Introduction

Postharvest losses and food safety of horticulture produce are the major concern for the developing countries; particularly in terms of economic value [1]. Citrus is non-climacteric fruits; considered as one of the major produced and exported fruit genera worldwide (more than 100 countries), specially grown in China, Brazil, America, etc. [2, 3]. Citrus cultivation covers approximately 11.42 million hectares, with a total production of 179 million tons production of citruis fruits. In 2017, the production of different citrus fruits such as oranges (73.3 million tons), mandarins (33.4 million tons), lemons (17.2 million tons), grapefruits (9.5 million tons), and others (13.5 million tons) were recorded [4]. China is the leading country for the production of citrus fruits with (82.7 million tons) annually followed by Brazil (18.14 million tons) and India (1.053 million tons) respectively [4,5,6,7]. Generally, citrus fruits are grown for consumption and processing, mainly to produce juice. According to [2, 8], 76% of juice oranges consumed worldwide; which generates economic benefits and employment. The citrus fruits are highly acknowledged by the consumers owing to their taste, flavor, aroma, and health benefits. It is an excellent source for nutrients, ascorbic acid (Vitamin-C), phenolic compounds, hydroxycinnamic acid, flavonoid compounds, anthocyanins, and other bioactive compounds which leads to improving antioxidant, antimicrobial activity, retarded of cardiovascular disease and cancer risk [9, 10]. The antioxidant, anti-inflammatory, anticancer, neuroprotective, and cardiovascular protective properties of the citrus fruits mainly due to the presence of bioactive compounds such as phenolic acids, flavonoids, ascorbic acid, ferulic acid, naringin, hydrocinnamic acid, cyaniding glucoside, alkaloids, limonoids, coumarins, carotenoids, essential oils, and others [11,12,13]. Various species of the citrus genus have been grown globally such as oranges, bergamot orange, mandarins orange, tangerines, lemons, limes, grapefruits, pummelos, citrons, kumquats, clymenia, desert lime, ginger lime, hyuganastu, kabosu, kawachi bankan, koji orange, mangshanyegan, round lime, satsuma, sudachi and other hybrid varieties [7, 10]. The properties and chemical compsoitions in citrus are varied according to the varieties and species. For example; mandarians/ornage/kinnow fruits are rich in vitamin C, antioxidants, dietary fibers, fibers, potassium, flavonoid and tangeretin; grapefruits exhibited higher antioxidants, anthocyanins and vitamin C; pummelo/shaddock had good amounts of vitamin C, antioxidants, fibers, and flavonoids; lemon posses vitamin C, flavonoid and antibacterials activity etc. [11]. In the present review, authors have briefly discussed about the postharvest physiological problem in citrus origin of fruits and their postharvest management by using nano-formulation. The review also congregates the information about the technologies i.e., high pressure homogenization, ultrasonication, microfludizer etc. used to develope nano-formulations enriched with natural active ingredients i.e., plant extracts, essential oils etc. and their effects on shelf-life of citrs fruit. In addition, the natural plant-based materials such as plant extracts and essential oils could be considered as alternatives of fungicides at commercial scale.

Post harvest issues

Citrus fruits, unlike climateric fruits (e.g., apple, pear, tomato, and melon,), lack a ripening associated increase in respiration and ethylene generation. Generally, citrus fruits have long shelf-life when compared to other tropical fruits, but if not handled and stored properly during post-harvesting they will become unfit for marketing and consumption. In developing countries, postharvest losses might reach 30% of total production and 50% in less developed countries [10]. The higher oxidation and transpiration are the main causes for higher deterioration effects, loss of nutritional value and firmness, and appearance in citrus fruits. These factors influence the higher moisture loss and respiration rate of fruits and vegetables, which are resposble for the microbial degrartion [14].

Physiological disorder and diseases incidence of citrus fruits

Postharvest losses in citrus fruits are caused by improper handling and storage factors including physiological disorders, mechanical, and physical damage, rot, and fruit senescence. These factors are also responsible for degrading the quality of fresh fruits and are unsuitable for consumption due to oxidations thereby casing food wastage. Generally, the mandarins have the shortest shelf-life among the citrus fruits; it has 15–30 days of shelf-life at ambient conditions. [15,16,17] recommended temperature 5–8 °C with 90–95% of relative humidity to preserve the postharvest shelf-life of mandarins for a longer period. Table 1 summed up the recommended conditions and shelf-life of citrus-origin fruits. Moreover, 30–50% of postharvest losses of citrus fruits genus have been accounted due to physiological disorder, higher respiration rate, and water transpiration; these causes are mainly dependent on the inadequate storage temperature and relative humidity or other environmental conditions [10, 18]. Approxmately, 20 different types of postharvest diseases are responsible for the spoilage of the citrus fruits. Fungal pathogens are the principal cause of wastage of citrus fruits and economic losses by deterioration effects [19, 20]. The microbiological spoilage is considered the primary cause for the spoilage of the horticulture produces, they reduced the quality and safety of the produced by increasing respiration, oxidation, enzymatic activities, weight loss, and suppression of ethylene production [21]. Furthermore, fungal and mold growth in citrus fruits poses serious threat to humans due to production of prolific mycotoxins such as citrinin, patulin, and tremorgenic compounds [22,23,24]. Among fungal pathogens P. digitatum, P. italicum and G. candidum are the major cause for the spoilage and are responsible for postharvest disease incidence in citrus fruits and economic losses as well [2, 25]. P. digitatum is responsible for the 90% postharvest losses in citrus fruits [2, 26]. Both Penicillium molds are wound pathogens that infect citrus fruits via rind wounds and produce spots on the fruit surface after 2–3 days due to physiological and storage conditions [27, 28]. There are very few studies available on the infection mechanism of P. italicim; they do not produce any secondary metabolites [29]. In addition, another important fungal pathogen is known as Guignardia citricarpa which causes black spot/sour rot disease in citrus fruits [30, 31]. The pathogenicity of G. citricarpa in citrus fruits is due to the secretion of extra-cellular endo-polygalacturonases; which are responsible for the rapid breakdown of tissues and cause postharvest diseases [32]. Similarly, Alternaria species such as Alternaria citri are responsible for the black rots in citrus fruits and cause postharvest problems [33, 34]. Moreover, mechanical damage during harvest and postharvest handling often triggers citrus fruit’s high sensitivity towards physiological abnormalities such as brush burn, zebra skin, and oleocellosis in the peel, which are induced by rind abrasion, hard handling, or thorn punctures [35, 36].

Table 1 Storage conditions and shelf-life of citrus fruits [18, 37]
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Conventional and alternative management strategies for improving shelf-life

Postharvest management strategies such as cold storage, modified atmosphere packagings (MAPs), controlled atmosphere packaging (CAP), thermal, non-thermal edible coating, and nanoemulsion and nano-based formulations are required to improve the shelf-life and postharvest characteristics of citrus fruits [38, 39]. Nowadays, consumer preferences have been shifting towards eco-friendly solutions as alternatives to synthetic and plastic-based packaging, including edible coating to ensure food safety and quality of fresh citrus fruits with higher shelf-life and organoleptic characteristics. The conventional packaging system provides better gas and water barrier properties but they directly affected the environment by emission of green house gases and human health due to its non-biodegrabale nature and may acute toxicity. Citrus fruits have a mechanism that is very similar to oxidative stress, and an edible coating/formulation could be a great way to reduce oxidative stress and extending the postharvest shelf-life of citrus fruits [40, 41]. Various types of treatments such as irradiation (UV, X-rays, gamma, hot water treatment, organic and inorganic salts, biocontrol agents, nanomaterial’s, nanoemuslions, natural plant-based products, and edible coating can be used to reduce the blue and green mold in citrus fruits [2, 28, 42]. The use of fungicides for postharvest treatment of citrus fruits has increased in the last decade; posing environmental and health risks due to their remaining traces on the fruits’ surfaces and non-biodegradable nature. The concept of edible packaging is an alternative to overcome the use of chemical and synthetic fungicide for the postharvest treatments of citrus fruits [6, 36, 43, 44]. In this regard, the biopolymer-based materials such as polysaccharides, proteins, lipid, wax, essential oils, and nano-particles can be used to develop nanoformulations for extending the shelf-life of food products such as fruits and vegetables, dairy, bakery, meat, and meat products, etc. These formulations can be used alone or in combination with each others [45,46,47,48,49,50,51,52,53].

In addition, numerous plant-derived natural and active agents such as terpenoids, alkaloids, phenolic acids, aldehydes, essential oils, plant extracts, organic compounds of microbial origins, and animal-derived compounds can be used for incorporation in nano-formulations to extend the shelf-life of horticulture produces while maintaining their physico–chemical and organoleptic characteristics [21]. The primary goal of developing edible formulation as a postharvest technology around the world is to preserve the quality attributes of citrus fruits while reducing postharvest losses between harvests to consumption [20]. The natural active materials i.e., plant extracts, essential oils, phenolic compounds, and others phytochemicals such as α-terpineol, terpinen-4-ol, linalool, and limonene essential oils (EOs) [54], citral [55, 56], citronellal, carvacrol, thymol [57], hermal extracts [58, 59], trans-anethole, anise oil, cuminaldehyde, perillaldehyde [60], flavonoids, alkaloid, saponins, terpenoids, tannis, polyphenols, anthocyanin, essential oils [61,62,63,64,65,66,67,68,69,70], punicalagin [71], cinnamic acid, cinnamaldehyde [71, 72], carnosic acid, carnosol, hispidulin [73], tannic acid [74], thymol, carvacrol, geraniol, eugenol, octanal, citral essential oil [69, 75,76,77,78], garlic [79], neem [80], Withania somnifera L., Acacia seyal L. [62], mustard, radish [81], chilli pepper, ginger extract [82], limonene, β-linalool, α-terpineol, citral, octanal, [30, 83, 84], plant extracts (Cistus villosus, C. siliqua, and H. umbellatum, Cistus L. species) [85, 86], garlic, neem, mint, basil leave extracts [87,88,89], Anvillea radiata, Thymus leptobotrys, Asteriscus graveolens, Bubonium odorum, Ighermia pinifolia, Inula viscosa, Halimium umbellatum, Hammada scoparia, Rubus ulmifolius, Sanguisorba minor and Ceratonia siliqua [90] and eugenol [23, 91,92,93] are good agents for incorporation in formulations; which act as antifungalactive agents and help to reduce the growth of Penicillium digitatum, Penicillium italicum, Aspergillus niger, Fusarium sp., Mycelial and Botrytis cinerea in citrus and other horticulture produces.

Role of nanotechnology

Currently, nanotechnology is considered as an economically viable tool to extend the shelf-life of fresh foods. Owing to their higher surface area per mass compared to larger particles opens up a new avenue for developing more stable and biologically active nanoformulations to extend the shelf-life of fresh foods. It is one of the promising technologies for developing nano-formulations to extend the shelf-life of fruits and vegetables while also delivering active ingredients like colorants, flavoring agents, antimicrobial agents, preservatives, and nutraceuticals [94,95,96,97]. The nanomaterials and nano-formulations are more potential to extending shelf life of fruits and vegetables due to nano size range, which influenced the mechanical, thermal, barrier and anti-microbial properties edible packaging. The use of synthesis technologies in nanotechnology is showed good intermolecular interaction between biopolymers and active agents, which influenced the propertis of packaging materials and maintain postharvest quality attributes of fruits and vegetables. The various additives such as silver nitrate (Ag), gold (Au), zinc (Zn) copper (Cu), titanium di-oxide (TiO2) and othernanoparticles (NPs) have been used in nano-formulations for extending shelf-life of various foods [98]. Generally, the NPs described as colloidal and solid particles with 10–100 nm of size; below 100 nm size of NPs showed excellent antimicrobial acivity [99, 100]. In recent year, silver nanoparticles have been received more attenstion due to its antimicrobial activity and application in food processing sector [94, 101,102,103]. According to [34], the use of silver nitrate nanoparticles has the potential to reduce the growth of green and blue molds in citrus fruits during storage and extend shelf-life. Therefore, the use of silver nitrate has negative impacts on the nervous sytem and gastrointestinal tract. On other side, the use of food grade TiO2 nanoparticle is safe for the consumption on daily basis is 0.2–0.7 mg/kg of body weight per day throughout the life [104]. TiO2 nanoparticle recommended for the use in food by the United States Food & Drug Adminstration (1966) due to its non-toxic in nature with good antibacterial activity and film forming ability [105]. It has excellent ability to extending the shelf-life of food products by producing excellent barrier properties against gas and water transpiration, reduced particle size of the food packaging and microbial load [106,107,108].

Approaches to developing nanomaterials

The different types of methods can be used to synthesize the nanoparticles to obtained nanoformulations using two approaches such as bottom up” and top–down” approaches (Fig. 1). The top-down methods is a destructive approach for the synthesis of nanoparticles; in this the bulk materials reduction in components using a nanometric scale [109]. In case of bottom-up method (productive method), the nanomaterials are synthesized from elementary level. These approaches include methods of nanoparticles synthesis such as sol gel method [110], chemical vapor deposition [111], spinning [112], pyrolysis [113], biological synthesis (bacteria, fungi, yeast, viruses, plants) [114,115,116], thermal decomposition [117], mechanical milling [118], nanolithography [119], sputtering [120], and laser ablation [121] methods.

Fig. 1
figure 1

Schematic of methods for the synthesis of metal-based nanoparticles [109,110,111,112,113,114,115,116,117,118,119,120,121]

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Next generation formulations: blending of nanotechnology and conventional approaches

Nanobio edible coating

The edible coating as an eco-friendly approach may hold promise to shelf-life extension of horticulture produces while maintaining food safety and quality [38]. In addition, the edible/nano coating is safe for consumption, biodegradable/biocompatible in nature with antioxidant and antimicrobial actions. It also acts as carrier of active ingredients to improve the functionality of coating materials as well as fruits and vegetables [48, 49]. Nano bio coatings possess barrier properties against water and gas transpiration which resulted to extending shelf-life of produces by retarded respiration rate, control ethylene production, reduced weight loss, microbial contamination, and maintained firmness and other postharvest characteristics, which make it acceptable in market [45,46,47]. The concept of edible coating was started with application on citrus fruits (ornages and lemon) in twelfth and thirteenth century in china to extending the shelf-life using wax-based materials. The first commercial wax coating was introduced in 1930 for pears and apples [49, 122]. Various types of matrix or biopolymers such as polysaccharide (cellulose, starch, chitosan, pullulan, pectin, alginate, carrageenan, and others), proteins (corn, casein, soy protein, whey protein, wheat gluten, rice bran, and keratin, etc., and lipid wax/oils) are used to develop biodegradable edible coatings /to extend the shelf-life of food products; these components can be used alone or in combination to develop edible packaging [123,124,125]. The plasticizers (glycerol, sucrose, sorbitol, propylene glycol, fatty acid, polyethylene glycol, and monoglycerides) and emulsifiers [polysorbates (tween), soy lecithin, ester of fatty acids, ethylene glycol monostearate, fatty acids, sucrose esters, and sorbitan monostearate] are also used to improve the mechanical and physical properties of the edible packaging; these materials act as carriers of food additives such as antioxidants, colorants, flavors, nutraceuticals, nutrients, antimicrobial and antifungal agents for improving the properties of edible packaging as well as food products by decreasing intermolecular force between the matrix and additives [36, 48, 49, 126,127,128,129,130,131,132]. Plasticizers are low molecular weight components and polar in nature. The edible coating/film maintained the integrity of the food products and protect from mechanical, physical, biological, textural properties and oxidative stress [133,134,135,136,137]. It is also effective in barrier environmental moisture, aromas, flavor, gases, water, and oxygen barrier properties [124, 133]. Numerous researchers have been examined the postharvest shelf-life and physiochemical attributes of citrus fruits using edible formulations for example; [138] was improved the shelf-life of orange fruits using forultions of edible coating i.e., chitosan, locusts bean gum comprised with pomegranate peel extract by reducing the growth of P. digitatum molds. The sodium alginate, citric acid, sucrose formulations with or without Ficushirta fruit extract were also found effective to reduce the decay incidence, weight loss, respiration rate, and enzymatic activities of ‘Nanfeng’ mandarin fruits during the storage period [139]. The antioxidant activity of the ‘Nanfeng’ mandarins was improved by stimulation the accumulation of phenolic contents and defense enzymes such as SOD, PPO, POD, CAT, CHI and PAL, etc. On the other side, the clay-chitosan formulation was also found potential to reduce the growth of P. digitatum in ‘Thomsan navel’ oranges [140, 141]. Youssef and Hashim [142] has applied different formulations and treatments on citrus fruits for improving their postharvest shelf-life by reducing the activity of fungal pathogens; the essential oils such as cinnamon oil & eucalyptus oil, calcium chloride, bavistin, paraffin wax, paraffin wax + bavistin (0.1%) formulations were applied. The paraffin wax-based formulation with bavistin was found effective to extending the shelf-life of citrus reticulata blanco fruits up to 73 days by minimizing postharvest decay incidence and diseases. On other side, the treatment of 1-MCP was also found effective to control the growth of blue mold rot, supressed ethylene production and postharvest pitting [143, 144]. Evidently, it has been proved that the biopolymer based edible coating, composite, and nano-formulations with addition of nanoparticles, essential oils, and other natural source such as plant extract, antioxidant, antimicrobial and antifungal agents are potential to extending the shelf-life of citrus fruits by maintain their physiochemical & postharvest characteristics and retarded the growth of blue and green molds during storage conditions.

Nanoformulations based on essential oils (EOs)

Essential oils (EOs) are natural plants products can be extracted from plant sources and are utilized as preservatives, flavorings, and stabilizers in the food and pharmaceutical industries. The essential oil possesses antimicrobial, antioxidant, antifungal activities due to presence of secondary metabolites and other volatile compounds [145, 146]. Owing to their antimicrobial, antioxidant properties, EOs can also extend the shelf-life of food products. In recent decades, essential oils among the natural products were extensively used as stabilizers, antimicrobial and antioxidant agents to incorporate into food products and packaging materials [147]. In food sectors they can be used in alcoholic & non-alcoholic beverages, gelatins, sweet, soft drinks, milk & dairy products, ice-cream, soft drinks, baked foods, cakes, and candies as flavor and stabilizer agents to enhance the organoleptic and physicochemical properties; on other hands, they can be also used in pharmaceutical sector to improve the taste and flavor of drugs [145, 148]. In vivo and in vitro experiments indicated that the EOs is effective against the food borne pathogens [146]. Previously, the antifungal activity of the essential oils such as (citrus, cinnamon, lemongrass, thyme, oregano, tea, cumin, birch, and bergamot) against the citrus pathogens (P. digitatum and P. itallicum) has been confirmed through in vivo and in vitro studies [36, 149]. Generally, EOs are environmentally friendly, non-toxic, and biodegradable in nature; they are generally recognized as safe (GRAS) for human consumption and are frequently employed in postharvest citrus fruit management as antifungal agents, as well as edible coatings and formulations [150, 151]. The use of EOs in edible coating and formulations is considered an effective technique to prevent the postharvest decay and quality attributes of fruits and vegetables as well as other food products by minimizing phenomena of lipid oxidation or they can also be used to reduce or replacethe usage of chemical and synthetic additives [146, 152]. Various technologies and methods such as liposome, polymeric particles, ultra-sonication, nano-emulsion, and solid-lipid nanoparticles have been used to incorporate or encapsulate the EOs in the food matrix and edible packaging [148, 153, 154]. These techniques coud improve the stability and efficiency of EO in the matrix by reducing the interaction between unstable, volatile compounds and external factors.

As per the European council (EC) regulation no. 1907/2006, should be stands on the improving protection of human health and the risk of environment degradation by reducing the use of chemical and substances. The framework EC 1935/2004 amended as 2023/2006, covers all the primary materials comes in to food contact including active materials, adhesives, ceramic and plastics materials. Whereas, the plastic food contact materials (EC 10/2011) stated that, the materials should be nano forms as per specification of Annex-I of the regulations to avoid the migration of substance. Therefore, EU 450/2009 stated that, the addition of active agents in nanoformulations for food packaging applications able to release or absorb substance in food packaging [155].

Several researchers have incorporated various types of EOs such as Thymus capitatus [156], sunflower [157], citral and eugenol [158], cilantro & coriander [159], clove & clove bud [160, 161], Ziziphora persica [162], cinnamon, palmarosa, lemongrass [163,164,165,166,167,168,169], nettle [170], thymol [171, 172], viride [173], anethum graveolens [174], mentha [175], oregano [176,177,178,179], lemon [180, 181], nigella sativa [182], rosemary [181], neem essential oil, and moringa oil [183,184,185,186] as an active agents in edible coatings and films to shelf life extension of fruits and vegetables. The addition of essential oils in biopolymer based nanoformulations influenced the flavor and aroma of the fruits and vegetables help in control release, which improving or maintaining the antimicrobial and antioxidant effects for longer period. Many researchers reported that the nanoformulations based on EOs are novel methods to extending the shelf-life of fruits and vegetables by improving their postharvest characteristics [187]. For example, Rizzo and Muratore [148] reported that the incorporation of essential oil with packaging materials improved the UV barrier and biological activity, and increased surface hydrophobicity of the food packaging; that resulting in the extending shelf-life of food products due to release of antioxidant and antimicrobial agents. The combination of eco-friendly packaging with essential oils extracted from natural sources and agro industrial waste are a sustainable approach in food packaging and processing sector [188, 189]. Neverthelss, essential oil has been reported by several studies as a potential inhibitor of postharvest infections and molds, however there are few reports on the use of lemon grass, clove, neem oil, and eucalyptus oil on mandarins (citrus) [184]. Nowadays, the food packaging industries facing the challenges related to negative impact of essential oils on the natural flavor of the fruits and vegetables, which reduced consumer acceptability. Therefore, the masking of unpleasant aroma of the essential oils should be control using different types of masking techniques such as use of hydrocolloids, blending of essential oils and emulsifiers etc.

Nanoformulation based on plant extracts

The use of chemical and synthetic antioxidant has been barred by the regulatory bodied due to their health effects [190, 191]. Recently, scientists have focused on natural sources, such as plant extracts, as antioxidants and antibacterial agents to use in active edible packaging to improve food quality and integrity [192, 193]. Plant extracts from fruits and vegetables are used as natural bioactive compounds as antioxidant additives in the food and pharmaceutical industries [194]. Many studies have revealed that incorporation of natural plant extracts in biopolymer based edible packaging exhibited excellent antioxidant activity and reduced UV light transmission [194,195,196,197,198,199,200]. The incorporation of plant extracts as antioxidant agents influences the functional properties of edible packaging as well as food products due to molecular interaction between natural antioxidant and biomaterials. Previous studies confirmed that the addition of tea extract in gelatine based edible film established the hydrogen bonds with base materials to reduce free hydrogen. On other hand, [196] reported that the water barrier property of the edible film could be reduced by incorporation of plant based natural extract. Some researchers reported that the edible coating enriched with plant extract or natural antioxidant agents help to improve the postharvest characteristics and shelf-life of fruits and vegetables by reduction of weight loss and respiration rate [197, 201, 202]. Similarly, addition of plant extract such as propolis [197, 203], clove [204], rosemary [205], ginger, ginger tea, grape seed, ginko leaf [199, 202, 206], pomegranate peel [45,46,47, 125, 207,208,209], tartary buckwheat extract [210], olive leaf extract [211], cocoa (leaf and pod) extract [212], curcuma [198], beetroot, carrot [194], ginseng [196], moringa oleifera extract [213, 214], borage extract [195], pineapple fruit: peel, pulp and core etc. Bitencourt [201] as natural antioxidant agents to improve the properties of edible coating in different types of fruits and vegetables. The antioxidant and antimicrobial potential of these plants extracts; results mainly from bioactive compounds such as phenolic and flavonoids compounds and their additives, antagonistic and synergistic effect; they also help to provide strong free radical scavenging activity to protect the food from microbial load and free radicals [192]. The natural extracts as active agents can be incorporated in edible packaging directly, encapsulation using wall materials and through nanoparticles [215]. It is well documented; the citrus origin fruits can be affected by the green molds (P. digitatum) and blue molds (P. italicum); they are responsible for cause postharvest decay and disease incidence in citrus fruits [2]. The plant extract in combination with an edible coating could operate as a light barrier, preventing ascorbic acid degradation and controlling color browning in citrus fruits by inhibiting the growth of blue and green molds [216]. The use of plant extract and natural bioactive compounds extracted from plant sources are potential alternatives to solving the problems of food processing industries by replacing traditional packaging, chemical and synthetic additives used for color and flavour [217]. The plant extracts improved the properties such as antioxidant, phenolic activity, anti-browning, and antimicrobial activity of the edible packaging as well as extending postharvest shelf-life in citrus and other fruits and vegetables [218,219,220,221] due to improving barrier, mechanical and biological properties of edible film and coating [215, 222,223,224,225,226,227]. The incorporated extract of Ficus hirta fruits with sodium alginate and clove extract with carboxy-methyl cellulose were found improved shelf-life of ‘Nanfeng’ mandarin and ‘Xinyu’mandarin by enhancing the free radical scavenging activity and defence enzymes [139, 228]. The incorporation of Ficus hirta fruits extract improved the antimicrobial activity of material to control blue mold on mandarins. Similarly, the citrus fruit (Xinyu tangerine) shelf-life was extended by using chitosan based edible coating enriched with fruit extract of Ficus hirta Vahl [229]. These results showed that the integration of natural extract of Ficus hirta Vahl fruits with chitosan coating was found to be efficient in reducing the growth of the fungus strain P. italicum in citrus fruits during cold storage (5 °C). The coating also activates the activity of defence enzymes or maintains postharvest quality of citrus fruits. Chen et al. [230] reviewed and concluded that the use of natural plant extract (neem extract, oregano extract, clove extracts etc.) as herbal coating is a sustainable approach to extending the shelf-life of fresh produce by improving their physicochemical and organoleptic characteristics. Similar to essential oils, the incorporation of higher amount and concentration of natural plant extract can impart bitter taste and off flavour of the edible coating; which can also impair the acceptability of fruits and vegetables [40, 130, 231]. Based on the previous literature, it can be concluded that the inclusion of higher amounts of plant extracts and bioactive compounds can affect the organoleptic properties of the citrus fruits. The strategies for stabilizing phenolic and bioactive compounds extracted from plant sources in edible packaging for release control of bioactive compounds from food packaging system should be optimized for citrus origin fruits [232, 233].

Antifungal properties of nanoformulations

The antifungal activity of TiO2 nanoparticle against Penicilium in edible coating and film has been reported by [234]. Moreover, TiO2 has higher tendency to form aggregates and lower capacity to homogeneously disperse in organic media [108]. On the other hand, [235] reported that the incorporation of silver NPs in CMC and guar gum nano-formulations significantly extending the shelf-life of kinnow mandarins during storage at 4 °C and 10 °C for 120 days. The nanoformulation has the potential to considerably improve postharvest properties by inhibiting the growth of total aerobic psychrotrophic bacteria, yeast and molds. Table 2 summarizes the information about previously used nanoparticles to developing nanoformulations and their effects on postharvest characterstics and shelf-life of citrus fruits.

Table 2 Applications of nanoformulations—as a fungicide for inhibition of fungal growth of citrus fruits
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The nanosystem is generally used for functional modification of formulations that integrate to form polymeric nanoparticle, nano emulsion, solid lipid nanaoparticle, nanofibers and others [96]. Techniques such as high energy emulsification and low energy emulsification can be used to develop the nano-formulationnano-formulations [245,246,247,248,249]. The high energy emulsification techniques include high energy homogenization; ultra-sonication and microfludization [98, 250,251,252,253,254] are potential to form nanoemulsion with nano size. In addition, the combined nanotechnology and essential oils significantly improved the antifungal activity of nanoformulations and poetntail in control release mechanism. This mechanism improved the antifunla activity and stability of the matrix for longer period and their application resulted higher quality attributes in fruits and vegetables for longer period. Figure 2 shows the scientific representation of high energy techniques such as high-pressure homogenization, ultrasonication, and microfludization for developing nano-formulationnano-formulations. These techniques of emulsification help to generate the nanoparticles, influencing zeta potential, and poly-dispersity index of the formulations; which resuls in droplet size of nanoemulsion [223, 255, 256]. The high-pressure homogenization process can be used to yield droplet size of nano emulsion up to 1 nm. The microfludization process considered as efficient techniques for developing nano-formulation. For example; [257] reported that the microfludization technique produced the nano size of fish oil powder emulsion in the range between (210–280 nm). In case of ultra-sonication process; it is considered as very reliable techniques to developing nano-formulationnano-formulations using sonicator probe. Furthermore, the low energy emulsification techniques include phase inversion [252], spontaneous emulsification [250], solvent evaporation [258], and hydro gel technique [259]. Recently, the application of nanotechnologies has been drastically increased to extending the post harvest shelf-life of the fruits and vegetables. In addition, the high pressure technologies such as high pressure homogenization, ultrasonication and microfludization improved the encapsulation efficiency of the active agents by intermolecular interaction between biopolymers and active agents and resulted in reduction particle size of the matrix with improveing higher stability, antioxidant and antimicrobial properties. The several researchers have used the high-pressure techniques such as homogenization, sonicator, and micro-fluidization to developing nano-formulations for extending the shelf-life of fruits and vegetables for example; [260] improved the poly-dispersity, zeta potential and reduce the nano droplet size of chitosan-based nano-formulation using ultra-sonicator (300W, 60 °C per second).

Fig. 2
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Schematic of high-pressure techniques for nano-formulations

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The size of flax seed oil and surfactant-based formulation was reduced below 70 nm using high intensity ultrasonication [261]. The increasing time of ultrasonication is directly dependent on the input energy, which tends to generate the nano droplet size of the formulations by disrupt their particle size [262]. On other hand, the strawberry fruit shelf-life was extended using Chitosan based nano-formulationnano-formulation enriched with lemon essential oil; prepared using single pass of microfludization at 165 MPa [180]. Similarly, [263] also developed chitosan-based nano-formulationnano-formulation using ultra-sonication techniques at 60 °C for 30 min to extending the shelf-life of loquat fruits. The high-pressure homogenization and ultra-sonication technology was used to formulated capsaicin nanoemulsion added with tween 80 as surfactant; the nano-formulation size was below 65 nm and it also showed potential antimicrobial activity against E. coli and S. aureus [264]. Akbas et al. [265] also produced ginger essential oil-based nano-emulsion using microfludization technique to develop gelatine-based nanoformulation.

Several researchers have developed chitosan-based nano-formulationnano-formulation using curcumin [266, 267], gelatin-gum arabic based nano-formulation with incorporation of jasmine essential oil [268], resveratrol and curcumin in grape seed oil [269], incorporation of polyphenolic compounds in polymeric nanoparticles [270,271,272,273,274], and cassava-based nano-formulations enriched with lycopene [275]; these nano-formulations exhibited excellent antioxidant an antimicrobial activities. Another side, the researchers also incorporated various types of nanoparticles with essential oils such as peppermint [276], cinnamaldehyde [277], thymol containing EOs of Lippia sidoides in chitosan-gum NPs [278], and zein sodium caseinate NPs [279], oregano EOs in chitosan nanoparticles [280] to develop nano-formulation for the application as edible coating and packaging materials for food commodities [281]. For example, [282] improved the antibacterial and antifungal activities of the essential oils (eugenol & cinnamaldehyde) based on nano-formulations against Salmonella and Listeria using poly (d, l-lactide—co-glycolide) nanoparticles. On the other hand, liposome-based nanoparticle with Origanum dictamnus essential oil was exhibited to controlling the growth of gram positive and gram-negative microbes [283]. On basis of the scientific evidences, it can be concluded that the nano-formulations containing essential oils and their derivatives are highly potential to reduce the microbial load. The outcomes of the previous studies also showed that the high-pressure technologies such as microfludization and ultra-sonication are superior and effective technologies for the developing nano-formulationnano-formulations compared to conventional homogenization by distribution of particle size of the materials [251, 284]. On another side, [285] reported that the combination of ultra-sonication and microfludization technologie that significantly improved the emulsifiying and thermal stabilit of the pectin; also exhibited the good encapsulation efficiency of the vitamin D3.

Effects of edible nanoformulations on postharvest shelf-life of citrus fruits

Various researchers have investigated the use of edible formulations based on biopolymers with natural plant extracts, essential oils, and nanoparticles as antioxidant and antimicrobial agents on citrus fruits at various storage conditions; they reported that edible formulations have the potential to extend the postharvest shelf-life of citrus fruits while maintaining overall physicochemical and organoleptic quality attributes (Fig. 3). Table 3 summarizes the previous applications of edible coating/formulations and their effects on shelflife extensions of citrus fruits.

Fig. 3
figure 3

Functionality of coating formulations on citrus fruits

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Table 3 Shelf-life of citrus fruits influence by edible nanoformulations as postharvest treatments
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Physiological loss in weight (PLW)

The weight loss in the citrus fruits during storage are the major cause for unacceptability and food waste; the respiration and transpiration of the water content is main reason for weight loss, which can be determining by changing the water vapor pressure between environment and fruits [309]. The use of edible coatings on citrus fruits reduced weight loss by slowing down the rate of respiration and water exchange through transpiration [287, 294, 296, 310,311,312,313]. The main objective is to reduce the physiological loss in weight of citrus fruits to maintain their postharvest quality attributes and consumer acceptability [134, 314]. Several studies have found that using edibles with natural plant extracts, essential oils, nanoparticles, and other bioactive compounds as active ingredients can reduce physiological weight loss in citrus fruits by slowing respiration and lowering water loss [34, 139, 228, 235, 286,287,288, 315]. Barsha et al. [302] reduced weight loss of ‘Navel’ oranges by coating them with a Chitosan-based edible coating supplemented with bergamot thymes oil, tea tree oil, and lowering water transpiration and respiration rate during storage at 25 °C. The pectin-based edible coating with incorporation of essential oil (0.5%, 1.0%, and 1.5%) has been used for extending the shelf-life of ‘Valencia’ orange [316]. The application of commercial wax was used as an edible formulation which was found effective to minimze the respiration rate and water transpiration which resulted lower mass loss in citrus fruits [317].

Total soluble solids (TSS)

TSS expressed the presence of carbohydrate content in fruits and vegetables. During the storage conditions of fruits and vegetables, the TSS content has been increased due to hydrolysis of carbohydrate and conversion into sugar, High TSS is a result of higher respiration rate, microbial contamination, and ethylene biosynthesis [47, 318]. The edible coating is an effective way to maintain the TSS of the citrus fruits during storage condition due to controlling respiration rate and metabolism of sugar into organic acids [319], which resulted in minimizing the electrolytic leakage of the citrus fruits. The biopolymer based edible coating functionalized with natural plant sources such as essential oil, plant extract, phenolic and bioactive compounds etc. are potential to prevent the increasing TSS by reducing weight loss, respiration rate and ethylene biosynthesis [286, 287, 290, 297, 320]. The various researchers have scientifically proved that the application of biopolymer-based formulations could be potential to maintained the TSS of citrus fruits for example; “Newhall” naval orange TSS maintained by using CMC based active and non-toxic formulations during storage period at 5 °C [321]. The wax-based commercial formulation enriched with citral and octanal was found potential to maintained TSS of citrus fruits at 25 °C storage condition [55, 322].

Titratable acidity/pH

Acidity is one of the most important features that indicate the taste and quality of citrus fruits [323]. Furthermore, the pH of the fruits and vegetables indicated the acidic and basic nature of the fruits and vegtables; which is always in the opposite direction of acidity [45]. The pH of fruits is a direct result of free hydrogen ions, and acidity is the method through which hydrogen ions are released [324]. The biopolymer-based edible formulation contains essential oils, plant extract, antioxidant agents and nanoparticles are the effective approaches to mainteinedthe acidity and pH of the citrus origin fruits during the storage period due to reducing the accumulation of organic acids and their conversion in respiratory substrates in TCA and glycolysis cycles [45, 286, 290, 291, 297, 301]. The various researchers have reported the scientic evidences on these aspects for example; [226] reported that the application of polysaccharide (cactus) based edible coating maintained higher acidity of the citrus fruits (kinnow mandarins) during storage. Similarly, [321] also maintained the acidity of the ‘Nehwall’ orange using formulation of carboxymethyl cellulose (CMC) with Impatiens balsamina extract at 5 °C with 90–95 relative humidity. Similarly, the ‘Navel’ oranges acidity was also maintained by [325] using chitosan-cinnamaldehyde based edible coating at 10 °C throughout the storage period by reducing the losses of fructose, citric acid, and glucose contents. This might be possible due to creation of barrier properties by edible coating between surface of citrus fruits and storage environment. Furthermore, the lipid-based nano-formulationnano-formulation enriched with nanoclay and essential oil of orange peel was also found effective to controlled degradation of titratble acidity of blood orange as compared to control samples [295].

Respiration/ethylene

The inadequate amounts of ethylene and respiration rate are main cause for the degradation of quality attributes and shelflife of citrus fruits during storage; they are responsible for color browning, weight loss, deterioration, and off flavor [10, 326]. Additionally, the environmental conditions such as temperature and humidity are responsible for increasing respiration rate; which increases temperature inside the fruits [287, 327]. Specially, in citrus fruits the ethylene biosynthesis increased the stimulation of chlorophyllase; which process responsible for break down pectin methyl esterase and chlorophyll content. The texture quality and color attributes are loss by the breakdown of pectin methyl esterase and chlorophyll contents [287]. The edible coating on citrus fruits leads to control respiration rate and minimized ethylene biosynthesis of citrus fruits during different storage conditions; which is resulting to maintain fiemness, color attributes and other postharvest characteristics of citrus fruits for longer time [296, 310, 312, 328]. Numerous studies have reported that the edible coating could restrict the gas exchange and ethylene biosynthesis [329,330,331]. The previous studies confirmed the aptness of the edible coating on citrus fruits for example; Velásquez et al. [317] reported that the HPMC-lipid based edible composite coating enriched with food additives was reported potential substitute for citrus commercial wax as antifungal and non-toxic formulation to improving the storability and appearance of citrus fruits. The efficiency of essential oil (0.5%, 1.0%, and 1.5%) with pectin-based edible coating was found effective to extending the shelf-life ‘Valencia’ orange at 23 °C by maintaining the postharvest characteristics, minimize lipid-oxidation and weight loss. Several researchers have applied different types of edible coating and nano-formulations such as bee wax, coconut oil [287], carnauba wax + organooclay [294], chitosan + essential oils (bergamot, thyme oil, tea tree oil) [302], sodium alginate + extract of Ficus hirta fruit [139] and nano-particles based formulations [236, 237]; and such treatments were found effective to control the respiration rate and ethylene biosynthesis of citrus fruits during storage periods. These treatments are also effective to reduce the decay incidence, weight loss, reduced microbial/fungal load and enzymatic activities of citrus fruits during.

Color

Color is an important factor for visual appearance of the fruits and vegetables; it is first preference of the consumers to choose the fruits and vegetables for consumption [327, 332]. Generally, the color browning is a result of degradation of chlorophyll content, higher ethylene biosynthesis and respiration rate [333, 334]. The enzymatic activity and granuataions are responsible for browning and reduction of color properties of citrus fruits [40]. The recent studies have shown that the edible coating as eco-friendly approach is considered to maintain the color attributes of the citrus fruits by reducing enzymatic activation, degradation of chlorophyll contents, reducing respiration rate, and ethylene biosynthesis [312, 335]. The shellac and bee wax-based edible coatings enriched with food additives such as potassium sorbate, sodium benzoate and sodium propionate and their compositions have also been applied on Ortanique mandrains [336], and Clemenules mandrains [337] to extend their shelf-life by controlling weight loss, color properties, and other quality attributes at 5 °C and 20 °C respectively. The formulation containing wax, citral, and octanal was reported potential approach to maintained the color attributes of citrus fruits at 25 °C storage temperature by minimizing enzymatic browning (polyphenol oxidase, peroxidase), ethylene biosyntheis and respiration rate [55, 67, 287].

Firmness

Firmness is a most important quality attributes of the fruits, which directly influences the marketing and consumer appealance; it can be measured using texture analyzer and sensory analysis methods [295, 338]. The firmness of the any fruits can be degrading by insoluble proteopectin to more pectin and pectic acid. Furthermore, in case of citrus fruits the degradation of insoluble proteopectin process is very slow as compared to other climatric fruits [339]. Moreover, the edible and non-toxic formulations containing of plant nautral sources (essential oils, plant extracts, and antioxidants) and nanoparticles are an alternative and sustainable approaches to extend the shelf-life of citrus fruit by maintained their texture and firmness properties by retarded the respiration rate, weight loss and ethylene biosynthesis [140, 294, 295, 298, 302, 317, 320, 330, 331, 340,341,342]. Previously, researchers have applied various types of edible coating and formulations like bee wax to maintain the firmness of the citrus fruits. On the other side, Barsha et al. [302] investigated the effects of chitosan coating containing bergamot, thyme, and tea tree oil on ‘Naval’ oranges during storage. The coating treatments were found effective to improving the postharvest quality of oranges at 25 °C. No significant changes were observed in the development of quality parameters of orange fruits throughout the cold storage using coating treatment but reduced loss of physiological weight and firmness was observed. Kaewsuksaeng et al. [310] also reported that the application of polysaccharide based edible formulations (chitosan/CMC) was more effective to maintaind the firmness of citrus fruits such as ‘Or’ & ‘Mor’ mandarins, ‘Navel’ oranges and ‘Star ruby’ grape fruits compared to commercial polyethylene wax. The edible coating derived from shellac wax was also reported to maintain the firmness of oranges by [297].

Antimicrobial/antifungal

The microbial spoilage is main factor to reduce the shelf-life and quality attributes of the citrus fruits by increasing lipid oxidation, ethylene biosynthesis, and higher respiration rate. They also produce mycotoxins such as citrinin, patulin and tremorgenic compounds in citrus fruits [21, 22, 24, 343]. P. digitatum and P. italicum are the major mold pathogens responsible for postharvest diseases in citrus fruits [2, 25]. On basis of the scientific evidence, it has been reported that the application of edible formulations enriched with plant natural sources and nanoparticle could be beneficial in reducing the growth of bacteria and molds (green and blue) in citrus fruits in citrus fruits (Fig. 4). The addition of active agents such as plant extract, essential oils and nanomaterisal significantly enhanced the antimicrobial and antioxidant activity of nanoformulations due to presence of bioactive compounds such as phenolic and flavonoid content. These bioactive compounds inhibit the growth of free radicals, minimizing the oxidation and enzymatic activations in citrus fruits during the storage period. In addition, the nano range of biopolymer based formulation compatibale with the active agents and controls their release mechanism for a longer period which resulted higher antioxidant and antmicrobial agents [296, 312, 344, 345]. [346] reported that the composite edible film developed with hydroxypropyl methyl-cellulose (HPMC) and lipid showed antifungal activity against P. digitatum and P. italicum in citrus fruits. They also reported the salt such as potassium sorbate, sodium benzoate is also effective against both the pathogens. Similarly, [320] also investigated the antifungal efficiency of hydroxypropyl methyl-cellulose and lipid components (shellac and bee wax) based edible formulations on clementine mandarins, hybrid mandarins and oranges. The composite formulation was found effective against green and blue molds of citrus. The additive sodium benzoate with hydroxypropyl methylcellulose and lipid components was found to be most the effective strategy to prevent postharvest decay of citrus and inhibit fungal pathogens. On other hands, HPMC-lipid based edible coating enriched with food preservatives (potassium sorbate, sodium benzoate and sodium propionate) and their compositions were also applied on ‘Valencia’ orange to retarded the fungal activity against green and blue molds (P. digitatum and P. italicum) and extending the shelf-life of oranges at 5 °C for 60 days by maintained the postharvest characteristics. The HPMC-lipid based edible coating enriched with food additives was reported to be an ideal substitute for citrus commercial wax as antifungal and non-toxic formulation for improving the storability and appearance of citrus fruits [317]. The HPMC-lipid (shellac and bee wax) based edible coating enriched with food additives such as potassium sorbate, sodium benzoate and sodium propionate and their compositions were also applied on Ortanique mandrains [336] and Clemenules mandrains [337] to extending their shelf-life by controlling weight loss, maintaining firmness, visual appearance, colors, and growth of fungal pathogens. The Ortanique and Clemenules mandrains were stored up to 8 week (5 °C) or 1 week (20 °C) and 30 days (5 °C) or 7 days at 20 °C respectively. On other hands, [321] formulated non-toxic formulation using carboxymethyl cellulose (CMC) enriched with Impatiens balsamina extract as antifungal agents to extending the shelf-life of “Newhall” naval orange during the storage period at 5 °C with 90–95 relative humidity. The treatment of developed formulation was found potential to improving the appearance and postharvest characteristics of orange by reducing the weight loss, respiration rate, maintained TSS, acidity, ascorbic acid, reduced undesirable effects by minimizing lipid per-oxidation. Moreover, the treatment of CMC + Impatiens balsamina extract based coating was also found effective to increased free radical scavenger activity, defense enzymes such as peroxidase, superoxide dismutase, chitinase and β-1, 3-glucanase respectively. On other side, “Newhall” naval orange was treated by 95% ethanol-based clove extract (100 mg/mL) and stored at 7 °C (90–95% RH). The treatment of ethanolic clove extract was effective to maintain physiological factors of orange including reduced weight loss, decay rate, maintained TSS, acidity, ascorbic acid content. The activity of defense enzymes such as superoxide dismutase and chitinase was effectively enhanced [321]. This study also suggested clove extract as substitute of synthetic fungicide to extend the shelf-life and storability of naval oranges. The Opuntia cactus-based coating was also used by [226] to extend the shelf-life of mandrain during storage at 5 °C for 35 days by maintain their physicochemical and physiological characteristics.

Fig. 4
figure 4

Antifungal postharvest strategies for citrus fruits [2, 20, 28, 48, 49, 347, 348]

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The polysaccharide derived biopolymer-based edible coatings was also reported potential to extend the shelf-life of citrus fruits. For example: the treatment of chitosan-based edible coating on naval oranges showed promosing results [349]. This study found that treating orange fruits with 2% chitosan-based coating reduced disease incidence and lesion diameter when compared to control fruits treated with a 0.5% glacial acetic acid solution which is mainly by increasing defense enzyme activity, inhibiting catalae activity, lowering ascorbate content. The activity of ascorbate peroxidase of naval oranges was slightly induced by chitosan based edible coating during 14–21 days of storage. Furthermore, the inhibition activity of chitosan-based coating on naval orange fruits against pathogens was remarkably (P < 0.01) improved at ambient temperature. The preharvest treatment as spray of chitosan-based coating was also effective to resistance against gray mold [350].

The effects of chitosan-based edible coating with or without incorporating essential oils on citrus fruits were investigated by various researchers. For examples; [302] investigated the effects of chitosan coating containing bergamot, thyme, and tea tree oil on ‘Naval’ oranges during storage. The coating treatments were found effective to improve the postharvest quality of oranges at 25 °C. The chitosan coating enriched with tea tree oil was most effective coating treatment to reduce 50% microbial growth and decay as compared to other treatments and uncoated. The treatment of chitosan-based coating could induce the resistance against black spot disease caused by Guignardia citricarpa, in oranges by regulating the level of hydrogen peroxide, ascorbateglutathione cycle and antioxidant enzymes; also, potential to inhibit the growth of P. digitatum, P. italicum, Geotrichum citri-aurantii, and B. cinerea in citrus fruits during storage periods [31, 340, 341, 349, 351, 352]. El-Mohamedy et al. [353] reported oligo-chitosan based coating treatment effective against Colletotrichum gloeosporioides in citrus fruits, which is responsible for the sensory and nutritional quality of fruits. Oligochitosan treatment also helps to enhancement of ascorbate, phenolic & flavonoids contents, lignin, hydrogen peroxide and glutathione. Deng et al. [354] was incorporated Mycoparasite and Verticillium lecanii in chitosan-based coating to protect the citrus fruits from green mold at the cellular level. Chitosan based coating was found capable to reducing the growth of Penicillium digitatum on citrus fruit. The salicylic gel, aloe vera gel [291] and carnauba wax coated with mononitrile nano clay [295] was also reported effective way to maintained the physico-chemical quality and reduced the microbiological load of ‘Thomsan Navel’and ‘Blood’ oranges at 4 °C and 7 °C during storage.

The pectin-based edible coating with incorporation of essential oil (0.5%, 1.0%, and 1.5%) was found effective to extend the shelf-life of Valencia orange [355], they reported that, incorporation of 1.5% of essential oil-based with pectin has potential to extend the shelf-life of orange at 23 °C by maintaining the postharvest characteristics, minimize lipid-oxidation and weight loss. The essential oil applied in a commercial packaging line and oranges were stored up to 7 days at 25 °C with minimum losses of weight loss (0.9% compared to control ones. Numerous researchers incorporated essential oils and applied on citrus fruits to extending their shelf-life for example; [64] applied essential oil (L. scaberrima) as fungicides to prevent the shelf-life of ‘Tomango’ oranges by maintain the postharvest characteristics. The applied essential oil coating was inhibited the growth of P. digitatum. Similarly, the lemon shelf-life was extended by using carvacol and thyme essential oil by retarded the growth of P. digitatum and P. italicum and maintains the postharvest characteristics and visual appearance of lemons during the storage period [317, 356]. The incorporation of citral and octanal with the commercial wax coating was potential reported to extending the postharvest storability of citrus fruits during the storage period at 25 °C [55, 67, 357]. They reported that two treatments, having combination of wax and citral (10 × mfc) and another having combination of wax and octanal (2 × MFC) were effective to inhibit the growth of P. digitatum (green mold); also significantly increased the antioxidant activity and vitamin C, minimized enzymatic activity, maintained TSS, acidity, pH, color of citrus fruits during storage. On other hands, the formulation of essential oil (Cinnamomum zeylanicum) and commercial wax (shellac, carnauba, paraffin, and polyethylene) were used to control blue and green molds of citrus; the C. zeylanicum essential oil formulation with shellac and carnauba wax was reported most potential to improve the shelf-life and postharvest quality attributes of citrus during storage at 23 °C compared to other formulations [150]. This might be possible due to permeable to gas, solubility of formulation, biocomapatability between wax and essential oil compounds. The nano-zinc oxide-2S albumin protein formulation significantly reduced the growth of Candidatus Liberibacter asiaticus [358].

The biobased films made from sodium alginate and locust bean gum were found to have the capability to safeguard Wickerhamomyces anomalus viability while inhibiting P. digitatum growth. Furthermore, these formulations were applied on ‘Valencia’ orange to prevent their postharvest quality attributes by reducing the growth of yeast and green mold [298]. Cellulose (methyl, CMC, HPC cellulose) based coating treatments are potential to control decay of ‘Pineapple’ and ‘Valencia’ oranges for first 2–4 weeks at 16 °C temperature condition. The methyl cellulose formulation was able to control decay similar to commercial shellac coating (2000 mg/L). Orange’s treated with the formulation composed of natural seal, methyl cellulose and yeast candida (guillermondii) maintain the colony forming units/cm in ‘Valencia’ orange for 3 weeks. The alginate and gellan based edible formulation were found effective to extending the shelf-life of Fortune mandarins throughout the storage period by maintaining the quality attributes and organoleptic characteristics [328]. Coating on citrus fruits following a layer by layer (LBL) approach such as using cellulose derivatives, methylcellulose, hydroxypropyl cellulose, carboxymethyl cellulose and chitosan coatings, were investigated, and the results showed that carboxymethyl cellulose as internal layer and chitosan as external layer gave the best performance for keeping mandarins unaffected from microbial growth [15]. They also used several formulations of carboxymethyl cellulose, with steric acid, oleic acid, glycerol, and the results were compared with commercial wax. The chitosan-based coating enriched with clove essential oil showed antifungal activity against green mold (P. digitatum) and inhibited growth of mycelial in citrus fruits during storage. Shao et al. [299] was also reported that the chitosan (1%) without addition of clove essential oil is more effective compared to chitosan coating enriched with essential oil for inhibition of mold growth in citrus fruits. Orri mandarins shelf-life was extended using potato starch based edible formulation enriched with sodium benzoate as antifungal agents. The optimized starch-based formulation significantly reduced the growth of blue and green mold such as P. digitatum, P.italicum and G. citri-aurantii as compared to control [348].

Sensory characteristics

Sensory analysis is a process to evaluate quality attributes of the fruits based on their texture, flavor, aroma, and visual appearance [359, 360]. It is considered as a key influncer on consumer preferences to accept or reject fruit produce based on their selectable parameters [361]. The several factors such as ripening index, respiration rate, climatic conditions, water activity, microbiological contamination and enzymatic activity are known to affect the quality attributes of the fruits. Thus, the edible coating and nanoformulations are the alternative way to be used as packaging or coating to citrus fruits to improving their sensory characteristics by maintaining their overall postharvest quality attributes, reducing microbiological load, weight loss, maintain firmness and reducing enzymatic browning [362, 363]. The inhibition of weight loss and respiration rate in cuitrus fruits by using edible/noano formulations significantly reduced the risk of pathogens contamination, enzymatic browning, utilization of organic acids, PPO/POD and maintained the higher color attributes with retention of freshness and color/aroma for a longer period during the storage period [45,46,47,48]. Studies have reported that the edible coating and formulations can improve or maintain the sensory characteristics of citrus fruit by reducing enzymatic browning and microbial spoilage [291, 295]. The citrus (Kinnow mandrain) fruits postharvest shelf-life was improved using polysaccharide (extracted from opuntia cactus) based edible coating during 35 days of storage period [226]. Based on sensory score and overall quality attributes the higher acceptability was found by citrus treated with 2% of opuntia cactus polysaccharide based edible coating compare to other treatment and control samples at end of the storage period. The alginate and gellan-based edible formulations were found effective to extending the shelf-life of Fortune mandarins throughout the storage period by maintain quality attributes and organoleptic characteristics [328]. El-Mohamedy et al. [353] also revealled that the sensory charactersitics of citrus fruits were maintained after treating with oilgo-chitosan based coating treatment. Similarly, [310] reported that the application of polysaccharide-based edible formulations (chitosan/CMC) were more effective to maintain the sensory characteristics such as color, aroma, texture, and visual apparnace of ‘Or’ & ‘Mor’ mandarins, ‘Navel’ oranges and ‘Star ruby’ grapefruits compared to commercial polyethylene wax by reducing the respiration rate, weight loss, color broening, and microbial load during their storage.

Conclusion and future perspective

Being a non-climacteric fruits, citrus has a short shelf-life due to higher growth of blue and green molds. The edible coating/nano-formulations are the effective and sustainable approaches to extend the postharvest shelf-life of citrus fruits by retarding mold growths, control respiration rate, ethylene biosynthesis, and weight loss. The nano edible formulations are being developing using different types of biopolymers as alone and blend with each other’s. The poor water barrier and gas barrier properties of polysaccharide and protein based biopolymers are the main disadvantage. Therefore, the composite/blending (binary/ternary) of these biopolymers exhibited good properties and potential for shelf life extension of citrus fruits. Furthermore, the plant-based sources such as plant extracts, essential oils, and nanoparticles can also be used to develop antifungal formulations as an alternative of fungicides for extending the shelf life of citrus fruits by retarded the growth of fungal pathogens. There are very limited reports have been available on the masking of unpleasant aroma/flavor of the essential oils use in the coating formulations for fruits and vegetable applications. Based on these findings, authors suggested research on the different types of masking techniques such as use of hydrocolloids, blending of essential oils and emulsifiers to masking the flavor and aroma of essential oils in the nano-formulations. Many researchers have reported, essential oils as potential inhibitors of postharvest pathogens and molds, yet the reports on the use of lemon grass, clove, neem oil and eucalyptus oil on mandarins (citrus) are limited. The silver nanoparticles are widely used for developing antifungal formulations to inhibit the growth of blue and green molds. In addition, the use of silver nitrate NPs may have negative impacts on the nervous system and gastrointestinal tract. Moreover, the NPs such as titanium dioxide (TiO2) have been reported safe for human consumption, which has also shown antifungal activity against P. digitatum and P. itallicum.

Further research should focus on finding biopolymers that are compatible with additives like plant extracts, essential oils, and nanomaterials, and that improve the antifungal activity of coating materials and citrus fruits. The biopolymers extracted from different types of fruits and vegetables by products such as kernels, seed, and peel can be considered as alternatives and sustainable ways to reducing the cost of coating formulations. The study on genetic variation on acidity of citrus fruits will also explore new area of research for scientific community. There are several commercial industries developing the coating formulations for fruits and vegetables prevention but they are using different types of fungicides to avoiding the fungal growth in fruits and vegetables. The effects of essential oils such as lemon grass, clove, neem oil and eucalyptus oil in edible/nano coating on the postharvest shelf life of citrus fruits needs more elaboration. Furthermore, more depth research is required to exploring and find out natural active agents as an alternative of fungicides to reducing the environmental as well as health effects. Therefore, the control release mechanism, toxicity and masking of unpleasant aroma/flavor of the essential oils in nano formulations should be explore in future with more details.