Introduction

In recent years, a growing interest of consumers in high-quality fresh fruit and vegetables has been observed [129]. This is mainly due to a better knowledge of the health benefits of produce consumption, as well as to the increased awareness of their nutritional content and functional traits [148]. Nevertheless, fruit and vegetables are perishable products whose safety, sensory and nutritional/functional qualities may be highly reduced if appropriate postharvest techniques are not applied, leading to a shortened shelf life [149, 174]. In addition, such reduced shelf life of fresh fruit and vegetables may result in postharvest losses with socioeconomic and environmental implications [100, 129]. For instance, in the year 2010, the United States Department of Agriculture reported a 10.5% loss of the total fruit and vegetables available at a retail level in the United States of America (USA) and 24.5% at the consumer level, leading to total economic losses of $34.8 billion [34]. Therefore, with the aim of improving the convenience and safety of horticultural products, without altering their sensory attributes, the world horticultural industry is constantly searching for innovative and sustainable processing, handling and storing techniques [10, 106].

Although it is well-known that temperature is the most important postharvest factor to be considered to preserve quality and safety in fresh horticultural products [86, 174], storing them in an improper atmosphere may result in some physiological disorders [171]. In this sense, ethylene (C2H4) is a plant hormone—physiologically active in trace amounts (even less than 0.001 μL L−1)—whose postharvest actions are diverse and vary between produce [119, 133, 165]. Nonetheless, overall, the major effects are related to an accelerated ripening process in climacteric fruit and to an accelerated senescence of non-climacteric produce, affecting the quality and leading to a reduction in postharvest life [98, 169]. Furthermore, it has been found that exposure to C2H4 increases postharvest rotting (fungal development) in non-climacteric produce [131]. Therefore, C2H4 is a critical factor in postharvest management of fresh horticultural commodities, especially during long-term storage and long-distance transport [104, 179]. According to Kyriacou and Rouphael [99], prevention of detrimental C2H4 action may be achieved through the use of scavenging technologies. Meanwhile, Wills [165] point out that the use of C2H4 scavenging tools is a low-cost option for preserving fresh produce quality. In addition, C2H4 scavengers seem to be a promising tool to minimise the need for refrigeration and hence to reduce energy consumption [166].

C2H4-scavenging products may be used to remove C2H4 from storage facilities and transport vehicles, as well as from the in-package atmosphere [32]. Nevertheless, since packaging is a major critical factor in postharvest management for preserving quality and safety of perishable horticultural commodities [107, 172], the most common application of C2H4-scavenging technology is in food-packaging systems [25, 175]. In fact, at the time of data collection, it was found that most research deals with C2H4-scavenging tools as an extension of modified atmosphere packaging (MAP) technology for fresh produce (Table 1), although some trials have also been carried out on produce stored under controlled atmosphere (CA) conditions (Table 2).

Table 1 Overview of KMnO4-based C2H4-scavenger trials and effects on fresh produce packaged in modified atmospheres
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Table 2 Overview of KMnO4-based C2H4-scavenger trials and effects on fresh produce stored under controlled atmosphere conditions
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Nevertheless, CA storage can be expensive since the facilities are usually fairly complex and the maintenance of the optimal gas levels demands a high surcharge [56]. Therefore, compared to CA technology, the active MAP technology is an attractive option to delay the rate of fresh fruit and vegetable physiological processes, extending their shelf life [75, 108].

Unlike traditional inert packaging, an active package is engineered to achieve an intentional and dynamic modification of the atmosphere within the package by means of scavenging or releasing technologies [75, 105]. Although the active MAP technology for fresh commodities is not new [176], it has gained popularity in recent years. Furthermore, in 2014, active packaging comprising C2H4-scavenger tools was described as an emerging technology [56]. Nowadays, one of the most commonly used C2H4-scavenging technology is based on potassium permanganate (KMnO4) [4].

KMnO4-based C2H4-scavenging systems for fresh fruit and vegetable packaging have been extensively studied [116, 170, 179]. Nonetheless, to the best of our knowledge, there is not a previous comprehensive review discussing applications of KMnO4-based C2H4 scavengers in food packaging and their effects on quality aspects of fresh produce. In fact, there is a lack of attention on the influence of storage conditions and commodity physiology on the C2H4-scavenging performance of the said scavenging systems. Thus, the present review compiles studies concerning the KMnO4 application as a postharvest tool to remove C2H4 from the surrounding atmosphere within the package. There, emphasis is made on reviewing KMnO4-based C2H4-scavenger effects on quality attributes of both climacteric and non-climacteric produce. Furthermore, special attention is given to factors that may affect the scavenging performance of such C2H4 removal systems.

State-of-the-Art of Potassium Permanganate-Based Ethylene Scavengers

C2H4 is a gaseous simple carbon–carbon double bond molecule that is commonly found in the environment. C2H4 is endogenously produced by plant tissues and some microorganism metabolism, but it can also occur as a result of combustion of hydrocarbons [91]. In fact, C2H4 is the most produced organic compound throughout the world [16]. Hence, because the postharvest life of many fresh fruit and vegetables is highly limited by means of their own metabolic processes and C2H4 exposure since they are still alive and respond to the environment factors, much effort has been devoted to protecting horticultural products from the negative effects of C2H4 exposure [91, 110, 178].

Among the available technologies for controlling C2H4, 1-methylcyclopropene (1-MCP) is an C2H4 action inhibitor that works by competitively blocking fruit C2H4 receptors [165]. As an effective C2H4 antagonist, 1-MCP has been globally recognised as a useful agro-food industry tool [161]. Nonetheless, it can only be conveniently used in certain horticultural products since pathological and physiological disorders (e.g. CO2-related injuries and chilling-related disorders) may be increased in both non-climacteric and climacteric fruit after 1-MCP exposure [102, 162].

C2H4 action may also be avoided by removing C2H4 surrounding fresh commodities. It has been proven that C2H4 scavengers are effective in removing C2H4 from the in-package atmosphere of fresh fruit and vegetables [75, 116]. C2H4-scavenging technology involves physical phenomena (overall, adsorption) and/or chemical reactions (usually, C2H4-oxidising agents are used) [45].

C2H4 adsorption may be performed by means of materials with active surfaces such as activated carbon, zeolites and some clays (e.g. pumice, cristobalite and clinoptilolite) [91, 97, 154]. Such materials can be contained into a C2H4-permeable sachet, or fine particles of those minerals can be dispersed into the packaging film (e.g. polyethylene films) [158, 172]. Nevertheless, active films incorporating mineral powders have shown low C2H4 removal efficacy because of low amount of mineral that can be incorporated without affecting the mechanical properties of the package [83]. In addition, they are opaque and grainy materials and incorporated mineral powders affect film permeability letting carbon dioxide (CO2) gas out more rapidly and letting oxygen (O2) enter more readily than regular plastic films [97].

Another scavenging approach is based on chemical processes [99], although in some cases such an approach is not profitable. For instance, palladium (Pd) and titanium dioxide (TiO2) are catalytic materials that can efficiently remove C2H4 [87, 153], but Pd is expensive and the reaction between TiO2 and C2H4 is ultraviolet light dependent [154, 175].

Over the years, the most well-known, inexpensive and commonly used C2H4-scavenging technologies have been those based on KMnO4, a strong agent that chemically scavenges C2H4 by an oxidation process [116, 142].

KMnO4-based C2H4-scavenging technology has been widely used for almost 50 years. The first report on the KMnO4 application in C2H4-scavenging systems for fresh fruit preservation dates from 1967 [67]. Early studies were on the storage of apples and bananas, where Forsyth et al. [67] and Scott et al. [139], respectively, found that KMnO4 can be useful to delay the ripening process. Since then, large research on the effects of KMnO4 on fresh horticultural products has been developed (Tables 1 and 2). Research results for 19 climacteric fruit and 5 non-climacteric fruit and 3 vegetable products were identified through searching in the Scopus and Web of Science search engines.

Most research has been largely centred on climacteric fruit, particularly in cultivars of apple and banana species, which in addition to produce a high C2H4 rate, they are highly C2H4 sensitive [24]. Moreover, KMnO4-based C2H4-scavenging products can be also used to protect floral products from the C2H4 detrimental damage (e.g. flower abscission or wilting, epinasty of buds and flowers and leaf shedding), maintaining their good appearance and extending their shelf life [8, 92].

Nowadays, several KMnO4-based products engineered for C2H4-scavenging applications are commercially available. As presented in Table 3, commercial C2H4 scavengers have been developed in different shapes (beads, cylindrical and irregular pellets and powder) containing 3.5–12% w/w KMnO4.

Table 3 KMnO4-based C2H4 scavengers currently available in the market
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According to the available information, currently available products in the market have a C2H4-scavenging capacity ranging from 3 to 6.5 L kg−1. Furthermore, there are some commercial KMnO4-based products such as Bi-On® SORB (Bioconservacion S.A., Barcelona, Spain), Chemisorbant (Purafil, Inc., Doraville GA, USA), MM-1000 MULTI-MIX® MEDIA (Circul-Aire Inc., Montreal, Canada) and Sofnofil™ (Molecular Products Limited, Essex, UK) that are not specifically intended for the removal of C2H4 but are marketed as broad-spectrum contamination control materials for air purification in industrial applications.

Ethylene Removal Reaction and Application Form of the Potassium Permanganate-Based Ethylene Scavengers

As presented in Tables 1, 2, 3, KMnO4 is usually supported on a porous C2H4 adsorber substrate with a large surface area, typically alumina, to facilitate the redox reaction [120, 154]. These C2H4-scavenger products work through an adsorption–oxidation mechanism, where C2H4 is physically adsorbed by the porous medium and then oxidised by KMnO4 [4]. KMnO4 oxidises C2H4 by attacking its double bond [101]. The C2H4 stoichiometric oxidation reaction (Eqs. 1–4) was previously described by Keller et al. [91]:

$$ 3{\mathrm{CH}}_2{\mathrm{CH}}_2+2{\mathrm{KMnO}}_4+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{MnO}+3{\mathrm{CH}}_3\mathrm{CHO}+2\mathrm{KOH} $$
(1)
$$ 3{\mathrm{CH}}_3\mathrm{CHO}+2{\mathrm{KMnO}}_4+{\mathrm{H}}_2\mathrm{O}\to 3{\mathrm{CH}}_3\mathrm{COOH}+2{\mathrm{MnO}}_2+2\mathrm{KOH} $$
(2)
$$ 3{\mathrm{CH}}_3\mathrm{COOH}+8{\mathrm{KMnO}}_4+\to 6{\mathrm{CO}}_2+8{\mathrm{MnO}}_2+8\mathrm{KOH}+2{\mathrm{H}}_2\mathrm{O} $$
(3)

Thus, the overall stoichiometric oxidation reaction is

$$ 3{\mathrm{CH}}_2{\mathrm{CH}}_2+12{\mathrm{KMnO}}_4\to 12{\mathrm{MnO}}_2+12\mathrm{KOH}+6{\mathrm{CO}}_2 $$
(4)

As the reaction proceeds, a gradual colour change from purple to dark brown occurs because the permanganate anion (MnO4–) is reduced into manganese dioxide (MnO2) [110, 168]. This phenomenon can be clearly noted in Figs. 1 and 2. These figures show the C2H4 breakthrough measurement on a KMnO4-based C2H4 scavenger (Bi-On® R12, Bioconservacion S.A., Barcelona, Spain) held under a continuous C2H4-enriched air flow (5000 ppm at 120 mL min−1) at room temperature (22 °C) and the changes in KMnO4 concentration (%) and colour that occur in the material over time.

Fig. 1
figure 1

Changes in C2H4 uptake capacity (a) and KMnO4 concentration (b) and visual colour change image (c) of a KMnO4-based C2H4 scavenger (Bi-On® R12, Bioconservacion S.A.) material over time

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Fig. 2
figure 2

Evolution of colour parameters a*, b*, C* and H° over time for a KMnO4-based C2H4 scavenger (Bi-On® R12, Bioconservacion S.A.) held under a continuous C2H4-enriched air flow

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KMnO4-based C2H4 scavengers are mainly used in sachet form for food-packaging application but can also be found in different presentation forms such as blankets, filter tubes or filter machines that can be placed in warehouses or transport chambers, incorporated into household refrigerators systems, or directly incorporated into the packaging material [83, 110, 120]. For instance, Taboada-Rodríguez et al. [152] reported that barrier properties of the cardboard material usually used in food packaging could be improved by applying a coating made of polylactic acid and KMnO4 supported onto sepiolite. However, KMnO4 is not typically integrated into food contact surfaces of food packaging because of its toxicity and colour [25, 68].

In a commonly individual C2H4-scavenging packaging system (Fig. 3), a sachet containing the C2H4-scavenger product is placed within the produce package [83], at the upper part of the package [113, 142]. At this point, it should be clarified that the most suitable position to attach a scavenger—or a release—sachet will depend on the density of the gaseous compound to be scrubbed or released, respectively. The density of a gas can be deduced from the ideal gas law (Eqs. 5–8):

$$ P\times V=n\times R\times T $$
(5)
$$ P\times V=\frac{m}{M}\times R\times T $$
(6)
$$ \frac{P\times V}{R\times T}=\frac{m}{M} $$
(7)
$$ d=\frac{P\times M}{R\times T} $$
(8)

where P is the atmosphere (atm), M is the gas molar mass (g mol−1), R is the gas constant (0.082 atm L (K mol)−1), and T is the temperature (K).

Fig. 3
figure 3

Schematic representation of an C2H4-scavenger sachet used in horticultural produce packaging

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Therefore, since C2H4 tends to rise to the top of the package because it is slightly less dense than air (for instance, the C2H4 and air density are 1.16 and 1.20 g mol−1, respectively, at 22 °C and 1 atm), the upper part of the package seems to be the most suitable position to attach an C2H4-scavenger sachet. Then, the sachet material must be highly permeable to the C2H4 gas molecule, allowing C2H4 uptake from the in-package atmosphere through a convection and diffusion mechanism [97, 175]. Some typically used sachet materials reported in the literature are polyethylene (PE), muslin cloth or cellulose [58, 114]. In general, the commercially available active packaging agents such as KMnO4-based products are typically packed into heat-sealed Tyvek® film sachets [172]. DuPont™ Tyvek® is a high-density PE film (Fig. 4) that meets the US and European Union regulations for food contact applications [57].

Fig. 4
figure 4

Scanning electron microscope image of DuPont™ Tyvek®

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Human Health and Environmental Concerns

Although there are already several registered commercial KMnO4-based products (Table 3), the active packaging use in the European market is just beginning to increase [158]. Gaikwad and Lee [68] and Vilela et al. [158] attribute such behaviour to low consumer acceptance and to a strict legislation. Despite the fact that KMnO4-based C2H4 scavengers are commonly placed into individual devices to keep products free from contamination and avoid human health risks associated with KMnO4 poisoning, there is still concern about possible KMnO4 migration. In this sense, it is highly recommended to avoid direct exposure to KMnO4, particularly, at concentrated solutions and to undiluted KMnO4 [78]. Besides that, KMnO4 can be harmful if it is swallowed, being classified by the Globally Harmonized System of Classification and Labelling of Chemicals as harmful if swallowed under the code H302 [156]. Therefore, it is true that KMnO4-based products must be carefully handled. Nonetheless, it is well documented that the risk of KMnO4 intoxication is not common, being approximately 142.9 mg kg−1 (10 g of KMnO4 per person weighing 70 kg) the lethal adult dose and 750 mg kg−1 the oral LD50 rat dose [38, 79]. In addition, KMnO4 is not listed on the World Health Organization (WHO) Acute Hazard Rankings [80]. Furthermore, KMnO4 is commonly used at low concentrations in highly diluted solutions as antiseptic and antifungal drug and it is included in the WHO Essential Medicines List [163]. KMnO4 as an indirect food additive is regulated by the Food and Drug Administration (FDA) under the Code of Federal Regulations (21 CFR §175.105 2018) [39]. General US FDA requirements for food contact applications are also established through the Code of Federal Regulations (21 CFR §177.1520 2018 and 21 CFR §177.2010 2018) [40, 41], while the European Union has specific requirements related to active packaging and its non-active parts that are regulated by the Regulation 1935/2004/EC and Regulation 450/2009/EC [63, 64]. Particularly, Regulation 450/2009/EC establishes a legal basis for active packaging safety and marketing [83]. The European Union and US regulation aspects concerning active and intelligent food packaging have been extensively discussed by Dainelli et al. [50] and Restuccia et al. [127].

On the other hand, the high oxidising power, either in heterogeneous conditions or without solvents, makes permanganate and its compounds of particular environmental concern. However, the permanganate oxidation process is considered to be eco-friendly [51]. In fact, permanganate (specifically, KMnO4) has been defined as a green oxidant in organic chemistry since the inorganic coproducts (particularly, MnO2) can be re-oxidised to permanganate, providing a sustainable approach [141, 147]. Therefore, as a green oxidant, KMnO4 has been widely used in industrial and agricultural processes, including air purification (by removing C2H4, pollutants such as sulphur dioxide (SO2), hydrogen sulphide (H2S), aldehydes and other volatile organic compounds), water treatment to remove pollutants such as algal toxins and pharmaceuticals, as a bleaching agent in textile industry and as an antimicrobial agent in pesticide products [51, 147, 154].

KMnO4 is not listened on the WHO Environmental Health Criteria Documents [164]. However, since KMnO4 is typically used at full scale for water treatment applications [61, 73], it is regulated by the United States Environmental Protection Agency under the chemical code number 068501 because high doses can be harmful to aquatic life [60]. KMnO4 is classified by the Globally Harmonized System as a very toxic agent to aquatic organisms under code numbers H400 and H410 [156]. Hence, KMnO4-based products should not be released into the environment (prevent from reaching water bodies such as lakes and rivers, soil and sewage systems). In addition, since KMnO4 is a strong oxidant agent, it can enhance combustion of other substances and can also release irritating or toxic fumes in a fire; thus, KMnO4-based products should be stored, disposed and separated from reducing, flammable and combustible substances [78]. Nevertheless, when KMnO4 is supported onto a solid material, the aforementioned MnO2 re-oxidation process is interfered [141]. The above gives rise to non-toxic and chemically inert by-products (Eq. 4) that do not represent an environmental concern and can be disposed of as normal waste [91, 135]. Therefore, KMnO4-based C2H4 scavengers can be considered an environmental-friendly tool that can be used in active packaging technology for horticultural products, with proper handling and relevant safety measures.

Effects of Potassium Permanganate-Based Ethylene Scavengers on Postharvest Quality of Fresh Produce

As earlier mentioned, significant effort has been made to elucidate KMnO4 effects on quality attributes of both climacteric fruit and non-climacteric produce. Tables 1 and 2 show how KMnO4-based products may be used to delay some physicochemical changes associated with produce ripening and senescence. However, cultivar, maturity stage, storage period and storage conditions, as well as the presence of exogenous C2H4 during storage or transportation, are some factors that may counteract the effect of these KMnO4-based tools [117]. For instance, in climacteric fruit, the rate and intensity of C2H4 production are higher at the ripe stage, regarding mature and half ripe stages [118]. The latter fact can be observed in the studies carried out by Bhutia et al. [19] and Amarante and Steffens [6, 7], where the effect of KMnO4-based C2H4 scavengers on ‘Kallipati’ sapote, ‘Gala’ apple and ‘Royal Gala’ apple was evaluated at different ripening stages. The results indicated that the C2H4 scavengers may retard the physiological and physicochemical processes in sapote and ‘Gala’ apple independently of the maturity stage, although the effects were more pronounced in fruit harvested and stored at a mature stage [6, 19]. Meanwhile, the influence of the maturity stage on the efficiency of C2H4 scavengers was found to be more marked in ‘Royal Gala’ apple [7]. Amarante and Steffens [7] reported that, despite having removed the total amount of C2H4 inside the apple package at each maturation stage, it was only possible to delay the ripening process in fruit harvested at less advanced maturation stage, while in fruit with a higher maturity stage, no significant effect was observed. The authors attributed such results to the fact that the C2H4 production rate increases along with the ripening process, causing a higher accumulation of C2H4 inside the package and hindering the C2H4 removal efficiency. Thus, the effect of C2H4 scavengers to delay the ripening process and extend the fruit and vegetable shelf life is more evident when they are harvested at the mature green stage. Therefore, it is recommended to harvest fruit and vegetables at the latter maturation stage or before the autocatalytic production of C2H4 in order to obtain the maximum benefit from the C2H4 scavengers.

In addition, because the responses to KMnO4-based C2H4 scavengers may vary between cultivars and species, in the following subsections, apple, banana, mango and tomato are used to illustrate the range of responses to KMnO4-based C2H4 scavengers since most of the literature available focuses on the study of such products. Nonetheless, an outline of the general responses of other climacteric fruit and non-climacteric produce to KMnO4-based C2H4 scavengers is provided to illustrate the potential for use and both benefits and limitations that may be reached by using KMnO4-based technology.

Apple

KMnO4-based C2H4 scavengers within apple packages at low storage temperature (0–4 °C) have shown positive effects on quality parameters, being effective to delay maturation process and then prolonging its shelf life up to 8 months [31]. Particularly, in ‘Gala’, ‘Royal Gala’, ‘Golden Delicious’ and ‘Brookfield’ cultivars, the incorporation of a KMnO4-based C2H4 scavenger into the storage container has been shown to decrease pulp firmness loss and to delay shell yellowing (Tables 1 and 2). Shorter et al. [143] and Knee and Hatfield [95] reported that bitter pit and superficial scald was reduced in ‘Granny Smith’ and ‘Golden Delicious’ apple, respectively, by scavenging C2H4 with KMnO4-based tools.

On the other hand, the titratable acidity (TA) and soluble solid content (SSC) behaviours throughout the ripening process depend on the variety in question. Therefore, KMnO4 effects on TA and SSC parameters are different in each variety. The latter fact may be explained since each variety has its unique characteristics. For example, the storage of ‘Golden Delicious’ apple under MAP conditions together with a KMnO4-based C2H4 scavenger at low temperature led to a lower TA decrease and a lower SSC accumulation [137]. Meanwhile, the TA decrease in ‘Gala’ variety was delayed, and SSC was not affected [6, 29]. The opposite effect was observed in ‘Royal Gala’ apple with the SSC increase minimised while the TA behaviour was unaffected [7].

In general, it can be concluded that KMnO4-based C2H4 scavengers are a useful tool to delay the de-greening process and pulp firmness loss in apple fruit, as well as to reduce flavour changes or variations in sugars and free organic acid contents throughout storage at low temperatures. Furthermore, physiological disorders may also be decreased in refrigerated apples by using KMnO4-based C2H4 scavengers.

Banana

Very good results can be obtained by removing C2H4 from the surrounding fruit atmosphere, especially for long-distance transportation. In this sense, Wills et al. [170] reported that the green life of ‘Cavendish’ banana can be increased of about 27 days at 15 °C, 17 days at 20 °C and 11 days at 25 °C by reducing the C2H4 concentration from 1.0 to 0.01 μL L−1.

From different research studies, the incorporation of KMnO4-based tools inside packages, storage rooms or banana reefers has shown to be effective in delaying the chlorophyll degradation process—and then, the shell yellowing process—reducing weight loss and slowing down the softening process during storage [69, 136, 155, 177].

On the other hand, as mentioned above, the TA and SSC changes are different depending on the characteristics of the product. In this sense, several authors pointed out that both TA and SSC factors tended to increase throughout the storage period of banana fruit [136, 155, 177]. Meanwhile, García et al. [69] found that SSC increased while TA tended to decrease during the baby banana storage period. In agreement with the authors, a similar SSC and TA behaviour was observed in ‘Prata Aña’ banana by Prill et al. [121]. However, in general, it has been observed that changes in TA and SSC can be delayed by incorporating KMnO4-based materials into banana packages regardless of whether the TA and SSC tend to increase or decrease [69, 136, 155, 177]. Moreover, the SSC/TA ratio tends to increase in banana fruit throughout storage, but García et al. [69] and Tourky et al. [155] reported that the said increasing trend was reduced in baby banana and ‘Williams’ cultivar, respectively, by packaging fruit with KMnO4-based tools.

The banana pulp/peel ratio is a quality parameter that indicates differential changes in the moisture content of skin and pulp. This parameter tends to increase throughout the maturation process due to a differential change in osmotic pressure driven by the sugars increase in the pulp [155]. Nevertheless, Chauhan et al. [43] reported that the pulp/peel ratio changes in ‘Pachbale’ bananas can be delayed if the fruit is packaged into a PE bag together with a KMnO4-based C2H4 scavenger (5 g of scavenger per kg of fruit) and stored 18 days at 13 ± 1 °C. Similarly, ‘Williams’ banana stored in PE bags together with a KMnO4-based C2H4 scavenger (20 g of KMnO4 per 3 hands) showed a slight delay in the increase of pulp/peel ratio during 45 days at 20 ± 2 °C [177]. Nevertheless, the incorporation of a similar C2H4-scavenger tool inside a ‘Raja Bulu’ banana package (approximately 60 g of scavenger per kg of fruit) did not affect the pulp/peel ratio during 18 days at 27–30 °C [136].

Based on the above, KMnO4-based C2H4-scavenger sachets are a useful tool in conjunction with the passive MAP technique to preserve the green life of banana fruit during transportation. Nonetheless, the search for innovative and promising packaging technologies continues. In this sense, a new kind of technology consisting of membranes made of alumina nanoparticle (ANP)-incorporated alumina nanofibres (ANF) loaded with KMnO4 was recently developed by Tirgar et al. [154]. In order to evaluate the effectiveness of the developed membranes, the authors performed a shelf life study by packing banana fruit into bags containing the developed membrane. Results showed that KMnO4 incorporated into ANP-incorporated ANF membranes has the potential to remove a great amount of C2H4 in the package and to delay the incidence of dark brown regions on the skin and flesh softening of banana fruit. However, it must be taken into account that every food contact material must comply with a strict series of requirements before considering their commercial application [50, 127].

Mango

The KMnO4-based C2H4-scavenger effects have been also studied on mangoes. Ezz and Awad [65] studied its effects on ‘Hindi Basennara’ and ‘Alphonse’ mangoes packaged under MAP conditions and stored at 8 °C. The results showed fewer weight losses in both mango varieties, but no effect on firmness loss was observed. Similarly, Jeronimo et al. [84] studied the effect of KMnO4-based C2H4 scavengers on ‘Tommy Atkins’ mangoes at 13 ± 1 °C. However, no significant effect was observed on fruit firmness loss nor on weight loss. At the end of the aforementioned studies, higher TA/ascorbic acid values and lower SSC were observed in the three studied mango cultivars regarding that fruit packed without an C2H4-scavenger tool. Furthermore, Jeronimo et al. [84] noted that C2H4 removal from the package slowed fruit metabolism resulting in lower SSC and TA changes. Hence, C2H4 scavengers are an effective tool to complement the MAP for maintaining the quality of the mango fruit.

Tomato

The use of KMnO4-based C2H4 scavengers has also shown good results in tomatoes, regarding quality maintenance during storage. Mujtaba et al. [114] found that packaging ‘Rio Grandi’ tomato into PE bags together with a KMnO4-based C2H4 scavenger can decrease SSC/ascorbic acid increment, induce TA to increase and retard lycopene biosynthesis, without effects on fruit pH. Moreover, the use of a KMnO4-based C2H4 scavenger in ‘Chonto’ tomato packaging can help to achieve lower weight and firmness losses and to slow SSC increment while TA is not affected [132].

Other Climacteric Fruit

The effect of KMnO4-based C2H4 scavenger has been also evaluated on other climacteric fruits such as papaya, quince fruit, fig, kiwi, sapote and sugar apple (Table 1). Particularly, the incorporation of a KMnO4-based C2H4 scavenger inside the ‘Sunrise Golden’ papaya package can help to reduce pulp consistency loss and the SSC increment, in addition to minimising peel colour changes [144]. Based on similar observations, Bal and Celik [12] reported that ‘Hayward’ kiwi maturity process can be delayed by fruit packaging in conjunction with a KMnO4-based C2H4 scavenger, which showed to be helpful to decrease firmness loss and TA changes, delay SSC increment and delay chlorophyll degradation.

Although a lower β-carotene content was observed by incorporating a KMnO4-based C2H4 scavenger into the packaging of ‘Isfahan’ quince fruit, less firmness loss was observed [3]. Meanwhile, SSC increment, TA decrease, weight losses and firmness losses can be delayed during the storage of ‘Kallipati’ sapote by using KMnO4-based C2H4 scavengers [19]. In sugar apple fruit, lower weight loss, lower SSC and TA increase and a mild pH decrease were found after the incorporation of KMnO4-based C2H4 scavengers [44].

As can be seen from previously reported studies, the application of KMnO4-based C2H4 scavengers in climacteric fruit packaging has mainly resulted in positive effects regarding quality maintenance. Nevertheless, some studies have not reported significant effects after incorporation of a KMnO4-based scavenger into the packaging. Sá et al. [130] did not find significant C2H4 removal effects on melon. In addition, only a slight decrease in colour changes and a reduction of ascorbic acid biosynthesis were observed in guava fruit after the incorporation of KMnO4-based C2H4 scavenger into the package [58]. Therefore, these studies suggest that this postharvest technique is not completely justified in guava and melon, although more research is needed to elucidate these issues.

Non-climacteric Produce

Few significant information concerning KMnO4-based C2H4-scavenger effects on non-climacteric products is available (Tables 1 and 2). Nonetheless, good results have been found. For instance, cherry storage in conjunction with a KMnO4-based C2H4 scavenger resulted in fewer weight and firmness losses, while a delay in both SSC and TA changes was also observed [59]. The ‘Precoce de Itaquera’ loquat shelf life was extended with the incorporation of a KMnO4-based C2H4 scavenger into the package [35]. In another study on ‘Teresópolis Gigante’ cauliflower, it was possible to delay the chlorophyll degradation process by using an C2H4 scavenger [28]. Furthermore, ‘Marathon’ broccoli softening process was delayed when it was stored together with a sachet containing KMnO4 [54]. As can be noted, KMnO4-based C2H4 scavengers seem to be a useful tool to delay senescence processes of non-climacteric produce, but there is an insufficient body of trials to speculate on the potential of the technology on such produce.

Factors Affecting Potassium Permanganate-Based Ethylene Scavenger Efficiency/Performance in Packaging Technology

The C2H4 removal efficiency of KMnO4-based C2H4-scavenger materials mainly depends on temperature, humidity and the amount of material used [25, 168]. Furthermore, the success of a postharvest technology to ensure good quality maintenance without compromising the safety and nutritional value of produce depends on the good understanding of the produce biology and physiology [105, 157]. Then, it is important to consider the latter factors in order to achieve an optimal performance of the C2H4-scavenger material.

Ethylene Scavenger Dose

Martínez-Romero et al. [110] pointed out that the cost–benefit could be optimised by adjusting the appropriate scavenger dose to remove C2H4 during the storage period of the shelf life of the target produce. Particularly, the application of KMnO4-based C2H4-scavenger materials in sachet form is well suited for short storage periods. The use of such sachets is not recommended during long-term storage of high C2H4-producing commodities since the C2H4 scavenger may be rapidly saturated, requiring frequent sachet replacement [165].

In this sense, some authors have evaluated the effect of KMnO4-based C2H4 scavengers by using different doses in order to determine the amount of scavenger material needed to remove as much C2H4 as possible from the in-package atmosphere. Brackmann et al. [29] needed to incorporate three sachets (each one containing 9 g of KMnO4-impregnated material per 18 kg of fruit) inside the package of ‘Gala’ apples in order to reach an C2H4 concentration of 1.4 μL L−1 after 15 days of storage at 4 °C. Meanwhile, an C2H4 concentration of 51.3 μL L−1 was found inside the apple packages without C2H4 scavengers, and an C2H4 concentration of 3.78 μL L−1 was reached by using just one C2H4-scavenger sachet. Amarante and Steffens [7] found no significant differences concerning the C2H4 in-package concentration nor on the ‘Royal Gala’ apple quality (shell colour, pulp firmness and SSC) after 2 months of storage at 0 ± 0.5 °C and 90–95% relative humidity (RH) using either one or two C2H4-scavenger sachets (10 g of KMnO4-impregnated material each one per 18 kg fruit).

On the other hand, Santosa et al. [136] studied the effectiveness of three different doses of a KMnO4-based C2H4 scavenger (10, 30 and 50 g of scavenger material containing 7.5% KMnO4, per 6 fruit units) during storage of ‘Raja Bulu’ banana fruit up to 18 days at 27–30 °C. They found no significant differences in physical quality attributes (such as shell colour and weight loss) by using either 30 or 50 g of C2H4 scavenger. Therefore, they concluded that 30 g of C2H4 scavenger per 6 fruit is enough to delay the ripening process ‘Raja Bulu’ banana. In another study, García et al. [69] evaluated the baby banana response to different concentrations of support material and KMnO4. They reported 1% w/w vermiculite and 1.5% w/w KMnO4 (percentages based on fresh fruit weight) as the most favourable combination to maintain the baby banana quality at 18 °C and 70–80% RH for 16 days. The above percentages are similar to those reported by Salamanca et al. [132] for ‘Chonto’ tomato. The authors reported that the lowest weight loss and SSC, as well as the highest firmness, can be achieved by storing ‘Chonto’ tomato together with 5% zeolite and 1.5% KMnO4 (percentages based on fresh fruit weight) during 28 days at 18 °C (85% RH).

Furthermore, Akbari and Ebrahimpour [3] reported that it is possible to delay the pulp firmness loss of quince fruit, colour change and accumulation of β-carotene by using 2.5 g of KMnO4 per kg of fruit during 150 days at 0 °C (85–95% RH). Nevertheless, it was possible to retain a higher quince fruit firmness when 5 g of KMnO4 per kg was used. In addition, Silva et al. [144] reported that ‘Sunrise Golden’ papaya can be preserved in good condition up to 25 days at 10 °C (85–95% RH) by incorporating a KMnO4-impregnated vermiculite sachet (containing approximately 1.7 g of KMnO4 per kg of fruit) inside the packaging.

As observed, suitable doses of C2H4 scavenger vary depending on the horticultural product. Accordingly, the C2H4-scavenger dose required to achieve a calibrated decrease in C2H4 concentration together with an increase in the commodity shelf life depends mainly on the C2H4 production of the packaged produce, as well as on the C2H4 uptake capacity of the C2H4-scavenger material [91, 110]. On the other hand, both the C2H4 production and the C2H4 sensitivity of produce are mainly influenced by maturity stage, packaging characteristics (including O2/CO2/C2H4 in-package concentrations) and storage conditions [19, 35, 149]. Thus, further studies are still needed to determine the minimum required C2H4-scavenger concentration for a specific product bearing in mind all of the above factors.

Ethylene Removal Capacity of the Scavenger Material

As mentioned above, the C2H4 removal capacity of the scavenger material must be known in order to set the most appropriate C2H4-scavenger dose. However, the C2H4 removal capacity of the products is not always indicated by the manufacturers (Table 3). Nonetheless, it can be assayed by passing a continuous C2H4-enriched air flow through a cylindrical reactor containing the C2H4-scavenger material sample, in a similar way as is described by Terry et al. [153] and de Chiara et al. [52]. The assay is conducted until the reactor outlet C2H4 concentration reaches the inlet C2H4 concentration (breakthrough measurement). Finally, the C2H4 uptake capacity is usually estimated based on the ASTM D6646-03 standard (Standard Test Method for Determination of the Accelerated Hydrogen Sulfide Breakthrough Capacity of Granular and Pelletized Activated Carbon) by the American Society for Testing and Materials [11], which was developed for hydrogen sulphide, with some adaptations as follows:

$$ \frac{\mathrm{g}\ {\mathrm{C}}_2{\mathrm{H}}_4}{{\mathrm{cm}}^3\ {\mathrm{C}}_2{\mathrm{H}}_4\ \mathrm{scavenger}\ \mathrm{product}}=\frac{\left(\frac{c}{100}\right)\times F\times t\times \left(\frac{1\;\mathrm{L}}{1000\;{\mathrm{cm}}^3}\right)\times \left(\frac{1\;\mathrm{mol}}{22.4\;\mathrm{L}}\right)\times \left(\frac{28.05\;\mathrm{g}\ {\mathrm{C}}_2{\mathrm{H}}_4}{\mathrm{mol}}\right)}{V} $$
(9)

The equation can be simplified to:

$$ \frac{\mathrm{g}\;{\mathrm{C}}_2{\mathrm{H}}_4}{{\mathrm{cm}}^3\ {\mathrm{C}}_2{\mathrm{H}}_4\ \mathrm{scavenger}\ \mathrm{product}}=\frac{\left(1.25\times {10}^{-3}\mathrm{g}\;{\mathrm{cm}}^3\right)\times C\times F\times t}{V} $$
(10)

where C is the inlet C2H4 concentration in the air stream (volume %), F is the total C2H4/air flow rate (cm3 min−1), t is the time to breakthrough (min), and V is the volume of the C2H4-scavenger product contained in the reactor (cm3).

Nonetheless, since the C2H4 uptake capacity is usually expressed in L kg−1 units, the following equation can be obtained by replacing the volume variable (V) per weight (W) in Eq. 10, and then multiplying by the C2H4 density in order to change grams units per litre:

$$ \frac{\mathrm{L}\;{\mathrm{C}}_2{\mathrm{H}}_4}{\mathrm{kg}\ {\mathrm{C}}_2{\mathrm{H}}_4\ \mathrm{scavenger}\ \mathrm{product}}=\frac{\left(1.25\ast {10}^{-3}\;\mathrm{g}\;{\mathrm{cm}}^3\right)\ast C\ast F\ast t}{W\ast \left(1.261\;\mathrm{g}\;\mathrm{L}\ {\mathrm{C}}_2{\mathrm{H}}_4\right)} $$
(11)

where W is the weight of the C2H4-scavenger product sample.

At this point, it is important to clarify that although the C2H4-scavenging capacity is an important feature to consider, it does not determine if one product is better than the other is. The most suitable product to remove C2H4 surrounding a commodity should be chosen based on the specific fruit or vegetable to be protected, the storage conditions and the desired market period since each produce has its own biological and physiological features [157].

Packaging Material

The interaction between the product and the packaging material is very important since the atmospheric composition in the package depends on the produce type, packaging material and storage temperature [49]. MAP is characterised by retarding the maturation and senescence in fresh produce by slowing down their respiration and C2H4 production rates [108, 174]. According to Janjarasskul and Suppakul [83], the common approaches to decrease C2H4 production are reducing storage temperature and balancing O2:CO2 concentrations in MAP. Although, in turn, with the aim of reaching the optimum O2:CO2 ratio in the shortest period of time, the respiration rate and the C2H4 production of the produce will define the best suitable film according to its permeability characteristics [49, 108, 118].

In most studies regarding the use of the KMnO4 effect on the quality of fresh commodities, the produce is packaged into commercially available polymer films (PE; polyvinyl chloride (PVC); polystyrene terephthalate (PET)) of 10–60 μm thicknesses (Table 1).

The effect of KMnO4-based C2H4 scavengers on packaged products under different films has been evaluated by some authors, who have obtained interesting results. Akbari and Ebrahimpour [3] evaluated the effect of non-perforated PE bags and perforated PE bags as ‘Isfahan’ quince fruit packaging in conjunction with a KMnO4-based tool during postharvest storage at 0 °C and 85–90% RH. They found that the overall quality was better maintained when quince fruit samples were stored in non-perforated PE bags for 75 days. Nevertheless, the fruit firmness was well maintained up to 150 days by using perforated bags. In contrast, Zewter et al. [177] reported that the chemical quality of bananas packaged in perforated bags together with KMnO4 sachets and maintained at 14–18 °C (33–58% RH) was longer maintained (24 days) compared to samples stored in non-perforated bags (20 days). Such different results can be explained since the banana fruit has a high respiration rate and a high C2H4 production leading to higher gas permeability requirements of the film [177].

Furthermore, the activity of enzymes responsible for cell wall degradation, colour changes and C2H4 production may be slowed down by reducing the O2 and C2H4 levels [119]. Moreover, CO2 acts as an antagonist of the C2H4 action, while high CO2 concentrations inhibit the activity of the enzymes 1-aminocyclopropane-carboxylate synthase and 1-aminocyclopropane-carboxylate oxidase, which catalyse the C2H4 synthesis pathway [96, 119, 166]. Therefore, C2H4 removal combined with the incremented CO2/reduced O2 reached by using MAP results in a synergistic system that causes a lower metabolic rate of the fruit or vegetable [83, 177].

Storage Temperature

KMnO4-based C2H4 scavengers may delay ripening under different storage temperatures as observed in Tables 1 and 2. It is well known that adsorption of any molecule depends on several parameters, including temperature, adsorbate (molecule to be adsorbed) concentration, pressure and RH [68]. Nevertheless, there is a lack of studies concerning temperature impact on the efficiency of KMnO4-based C2H4 scavengers. Lidster et al. [103] reported that high temperature increases the C2H4 removal efficiency of both KMnO4 crystals and KMnO4 supported on aluminium oxide. However, Campos et al. [35] achieved the total removal of C2H4 by using KMnO4-based C2H4 scavengers during loquat fruit storage at both 6 and 18 °C. Nevertheless, it should be noted that the C2H4 production of loquat fruit was lower at 6 °C than at 18 °C. Therefore, since the fruit C2H4 production is also reduced at low temperatures, the C2H4 removal efficiency reduction under low temperatures is not a big problem to deal with.

On the other hand, since the C2H4 production and the product C2H4 sensitivity is influenced by temperature, the optimum storage temperature depends on the commodity to be packaged [165]. Campos et al. [35] and Ezz and Awad [65] stated that better results could be obtained when the produce is stored under MAP at low temperatures. Such observation may be explained since low temperatures help to decrease the produce metabolism leading to less C2H4 production, which facilitates a complete C2H4 removal from the packaging [6, 65]. Conversely, although low-temperature storage is the most common tool for delaying ripening, some produce (i.e. some varieties of peach, grapefruit, lemon and lime, cucumbers, summer squash, eggplants and green beans) are susceptible to chilling injury under cold storage [74, 85]. Therefore, the appropriate storage temperature must be selected depending on the produce characteristics since some fruit and vegetables may show physiological disorders at unappropriated storage temperatures for them [119].

Additionally, temperature plays an important role in the permeability of polymeric films. Specifically, the exposure of the film to high temperatures decreases O2 permeability and increases CO2 permeability [134].

Relative Humidity

High RH is needed to prevent the dehydration and weight loss in fruit and vegetables because transpiration significantly affects the physical and chemical properties of horticultural crops [85]. In addition, RH is another extrinsic factor that may affect the C2H4 adsorption efficiency, as previously stated. According to Chopra et al. [45], H2O molecules can compete for adsorption sites with C2H4. However, the H2O presence is required to carry out the C2H4 oxidation process [91]. In this sense, Janjarasskul and Suppakul [83] suggested that the moisture released from the transpiration and respiration of the produce helps in the activation of KMnO4.

In a recent study, Spricigo et al. [150] evaluated the C2H4 removal capacity of silica and alumina particles impregnated with KMnO4 at different RH levels (45, 60, 75 and 90%) for 1 h. They found that the C2H4 removal capacity of KMnO4-impregnated alumina particles tend to increase as the RH increase. On the other hand, they also observed that the C2H4 removal capacity of KMnO4–silica particles increased as the RH increased from 45 to 75% RH, while at 90% RH, a removal capacity decrease was observed. The C2H4 adsorption reduction of the KMnO4–silica particles at high RH may be due to the competition between C2H4 and H2O molecules for binding sites since silica is a high H2O adsorber material and H2O molecules are adsorbed faster than those of C2H4 [45].

Other Considerations

The scavenging capacity and efficiency/performance of KMnO4-based C2H4 scavengers strongly depends on the support material properties and on the KMnO4 concentration [68, 97]. In fact, a wide range of properties determine the ability of a material to act as an appropriate KMnO4 support, such as pore structure, surface area, bulk density, surface chemistry and KMnO4 uptake capacity [4]. Several studies have demonstrated that support materials with higher surface areas, lower bulk densities and higher KMnO4 uptake capacity are more efficient in reducing C2H4 levels [150, 154]. In addition, it should be considered that the support material could be able to absorb C2H4 from the atmosphere, but also it could absorb other organic compounds, negatively altering the fresh commodity characteristics [68, 83, 116]. However, we will not delve into this issue since a comprehensive review, specifically dedicated to supporting materials commonly used for KMnO4-based C2H4 scavengers, has recently been published [4].

Conclusions and Future Perspectives

The physicochemical parameters, sensory quality and nutritional properties of fresh fruit and vegetables can be preserved for a longer time by combining modified atmosphere packaging and C2H4-scavenging systems such as those based on KMnO4. Furthermore, reducing C2H4 action could help to reduce postharvest losses, improve food safety and help economic sustainability. Therefore, an increasing interest in C2H4 scavengers has been paid in the last years. Related companies have invested intense research on this issue in order to develop new materials and technologies to improve the efficiency of the current C2H4 scavengers. At this point, special attention has been given to the application of nanotechnology. In addition, it is possible to chemically, or physically, modify the support material structure to improve the KMnO4 incorporation. Nevertheless, although the C2H4-scavenging technology has been widely used during the last decades, and some commercial products are already available, an important research effort at both laboratory and industrial scales is still needed. More specific technical data concerning application conditions of the different C2H4 scavengers is required. It is necessary to study the optimal use conditions of each material, such as the recommended temperature and RH to obtain the maximum benefit from the scavenger. Additionally, it is necessary to demonstrate the C2H4-scavenger performance under other critical storage parameters taking into account the metabolic rate of the packaged produce, which depends on the kind of commodity, variety and maturity stage and also on the packaging material. Especially, efforts at understanding and modelling the C2H4-scavenging dynamic processes of KMnO4-based products are needed. On the other hand, consumers have a lower level of awareness on the KMnO4-based C2H4-scavenging technology efficacy and safety, resulting in lower acceptance, and then affecting its commercialisation. Therefore, the continuous improvement of consumer awareness is needed.