Sustainable Pistachio Dehulling Waste Management and Its Valorization Approaches: A Review

In countries such the USA, Iran, and Turkey, pistachio nut is considered one of the most economically valuable agricultural products. Pistachio production and related dehulling processes generate a large quantity of organic waste, containing green hull, cluster woody part, shells, and leaves. The inadequate conventional management of such wastes calls for sustainable and economical strategies not only to enhance resource efficiency but also to create value. Pistachio residues have a high content of total extractives and essential oils and a considerable amount of phenolic compounds that explain their good antioxidant activities and other potential human health benefits. Furthermore, considering the generation volume (about 660,000 tons) and lignocellulosic structure, pistachio residues can also be sustainably used to produce value-added products, such as biofuels, phytochemicals, activated carbon, and other potential bioproducts such as filamentous fungi as protein enriched biomass, single-cell protein (SCP), and volatile fatty acids. In general, recent studies have not comprehensively investigated all value-added potential products. This review provides a thourough insight into the present pistachio processing industries, and pistachio waste chemical composition and characteristics. Furthermore, the applications of pistachio residues as a renewable source for the production of potential value-added products by various thermochemical (pyrolysis, gasification, and liquefaction), physicochemical (solvent extraction, ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and extraction by pressurized liquids (PLE)), and biological (anaerobic digestion (AD) and fermentation (solid-state and submerged)) processes are presented including an analysis of the advantages and disadvantages of such methods. In this regard, production of new products such as edible filamentous fungi and antioxidant, and their market appeal has been briefly considered.


Introduction
A growing number of clinical studies have revealed the potential health benefits of pistachio nuts based on their high content of potassium, phytosterols, vitamin K, g-tocopherol, and lutein compared to other nuts, their heart-healthy fattyacids profile, protein, dietary fiber, and some phytochemicals, including phenolic acids and xanthophyll carotenoids [1][2][3][4].Considering the aforementioned nutritional and health benefits, pistachios, consumption is increasing worldwide.
Based on the latest statistics, the USA and Iran are world leaders in the production and cultivation of pistachios with a sum of about 700,000 metric tons in 2019 (more than 70% of the world's pistachio production) [5][6][7].Figure 1 and Table 1 demonstrate the world's top pistachio production and producing countries in 2019.
Pistachio (Pistacia vera L.) contains an edible seed in the center protected by a hard lignified shell and a green hull [9].After harvesting, dehulling and downstream process to obtain pistachio kernels, green hull, cluster woody part, shell, and leave are the most common types of waste generated.Pistachio by-product mixture (PBM), the largest portion of total pistachio residues, refers to a mixture of leaves, clusters, and green hulls resulting from the dehulling process [10].
PBM is biodegradable and prone to spoilable, turning black, and moldy in a matter of days if spread over land untreated.The molesting insects and extensive soil and water pollution, for example, increases the distribution of aflatoxin by overwintering spores of Aspergillus sp.around the pistachio orchard and terminals, are some issues associated with dumping PBM.Although some farmers and pistachio growers apply PBM as fertilizer without any treatment, increases in the amount of aflatoxin contamination in the pistachio production chain due to the movement of Aspergillus sp.spores [8][9][10].Although a small portion of the green hull is used for animal feeding, tannins and polyphenolic compounds in the hulls reduce its acceptance and digestibility as animal feed [11•].
As above mentioned, Like most other agricultural wastes, pistachio wastes are not managed by circular bioeconomy approach which is proposed more efficient sustainable biobased renewable resource management [12,13].On the other hand, because of growing volume of these waste, limited fossil fuels, and chemical sources, sustainable management of agricultural wastes is a critical issue in the world which lead humans to find a sustainable manner for energy and biobased generation [14,15].For this purpose, the concept of biorefinery, thermochemical, and extraction methods is used to convert biomass to chemicals, energy, and other bio-based products [16,17].Table 1 Top pistachio producing countries in 2019 [8] Hence, pistachio residues need to be considered potential raw materials for producing value-added products such as phytochemicals, biofuels, protein-rich food or feed, fungal biomass protein, bio-oil, and activated carbon rather than just excretion or feeding them, to avoid transmission of hazardous materials and environmental pollution [18, 19, 20••, 21].
The present study opens with a thorough review of pistachio composition, production and processing, and waste generation.The main attention was then focused on thermochemical, extractive, and biological production of valueadded products from pistachio residues.There is a general lack of a comprehensive literature review in this area.Moreover, the potential of PBM as feedstock to waste-based biorefineries is investigated.

Pistachio Dehulling Process and Residues
In most countries, it is common practice that pistachios are harvested as mature (except in Turkey that pistachios are harvested as un-matured and matured) [22] by knocking them off the tree followed by shaking onto sheets or mechanical shakers.This process is called dehulling, through which the soft hull, leaves, and clusters are separated from nuts [23][24][25].From 3 kg of pistachio harvested that enters the processing unit, 1 kg of dried nuts is produced, and about 66% ends as residuals [11•].
PBM forms more than 90% of total pistachio residues.Pistachio green hull (PGH), as a large portion of pistachio (approximately 33% of dry matter weight), is the main contributor to (about 59%) PBM [24,25].After dehulling in the pistachio terminals, separating unpeeled and unripe pistachios, washing, and drying to the appropriate moisture level are the following processes [11•].
Due to the high moisture content of PBM, its improper management results in environmental and ecological challenges [26].However, based on the energy and nutrient content point of view, the pistachio dehulling residues is an ideal source for extracting or producing valuable products.

Pistachio Residues Composition and Usage
Pistachio nuts are a rich antioxidant and anti-inflammatory source due to their high content of phenolic compounds such as anthocyanins, flavonols, flavanones, cardioprotective constituents, and phytosterols tocopherols varying based on the species of pistachio [27,28].Similar to pistachio nuts specification, the composition of pistachio residues changes with the species of pistachio [29], variety of pistachio [30], and method of harvesting and storage [31].Several studies have reported the chemical composition of various types of pistachio residues [10, 11•, 32•, 33].As presented in Table 2, carbohydrate, phenolic compounds, and protein are the most important components found in different streams of pistachio residues.
In addition to the chemical composition listed in Table 2, there are other valuable minerals and vitamins in PGH such as phosphorus, nitrogen, potassium, sodium, calcium, magnesium, iron, manganese, zinc, and copper, which are generally less than 10% [11•, 33].
Dietary fiber is the portion of plant-derived food consisting of structural carbohydrates and lignin that cannot be completely broken down by human digestive enzymes or readily absorbed in the small intestine [26,35].These fibers form the major structure of lignocellulosic structure.Although dietary fibers are generally more slowly digested than non-fiber carbohydrate (NFC), they can potentially be converted into value-added products [36][37][38].PBM can be considered lignocellulosic biomass [10].Due to the lignocellulosic nature of pistachio residues and their rich organic matter content, their potential applications and products have been investigated.Biogas production [39], compost and fertilizer [40][41][42], animal feed [43], biosorbent [44,45], phytochemicals [46], and pigments [28] from PGH, bio-oil [47], and activated carbon [48] from pistachio shell and also preparing the substrate for mushroom from the mixture of shells and PGH [49] are some of these studies which the valorization approaches discussed later.Another use of pistachio waste is as animal feed.Paydaș et al. [50] investigated the effects of the addition of pistachio shells on different levels of corn silage.As a result, by ensiling up to 9% pistachio shells along with corn silage reduction in ruminal methane production in ruminants can be achieved.Although pistachio waste can be used as additive in conventional ruminant feed, the presence of high levels of copper and phenolic compounds restricts its inclusion proportions in ruminants' diet.These by-products also contain tannins that limits pistachio waste application in ruminant feeding due to toxicity or undesirable interactions with protein, carbohydrates, minerals, or rumen microorganisms [51,52].Therefore, using other value creation approaches than application in animal feeding can be more promising.

Pistachio Dehulling Waste Valorization Approaches
Based on the previously noted structural and compositional characteristics of pistachio dehulling waste, there are various methods which have the potential to convert these residues into valuable products bringing environmental and economic advantages while alleviating issues revolving around pistachio waste management.Considering Fig. 2, approaches of creating added value from pistachio wastes have been determined.

Thermochemical Methods
Thermochemical conversion biomass into fuels happens through chemically catalyzed thermal decomposition.Pyrolysis, gasification, and liquefaction are such thermochemical processes used in the valorization of waste.Pyrolysis is performed in the absence of oxygen at medium and high temperatures.Bio-oil, biochar, and some light gases such as hydrogen are the most common products of the pyrolysis process.The type and distribution of products vary depending on the operational conditions and the composition of the residues.For example, higher temperatures yield more gaseous products.Pyrolysis and gasification are significantly different in terms of yield type and the ratio of products.Although pyrolysis is an affordable technology, some pollutants such as SO x , NO x , hydrogen sulfide, and ammonia are produced during the process [70][71][72][73][74].
Okutucu et al. [75] investigated the feasibility of the application of pistachio shell as feedstock for the production of fungicidal oil (pyrolysis bio-oil) and a precursor for the production of activated carbon by pyrolysis at temperatures of 300-600 °C.Fungicidal activity of the bio-oil against four different types of fungi (T.viridae, A. niger, T. rubrum, and C. versicolor) was proven effective at concentration 10-50 mg.ml −1 .Compared to the fungicidal activity of the bio-oil produced from peach stone, which only had acceptable activity against C. versicolor [76], the generated antifungal from pistachio shell has showed more efficiency.The surface area and micropore volume of the activated carbon produced from the char were found to be 708 m 2 .g −1 and 0.280 cm 3 .g−1 , respectively.In addition, Yang et al. [77] investigated activated carbon production from pistachio shells by activating the resulting chars following pyrolysis.The pyrolysis temperature was 500 °C, and residence time at this temperature was 2 h.The aforementioned study also investigated the effects of CO 2 flow and heating rates during activation.The surface area and micropore volume of the activated carbon prepared at 800 °C were found to be 1064.2m 2 .g−1 and 0.210 cm 3 .g−1 , respectively.Comparing the results of this study with other similar agricultural wastes, the surface area of biochar obtained from pistachio shell was much higher than that of walnut and hazelnut shell [78,79].In summary, pyrolysis chars of pistachio shells has a good potential for being used as a solid fuel or as raw material to produce activated carbon and adsorbent.Despite pyrolysis chars, gas products and bio-oil are not attractive as fuel due to the low calorific value and high oxygen content, respectively.In addition to the various studies on pistachio shell pyrolysis, some researchers have performed pyrolysis on PGH.Jalayeri et al. [80] investigated the use of biochar derived from PGH for Cu (II) removal from aqueous solutions.The PGH was pyrolyzed at 450 °C for 1 h in the absence of oxygen.The maximum amount of Cu (II) ions adsorbed by biochar was reported 19.84 mg.g −1 .
Moreover, Karatas et al. [81] studied the gasification of walnut and pistachio shells.It was shown that the lower heating value of the pistachio shell is higher than that of the walnut shell by 8-12%, and it increases from 3.2 to 5.4 MJ.Nm −3 with the reduction in equivalence ratio from 0.4 to 0.2 for pistachio shell.
Besides the above methods, hydrothermal liquefaction has been used as a method for high biomass conversion yields in the production of bio-crudes [82].Hongthong et al. [83] investigated co-processing of common plastics with pistachio hulls via hydrothermal liquefaction at 350 °C over 15 and 60 min time intervals in blends of up to 20 wt% plastic to pistachio hulls.Under optimal conditions, nylon-6 and PET broke down almost entirely, and high yields of up to 35% bio-crude were achieved.

Physicochemical Methods
Physical, chemical, or combination of these methods are used to produce valuable products from pistachio dehulling wastes.The products derived from pistachio dehulling wastes through these methods are highlighted in this section.
Among bioactive compunds in pistachio dehulling wastes, polyphenols have attracted special attention for their antioxidant, anti-inflammatory, and other biological activities.Liquid-solid extraction and liquid-liquid extraction (solvent extractions) are the main conventional extraction methods of different compounds from biomass.The extraction of phenolic and antioxidant compounds is commonly done using simple and traditional methods such as Soxhlet with organic solvents or conventional hydrodistillation.For example, Mohammadi et al. [84] investigated the extraction of essential oils from pistachio shells and clusters by the hydrodistillation method.Essential oils extraction efficiency from cluster and shells was 2.10% and 0.13%, respectively.The major compounds identified in shells were d-limonene, α-thujene, and terpinolene, while the major components of clusters were α-thujene and α-pinene.The total phenolic content in shells and clusters were 958 and 796 mg gallic acid (GA)/100 g dry material, respectively.In another study, the phenolic content of water extract of PGH was reported to contain about 197 mg GAE/g dry weight [85].
However, as presented in the literature, these methods have low extraction efficiency, long extraction time, and their high temperature leads to the degradations of the bioactive and unstable ingredients [86][87][88].The enhancement of the extraction efficiency, reduction of solvent consumption, extraction time, and the use of greener solvents have led to modern or nonconventional techniques such as ultrasoundassisted extraction (UAE), microwave-assisted extraction (MAE), and extraction by pressurized liquids (PLE) [89,90].
The MAE uses microwave energy that includes internal heating to provide faster extraction, lesser or no solvent consumption, and lower external temperature when compared to traditional heating [91,92].Özbek et al. [93] investigated the extraction of non-polar compounds from pistachio hulls by MAE and Soxhlet methods.The extraction yields were 9.81 and 9.50% for MAE and Soxhlet methods, respectively.The results revealed that the total phenolic compounds, antioxidant activity, and tocopherol content of the MAE extract were significantly higher than that of the Soxhlet extract.In another study, Özbek et al. [94] investigated the extraction of phenolic compounds from pistachio hull using microwaveassisted solvent extraction (MASE).In optimal conditions, 62.24 mg gallic acid equivalent (GAE).g−1 dry hull of total phenolic compounds was obtained.In addition, in this study, MASE was proven to give higher extraction yield, phenolic compounds, and antioxidant activity than conventional solvent extraction.Furthermore, pistachio shells have been evaluated for natural antioxidants extraction via various extraction methods and solvent such as methanol, ethanol, ethyl acetate, UAE, and MAE among which microwaveassisted ethanol extract turned out to be the most promising [95].
The UAE method is based on ultrasonic waves, which lead to acoustic cavitations production.Although UAE is known as a cheap and effective alternative method compared to MAE and conventional methods, its low efficiency has been claimed in some studies [96,97].For example, Mokhtarpour et al. [98] studied the phenolic profile and tannins from pistachio by-products by four different solvents and UAE methods.Using acetone resulted in higher total phenol compared to methanol, ethanol, and water.In addition, higher tannin content was extracted by ethanol compared to methanol and water.In another experiment, no difference was observed in the extraction of total phenols and tannins by UAE and 12 h solvent extraction without ultrasonic.Despite the results of the mentioned study, the findings of Rajaei et al. [57] indicated that the yield of phenolic compounds extracted by UAE was higher than maceration extraction and MAE methods.Generally, the amount of total phenolic content in pistachio dehulling waste, regardless of the extraction method, is much higher than some conventional agricultural wastes such as rice straw (486 mg (GA)/100 g dry material) [99] and many edible vegetables and fruits considered rich sources of phenolic compounds [100].
Pectin is another promising product which can be obtained from PGH by extraction.Pectin is widely used in food industry for gelling and as emulsifier.Based on the degree of esterification of pectin (high and low methoxyl pectin), PGH pectin obtained by acidic extraction method is classified as low methoxyl pectin, which can be applied as a food ingredient for gel formation in low calorie products [67].In addition to the acid extraction method, UAE and MAE have also been investigated to extract pectin from PGH.In short, PGH is a good source of high purity pectin [66,67,100].
Anaerobic digestion (AD) is the natural process in which bacterial consortia decompose organic materials in the absence of oxygen under controlled operational conditions.During AD, organic material is hydrolyzed into sugars, amino acids, and fatty acids resulting in the final main metabolites of methane and carbon dioxide called biogas as a renewable source of energy [104, 105 ••, 106].In addition to biogas, a nutrient rich digestate is generated which has the potential to be used as fertilizer [107].A few studies on biogas production from pistachio dehulling wastes show the possibility of biogas production as a renewable energy source.For example, Demirer [108] investigated the first report on biogas production potential of pistachio dehulling waste.The results showed that 134 mL of biogas (62.6 mL methane) was produced per gram of dehulling solid waste.In another study, Celik and Demirer [39] investigated the anaerobic treatment capability and potential of biogas production from pistachio residues.In this regard, thermal and chemical (acidic and alkaline) pretreatment methods were evaluated and applied to pistachio residues.Based on the acquired results, thermal pretreatment has shown the best performance and the highest methane yield with 213.4 ml CH 4 .g−1 COD.
By comparing the results of the AD of pistachio dehulling waste to other conventional agricultural wastes such as wheat and rice straw, it seems that pistachio wastes have less potential for biogas production.For example, Baetge et al. [109] investigated the production of biogas from rice straw generating 449 mL of biogas per gram of organic dry matter.Also Xi et al. [110] studied biogas production from wheat straw obtaining 337 ml biogas per gram of total solid.In orger to improve biogas production from pistachio waste, strategies such as co-digestion can be considered.
Similar to AD, aerobic fermentation converts lignocellulosic wastes such as pistachio residues into valuable metabolites.Fermentation of pistachio residues has been investigated in a few studies.Recently, Göncü et al. [111] evaluated the bioethanol production from pistachio shells by applying ozone, hot water, and combined (ozone and hot water) pretreatment.This study showed that combined pretreatment was more effective than single where through a post enzymatic hydrolysis, 1.21-2.33g.L −1 ethanol was obtained at the end of the fermentation by Saccharomyces cerevisiae, which presents a fermentation efficiency of 42-55%.Pretreatment methods are usually applied for opening the lignocellulosic biomass structure and increasing the accessibility to the inner structure by enzymes and microorganisms during the AD or fermentation process by eliminating lignin or other structural components [112][113][114].Physical (chipping, milling, cavitation, etc.) [115], chemical (alkaline, acidic, oxidizing agents, etc.) [116], physico-chemical (steam explosion, hydrothermal, etc.) [117], and biological (enzymatic, etc.) [118] approaches are some of the most common pretreatment methods with only a few practiced during bioconversion pistachio waste.Due to the composition of PBM, compounds such as furfural and 4-hydroxy-3,5-dimethoxybenzaldehyde, can be released during harsh pretreatment that acts as inhibitory products to fermentation [32•].
Solid-state fermentation (SSF) is a fermentation method by which the growth of microorganisms happens in the absence of free water [119].In this case, Karimi et al. [120] and Abbasi et al. [121] determined the effect of SSF on pistachio hulls' antioxidant activity using fungi.It was claimed that SSF is not an effective method for improving the antioxidant activities of pistachio hulls.
Although the production of some bioproducts from pistachio residues has been investigated, there are various unexplored areas for pistachio residues bioconversions.Acetonebutanol-ethanol (ABE) [122], volatile fatty acids (VFAs) [123], bio-hydrogen [124], single-cell proteins [20 ••, 125], fungal biomass protein such as filamentous fungi [125][126][127], etc., are some of the potential bioproducts that can be considered when considering value creation from pistachio residues.One of the bioproducts that has recently received attention are edible filamentous fungi, that have the ability to produce a variety of enzymes that enable them to growth on various complex organic substrates.The product of such biological process is a protein-rich biomass that can be used as human food and animal feed.Fungal cultivation can also generate other products such as ethanol, chitin, chitosan, and pigment [126, 128••, 129].According to the characteristics and chemical composition of pistachio waste, obtaining novel bioproducts as fungal biomass or antioxidants instead of conventional bio-based products can be considered an attractive alternative.

Comparative Evaluation of Valorization Approaches
It is known that biomass pretreatment considered in different valorization practices is the energy and cost hotspots of the system.Acidic or basic, advanced oxidation processes (AOPs) such as ozonolysis and Fenton, biological and organosolv pretreatment are some common examples of such pretreatment methods [130][131][132][133].These methods have also been considered in industrial scale practices.For example, ethanol-based organosolv is used in Lignol Innovations in Canada, an integrated biorefinery producing ethanol, lignin, furfural, xylose, and acetic acid [134].Regardless of the pretreatment stage, the main mentioned valorization approaches, i.e. thermochemical, physicochemical, and biological, are comparable to each other in terms of technical, economic, energy demand, and environmental aspects.These approaches can be used individually in an integrated approach in order to produce specific value-added products from pistachio waste.However, as summarized in Table 3, there are various advantages and disadvantages to each method.
Considering the generation volume of pistachio waste in specific areas, this waste can be considered one of the most abundant low-priced organic substrate for bioconversion purposes in a biorefinery concept fort the production of biofuels and other added value products [141].However, if an integrated biorefinery concept is not considered, taking into account the benefits and shortcomings presented, and regardless of the specific cases where a unique product is aimed at, production of new products that open up new markerts and economical apportunities compared conventional well-established products should be more promising from dense agricultural wastes such as pistachio wastes.In this regard, production of single-cell protein (e.g.edible filamentous fungi), antioxidant, pigment, etc., may be more appealing as they can be produced on a small scale with rather small investment and low market rivalry as a green alternative products.In addition, considering the potential products, the negative or low value of the substrate, physicochemical and biological processes present a wider scope of products with considerably low processing cost.

Future Prospectives
Considering the trend in population growth and future demands for food, feed, and energy, production and application practices should be focused on the efficient use of resources and the recovery of nutrients from waste in a circular economy concept.When it comes to organic-rich waste streams, waste-based biorefineries are the true realization of such a goal through which nutrients are recovered into a range of value-added biochemicals and biomaterials.
About 1,000,000 metric tons of pistachio is produced per year around the world resulting in the generation of 660,000 metric tons of waste.Considering the rich dietary fiber, antioxidants, and phenolic compounds content of this residue, it has great potential to be used as feedstock for a waste-biorefinery.In order to have an efficient high-yielding conversion, a detailed understanding of pistachio residues characteristics is necessary.However, based on little information available on pistachio residues content and available state-of-the-art conversion processes, it can be predicted that pistachio residues can become a key player in providing antioxidants, phytochemicals, activated carbon, and biofuels.More detailed studies on the applications and processing of pistachio residues, on the other hand, are needed to demonstrate its potential and provide sufficient knowledge and feedback on the development of industrial-scale processes on a pistachio processing waste platform.
Although the potential for optimising the bioconversion of pistachio residues as lignocellulose biomass has been demonstrated in a few available literatures, further research into biological processes such as fermentation has yet to be conducted.Also, it can necessarily require pretreatment methods for improved digestibility.Therefore, further research into pretreatment methods is essential in order to evaluate more cost-effective options that lead to higher product yield and lower overall costs.

Conclusion
Pistachio residues and by-products such as dehulling waste or shells contain a significant amount of valuable chemical compounds and nutrients.Due to the high content of phenolic compounds and extractive in pistachio waste, production of antioxidant and other human health benefiting from such waste stream has attracted great research and industrial attention.Pistachio waste has great potential to be used as feedstock in biorefineries for the production of other valueadded products such as biofuels, bio-oil, carbon active, and novel products such as edible fungal biomass and volatile fatty acids.Thermochemical, physicochemical, and biological valorization approaches have been considered in this regard.Although all applied pistachio waste management practices and products have certain degree of feasibility and appeal, considering the trade off between the advantages and shortcomings, novel products such as antioxidant and single-cell proteins have the potential to be further explored.Generally, the valorization of such an organic-rich resource, which is commonly regarded as an agricultural waste, will be a prizing resource recovery strategy not only to manage waste but also to create value.However, pistachio waste valorization methods practiced until the present are limited, leaving the field open for extended future research.

Fig. 1
Fig. 1 Distribution map of world's top pistachio producing countries in 2019 adopted from [5]

Table 2
Chemical composition of pistachio by-