2
Ethylene-removing packaging
D. ZAGORY
Ethylene is a chemically simple, ubiquitous chemical that has diverse and
profound effects on the physiology of plants. Ethylene has so many different
effects on plants, is effective in such low concentrations, and its effects are
so dose-dependent, that it has been identified as a plant hormone. Though
many of the effects of ethylene on plants are economically positive, such as
induction of flowering in pineapples, de-greening of citrus and ripening of
tomatoes, often ethylene has been seen to be detrimental to the quality and
longevity of many horticultural products. For this reason, there has long
been interest in removing ethylene from the horticultural environment and in
suppressing its effects.
Some of the diverse ways in which to absorb, adsorb, counteract or
chemically alter ethylene have led to products designed to reduce its
deleterious effects. This chapter will briefly review the chemistry, physiology and agricultural effects of ethylene preparatory to describing the
research and commercial effort undertaken to incorporate ethylene control
agents in packages for horticultural products. Some of this effort has met
with commercial success, but much has not. However, with the rapid growth
of packaging of fresh fruits and vegetables, particularly fresh cut salads and
fruits, opportunities for such products are bound to increase. Therefore, it is
timely to review the basis and activities relating to these products to better
elucidate the possible forms that they can and will take and to point out
some of the advantages and disadvantages of the various approaches likely
to emerge.
2.1 The chemistry of ethylene
The ethylene molecule is of the alkene type, being simply two carbons
linked by a double bond with two hydrogen atoms on each carbon. Such a
simple molecule can be synthesized through several different pathways and
is subject to many kinds of chemical reaction.
2.1.1 Synthesis
Ethylene can be synthesized both biologically and non-biologically. It is a
common component of smoke and can be found as a product of aerobic
combustion of almost any hydrocarbon. It is thus a common air pollutant, its
chief source being automobile engines.
Biological sources of ethylene include higher plant tissues, several species
of bacteria and fungi, some algae, and some liverworts and mosses. The
biosynthetic pathways for ethylene are diverse among these different
organisms. The pathway of synthesis from methionine has been described in
detail for higher plants (Yang and Hoffman, 1984). The pathways for
synthesis in bacteria appear to be diverse since any of several carbon sources
other than methionine will serve as precursors (Sato et al, 1987). Nitrogenfixing bacteria can reduce acetylene to ethylene (Dillworth, 1966). Approximately 25-30% of fungal species tested produce ethylene on appropriate
media (Fukuda et al, 1984; Hag and Curtis, 1968). The pathways of plant
and fungal ethylene synthesis appear to be distinct, as the inhibitor
rhizobitoxin blocks synthesis in plants but not in the fungus Penicillium
digitatum (Owens et al, 1971). The pathway of ethylene synthesis in nonvascular plants may be different from that in vascular plants (Osborne,
1989a).
Because this chapter is primarily concerned with methods of eliminating
ethylene, not producing it, it is not necessary to go into the details of
production by different organisms. This has been reviewed in detail
elsewhere (Abeles et al, 1992). The important point is that environmental
ethylene can be biologically produced by a wide range of organisms, both
visible and invisible, and such sources ought to be considered when devising
strategies to reduce ambient ethylene.
2.1.2 Degradation
Ethylene undergoes several types of degradation reactions. Because of its
double bond, ethylene absorbs ultraviolet (UV) radiation at 161, 166 and 175
nm (Roberts and Caserio, 1967). Ultraviolet photodecomposition of atmospheric ethylene is an important environmental ethylene sink (Scott and
Wills, 1973) and yields primarily hydrogen, acetylene, n-butane and ethane
(Noyes et al, 1964). Soil microorganisms can degrade ethylene and at least
one species, Mycobacterium paraffinicum, is thought to be an efficient
oxidizer of ethylene (Abeles et al, 1992).
Ethylene reacts with ozone to yield water, carbon dioxide (CO2), carbon
monoxide (CO), and formaldehyde (Scott et al, 1957). Ultraviolet light will
interact with oxygen (O2) in air to form ozone which breaks down ethylene,
but UV light will directly degrade ethylene as well. Thus, UV light will
effectively eliminate ethylene even in low O2 atmospheres (Shorter and
Scott, 1986). However, the reaction is inefficient at very low ethylene
concentrations such as those found in fresh produce environments so the
commercial potential of ozone as an ethylene scrubber is limited. Atomic
oxygen will also react with ethylene and can form an array of compounds
including ethylene oxide, ethane, CO, propylene, acetaldehyde, propanol,
butanol, hydrogen and dioxyketone (Leighton, 1961).
The double bond of ethylene makes it very reactive through a number of
reaction pathways. The double bond will undergo hydrogenation, in the
presence of any of several metal catalysts, to yield ethane (Morrison and
Boyd, 1966). Ethylene will react with halogens (chiefly chlorine and
bromine) through halogenation and hydrohalogenation reactions to form
dihaloalkanes. Thus, ethylene can be eliminated from air by passing it over
brominated activated charcoal to form dibromoethane (Talib, 1983). Brominated charcoal filters are relatively efficient removers of ethylene. Up to
90% of the bromine will react with ethylene. However, bromine also reacts
with water to form HBr and Br2 gas is released from the carbon filter. These
compounds are injurious to plant tissues and corrosive to stainless steel. In
addition, brominated activated charcoal is hygroscopic and will become wet
in humid conditions. Alternatively, ethylene will react with hydrogen halides
to form ethyl halides (Morrison and Boyd, 1966). Ethylene reacts with
concentrated sulfuric acid to form ethyl hydrogen sulfate or with water in the
presence of acids to yield ethanol (Morrison and Boyd, 1966).
Certain oxidizing agents react with ethylene to form glycols. The most
common of these oxidizing agents is potassium permanganate (KMnO4)
which oxidizes ethylene to ethylene glycol and thence to CO2 and water
(Morrison and Boyd, 1966). Potassium permanganate is often adsorbed onto
Celite, vermiculite, silica gel or alumina pellets. Permanganate scrubbers are
also effective in adsorbing air pollutants such as O3, H2S, SO2, NO and
NH3.
It is clear that ethylene is a very reactive compound that can be altered or
degraded in many ways. This creates a diversity of opportunities for
commercial methodologies for the removal of ethylene and, in fact, many
different methods have been used. However, many of the common reactions
undergone by ethylene require high concentrations of ethylene and/or high
temperatures and pressures. Therefore, many of the processes most commonly used to modify ethylene in the petrochemical industry are not
appropriate for the conditions generally found in a food package environment.
2.13
Adsorption and absorption
In addition to chemical cleavage and modification, ethylene can be absorbed
or adsorbed by a number of substances including activated charcoal,
molecular sieves of crystalline aluminosilicates, Kieselguhr, bentonite,
Fuller's earth, brick dust, silica gel (Kays and Beaudry, 1987) and
aluminium oxide (Goodburn and Halligan, 1987). A number of clay
materials have been reported to have ethylene adsorbing capacity. Examples
include cristobalite (> 87% SO2, > 5% AlO 2 , > 1% Fe2O3) (Kader et ai,
1989), Ohya-ishi (Oya stone) and zeolite (Urushizaki, 1986a). Oya stone is
mined from the Oya cave in Tochigi Prefecture in Japan. The cave has been
used to store fresh produce and is reputed to confer added storage life. The
salutary properties of the cave are thought to reside in the largely zeolitic
stone interior. To improve its ethylene adsorptive capacity, the Oya stone is
first finely ground with a small amount of metal oxide. The mixture is then
kneaded and heated to 200-9000C, then oxidized with ozone or electromagnetic radiation (Urushizaki, 1986b).
Some regenerable adsorbents have been shown to have ethylene adsorbing
capacity and have the benefit of being reusable after purging. Examples of
such adsorbents include propylene glycol, hexylene glycol (Rizzolo et al,
1987a), squalene, Apiezon M, phenylmethylsilicone, polyethylene and
polystyrene (Rizzolo et a/., 1987b).
Some adsorbents have been combined with catalysts or chemical agents
that modify or destroy the ethylene after adsorption. For example, activated
charcoal has been used to adsorb ethylene. In some cases, the activated
charcoal has been impregnated with bromine or with 15% KBrO3 and 0.5M
H2SO4 to eliminate the activity of the ethylene (Osajima et a/., 1983). A
number of catalytic oxidizers have been combined with adsorbents to
remove ethylene from air. Examples include potassium dichromate, KMnO4,
iodine pentoxide, and silver nitrate, each respectively on silica gel (Eastwell
et al, 1978).
Electron-deficient dienes or trienes, such as benzenes, pyridines, diazines,
triazines and tetrazines, having electron-withdrawing substituents such as
fluorinated alkyl groups, sulphones and esters (especially dicarboxyoctyl,
dicarboxydecyl and dicarboxymethyl ester groups), will react rapidly and
irreversibly with ethylene at room temperature and remove ethylene from
the atmosphere. Such compounds can be embedded in permeable plastic
bags or printing inks to remove ethylene from packages of plant produce
(Holland, 1992).
Metal catalysts immobilized on absorbents, such as platinized asbestos,
cupric oxide-ferric oxide pellets and powdered cupric oxide, will effectively
oxidize ethylene, but in many cases the reactions require high temperatures
( > 1800C). Clearly such systems would be inappropriate for food packaging
applications.
2.2 Deleterious effects of ethylene
Ethylene has long been recognized as a problem in postharvest handling of
horticultural products. Since the discovery in 1924 that ethylene can
accelerate ripening in fruits (Denny, 1924) it has become clear that ethylene
can be the cause of undesirable ripening of fruits and vegetables. It is now
recognized that ethylene, in very low amounts, can be responsible for a wide
array of undesirable effects in plants and plant parts. The physiological
effects of ethylene are so important, so diverse, and are induced by such
small amounts of ethylene that it is considered a plant hormone. The diverse
physiological effects of ethylene have been extensively reviewed elsewhere
(Abeles et ai, 1992) so only those effects that are deleterious to packaged
plant produce will be discussed here.
2.2.1 Respiration
Perishability of produce generally is well correlated with respiration rate.
Commodities such as asparagus, broccoli and mushrooms that have very
high respiration rates are very perishable, having postharvest lives measured
in days. Those commodities such as nuts, dates, dried fruits, potatoes and
onions that have very low respiration rates have postharvest lives measured
in months (Kader, 1985). Reduction of respiration rate increases postharvest
life and elevation of respiration rate generally decreases it. This is one of the
reasons why low temperature is so important in postharvest management.
Reducing the temperature rapidly reduces the respiration rate of the
product.
Ethylene accelerates the respiration of fruits, vegetables and ornamental
plants. In the case of climacteric fruit, ethylene can induce a rapid and
irreversible elevation in respiration leading directly to maturity and
senescence. In non-climacteric plant organs, ethylene induces a reversible
increase in respiration. In most cases, exposure to a few parts per million
(ppm) of ethylene leads to increased respiration and increased perishability.
2.2.2 Fruit ripening and softening
Ethylene has been referred to as a 'ripening' hormone because it can
accelerate softening and ripening of many kinds of fruit. Exposure of mature
fruit to ethylene leads to increased respiration, increased production of
endogenous ethylene, and softening of fruit tissues (Abeles et aiy 1992).
This is achieved through the direct or indirect stimulation of synthesis and
activity of many ripening enzymes by ethylene.
Some fruits, such as bananas and tomatoes, are often deliberately exposed
to high concentrations of ethylene (~ 100 ppm) to induce rapid ripening. In
most cases, for packaged fruits it would be desirable to prevent exposure to
ethylene and thereby prevent rapid ripening.
2.2.3 Flower and leaf abscission
Cell wall hydrolysis of specific cells at the base of leaves, petioles, petals,
pedicels and fruit leads to abscission of the distal organ (Abeles et al., 1992).
Ethylene has been shown to accelerate abscission for many, though not all,
plants and plant parts (Jankiewicz, 1985; Osborne 1989b; Reid, 1985a).
Ethylene causes flower and leaf abscission of many potted ornamental plants
(Cameron and Reid, 1983).
2.2.4 Chlorophyll breakdown
Ethylene increases the rate of chlorophyll degradation in leaf, fruit and
flower tissues (Aharoni, 1989; Knee, 1990; Kusunose and Sawamura, 1980;
Makhlouf et al, 1989). This can be of particular concern in the case of leafy
green vegetables such as spinach, immature fruits such as cucumbers and
squash, and flowers such as broccoli (Reid, 1985b). The presence of low
levels of ethylene can cause yellowing and reduced quality.
2.2.5 Petal inrolling in carnations
Low concentrations of ethylene (< 1 ppm) cause inrolling (or sleepiness)
of the flower petals of sensitive carnation varieties accompanied by a loss of
turgor in the petal tissues (Halevy, 1986). Some carnations are so sensitive
to ethylene that they have been used as ethylene bioassays. Such sensitive
varieties are often subjected to a pulse treatment with silver thiosulphate to
render them insensitive to the effects of ethylene (Cameron and Reid,
1983).
2.2.6 Postharvest disorders
Ethylene can be responsible for a number of specific postharvest disorders of
fruits and vegetables. Examples include russet spot (small oval brown spots,
primarily on the midrib) of crisphead lettuce, formation of bitter-tasting
isocoumarins in carrots, sprouting of potatoes, and toughening of asparagus
(Reid, 1985b).
2.2.7 Susceptibility to plant pathogens
Many postharvest plant pathogens are opportunistic microorganisms that
thrive on injured or senescent tissues. To the degree that ethylene accelerates
senescence and causes specific physiological disorders, it also enhances the
opportunities for pathogenesis. The growth of a number of postharvest
pathogens is directly stimulated by ethylene (Barkai-Golan, 1990; BarkaiGolan and Lavy-Meir, 1989; Kepczynska, 1993). In addition, several
postharvest plant pathogens produce ethylene (Barkai-Golan, 1990) and this
ethylene may compromise the natural defences of the plant tissues.
2.3 Interactions of ethylene and other gases
The activity and reactivity of ethylene depends, in part, on the presence of
other atmospheric gases. The user of packaging materials for the removal or
inactivation of ethylene should consider the presence and concentrations of
oxygen, carbon dioxide, ozone and ethylene and their interactions with each
other and with plant tissues.
2.3.1 Oxygen
Ethylene production, biosynthesis and explosiveness are all related to
ambient oxygen concentration. Most pathways of ethylene synthesis,
whether biological or chemical, are oxidative conversions or cleavages.
Although rice and some other aquatic plants have been reported to
synthesize ethylene in the absence of O2 (Ku et al, 1970), most plants
require O2 for ethylene synthesis. However, the oxygen affinity of ethyleneforming enzyme (EFE) is much less than that for respiratory enzymes. The
K1n for conversion of 1-aminocyclopropane-l-carboxylic acid (ACC) to
ethylene in apple is about 1.4% O2 (Banks et al., 1984; Bufler and Streif,
1986) but K1n values for other plants organs are generally 3-10% (Burg and
Thimann, 1959; Lieberman et al, 1966). In some cases, reduced O2 in a
package may more effectively reduce ambient ethylene through reduced
ethylene synthesis than ethylene-adsorbing capacity built into the package.
However, reduced O2 apparently slows the conversion of ACC to ethylene,
resulting in accumulation of ACC (Burg and Thimann, 1959; Hansen, 1942;
Imaseki et al, 1975; Jackson et al, 1978). Upon exposure to higher O2
concentrations, the accumulated ACC will be rapidly converted into
ethylene so low O2 must be maintained continuously to maintain low
ethylene concentrations.
The combustion of organic materials requires O2 and results in ethylene as
one of the combustion products. Ethylene at concentrations between 3.1 and
32% by volume, is explosive in air (Reid, 1985b). Neither of these
conditions occurs in packages.
232
Carbon dioxide
Carbon dioxide may stimulate, inhibit or have no effect on ethylene
synthesis, depending on the plant tissue (Abeles et al, 1992) and the
concentration of CO2. More importantly, CO2 renders normally sensitive
plant tissues insensitive to the effects of ethylene, thereby preventing
abscission (Wittenbach and Bukovac, 1973), floral senescence (Nichols,
1968), chlorophyll loss (Aharoni and Lieberman, 1979) and growth
(Chadwick and Burg, 1967).
2.3.3 Ozone
As was mentioned above, ozone oxidizes ethylene to simple breakdown
products and has been used experimentally to remove ethylene from produce
storage areas. However, ozone would not normally be found or introduced
into a food package.
2.4 Ethylene sources in the environment
Ethylene is ubiquitous at low levels in the environment. It is a common
pollutant that can be detected with sensitive instruments. As most methods
of adsorbing or decomposing ethylene have finite capacities for activity, it
seems prudent to reduce environmental ethylene to avoid saturating the
environment of the package with ethylene. Ethylene can come from many
sources both within and outside the package.
Although there are no national standards for environmental ethylene,
California, USA standards recommend human exposures to no more than
0.5 ppm for 1 h or 0.1 ppm for 8 h (Anon., 1962). Such levels are below
damage thresholds for all but the most sensitive horticultural commodities.
2.4.1 Combustion
Ethylene is a common breakdown product of virtually all aerobic combustion processes. Burning agricultural wastes, wildfires, diesel- or propanepowered forklifts, cigarette smoke, truck and auto exhaust, and industrial
stack emissions are all common sources of ethylene. In addition, the heat
generated by combustion (from forklifts, for example) can raise the
temperature of the product sufficiently to stimulate production of productgenerated ethylene.
Ambient atmospheric levels of ethylene are normally in the range of
0.001-0.005 ppm (Abeles et al., 1992), however, urban air levels as high as
0.5 ppm have been measured (Scott et al, 1957). Such high levels are
sufficient to have physiological effects on some fresh produce. Removing
agricultural sources of ethylene and insulating storage rooms from ethylene
air pollution can significantly reduce ambient ethylene.
2.4.2 Plant sources
Growing plants do not normally produce enough ethylene to alter ambient
atmospheric levels of the chemical. In closed areas, such as storage rooms,
packing houses, shipping containers, greenhouses and warehouses, plantgenerated ethylene can be significant (Abeles et al., 1992). Sensitive
products should not be held or stored in proximity to ethylene-generating
products or product-ripening rooms.
2.4.3 Ripening rooms
Bananas and tomatoes are routinely ripened by exposure to 50-100 ppm
ethylene in large sealed rooms. When such rooms are vented, the dispersal
of ethylene can be significant. When ripening rooms are built into produce
storage or distribution warehouses, the ethylene can come in contact with
other products being held in the warehouse. If that produce were packed in
ethylene-adsorbing packaging, the ethylene at such levels might saturate the
packaging and render it ineffective.
2.4.4 Fluorescent ballasts and rubber materials
The ballasts that hold fluorescent lights are sources of ethylene. In addition,
rubber materials exposed to heat or UV light can release ethylene (Reid,
1985b).
2.4.5 Microorganisms
Although several soilborne microorganisms produce ethylene, others degrade it. The net effect appears to be that the soil serves primarily as a sink
for ethylene.
Postharvest plant pathogens growing on stored products in enclosed
holding areas can be important sources of ethylene. AU infested foodstuffs
should be immediately discarded.
2.5
Commercial applications in packaging
Several of the technologies described above have been incorporated into
packaging materials that are either commercially available or are likely to
become available in the near future. As is common in the commercial sector,
some of the claims for ethylene ad-/absorbing capacity for these packaging
materials have been poorly documented and thus the efficacy of the
materials is difficult to substantiate.
Most substances designed to remove ethylene from packages are delivered
either as sachets that go inside the package or are integrated into the
packaging material, usually a plastic polymer film or the ink used to print on
the package.
2.5.1 Potassium permanganate-based scavengers
Many vendors offer ethylene adsorbers based on KMnO4 immobilized on
any of several minerals. These products are available in sachets for packages
and on blankets that can be placed in produce-holding rooms. Potassium
permanganate is not integrated into food-contact packaging because of its
toxicity. However, sachets could be used inside produce packages and have
been shown to effectively scavenge ethylene from packages of bananas,
persimmons, kiwifruit, avocados (Ben-Arie and Sonego, 1985; Fuchs and
Temkin-Gorodeiski, 1971; Hatton and Reeder, 1972; Krishnamurthy and
Kushalappa, 1985; Liu, 1970; Maotani et al.9 1982; Scott et al9 1970).
Typically, such products contain ~ 4-6% KMnO4 on an inert substrate
such as perlite, alumina, silica gel, vermiculite, activated carbon or celite
(Abeles et al, 1992). The performance and useful lives of these scavengers
depends on the substrate surface area and the content of reagent (KMnO4).
Formulations differ in density and surface area of substrate and the loading
of reagent.
Some suppliers of KMnO4-based ethylene scavengers are listed in Table
2.1. This table is not a complete listing of all companies supplying such
products but only those known to the author at the time of writing.
2.5.2 Activated carbon-based scavengers
Various metal catalysts on activated carbon will effectively remove ethylene
from air passing over the bed of carbon. Commercial units, known as
swingtherm ethylene converters, are based on such a system. However, they
require heat and movement of gases and so are not applicable to packaged
produce. Activated charcoal impregnated with a palladium catalyst placed in
paper sachets effectively removed ethylene in an experiment on maintaining
quality of lightly processed kiwifruit, banana, broccoli and spinach (Abe and
Watada, 1991).
The Japanese company Sekisui Jushi has developed a product, Neupalon,
that is a sachet containing activated carbon and a water absorbent capable of
Table 2.1 Suppliers (USA) of potassium permanganate ethylene scavengers
Air Repair Products, Inc.
PO Box 1006
Stafford, TX 77477
Cams Chemical Company, Inc.
1001 Boyce Memorial Drive
Ottawa, IL 61350
Complete Control
PO Box 1006
Stafford, TX 77477
DeltaTrak, Inc.
PO Box 398
Pleasonton, CA 94566
Ethylene Control, Inc.
PO Box 571
Selma, CA 93662
ExtendaLife Systems
PO Box 55044
Hayward, CA 94545-0044
Loomix, Inc.
405 E. Branch Street
PO Box 490
Arroyo Grande, CA 93420
Purafil, Inc.
PO Box 80434
Chamblee, Georgia 30366
Purity Corporation
9539 Town Park
Houston, TX 77036
Note: Nippon Greener Co. is reported to use potassium permanganate by Abe
(1990) in his listing of ethylene absorbers.
absorbing up to 500-1000 times its weight of water. The company provides
data showing that Neupalon adsorbs 40 ml ethylene per m2.
Honshu Paper, also in Japan, has a product called the Hatofresh System
that is based on activated carbon impregnated with bromine-type inorganic
chemicals. They do not specify which bromine compounds are used. The
carbon-bromine substance is embedded within a paper bag or corrugated
box, which is used to hold fresh produce. They claim that the bag will
adsorb 20 ml ethylene per g of adsorbent. It is unlikely such bags could be
used in most developed countries due to the reaction of bromine compounds
with water, which can release toxic bromine gas.
Mitsubishi Chemical Company of Japan produces a product called SendoMate which is based on palladium catalyst on activated carbon which
adsorbs ethylene and then catalytically breaks it down. The product comes in
woven sachets that can be placed in packages of produce.
2.5.3 Activated earth-type scavengers
In the past several years a number of packaging products have appeared
based on the putative ability of certain finely dispersed minerals to adsorb
ethylene. Typically these minerals are local kinds of clay that are embedded
in polyethylene bags which are then used to package fresh produce. Many,
though not all, of the bags are marketed by Japanese or Korean companies,
though some are also sold in the United States and Australia.
The Cho Yang Heung San Co. Ltd. of Korea markets a film bag called the
Orega bag, based on the US patent of Dr Mitsuo Matsui (Matsui, 1989). Fine
porous material derived from pumice, zeolite, active carbon, cristobalite or
clinoptilolite is sintered together with a small amount of metal oxide before
being dispersed in a plastic film. Neither plastics containing chlorine such as
polyvinyl chloride or polyvinylidene chloride, nor plasticizers, are apparently suitable for these applications (Choi, 1991). The inorganic materials
have pores ranging from 2000 to 2800A and the resulting film is reported to
have the capacity to adsorb at least 0.005 ppm ethylene per hour per m2
(Choi, 1991). Adsorption of this small amount of ethylene may not be
helpful for some situations.
Another such film is described in a US Patent assigned to Nissho and Co.
Ltd. of Japan (Someya, 1992). This film incorporates finely ground coral
(primarily calcium carbonate), having pore sizes in the range of
100,000-500,000A. After incorporation in a polyethylene film, the coral is
claimed to absorb ethylene. However, no data have been presented to
support this claim.
A product called Ethad® has been developed by the Rubber Research
Institute of Malaysia; it releases ethylene in order to stimulate the production
of latex by rubber trees. The product is based on powdered zeolite in viscous
oil or grease. The zeolite is reported to adsorb 8% ethylene by weight
(Abeles et ah, 1992). Apparently Ethad® has not been used to adsorb
ethylene in packages.
Evert-Fresh Corporation markets Evert-Fresh bags in the USA. The bags
are, presumably, polyethylene with Japanese Oya stone dispersed within the
film matrix. Oya stone has putative ethylene-adsorbing capacity. Evert-Fresh
Corp. offers shelf-life data for several fresh commodities to demonstrate the
benefits of their bags.
A product called BO Film is marketed by the Odja Shoji Co. Ltd. of
Japan. It is a low-density polyethylene film extruded with finely divided
crysburite ceramic which is claimed to confer ethylene-adsorbing capacity
(Joyce, 1988).
There are many other similar bags being sold throughout the world offering improved postharvest life of fresh commodities due to the adsorption of
ethylene by the minerals dispersed within the film. The evidence offered in
support of this claim is generally based on shelf-life experiments comparing
common polyethylene bags with mineralized bags. Such evidence generally
shows an extension of shelf-life and/or a reduction of headspace ethylene.
Such data are unconvincing. Although the finely divided minerals may
adsorb ethylene, they will also open pores within the plastic bag and alter the
gas-exchange properties of the bag. Because ethylene will diffuse much
more rapidly through open pore spaces within the plastic than through the
plastic itself, one would expect ethylene to diffuse out of these bags faster
than through pure polyethylene bags. In addition, CO2 will leave these bags
more readily and O2 enter more readily than is the case for a comparable
polyethylene bag. These effects can improve shelf-life and reduce headspace
ethylene concentrations independently of any ethylene adsorption. In fact,
almost any powdered mineral can confer such effects without relying on
expensive Oya stone or other speciality minerals. Hercules Chemical
Company relied on this effect while using calcium carbonate to improve the
gas-transmission properties of their Fresh Hold breathable bags without
making any claims regarding ethylene adsorption (Anderson, 1989).
Although the minerals in question may have ethylene-adsorbing capacity,
the data supporting the commercial products incorporating these minerals
fail to demonstrate such capacity. Even if they do have ethylene-adsorbing
capacity, it is possible that they will lack significant capacity while
embedded in plastic films. The ethylene would have to diffuse through the
plastic matrix before contact with the dispersed mineral, thus greatly slowing
any processes of adsorption. Once the ethylene has diffused part-way
through the plastic film, venting to the outside may be nearly as fast and
effective as adsorption on embedded minerals.
In a study performed in Australia with BO film, the mineral in the bag
took up little ethylene (Joyce, 1988). Furthermore, in studies with pure
mineral granules of Cera-sutora A, the author found that the ethylene
sorption capacity of the material was only - 170 nmol/g after 15 h at 200C
(Joyce, 1988). This amount of ethylene sorption is insignificant.
In studies in the USA, the author tested four proprietary bags from Japan,
all containing dispersed minerals and all claiming ethylene-adsorbing
capacity. Weighed samples of each bag were placed in sealed jars with
sampling ports attached. A second set of jars were left empty. We injected
a known quantity of ethylene into each jar. Each day for seven days we
sampled the ethylene concentrations in each jar. We could detect no
differences in ethylene concentrations between the jars with film and those
without film. Our conclusion was that none of the four films adsorbed
measurable amounts of ethylene (Zagory et al.9 1988).
In the future, it would be useful if companies claiming ethylene-adsorbing
capacity for their products presented direct evidence for these claims. Shelflife studies and headspace analysis of ethylene concentrations do not support
claims of ethylene-adsorbing capacity. Direct measurement of ethylene
depletion in closed systems containing samples of the bags without any
produce to confound the results would be much more instructive. Furthermore, such studies should be done at low temperature and high relative
humidity to mimic the conditions under which they will be expected to
perform.
2.5 A
New and novel approaches to ethylene-removing packaging
There are some new and unusual approaches to developing ethyleneremoving packaging that deserve mention.
Perhaps the most promising new development in ethylene-removing
packaging is the use of electron-deficient dienes or trienes incorporated in
ethylene-permeable packaging. The preferred diene or triene is a tetrazine.
However, since tetrazine is unstable in the presence of water, it must be
embedded in a hydrophobic, ethylene-permeable plastic film that does not
contain hydroxyl groups (Holland, 1992). Appropriate films would include
silicone polycarbonates, polystyrenes, polyethylenes and polypropylenes.
Approximately 0.01-1.0 M dicarboxyoctyl ester of tetrazine incorporated in
such a film was able to effect a ten-fold reduction in ethylene in sealed jars
within 24 h and a 100-fold reduction within 48 h (Holland, 1992). The
tetrazine film has a characteristic pink color when it is new and turns brown
when it becomes saturated with ethylene so it is possible to know when it
needs replacing.
A new product called Frisspack has been developed in Hungary for use in
storage of fresh fruits and vegetables. The product consists of a chemisorbent of small particle size dispersed among the fibers in the early phase
of paper production. The result is a paper sheet with putative ethyleneadsorbing capacity. The nature of the chemisorbent and data supporting the
claim of ethylene adsorption are not available. No response was received
from the vendor following the author's request for information.
Although there are many packaging products claiming ethylene-removing
capabilities, few of the claims are backed up with reliable data. Standardized
procedures for demonstrating efficacy would aid the development of this
growing industry. In addition, a thorough understanding of the physiological
effects of ethylene and its importance in sealed permeable packages should
precede any use of these products. In many cases, the elevated carbon
dioxide levels common in modified atmosphere packages may obviate the
need for ethylene removal. In other cases, with very sensitive commodities
such as kiwifruit and carnations, ethylene-adsorbing capability may be
crucial in the maintenance of shelf-life and commercial quality.
Acknowledgements
Thanks for literature and helpful discussion are owed to: Linda Dodge,
Cheryl Reeves, Michael Reid, Michael Rooney, Mikal Saltveit and Kit
Yam.
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