Postharvest handling of ethylene with oxidative and absorptive means

Abstract

Fruit ripening is an unfolding of a series of genetically-programmed modifications and tend to be highly orchestrated irrevocable phenomenon mediated by ethylene. Phytohormone ethylene also leads to over-ripening, senescence, loss of texture, microbial attack, reduced post-harvest life and other associated problems during storage and transportation of fruits. Its harmful impacts on fresh fruits, vegetables, and ornamentals result in substantial product losses even up to 80%. Curbing of this inevitable menace is therefore need of the hour. Accrual of ethylene in packaging system should fundamentally be ducked to extend the shelf-life and uphold an adequate superiority of perishables in visual and organoleptic terms. The current review discusses about properties, factors affecting and impact of ethylene, intimidation of its impact at gene vis-à-vis activity level using gene-modification/inhibition techniques, chemical/physical in conjunction with other suitable approaches. It also entails the most commercially cultivated approaches worldwide viz. KMnO₄-based oxidation together with adsorption-based scrubbing of ethylene in thorough details. Future ethylene removal strategies should focus on systematic evaluation of KMnO₄-based scavenging, exploring the mechanism of adsorption, adsorbent(s) behavior in the presence of other gases and their partial pressures, volatiles, temperature, relative humidity, development of hydrophobic adsorbents to turn-up under high RH, regeneration of adsorbent by desorption, improvement in photocatalytic oxidation etc. and further improvements thereof.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05777-1.

Introduction

The process of fruit ripening is an unfolding of a series of genetically programmed modifications in terms of its biochemical, physiological, textural, sensorial and organoleptic attributes and tend to be a highly orchestrated irrevocable phenomenon. Phytohormone ethylene originated during beginning of ripening of fruits spearheads genes for manufacturing various ripening enzymes (Kumar et al. 2019). The origin of ethylene can be biological or non-biological environmentally. Higher plant tissues, few algae, certain mosses and numerous bacterial species constitute ethylene biologically (Vermeiren et al. 2003); conversely, major non-biological contributors of ethylene include fossil fuels’ incomplete incineration, fluorescent lights, burning of agricultural wastes, emissions from automobiles, smoke, and seepage of industrial polyethylene production plants etc. The estimated annual total global (atmospheric) ethylene emission is to the tune of 18 − 45 × 10⁶ tons, of which 74 and 26% arise due to natural and manmade sources, respectively. The demolition of ethylene takes place primarily in the troposphere of earth’s atmosphere where it reacts with •OH radicals (89%) and O₃ (8%). The estimated atmospheric lifespan of ethylene oscillates somewhere amid 2–4 days (Keller et al. 2013; Sawada and Totsuka 1986).

Ethylene is a natural ripening phyto-hormone necessarily required for natural ripening of fruits, though each fruit has its own dose requirements depending upon fruit type, variety, stage of ripening, as well as ripening behaviour (climacteric/non-climacteric). Fresh produce releases different concentrations of ethylene, and susceptibility to ethylene exposure also varies for various perishables. The gaseous hormone can alter the physico-chemical stability of commodities at the concentration 10–100 nL L⁻¹ (Aghdam et al. 2019; Sadeghi et al. 2021). On the other hand, it leads to over-ripening, senescence, loss of texture, microbial attack, less post-harvest life and other associated problems during storage and transportation of fruits. It is no exaggeration to say that ethylene is one of the most notorious contributors to post-harvest losses. Ethylene, chemically a simple molecule of the alkene type has multifarious effects on growth, development and storage life of many fruits, vegetables, and ornamental crops in a dose dependent manner (Saltveit 1999). It exerts its effect as physiological ripening agent at enormously low concentrations ranging from ppm (μL L⁻¹) to ppb (nL L⁻¹) under controlled conditions. It has been observed that even insignificant quantities of ethylene all through shipping and storage can lead to quicker deterioration of fresh fruits, vegetables, and ornamentals. The ethylene can harm perishables resulting in significant product losses oscillating between 10 and 80% (Kader 2003; Keller et al. 2013). The generation of ethylene during ripening and storage leads to rapid senescence, decay, loss of firmness and subsequent diminished shelf-life as ethylene acts as an elicitor of ripening-mediated variations. The evident symptoms of short shelf-life include decay, weight loss, deterioration of appearance, textural and nutritional quality attributes (Ulloa 2007). These changes are arbitrated by cell wall degrading enzymes (cellulases, pectinases), pathogenesis related (PR) proteins responsible for tissue softening and decay, color as well as flavor/aroma related changes. Inhibiting ethylene action can have fabulous benefits commercially during the storage of ethylene sensitive perishable commodities. For example, 1% wastage in a refrigerated cargo ship having a load of 8000 tons amounts to 80 tons loss of perishable produce, which is a mammoth amount (Keller et al. 2013). Thus, in order to control the post-harvest losses, decay and senescence, the management of ethylene concentration remains to be of utmost importance. The present review discusses various ways researched throughout the world to curb the menace of ethylene thus maintaining the freshness of perishables and minimizing post-harvest losses while storage and transportation with specific reference to oxidative and absorptive means.

Climacteric versus non-climacteric fruits

Fruits are generally classified into climacteric or non-climacteric types depending upon the pattern of respiration, and responsiveness to externally supplemented ethylene. The major differences between both can be enumerated as given in Table 1.

Table 1.

Ethylene mediated differences in climacteric and non-climacteric fruits

Source: Keller et al. (2013), Yahia and Carrillo-López (2019)

S. no.	Climacteric fruits	Non-climacteric fruits
1.	Can ripen after harvest	Cannot ripen postharvest
2.	Characterized by consecutive pre-climacteric (firm fruit; less respiration and ethylene generation; removing outside exposure of ethylene prolongs this phase) and climacteric periods	No such demarcation
3.	Climacteric fruits typically show abrupt and substantial upsurge in ethylene generation and respiration during climacteric rise	No such sudden increase and fruits commonly emit a considerably reduced level of ethylene
4.	Pre-climacteric period: Ethylene: 1–10 ppm/kg/h (μL/L/h); Climacteric period: ethylene evolution may reach up to 30–500 ppm/kg/h during ripening at 20–25 °C (depending upon fruit species)	Ethylene production levels usually range from 0.1 to 0.5 ppm/kg/h, during ripening at 20–25 °C
5.	Ethylene supplemented of an external source triggers autocatalytic ethylene generation thus, leading to acceleration of the climacteric period which is irretrievable, i.e. high ethylene production and rapid ripening with amplified respiration rate	Show augmented rate of respiration and ethylene production in response to external ethylene treatment but no acceleration in the time required for ripening
6.	Noticeable compositional and textural changes are escorted with increase in respiration. Once climacteric rise progresses, reducing/removing the external ethylene concentration cannot reverse ripening	Exogenous ethylene application cannot induce endogenous ethylene production levels and reducing/ removing the external ethylene concentration cannot reverse ripening
7.	Threshold: 0.1 ppm	Threshold: Below 0.005 ppm (5 ppb)
8.	Tomato, banana, mango, apple, avocado, passion fruit, muskmelon, apricot, plum, papaya, kiwifruit, pear, broccoli, fig, breadfruit, guava, nectarine, peach, persimmon, watermelon	Leafy vegetables, strawberry, grape, cherry, potato, cucumber, citrus fruits (lemon, orange, lime, grapefruit) pineapple, melon, peas, pepper, cacao, blueberry, olive

Effects and impact of ethylene

Inhibiting the action of ethylene or removing it from perishables’ surroundings can have incredible advantages in terms of commerce for the storage of ethylene sensitive perishables. Accumulation of ethylene in the packaging system should fundamentally be avoided/stopped in order to protract the shelf-life and uphold an acceptable quality of fruits and vegetables with regard to visual and organoleptic characteristics (Chaves and Mello-Farias 2006; Sadeghi et al. 2021). Modern technologies for foods like active packaging these days are engineered to confiscate unwanted elements (including ethylene) from the headspace of packaging via absorption, adsorption or scavenging. The said target is fulfilled by incorporating a sachet/blanket/filter etc. with commodity or a physical or chemical absorbent/adsorbent (agent) is placed directly in the packaging itself (Gaikwad et al. 2018; Sadeghi et al. 2021). The containment of ethylene for maintaining the freshness and lowering post-harvest losses of perishables is impetus and need of the hour. Many leading companies in the world offer newer ways to curb the menace of ethylene and save the produce from losses. According to the extensive review of Keller et al. (2013), the work related to ethylene curbing can be categorized in to two major ways: plant level actions for inhibiting ethylene production as well as action (genetic as well as chemical approaches), whereas another way concerns environment level actions (avoidance, inhibition, and removal). The inhibition includes controlled atmosphere (CA), modified atmosphere (MA), modified atmospheric storage (MAP) and hypobaric storage whereas the removal strategies comprise of ventilation, destructive oxidation and adsorption. The oxidation can be chemical mediated (KMnO₄, UV-C, ozone, photocatalysis etc.) and by biological means (use of biofilters, microorganisms etc.) (Keller et al. 2013). Further, various researchers have signposted that ethylene has capability to infuse through physical materials like cardboard boxes, wooden packaging, and concrete walls thus, additionally complicate the curbing process. Wills et al. (2000) have reported that at obviously higher concentrations, ethylene can undergo diffusion with ease from one storage room to another. Potassium permanganate-based scrubbing of ethylene is practiced worldwide by leading companies of the world. In order to enhance the scavenging capacity, KMnO₄ is usually embedded in various inert support materials such as minerals or nanoparticles in the form of permeable sachets (Sadeghi et al. 2021). Other approaches also have their own pros and cons.

In addition to ripening, ethylene is also involved in regulating various other vital processes during plant growth and development. These include fruit/vegetable ripening, seed germination, cell elongation, defence against pathogens, flowering, dormancy, senescence, geotropism and response to external stress factors (Gaikwad and Lee 2017; Gaikwad et al. 2020; Zhu et al. 2019). Apart from its role of fruit ripening (which makes the perishables an elixir) and as a base of ‘n’ number of branches of science viz. Physiology, Food Science, Biochemistry, Aroma and Flavor Science etc., ethylene has been considered problematic while post-harvest management of horticultural foodstuffs (fruits, flowers and vegetables). When ethylene gets attached to its receptor, it accelerates a series of reactions for ripening, texture softening and organoleptic changes at the level of synthesis as well as activity at enzyme level (cellulase, pectinase, ethylene synthesis etc.). It is involved in accelerating chlorophyll degradation and inducing yellowing of green tissues viz. leafy green vegetables (spinach), immature cucumbers and broccoli, flavour changes, conversion of starch to sugar, loss of acidity and textural changes (Zagory 1995). Table 2 is enumerating the specific effects of ethylene exposure on post-harvest shelf-life, disorders and marketable quality of fresh fruits and vegetables. Ethylene also boosts the chances of pathogenesis as a consequence of stimulating physiological ripening and senescence (Vermeiren et al. 2003).

Table 2.

Negative effects of ethylene in fruits and vegetables

Source: Kader (2003), Keller et al. (2013), Vermeiren et al. (2003)

S. no.	Commodity	Problem encountered	Reason
1.	Asparagus spears	Toughness	Accelerated lignin biosynthesis
2.	Watermelon	Reduced firmness	Increased activity of peroxidases
3.	Leafy green vegetables (spinach), broccoli, cucumbers, Brussel sprouts	Yellowing of green tissue	Chlorophyll degradation
4.	Lettuce, leafy vegetables, eggplants	Russet spotting	Germination
5.	Potatoes	Sprouting	Germination
6.	Carrots	Bitter taste	Formation of isocoumarins
7.	Kiwi fruit	Softening	Textural enzyme activation
8.	Fresh produce and flower bulbs	Decay	Inhibition of antifungal compounds creation: stimulation of fungal growing (Botrytis cinerea on strawberries); (Penicillium italicum on oranges)
9.	Garlic and onions	Odour	Flavour changes
11.	Vegetables and cut flower	Wilting	Water loss
12.	Apples	Scald and loss of crunch	Textural changes
13.	Citrus	Rind breakdown

Various ethylene control approaches

Historically in 1864, leakage of an illuminating gas (containing ethylene) in greenhouse resulted in premature senescence and defoliation of plants and trees growing near the gas lines. Ethylene (C₂H₄) is a small colourless gaseous molecule having molecular weight 28.05 g/mol (density 1.178 kg/m³ at 15 °C) and is lighter than CO₂ (44 g/mol) and O₂ (32 g/mol). Ethylene’s double bond makes it amenable to be modified or tarnished in various ways and offers a myriad of opportunities for profit-making methodologists to remove/curb ethylene levels (Chowdhury et al. 2017). Table 3 represents various approaches researched for curbing the menace of ethylene either at the level of synthesis or during storage and transportation. One of the main things to be kept in mind that once ripening of a fruit has progressed towards its climacteric stage, reducing the external ethylene concentration cannot reverse ripening. However, during the initial stage of fruit ripening, when internally very low ethylene concentration is present, either inhibiting the production and/or removal of ethylene can slow down the ripening of climacteric perishables to a larger extent (Saltveit 1999).

Table 3.

Various approaches for ethylene control

Source: Modified from Keller et al. (2013), Schaller and Binder (2017), Vermeiren et al. (2003)

S. no.	Type	Target(s)	Inhibitor/Active agent/Mechanism
1.	Genetic modifications of ethylene synthesis at gene level (Keller et al. 2013)	Ethylene biosynthetic genes: ACC synthase, ACC oxidase	Action at DNA level by recombinant DNA technology (RDT), CRISPRi (Clustered Regularly Interspaced Palindromic Repeats interference) gene technology (Pathak et al. 2018)
	Pros: Normal maturation; being used in case of tropical fruit species having a very limited shelf-life Cons: Genetically modified (GM) crops, ethical issues, food safety risks and associated environmental impact
2.	Genetic modifications of ethylene receptor synthesis at gene level (Keller et al. 2013)	Ethylene receptor gene	Isolation of the mutant gene from Arabidopsis thaliana (an ethylene-resistant plant) and its insertion into commercial crops
	Pros: Available ethylene cannot exert its effect due to lack of the binding site Cons: GM crops, ethical issues, food safety risks and the environmental impact
3.	Chemical inhibition of ethylene synthesis and signaling	ACC synthase	2-Aminoethoxyvinylglycine (AVG) and 2-amino-oxyacetic acid (applied in liquid form) (Schaller and Binder 2017)
		ACC oxidase	Aminoisobutyric acid and Co2+ ions (applied in liquid form) (Schaller and Binder 2017)
	Cons: Cost; cumbersome for large lots
4.	Chemical inhibition of ethylene receptor action (Schaller and Binder 2017)	Ethylene receptor	1-Methyl cyclo-propene (1-MCP) (competes with ethylene for receptor); 2,5-Norbornadiene, trans-Cyclooctene (all applied in gas form) Silver ions, silver nitrate and silver thiosulfate: (applied in liquid form)
	Pros: 1-MCP is being utilized as an ethylene analogue (competitive inhibitor) and effective for a small lot Cons: 1-MCP is costly and has specific requirements; not scaled up to the commercial levels Ag is toxic and restricted to lab level Note: A copper ion present in ethylene receptor helps coordinate the binding of ethylene with its receptor (Rodriguez et al. 1999) Silver ions may replace the copper ions in the binding site and interact with ethylene (Knee 1995)
5.	Ventilation	Removal of accumulated ethylene	One exchange per hour via diffusion (in general)
	Pros: Easy to do Cons: Cooling and humidification/dehumidification of ambient air will cost extra bucks for refrigeration; if external air has more concentration of ethylene than required for a commodity stored, it becomes problematic; literally unfeasible in modified and controlled atmospheric storages (Keller et al. 2013)
6.	CA storage, MA, MAP (modified atmospheric packaging) etc	Ethylene production and action	On the basis of the commodity being stored/packaged, less than 8% O2 and more than 1% CO2 (1 − 25%) are generally used (Kader 2004)
	Pros: Can extend the post-harvest life of the perishables Cons: Confined use, high cost; too low O2 level and/or too high CO2 concentrations, may encourage anaerobic metabolism and hasten senescence as well as spoilage (Beaudry 1999; Watkins 2000) Note: Maintenance of optimum concentrations of O2 and CO2: i.) slows down respiration (by around 2.5%) and ethylene production (by 50%); ii.) reduces ethylene action and development of decay-causing pathogens; iii.) delays ripening and senescence iv.) controls insects (Kader and Rolle 2004; Saltveit 2003)
7.	Scrubbing by KMnO4	Ethylene degradation	Oxidation: KMnO4 mediates oxidation of ethylene at ambient temperature to harmless entities (CO2 and water), concurrently releasing manganese dioxide and potassium hydroxide
	Pros: Most widely used; suitable to small volume storage rooms; most commercial companies rely on this technology Cons: Frequently decreased activity, residual KOH, moisture hindrance; rapidly exhausted and to be substituted with a fresh lot several times; very less scientifically documented; KOH recovery from manganese and carrier material is tough; when surface layer gets oxidized, ethylene diffusion into bead interior becomes challenging (Keller et al. 2013)
8.	Hypobaric storage	Ethylene synthesis	Less pressure than atmospheric while storage (Keller et al. 2013)
	Cons: Cost, low efficiency, lab scale only; restrictive safety considerations
9.	Ultraviolet (UV) -C	Ethylene	Oxidation
	Cons: Cost, human health due to ozone generation, complex requirements, very less efficiency (~ 7%) (Keller et al. 2013)
10.	Ozone	Ethylene	Oxidation at 184 nm (UV) is most efficient
	Pros: Ethylene removal along with control of postharvest diseases of fruit Cons: Very less efficiency; ozone toxicity; long-term exposure to ozone more than 1 ppm leads to irreparable harm and even death @5 ppm (Keller et al. 2013)
11.	Biofilters	Ethylene	Ethylene is trapped in filters and used as carbon source by microorganisms
	Cons: Low efficiency, lab scale success; very less documented; large area requirements (Keller et al. 2013)
12.	Adsorbents	Ethylene	Selective adsorption (Zeolite, silica, activated carbon, clay)
	Pros: High surface area, possibility of regeneration Cons: Moisture hindrance, selectivity, very less research, less adsorption rates when the concentration of ethylene to be adsorbed is less, opacity and undesirable packaging film color of zeolites (Sadeghi et al. 2021)
13.	Catalytic oxidizers combined with adsorbents (Vermeiren et al. 2003)	Oxidation of ethylene trapped via adsorbents	Catalytic oxidizers (potassium dichromate, potassium permanganate, iodine pentoxide and silver nitrate, each respectively) implanted on adsorbent (silica gel) e.g. Activated charcoal (adsorption) impregnated with bromine or with 15% KBrO3 and 0.5 M H2SO4 for (oxidation) of ethylene
	Cons: Less documented except KMnO4
14.	Photocatalytic oxidation	Ethylene oxidation via catalysis in the presence of photons	TiO2, hv C2H4 + 3O2 → 2CO2 + 2H2O Requires a semiconductor material (titanium dioxide/TiO2) as catalyst; energy obtained by direct light absorption Illumination of surface of the catalyst (with UV radiation), initiates photochemical oxidation thus, generating reactive oxygen species (ROS), by decomposing oxygen and water. The resulting ROS does further oxidize ethylene into carbon dioxide and water (Gaikwad et al. 2020)
	Pros: Conversion to non-toxic materials at near-ambient temperatures (carbon dioxide and water), TiO2 is stable, works at ambient temperature Cons: Moisture hindrance, initial high cost Note: TiO2 has high quantum yield; is resistant to photo-corrosion and chemicals, water insoluble and less toxic. TiO2 (band gap energy 3.2 eV) gets photoexcited at wavelengths less than 385 nm (Keller et al. 2013)

Factors affecting ethylene production and its response/action during storage and transport of perishables

Various studies indicated that although ethylene may exhibit little toxicity yet pose no potential risk to human health. Interestingly, ethylene was used as an anaesthetic for many years before the advent of modern anaesthetics, but replaced eventually due to its high explosion risk as ethylene in air oscillating between 2.75 and 28.6% at 0.1 MPa and 20 °C may explode (Keller et al. 2013). The minimum and maximum odour threshold levels are 299 ppm and 4600 ppm, respectively. Ethylene molecules are perceived by its receptor present in cell membranes of plants. The ethylene-receptor binding unlocks the receptor, which in turn results in a myriad of chemical reactions at cell and tissue level leading to genetic as well as colour, texture and other related changes which further enhance the production of ethylene (Keller et al. 2013). The latter authors reported that perishables are loaded usually ∼85% by volume, of the container capacity. Depending upon the perishable goods stored, the air circulation rates vary accordingly.

1. Temperature: Horticultural commodity has its own recommended temperature. At low temperature, there is substantial decrease in fruit metabolism leading to significantly sluggish response to ethylene, while the perishable commodities’ deterioration increases two to threefold for every 5 °C upsurge in temperature. The recommended temperature and relative humidity (RH) for short term storage of fruit and vegetables are (provided the concentration of ethylene should remain < 1 ppm in storage locations) as follow: cole crops, most of the leafy vegetables (0–2 °C; 90–98%), fruits and berries (temperate type) (0–2 °C; 85–95%), citrus, other subtropical fruits and various vegetables resembling fruits (7–10 °C; 85–95%), general root-type vegetables, melons, winter squash and most tropical fruits (13–18 °C; 85–95%) (Keller et al. 2013).

2. Relative humidity (RH): The relative humidity can significantly affect water loss, uniformity of fruit ripening, progress of decay and physiological disorders. While storing fruits, the most suitable RH arrays between 85 and 95%, although in case of vegetables, the corresponding range fluctuates between 90 and 100% (Kader and Rolle 2004).

3. Ethylene: At sea, remote sites and rural areas, ethylene concentration ranges to < 1 − 5 ppb while in urban and indoor areas, it may reach up to the level of 50 ppb. In sealed cold store, controlled atmospheric storage and mixed cargoes, the ethylene may be very high, which could result in deterioration of perishable commodities. Ethylene is able to pervade through cardboard boxes, wooden packaging, and concrete walls and to diffuse one storage compartment to another (Keller et al. 2013; Wills et al. 2000). The C₂H₄ concentrations greater than 0.1 μL L⁻¹ affect fresh produce storage life firmly and C₂H₄ concentration between 0.1 and 0.5 μL L⁻¹ is considered as threshold level to induce ripening in fruits like banana, melon, avocado and pear etc. (Blanke 2014).

4. Oxygen: Low oxygen (than 21%) leads to less respiration and ethylene production specifically in modified and controlled atmospheric storage as well as in modified atmospheric packaging (Keller et al. 2013).

5. Carbon dioxide: High CO₂ accumulated due to respiration may decrease ethylene production.

The concentration of C₂H₄ in the product’s vicinity is undesirable and remains as a factor of utmost importance to be controlled as it often leads to more rapid post-harvest deterioration of fresh produce. This happens especially while their storage and transport and generally leads to significant losses to perishables. Importantly, refrigeration (low temperature) and humidity slow down the decay, but are unable to arrest ethylene generation completely. They don't halt the production of harmful ethylene gas completely. The climacteric fruits are the major sources of C₂H₄ and capable of altering horticultural produce environments (Pathak et al. 2017). Cleavage, chemical modification, absorption and adsorption of ethylene molecule are employed in manufacturing the ethylene scavengers (Alves et al. 2022). Ethylene scavenging can be done by chemical or physical methods (absorption, adsorption and/or other oxidation mechanisms) from the surrounding environment to maintain good keeping quality of fresh fruits and vegetables for comparatively longer periods (Chopra et al. 2017; Yildirim et al. 2018). Chemical molecules like electron deficient dienes and trienes like benzene and pyridine (Alves et al. 2022) and resveratrol (Li et al. 2022) can also be explored for ethylene scavenging. As reported in the review of Álvarez-Hernández et al. (2018), in terms of value, advanced packaging captures nearly 5% share of the total packaging market worldwide, out of which 35% (nearly 1/3rd) owes to active packaging. However, C₂H₄ scavengers epitomize 3% of total market share of gas elimination packaging technologies (Gaikwad and Lee 2017; Wyrwa and Barska 2017).

KMnO₄-based oxidation and adsorption-based ethylene elimination

In further section, detailed discussion of KMnO₄-based oxidation and adsorption-based methods has been elaborated as the former is most commercially cultivated while the latter is one of the technologies of choice for research in the area of ethylene elimination.

KMnO₄-based oxidation

Ethylene scavengers effectively eradicate ethylene generated by packed fresh produce via absorbing/scavenging it, thus containing postharvest fatalities (Gaikwad and Ko 2015). By far the most successful and commercially viable scavenger of ethylene from horticultural products, used worldwide is KMnO₄-based oxidation on activated alumina support, though other adsorbents also find place either as support material or as ethylene adsorbent itself (Álvarez-Hernández et al. 2018; Gaikwad and Lee 2017; Keller et al. 2013). The commercially accessible scavengers can scavenge ethylene to the tune of 3 to 6.5 L/kg (Scully and Horsham 2007). Table 4 enlists various manufacturers involved in KMnO₄-based oxidation system providers, their trade names, support materials and other accessible information as per the available literature to have a thorough glimpse of scavenging-based ethylene removal methods.

Table 4.

Commercially available potassium permanganate based ethylene scavengers

Manufacturer	Trade name	Principal ethylene scrubber	Support material	Country	Final supplied form	References
Molecular Products Ltd	Ethysorb®	Potassium permanganate (3.5–5.5%)	Activated alumina	Essex, UK	Tube, beads, blanket	Álvarez-Hernández et al. (2019), Gaikwad et al. (2020), Molecular Products Limited (2013)
	#Sofnofil™	Potassium permanganate (< 6%)	Activated alumina	Essex, UK
#Purafil Inc	Purafil Chemisorbent Media	Potassium permanganate (≥ 4%)	Activated alumina and other binders	Doraville, Georgia, USA	Pellets in sachet	Purafil Inc. (2015)
	Purafil Select Media	Potassium permanganate (≥ 8%)
	Purafil Select CP Blend Media	Potassium permanganate (≥ 8%) with blend of purakol
Biopac Solutions	BIOPAC	Potassium permanganate	Porous material	West Burleigh, Australia	Sachet, filter	Gaikwad et al. (2020), https://www.biopac.com.au/ethylene-control/
DeltaTRAK Inc	Air Repair	Potassium permanganate (4%)	Activated alumina	California, USA	Mini-packets and blanket, sachet	Gaikwad et al., (2020), https://www.deltatrak.com; Vermeiren et al. 2003
#Circul-Aire Inc	Multi-Mix® MM-1000	Potassium permanganate	Activated alumina	Quebec, Canada		www.circul-aire.com (2006)
Dennis Green Ltd	Mrs. Green’s Extra Life	Potassium permanganate		USA	Cartridge/ disc for refrigerator	https://www.amazon.ca/Dennis-Green-Ltd-Produce-Preserver/dp/B0006GSLDQ; Vermeiren et al. (2003)
BioXTEND Co	BioX® / BioXTEND®	Potassium permanganate BioX 4 (4–4.5%); BioX 8 (8–8.5%)	Porous clay minerals	Fort Myers, Florida, USA	Sachet, granules, filters, module	Álvarez-Hernández et al. (2019), Gaikwad et al. (2020), https://bioxtend.com/
#Bry-Air (Asia) Pvt. Ltd	BrySorb™ 508/508BL	Potassium permanganate	Activated alumina	Gurugram, India	Beads	Bry-Air (Asia) Pvt. Ltd.; Gaikwad et al. (2020)
Isolcell Spa	PURETHYL	Potassium permanganate	Activated alumina	Laives, Italy	Granules	Gaikwad et al. (2020), https://storage.isolcell.com/wp-content/uploads/2018/08/PURETHYL.pdf
Prodew Inc	Prodew	Potassium permanganate granules		Marietta, Georgia, USA	Sachet, cabinet	Gaikwad et al. (2020), https://www.prodew.com/flyers/eth_flyer_web.pdf
AgraCo Technologies International	Extend-A-Life™ (filters); Produce Saver™ (sachets)	Potassium permanganate (8%)	Zeolite	LLC, PE, USA	Filters and sachets	https://www.agraconew.com/products/ethylene-filters (2014)
Ozeano Urdina SL	Ozeano ETH	Potassium permanganate (7.5%)	Alumina	Bizkaia, Spain	Sachet and filters	Ozeano Urdina (2013)
Bioconservación	Bi-On®	Potassium permanganate R8 (8%); R12 (12%)	Zeolite/natural clays	Barcelona, Spain	Cylindrical pellet, granule, sachet, tube	https://www.bioconservacion.com/
Befresh Technology	BEfresh	Potassium permanganate	Natural clays	Spain	Sachet, filter, tube	Befresh Technology (2018), Gaikwad et al. (2020)
Sensitech Inc	Ryan®	Potassium permanganate (6%)	Natural clays	Beverly, MA, USA	Sachet, filter, tube	https://www.virtualmarket.asiafruitlogistica.com/en/Ryan®-Ethylene-Absorption Filters-and-Sachets
KEEPCOOL	KEEPCOOL	Sepiolite mixed with potassium permanganate	Activated carbon	Moline de Segura, Spain	Sachet, filter	Gaikwad et al. (2020), https://keep-cool.es/en/how-it-works/ethylene-absorbing-filters/
Ethylene Control Inc	Super Fresh Media	Potassium permanganate (4–6%)	Zeolite (clinoptilolite)	Selma, CA, USA	Sachet	https://ethylenecontrol.com/sachets
Greenkeeper Iberia	GK3/GKZ4	Potassium permanganate GK3 (8%); GKZ4 (12%)	Natural zeolite (Phyllosilicate and aluminosilicate)	Toledo, Madrid, Spain	Sachet and sheet, tube, module	Álvarez-Hernández et al. (2019), Gaikwad et al. (2020), https://greenkeeperiberia.es/en/gk3-y-gk4/
KeepFresh Technologies	KEEPFRESH	Potassium permanganate	Natural zeolite	Malaga WA, Australia	Sachet/ sheet	Gaikwad et al. (2020), https://keepfresh.com.au/product-profile/
Retarder S. R. L	Retarder®	Potassium permanganate	Clay	Verzuolo, Italy	Sachet, filters, tube	Gaikwad et al. (2020), https://www.virtualmarket.fruitlogistica.de/en/Retarder-SRL,c44327
Miatech Inc	Eris Filter	Potassium permanganate	Non-woven polyester	Clackamas, Oregon, USA	Sheet as filter, blanket	Gaikwad et al. 2020; Miatech Inc. 2020a, b
Keep It Fresh		Oxidizing agents	Zeolite	California, USA	Bags, sachets, pads, curtains, and tubes	https://kif-usa.com/profile/

^#Used as generalized purifier and not specifically destined for ethylene by manufacturer

KMnO₄-based oxidation process is a sort of destructive approach allowing irreversible as well as continuous ethylene removal (Keller et al. 2013). One more thing should be kept in mind while dealing with ethylene scavenging mechanisms that natural convection and diffusion are the only driving forces involved in gas movement. Therefore, for enhanced oxidation of ethylene by KMnO₄, the latter is generally reinforced onto solid carrier materials which are inert, have minute particle size with a large surface area such as celite, activated alumina, vermiculite, silica gel, activated carbon, limestone, clay, zeolite, perlite, pumice, brick, or glass etc. (Shaabani et al. 2005; Spricigo et al. 2017). These inert materials adsorb/absorb ethylene and provide huge surface area for latter’s smooth interaction with KMnO₄ (Álvarez-Hernández et al. 2018; Gaikwad et al. 2020; Wills and Warton 2004). The concentration of KMnO₄ may vary from 2.5 to 12% as reported by various researchers. Typically, the average concentration of KMnO₄ remains about 4–6%. The scavenging capacity largely depends upon surface area of material and KMnO₄ concentration (Gaikwad et al. 2020; Zagory 1995). The oxidation of ethylene with potassium permanganate gets accomplished in a two-step process. On reaction, ethylene (C₂H₄) gets converted to acetaldehyde (CH₃CHO) via oxidation, which is then gets oxidised to acetic acid (CH₃COOH). Acetic acid can further be transformed to harmless entities like carbon dioxide and water in an oxidative reaction. The entire process can be represented as:

$${\text{3C}}_2{\text{H}}_4+{\text{12KMnO}}_4+{\text{2H}}_2\text{O}\to {\text{6CO}}_2+{\text{2H}}_2\text{O}+{\text{12MnO}}_2+\text{12KOH}$$

KMnO₄ being powerful oxidant, oxidises C₂H₄ to harmless CO₂ and H₂O in a cheap and easy way. KMnO₄-based scavengers can be used in an array of ways viz. in active packaging, storage, transportation and domestic refrigerators (Gaikwadet al. 2020; Keller et al. 2013). These scrubbers are available in the form you name e.g., sachets, bags, tube filters, blankets, labels or films etc., though sachets are the most widely used form due to their suitability for individual packaging (Janjarasskul and Suppakul 2018) and ease of application (Álvarez-Hernández et al. 2018). However, KMnO₄ cannot be placed in direct contact with foodstuffs owing to its toxic nature and insufficient enduring efficacy under high moisture conditions which remains a pre-requisite during storage of most of the fresh fruits and vegetables, KMnO₄ cannot be used in direct contact with foodstuffs (Gaikwad et al. 2019; Wyrwa and Barska 2017; Yildirim et al. 2018). Post oxidation, potassium permanganate changes its color from purple to brown due to consequent reduction of MnO₄ to MnO₂. Studies conducted by various researchers have revealed the effectiveness of these sachets in removing ethylene from packages of bananas, diced onions, apples, mango and tomato etc. (Vermeiren et al. 2003). Warsiki (2018) prepared chitosan and KMnO₄-based active packaging and used it for tomato ripening inhibition. The fruits packed with KMnO₄-based active packaging possessed high hardness compared to control at ambient storage, however, the tomato stored at refrigerated storage had lower hardness value when compared with respective control. Spricigo et al. (2017) studied the effect of particle size (viz. micro- versus nanoparticles) as support material; KMnO₄ content (2.5, 5 and 10%) and RH (45, 60, 75 and 90%) on KMnO₄-based oxidation. KMnO₄-based ethylene scrubbers (0.3 g sample used for each experiment) supported onto SiO₂ and Al₂O₃ (at 25 °C) oxidized 7.48 ml/L C₂H₄ after 1 h of exposure. Ethylene removal rates showed an upsurge with decreased particle size and increased KMnO₄ concentration, regardless of the support material used. The KMnO₄-based scavenging system can be utilized in conjunction with a controlled or modified atmosphere or in active packaging to confiscate the concentration of C₂H₄ accrued within a closed environment (Keller et al. 2013). Bhattacharjee and Dhua (2017) observed comparatively higher shelf-life of pointed gourd fruits stored at 29–33 °C with 68–73% RH (in polypropylene bags) with celite as support compared to silica gel for KMnO₄ as ethylene scrubber (4–8 g scrubber kg⁻¹ fruit). The baby bananas were stored (@18 °C; 70–80% RH) each with 17 g of KMnO₄ (kg⁻¹ fruit) supported on montmorillonite, vermiculite, kaolinite and zeolite as diverse support materials (García et al. 2012). Vermiculite was found to be the best while kaolinite occupied the bottom place (Álvarez-Hernández et al. 2018). In another experiment, polyolefin elastomer having nanoparticles of nanosilica and nanoclay impregnated with KMnO₄ showed increased ethylene absorption at higher concentrations due to higher KMnO₄ concentration. The nanoparticles were able to extend the shelf life of bananas up to 15 days at ambient conditions (Ebrahimi et al. 2021). The KMnO₄-loaded sepiolite mediated ethylene scavenging was utilized in conjunction with encapsulated thymol for inhibiting Botrytis cinerea in cherry tomatoes (Álvarez-Hernández et al. 2021). Ni et al. (2021) utilized the 1-MCP and molecular sieves loaded with potassium permanganate as ethylene scavenger for preserving Agaricus bisporus. The potassium permanganate loaded halloysite nanotubes (HNTs) onto low density polyethylene had higher ethylene scavenging effect and thus delays the changes associated with ethylene (Joung et al. 2021). Ahmad et al. (2023) found that combination of 1-MCP and hypobaric storage was effective in improving the post-harvest storage life of Shughri pears.

In most technologies dealing with oxidation,for effective elimination of ethylene, the air within the storage premise (room/transport vehicle) should essentially be circulated past the scrubbing material (Keller et al. 2013). The capability of KMnO₄ to lessen ethylene concentration from the atmosphere surrounding horticultural commodity (apple) was proved for the first time by Forsyth et al. (1967). Afterwards, many studies have been done but no scientific studies were performed regarding the potassium permanganate concentration and the effect of ethylene concentration (Keller et al. 2013). Blanpied et al. (1985) showed that for a 200 tonnes Empire apple store, the elimination effectiveness of KMnO₄ beads declined to 25% after twelve days of continuous use and thus needed regular replacements. However, they postulated that this decrease was mainly owed to the moisture which hindered competitively ethylene scavenging. Keller et al. (2013) postulated a dire 50% diminution of KMnO₄ efficiency when the relative humidity augmented from 70 to 90%. According to an exhaustive study of Wills and Warton (2004), if a commodity yielding 1 μl kg⁻¹ h⁻¹ ethylene is held at 20 °C with 90% RH, nearly 6 g absorbent per kg of commodity should be required to plummet the ethylene concentration by 90%, on the other hand, with a commodity generating 10 μl kg⁻¹ h⁻¹ ethylene under similar conditions, the amount of absorbent required should be 60 g of absorbent per kg of stored commodity. This implies that the potassium permanganate-based scrubbing appears feasible for perishables producing little ethylene. Coming to these simulations, for a 20 kg of packed commodity producing ethylene @10 μl/kg/h, the amount of KMnO₄ compulsory would be near about 1.2 kg, which will concomitantly release 0.8 kg of KOH as residual directly within the solid body (Keller et al. 2013).

Adsorption based ethylene removal

Ethylene adsorption is prompted by van der Waals forces among the adsorbent and the adsorbate molecules. Adsorption (surface phenomenon) of any molecule depends on quite a few parameters: adsorbent concentration, temperature, gas composition, and RH. Adsorption of ethylene can be accomplished on activated charcoal, crystalline aluminosilicates, bentonite, aluminium oxide, Fuller’s earth, brick dust, silica gel, clay materials (cristobalite and zeolite) etc. However, activated carbon, zeolite, carbon fibre and silica gel come under the category of standard physical adsorbents (Álvarez-Hernández et al. 2018). In addition, certain sorbent chemicals, like propylene glycol, hexylene glycol, squalene, phenylmethyl silicone, polyethylene and polystyrene, adsorb ethylene and they can be reused by regenerating and after purging (Vermeiren et al. 2003; Zagory 1995). The solid adsorbents when offered with certain alkaline treatments viz. making to react under a gas stream or with certain agents, can develop the selectivity to a specific adsorbate (Gaikwad et al. 2019). Various adsorbent materials used as such or as support for enhancing the scavenging capacity of KMnO₄ have been elaborated in Table 5.

Table 5.

Various adsorbent materials used as such or as support material for enhancing the scavenging capacity of KMnO₄

Support material	Surface area (m2g−1)	Pore diameter (Å)	Pros and cons	Representative example	References
Metal oxides
Silica gel: can be of two subtypes (Mesopore having pore diameter more than 20 Å)			Mechanism: adsorption as well as chemical modifications Pros i. amorphous material form of SiO2 ii. polymer of silicic acid with a surface rich in hydroxyl groups, or silanols (Si–O–H) iii. Non-toxic, GRAS, cheap, accessible iv. excellent thermal and chemical stability Cons: low C2H4 removal capacity	KMnO4 embedded onto SiO2 crystals prolonged the shelf-life of guava fruits (up to 7 weeks), under active packaging using LDPE film at 8 °C (Singh and Giri, 2014)	Álvarez-Hernández et al. (2018), Jal et al. (2004), Sneddon et al. (2014)
Low density	300–350	100 -150
Regular density	750–850	22–26
Activated alumina (Al2O3)	50 -500	60–150	Pros i. Activated Al2O3 is a semi-crystalline inorganic material composed mainly of aluminium oxide ii. High adsorption capacity and thermal stability iii. GRAS, inexpensive, non-toxic iv. Used in most of the commercial KMnO4-based C2H4 scrubbers Cons: Less scientific reports on postharvest life of fruits and vegetables	90% C2H4 removal after 2.5 h when 1.0 g Al2O3 beads containing 4% KMnO4 were exposed to 20 μL L−1 C2H4 at 20 °C at 60–70% RH (Wills and Warton 2004)	Álvarez-Hernández et al. (2018), Mallakpour and Khadema (2015)
Layer silicates and zeolites
Clays		Interlayer spacing of about 0.9 to 1.2 nm	Pros: i. Hydrous layered aluminosilicates composed of two layers: tetrahedral and octahedral ii. High surface area, high sorption, swelling, intercalation and cation-exchange with other cations with no change in structure iii. Eco-friendly, non-toxic, economical and recyclable iv. montmorillonite (MMT) most common. Others include pumice, cristobalite, cloisite, halloysite, Japanese Oya and clinoptilolite v. Can be incorporated into an ethylene-permeable sachet, or into the packaging film via extrusion	KMnO4-based C2H4 scrubber on clay support. 30 g of scrubber per 0.42–0.67 kg of banana delayed peel yellowing, reduced weight and firmness loss. Shelf-life increased up to 18 days at 27–30 °C (Santosa and Widodo 2010)	Gaikwad et al. (2018), Kaur and Kishore (2012), Tas et al. (2017)
Zeolite (Zeolite Y most popular)	900–3000	3–12	Pros i. Zeolite is a pure form of ancient volcanic ash ii. hydrophilic crystalline aluminosilicates with a negative framework; charges balanced with alkali/alkaline earth ions iii. three-dimensional structure, adsorbent, high surface area with large pore structures, cation exchange and molecular sieve ability, low cost and availability iv.) can be integrated into packaging films to allow high gas permeability Cons: i.) Moisture hindrance	Montmorillonite (10%) incorporated with low-density polyethylene eliminated 37% of ethylene present in package headspace after 50 h (Coloma et al. 2014)	Patdhanagul, et al. (2012), Yagub et al. (2014)
Activated carbon	300–4000 may go up to 5000		Pros: i. Non-crystalline porous forms of carbon obtained by pyrolysis of carbonaceous material ii. Commercial grade activated carbons have pore volume (10–25 Å) in diameter ii. Can be granular, powdered or fibre; most preferred is the granular form as it is multipurpose and easy to regenerate iii. High surface area, hydrophobic, lightweight and cheap	Best C2H4 adsorption is performed by granular form (over 80%) in comparison with powdered (over 70%) and fibre forms (over 40%) (Martínez-Romero et al. 2007)	Ben-Mansour et al. (2016), Gaikwad et al. (2018), Sneddon et al. (2014)

Although there are various KMnO₄ based commercially viable ventures in the field of ethylene removal, yet certain adsorbent-based ventures also do exist. Table 6 reviews various manufacturers involved in adsorbent-based ethylene removal system providers, their trade names, principal adsorbents and other accessible information as per the available literature. Also, the adsorbent materials have found place in some patents in the field of ethylene removal. These include Orega bags (US patent) consisting of pumice-tuff, zeolite, activated carbon etc. and synthetic resin film sheet (Nissho and Co., Japan; US patent) with crushed coral and calcium carbonate (Vermeiren et al. 2003).

Table 6.

Commercially available adsorbent based ethylene scavengers

Manufacturer	Trade name	Principal adsorbent used	Other strategy combined	Country	Final supplied form	References
#Circul-Aire Inc. (Multi-Mix®)	MM-3000	Activated carbon		Quebec, Canada	0.3 cm diameter cylinders	Circul-Aire Inc. (2006)
	MM-7000		Phosphoric acid
	MM-9000		Potassium hydroxide
DeltaTRAK Inc	Prime Pro EAP®	Polyethylene plastic cover with ethylene adsorbent		California, USA	Pallet	https://www.deltatrak.com
Peakfresh products	Peakfresh	Minerals impregnated in LDPE		Australia	LDPE Film (MAP)	Vermeiren et al. (2003), www.peakfresh.com/howitworks.htm
*Marathon Products	Ethylene filter products	Zeolite		USA	Sachet	Gaikwad et al. (2020)
Dry Pak Industries Inc	Dry Pak	Absorption		Encino, CA, USA	Sachet, filter, film	www.drypak.com/ethyleneAbsorbers.html
*Sekisui Jushi Corp	Neupalon	Activated carbon	Metal catalyst	Japan	Sachet	Gaikwad et al. (2020), Vermeiren et al. (2003)
Grofit Plastics	Biofresh	Mineral		Israel	Zipper bag, liners for cartons	Gaikwad et al. (2020), www.grofitplastics.com
Desiccare Inc	Ethylene Elimination Pack	Zeolite		Nevada, USA	Sachet	https://www.desiccare.com/ethylene-absorber-1; Vermeiren et al. (2003)
Evert-fresh	Evert-Fresh Green-Bags	Minerals (Zeolite)		USA	Bags of food-grade film	Gaikwa et al., (2020), https://www.evertfresh.com/; Vermeiren et al., Debevere 2003
It’s Fresh Ltd	Infinite™	Blend of clay and minerals	Printed onto flexible films	London, UK	Filters, transit sheets, pads, and labels	https://itsfresh.com/technology/
*Cho Yang Heung San Co. Ltd	Orega bag	Minerals		South Korea	Bags	Vermeiren et al. (2003)
*Mitsubishi Gas Chemical Co	SendoMate	Activated carbon	Palladium (Pd) catalyst	Japan	Sachet	Gaikwad et al. (2020)
*Honshu Paper	Hatofresh System	Activated carbon	Bromine	Japan	Paper bag or corrugated box	Vermeiren et al. (2003)
*E-I-A Warenhandels GmbH	Profresh	Minerals		Austria	Polymeric Film	Gaikwad et al. (2020), Vermeiren et al. (2003)
*Odja Shoji Co. Ltd	BO Film	Crysburite ceramics		Japan	Film (polymeric)	Gaikwad et al. (2020), Vermeire et al. (2003)
*Nippon Container Corp	FAIN			Japan	Polymeric Film	Gaikwad et al. (2020)

^#Used as generalized purifier and not specifically destined for ethylene by manufacturer

*The compilation has been done based on the quoted references; however direct website/address could not be recovered through google search

KMnO₄ (oxidation) and adsorption-based ethylene scavenging

The capability to perform under the atmosphere of high humidity is a must for an effective adsorbent of ethylene in order to simulate the conditions during storage and transportation of perishables. However, as reviewed extensively by Keller et al. (2013), published results on adsorption of ethylene by many adsorption/oxidation-based strategies on horticultural perishables stored under various environmental conditions like low ethylene and temperature, high relative humidity along with the presence of other volatile entities as well as ethylene adsorption isotherm and the adsorption capacity of the adsorbent are rare. Abe and Watada (1991) reported less ethylene accumulation and consequent reduction of softening in kiwifruit and banana and diminished loss of chlorophyll in spinach leaves, when palladium chloride catalyst infused charcoal was used as an ethylene absorbent at 20 °C; although, it did not give promising results in case of broccoli. Marzano-Barreda et al. (2021) used biodegradable active packaging containing synthetic zeolite Watercel ZF as ethylene scavenger for packing fresh broccoli florets. The adsorbent-based ethylene removal is different from potassium permanganate-based oxidation and the differences between KMnO₄ and adsorption-based ethylene scavenging systems has been compiled and presented in Table 7.

Table 7.

Difference between KMnO₄ (oxidation) and adsorption-based ethylene scavenging

Source: Keller et al. (2013), Liu et al. (2006), Vermeiren et al. (2003), Wills and Warton (2004)

S. no.	Charactristics	KMnO4-based ethylene scavenging	Adsorption-based ethylene scavenging
1.	Mode of action	Oxidation	Adsorption
2.	Base material	KMnO4	Activated carbon, clay, zeolite, silica, Fuller’s earth, Kieselguhr etc
3.	Nature	Oxidative scavenging	Recuperative
4.	Type of phenomenon	Chemical	Physical (2 dimensional surface phenomenon)
5.	Effect on ethylene	Broken down to CO2 and water	Only adsorbed and can be recovered by desorption
6.	Recovery of base material	Irreversible, once spent can not be regenerated	Reversible, can be reused post desorption
7.	Effect of ethylene concentration	Independent of ethylene concentration and depends upon the oxidative capacity of KMnO4 present	Depndent on ethyelene concentration; activity may decrease with decrease in ethylene concentration
8.	Commercial viability	Most widely used	Less used comparatively; more research is needed in this area
9.	Specificity	Specific	Non/less-specific
10.	Moisture hindrance	Yes, nearly 45% KMnO4 present in inner core of KMnO4-based beads sometimes may remain unused	Yes, mircopores may also adsorb water vapour, thus blocking surface area (to be used for ethylene adsorption)
11.	Requirement of support material	Yes	May itself act as support material as well as adsorbent
12.	Generation of monitorable waste	KOH	None
13.	Desorption	Not possible as ethylene gets converted to CO2 and H2O	Possible by temperature increase or by change in pressure
14.	Ethylene adsorption capacity (mmol/kg)	34–41	11–78

Activated carbon/charcoal

Activated carbon/charcoal (AC) is a sort of porous carbon, non-crystallinein nature and produced by pyrolytic treatment of carbonous materials (Ben-Mansour et al. 2016; Sneddon et al. 2014). Various forms of AC include granular, powdered or fibre form, though due to comparatively easy regeneration and versatile nature, the granular form is the most preferred. For preparation of AC, the carbon containing material is first carbonized at high temperature followed by activation. The activation step of carbon is performed to generate more pores vis-à-vis to change their pore volume, form and size etc. The latter step can be attained by physical as well as chemical methods (Álvarez-Hernández et al. 2018; Yang 2003). Activated carbon offers advantages owing to its hydrophobic behaviour, more surface area, being lightweight and low production cost (Ben-Mansour et al. 2016; Gaikwad et al. 2020). Most commercial grades of AC typically own a diameter ranging between 10 and 25 Å for pore volume along with surface area oscillating between 300 and 4000 m² g⁻¹, though the latter can reach as much as 5000 m² g⁻¹ for some ACs (Martínez-Romero et al. 2007; Yang 2003). Bailén et al (2006) stored Beef’ tomato (8 °C; 90% RH) in polypropylene bags (20 μm thickness) under modified atmosphere packaging. The packaging was loaded with sachets comprising granular AC (5 g) with surface area of 226 m² g⁻¹. The granular AC deferred the quality changes in colour, physiological weight, and overall firmness of tomato while storage and was able to reduce significantly C₂H₄ levels up to 2 weeks inside packages. The AC can be united or impregnated with other ethylene adsorbing/scavenging compounds like KMnO₄ to further enhance its usefulness. Mukti et al. (2018) used mangosteen rind powder (a waste with 180–355 µm size) for porous carbon preparation. The rind powder was carbonized at 575 °C for 3.5 h followed by pyrolysis/activation upto 850 °C and kept for 15 min under flowing nitrogen and steam. The highest surface area obtained was 1080 m² g⁻¹ thus falling in the category of mesoporous carbon with the ethylene adsorption capacity of 40.12 cm³ g⁻¹. In another experiment, rice husk was carbonized at 300 °C for 3 h to make it silica free followed by its activation with activating agents (viz. NaOH, ZnCl₂, and KOH) separately in 1:1 ratio at 900 ◦C under nitrogen flow. The KOH activated samples had high specific surface area (2342 m² g⁻¹) and large pore volume (2.94 cm³ g⁻¹) (Shrestha et al. 2019). Liu et al. (2006) reported that activated carbon has approx. 11–78 mmol/kg ethylene absorption capacity which can quench 64–1000 ppm ethylene at 30 °C, while palladium catalyst with activated carbon has corresponding ethylene absorption capacity of 7–71 mmol/kg and has similar ethylene quenching capacity at 30 °C.

Conclusion

Fruit ripening encompasses a progressive series of physiological, biochemical, sensorial, textural and organoleptic amendments. Ethylene responsible for fruit ripening is invariably produced in climacteric and non-climacteric plants from methionine. It is required for natural ripening of fruits and also leads to over-ripening, senescence, loss of texture, microbial attack, less post-harvest life and other associated problems during storage and transportation of fruits. Therefore, ethylene accumulation in packaging system should be avoided to lengthen the shelf-life of perishable commodities. KMnO₄-based oxidation is the most commercially cultivated approach worldwide. However, adsorption-based scrubbing is also a vital phenomenon which can be tapped alone or in combination with earlier existing processes for ethylene removal. Future research should focus on systematic evaluation of KMnO₄-based scavenging, exploring the mechanism of adsorption, adsorbent(s) behavior in the presence of other gases and their partial pressures, volatile organic compounds, temperature, relative humidity, development of hydrophobic adsorbents to turn-up under high RH conditions, improvement in adsorption by π-complexation, regeneration of adsorbent by desorption (temperature or pressure swing), and improvement in photocatalytic oxidation etc. Heat input and air flow patterns also need to be considered while designing ethylene removal strategy for a large-scale storage. Banking upon novel approaches including combination of one or more strategy which is economical as well as amenable to scale-up may revolutionize the perishables’ shelf-life and in turn global economy.

Supplementary Information

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Authors contributions

SK: Corresponding author, Conceptualization, literature collection, Writing original draft; Writing—review and editing. RK: Conceptualization, Writing original draft, literature collection, editing. BRB: Formal analysis, editing. Prerna Nath:Formal analysis, literature collection. RKS: Project administration, Conceptualization, Supervision. SM: Review, editing. AP: Review, editing. RS: Literature collection, review. AK: Review, editing.

Funding

Indian Council of Agricultural Research.

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All data generated or analysed during this study are included in this published article (and its supplementary information files).

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