 13.80 mm 22.3 mm 18 mm 14.23–19 mm	Ordoñez-Cano [86] Karaogul and Ugurtay [49] Kepekci et al. [60] Elhadef et al. [52] Hasheminya and Dehghannya [87] Rajaei et al. [58]
MBC	Staphylococcus aureus Bacillus subtilis	~500 µg mL−1 ~500 µg mL−1	Shahdadi et al. [39]

For detailed information by study, see Table S4. MBC = minimum bactericidal concentration; MIC = minimum inhibitory concentration.

The compounds present in PGH, such as quercetin and gallic acid, have been shown to increase membrane permeability. These compounds facilitate the partitioning of lipids that constitute the cell membrane, thereby altering the intracellular pH of bacteria by affecting ions flow. This disruption blocks the energy production and synthesis, leading to the loss of cellular content and ultimately causing bacterial death. Additionally, apigenin has been reported to inhibit the DNA gyrase enzyme and the activity of protein dehydratase. TAN are also known to damage cell membranes and inhibit enzyme production, thereby affecting the metabolic pathways of microorganisms (Figure 6) [39,52,58,87].

Figure 6. Mechanism of antibacterial activity of phenolic compounds against bacteria.

The antibacterial efficacy of PCs can be attributed to the phenol group in their molecular structure, which acts as an antiseptic by inactivating microorganisms through mechanisms such as cell lysis or biofilm inhibition [44,88]. Additionally, this functional group not only affects bacterial cells but also alters the fungal cell membrane, thereby impeding germination and mycelial development [45].

Shahdadi et al. [39] assessed the impact of aqueous extracts of PGH on foodborne bacteria, specifically Staphylococcus aureus and Bacillus subtilis. Their findings demonstrate that these extracts possess inhibitory properties against the bacteria, attributable to the presence of PA, FLA, and TAN. This indicates that PGH extracts are particularly efficacious against Gram-positive bacteria. Conversely, Elhadef et al. [52] investigated a representative spectrum of bacteria. They found that an extract concentration of 10 mg mL⁻¹ of PCs had a more pronounced effect on Gram-positive bacteria than on Gram-negative bacteria. Nonetheless, an inhibition zone was observed in both groups, suggesting that PGH extracts function as effective antibacterial agents that inhibit and control the proliferation of foodborne bacteria.

The observed more substantial effect of PGH extract on Gram-positive bacteria can mainly be attributed to differences in their cell wall structure [40,52,58,87]. Gram-positive bacteria are characterized by a thick peptidoglycan layer surrounding their cytoplasmic membrane, which sets them apart from Gram-negative bacteria. The cell wall of Gram-negative bacteria is more complex, consisting of an extra layer of lipopolysaccharides and lipoproteins that cover a thinner peptidoglycan layer and the cytoplasmic membrane [89]. Additionally, the presence of essential oils and ANA in PGH extracts may help interactions with the outer layer of Gram-negative bacteria, made up of glycolipids and lipoproteins. However, effective inhibition necessitates a synergistic action involving phytochemicals such as quercetin, gallic acid, and apigenin [2,33,52].

Effectiveness of Phenolic Compound Extraction on Antibacterial Activity

Several studies have recovered PCs using various extraction techniques, and their extracts were tested against Gram-positive and Gram-negative bacteria using the MIC method. Karaogul et al. [49] reported macerating PGH with distilled water, resulting in an extraction yield of 0.25% TPC. The extract was tested against bacteria, showing values of 485 mg mL⁻¹ for E. aerogenes, 121.25 mg mL⁻¹ for B. subtilis and S. aureus, 60.14 mg mL⁻¹ for P. aeruginosa, and 14.55 mg ml⁻¹ for K. pneumonia. Shahdadi et al. [39] performed boiling with distilled water to recover PCs, yielding a TPC of 24.54%. The extracts were tested against S. aureus and B. subtilis, with results of approximately 500 µg mL⁻¹ and 250 µg mL⁻¹, respectively. In the case of MBC, values of approximately 500 µg mL⁻¹ were obtained for both strains. Ozay et al. [53] extracted compounds by maceration in distilled water, yielding 1.76%. The bacterial assays yielded values of 1,615 mg mL⁻¹ for P. aeruginosa, 857.12 mg mL⁻¹ for E. coli and L. pneumophila, 807.5 mg mL⁻¹ for S. aureus, 428.56 mg mL⁻¹ for E. hirae, and 100.9 mg m^L−1 for E. faecalis. Rajaei et al. [58] utilized maceration to obtain PCs, reporting an extraction yield of 4.93% of TPC. The crude extracts demonstrated activity against B. cereus and S. aureus with MIC of 1 mg mL⁻¹ and 2 mg mL⁻¹, respectively. Conversely, the purified extracts exhibited MIC values of 0.5 mg mL⁻¹ for both strains. Finally, Ordoñez-Cano [86] employed solid-state fermentation with A. niger GH1 and ethanol/water/lactic acid (80/19/1, v/v/v) as the extraction solvent, yielding an extraction yield of 4.14% for TPC. The extract was tested against S. aureus, yielding a value of 1500 µg mL⁻¹, while no inhibition was observed against S. typhimurium and E. coli at concentrations ranging from 187.5 to 3000 µg mL⁻¹.

Furthermore, the disk diffusion method has been evaluated against Gram-positive and Gram-negative bacteria. Kepekci et al. [60] extracted PCs using the Soxhlet method with methanol as the solvent, yielding a TPC of 0.024%. They evaluated a concentration of 30 µL disc-1, obtaining values of 19 mm for L. monocytogenes, 18 mm for C. difficile, 12.3 mm for E. coli type 1, 11.7 mm for P. aeruginosa, 11.3 mm for E. coli ATCC25922, 10 mm for S. aureus, and 9 mm for E. faecalis. Elhadef et al. [52] extracted PCs from PGH via maceration with various solvents; distilled water showed the highest efficacy, yielding 1.76% TPC. The extract was evaluated at a concentration of 10 mg mL⁻¹, and the obtained values were 18.75 mm for B. cereus, 18.66 mm for S. aureus, 17.25 mm for L. monocytogenes, 16 mm for S. enterica, 15.75 mm for P. aeruginosa, and 15.25 mm for E. coli. The results indicate that a high extraction yield is not directly associated with higher inhibition efficiency in the disk diffusion method. However, in the MIC technique, higher extraction yields were observed to inhibit bacteria at lower concentrations. This highlights that the extraction techniques used to recover PCs from PGH were maceration, decoction, Soxhlet, and solid-state fermentation, with decoction and maceration yielding the best results when evaluating the extracts against bacteria. However, extraction yields obtained with the maceration technique varied across studies, suggesting that external factors also influence the extraction yield.

In conclusion, higher extraction yield was not associated with greater inhibition in the disk diffusion method. However, higher antibacterial activity in the MIC technique was associated with a higher PCs content in the extracts, not with the recovery technique used.

6. Future Perspectives

PGH is a by-product that contains valuable phytochemical compounds, which have been shown to act as antioxidants against free radicals and as antibacterial agents against Gram-positive bacteria. There is still much to learn about these PCs, which can be harnessed to develop products that recover the beneficial properties of these compounds. These products may be utilized in several industries: the cosmetics industry can incorporate them into personal care products; the pharmaceutical industry can use these antioxidant and antimicrobial compounds, which also exhibit other biological activities such as anticancer, antihypertensive, and antidiabetic effects; and the food industry can apply them in the production of encapsulated products, functional foods, or dietary supplements. Currently, our research group is investigating other agricultural wastes to develop functional foods with higher levels of phenolic compounds. This initiative aims to achieve some of the objectives in waste valorization, such as PGH.

However, the recovery of PCs will generate another residue corresponding to the PC-free PGH. The usefulness of this by-product, as livestock feed or crop fertilizer, needs to be evaluated. Since it contains a low quantity of PCs, it may not cause dietary problems or interact with the soil. Additionally, the PGH includes other important compounds that have been studied, such as essential oils and pectin, which, along with PCs, can be explored for potential biotechnological applications in microorganism cultivation or bioenergy, like biofuels, by utilizing the entire PGH.

Future avenues include conducting techno-economic assessments, safety and toxicity analyses, bioavailability investigations, and considering regulatory factors for food and pharmaceutical uses. Additionally, Life Cycle Assessment, environmental footprint, and emission rate evaluations can be employed to assess pistachio by-products in future research endeavors.

Using PGH encourages a circular economy around agro-industrial waste, emphasizing that these by-products can be a valuable source of natural compounds. As chronic degenerative diseases become more common—often linked to environmental factors, genetics, and human behavior—the importance of these post-harvest by-products and their potential for value creation becomes even more significant.