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From: E.J. Wright, M.C. Webb and E. Highley, ed., Stored grain in Australia 2003. Proceedings of the Australian Postharvest Technical Conference, Canberra, 25–27 June 2003. CSIRO Stored Grain Research Laboratory, Canberra. Carbonyl sulfide (COS) as a fumigant for stored
products: progress in research and commercialisation
Stored Grain Research Laboratory, CSIRO Entomology, GPO Box 1700, Canberra, ACT 2601 Abstract. Carbonyl sulfide (COS) is a potential new fumigant for stored products, patented worldwide by CSIRO Aus-
tralia. It may replace methyl bromide for many applications on durable commodities and be used as an alternative to
phosphine when there is a significant problem with insect resistance. Laboratory and field studies have shown that
COS is effective on a wide range of pests at all life stages, at reasonable concentrations (<100 g/m3) and exposure
times (1–5 days) at 10–25˚C. Quality studies indicated no adverse effects on canola oil composition or colour, quality
of bread, noodles or sponge cake made from wheat, malting or brewing characteristics of barley, or germination or plu-
mule length of wheat, oats, barley or canola. Sorption, penetration, desorption, decay of residues to natural levels, and
interaction with other materials have all been shown to be consistent with being a practical fumigant for most commod-
ities. Because COS in naturally present in a great variety of raw and processed foodstuffs, and is produced in mammals
during the metabolism of some sulfur-containing compounds, it is argued that the low residues of COS remaining post-
fumigation do not appear to pose a risk to human health. Similarly, the widespread and abundant natural occurrence of
COS in the air, soil, and live and decaying vegetation suggests that COS does not present a tangible environmental haz-
ard. COS has been trademarked in Australia as COSMIC®, and a global licence agreement has been signed for manu-
facture and distribution of COS.
Introduction
ties are midway between those of carbon disulfide (CS2)and carbon dioxide (CO2), both of which have been Carbonyl sulfide (COS) is a potential new fumigant for stored products. It was patented worldwide in 1992 byCSIRO Australia (Banks et al. 1993) and first announcedto the scientific community at the 6th International Performance as a fumigant
Working Conference on Stored Product Protection inCanberra, Australia in 1994 (Desmarchelier 1994).
Laboratory studies on toxicity
Carbonyl sulfide has the potential to replace methylbromide in several of its applications for durable Laboratory studies have shown that COS is effective commodities, and also to be used as an alternative to on a wide range of pests in all stages, including the phosphine when there is a significant problem with common stored-product species, at reasonable concentra- insect resistance. This paper summarises research tions (<100 g/m ) and exposure times (1–5 days) at 10– results, primarily from the fumigant research group 25˚C (Desmarchelier 1994; Plarre and Reichmuth 1996; within the Stored Grain Research Laboratory (SGRL), Zettler et al. 1997; Xianchang et al. 1999; Weller 1999).
and CSIRO’s progress towards commercialisation of this Of the stored-product pests tested, Sitophilus oryzae (L.) is the most naturally tolerant and in-depth studies weretherefore conducted on this species at CSIRO. Thesestudies provided a range of exposure time/concentration What is carbonyl sulfide (COS)?
regimes to give disinfestation level mortalities. Theyshowed concentrations of 20 g/m3 for an exposure time of Ferm (1957) and Svoronos and Bruno (2002) have 5 days to be a particularly good combination, although reviewed the chemistry of carbonyl sulfide (COS). Pure there are other possible combinations (Weller and Morton COS is a colourless and odourless gas with a low boiling point. Carbonyl sulfide is present in nature,including air, soils, oceans, vegetation, cereals and Haritos (2001) has shown the acute toxicity of COS is foods (Sze and Ko 1980; Ren 1997; Ren and Desmarch- dependent on carbonic anhydrase metabolism of COS to elier 2001). Many of its chemical and physical proper- hydrogen sulfide, the toxic agent of COS.
Carbonyl sulfide (COS) as a fumigant for stored products Field trials
sponge cake processed from fumigated wheat (Desmarch- Trials were undertaken to start the formal path towards registration, to validate laboratory predictions in the field, Malting and brewing trials were carried out on malting to demonstrate compliance with occupational health and barley fumigated with carbonyl sulfide in the field trials safety (OH&S) standards, to produce industry-sized mentioned above, by an independent laboratory in parcels of grain to allow industry assessment of quality Australia. Residues in products showed that COS was (baking, malting, brewing and residues), and to assess the present in malt at 0.0033 mg/kg (ppm, w/w), and in spent time required for killing insects, venting and clearing grain at 0.0007 mg/kg, but was not detectable in wort or beer made from either fumigated or unfumigated barley.
Trials on wheat (Desmarchelier et al. 1998), oats, These results indicate that COS residues in malting and barley, canola and field peas (Ren et al. 2002, 2003) have brewing products were comparable with levels present in been conducted in 50 t silo bins with COS (20–25 g/m3), products made from unfumigated barley from the same for 3–5 days of exposure over a range of grain tempera- source (Ren et al. 2003). These results are consistent with tures (10–27˚C) and on hay (Weller 2003b) in shipping those obtained using fate of 14C-labelled COS applied on containers. These studies showed complete control of grains and grain fractions (Annis et al. 2000). Finally, mixed-age cultures of Rhyzopertha dominica (F.), Sito- there was no effect of COS on malting and brewing char- philus oryzae, Oryzaephilus surinamensis (L.), Tribolium acteristics of barley, and commercial testing of the beer confusum (Herbst), Tribolium castaneum Jacquelin du made from fumigated barley showed that it was no Val, Ephestia cautella (Walker), Trogoderma variabile different from beer made from unfumigated barley. Ballion, Cryptolestes ferrugineus (Stephens), Liposcelis There was no significant effect of fumigation with bostrychophila Badonnel, Liposcelis decolor (Pearman), COS on either germination (7-day count) or plumule Liposcelis entomophila (Enderlein) and Ephestia kueh- length of wheat, oats, barley and canola (Ren et al. 2002).
niella Zeller. The results from the field trials were consis- The work on germination parallels an observed lack of tent with the laboratory data regarding toxicity to insects.
effect of COS on wheat at higher doses than required for The trials have not shown any actual or potential fumigation and the lack of effect on plumule length (Ren problems in the large-scale use of COS. In fact, they have et al. 1996). It is worth noting that at least one fumigant, shown that safe and effective fumigation should be well hydrogen cyanide, affects plumule length without within the competency of a skilled fumigator. Larger-scale reducing germination (Ren et al. 1996). trials of >500 t parcels of wheat, barley and canola arecurrently being conducted, primarily to optimise applica- Sorption and penetration
tion methods and to confirm worker/environmental safetyduring application, fumigation and aeration.
Of 32 different commodities investigated, including cereals, pulses, oilseeds, dried fruit, spices, nuts, and Effects on grain quality and viability
beverage crops, only 2 (kidney beans and sunflower seeds) Fumigation of canola with COS does not adversely had sorption rates incompatible with the use of COS as a affect either oil content or oil colour (Ren et al. 2002). This fumigant (Weller 2003a). Paddy rice and rice products is in agreement with Ren et al. (1997), who showed that have been looked at in detail, with paddy rice and rice fumigation in vitro of canola oil with COS at 100 g/m3 had flour sorbing COS at a higher rate than either brown rice no effect on lipid composition as assessed by Fourier trans- or white rice (Reuss and Annis 2002).
form infrared (FTIR) or ultraviolet spectroscopy. The result An increase in moisture content of a commodity was also consistent with that from a 14C-labelled COS results in an increase in the rate of sorption, however grains fumigated within the normal storage range (up to Detailed quality tests from the field trials on wheat 15% moisture content) displayed relatively low rates of showed no adverse effect on quality of bread, noodles or sorption (Weller 2003a). A reduction in the rate of Table 1. Chemical and physical properties of carbonyl sulfide compared to other fumigants.
sorption has been observed with increasing age of wheat obvious effect on the material or on the concentration of and on repeated fumigation (Weller 2003a).
COS remaining in, or available to, the gas space. Loss of In the field trials mentioned above, the distribution of COS on fresh concrete was rapid, but loss on aged COS in each trial became fairly even in less than one day.
concrete was minimal, similar to the sorption of CO2 At the end of the fumigation, compared with the initial described by Banks and McCabe (1988) (Y.L. Ren, pers.
concentrations, approximately two-thirds of the intergran- Interactions between COS and copper have been studied in both Australia and Germany (Plarre and Reich- Desorption and decay of residues to natural levels
muth 1996; Ren et al. 2000). These studies have found Desorption of COS from fumigated grains is quicker that pure COS or COS with <0.1% contamination by than that of methyl bromide or phosphine. Laboratory hydrogen sulfide (H2S) does not corrode copper, but COS data showed that the accumulated levels of COS, which significantly contaminated with H2S does. This indicates desorbed from grain fumigated at 45 mg/L, were <0.1 that the COS used for commercial fumigations must be ppm after 24 hours of airing (Ren et al. 2003). This level substantially free of H2S. This is now an important speci- was 100 times lower than the Australian experimental fication for the commercial production of COS. threshold limit value (TLV) of 10 ppm. The experimentalTLV for carbonyl sulfide was based on that of hydrogensulfide (10 ppm; ACGIH 2001), as it is assumed that the Consumer safety
effects of COS are due to the action of hydrogen sulfideresulting from partial decomposition of COS in the lungs COS is naturally present in a variety of raw and processed and after adsorption into the blood stream (Braker and foodstuffs, including cheddar cheese, cereals and oilseeds.
Mossman 1971). The TLV for hydrogen sulfide has The natural levels of COS in grains and oilseeds were subsequently been reduced to 5 ppm (ACGIH 2001).
found to be 0.02–0.07 mg/kg (Ren 1997; Desmarchelier et The field trials on barley showed that after 6 h of al. 1998; Ren and Desmarchelier 2001). Levels of COS forced ventilation (1 air change per hour), the in-bin varied with commodity, moisture content, temperature concentrations of COS were reduced from 3500 ppm to and the period of storage. The levels of COS ranged from <1 ppm. Similar results were obtained for wheat, oats and 0.02–1 mg/kg at harvest, increased during the first 4–5 canola, in that in-bin concentrations were reduced to low months of storage, and then began to decline, particularly levels by a combination of aeration and ventilation (Ren et at grain temperatures greater than 20˚C and moisture contents greater than 9.5% (Ren and Desmarchelier Post-fumigation residues of COS in the above trials were found to be below the Australian experimental COS has been shown to be produced in cheese from maximum residue limit (MRL) of 0.2 mg/kg after 4 h of either ascorbic acid or riboflavin at pH of 5.5 and temper- airing. In other work, COS residues in fumigated paddy ature of 24˚C, with more COS produced with increased rice, brown rice, white rice and rice flour were well below levels of cysteine (Aston and Douglas 1983). As a result, the MRL following fumigation for 5 days at an applied COS is likely to be present, or produced, in any proteina- dose of 20 g/m3, and subsequent airing (Reuss and Annis ceous food at acid pHs, and therefore likely to be ubiqui- 2002). Therefore, the field trials confirmed that COS can be ‘aired-off’ rapidly from the commodity after fumiga- COS is produced in mammals during the metabolism tion and, thereafter, that COS levels in fumigated grains of some sulfur-containing compounds (Peeples and Dalvi and seeds are indistinguishable from the levels in 1978; Johannson 1989) and is in turn metabolised by untreated commodities. This is both due to the fast desorp- carbonic anhydrase to hydrogen sulfide (Klason 1887).
tion of COS from commodities after fumigation and to the COS is also produced by hydroxyl radical attack on the very low, but natural, presence of COS in grains and seeds sulfur-containing amino acids cysteine and methionine (around 0.05–0.1 mg/kg). However, it is worth noting that (Giroux and Lacroix 1998). Reactive oxygen species such for commodities with high sorption, a slight increase in as hydroxyl radicals are a normal by-product of respira- the sulfur levels of the commodity is apparent by X-ray tion (White and Coon 1980), and are generated as a part of fluorescence (Weller 2003a); this is being examined more normal inflammatory processes (Conner and Grisham closely to determine the exact fate of these sulfur residues.
1996). Because of the ubiquitous nature of hydroxyl radi-cals, all human tissues are exposed to COS at some level.
Interaction with materials
Thus, COS is a naturally occurring compound to which allplants and animals are already constantly exposed. There- A number of materials, including hard and soft timbers, fore, one can conclude that low-level residues resulting paper, iron, steel and galvanised sheet, polyvinyl chloride from the fumigation of grain with COS, and any reaction (PVC), polyethylene, and brick, were exposed to high products remaining as residues, are identical to those to concentrations (>45 g/m3) of COS at high temperature which humans are naturally exposed both through their (45–60˚C) and relative humidity (65–100%) without food supply and through normal biological processes.
Carbonyl sulfide (COS) as a fumigant for stored products Consequently, such residues, at low levels, do not appear tion of COS. The first commercial-scale manufacture is expected to begin shortly. It is hoped that COS will beregistered for use on grain in Australia by 2005, with othercountries following. Environmental safety
COS, together with other biogenic volatile sulfur Acknowledgments
compounds, is produced naturally in soils (Staubes et al.
1989), marshes (Steudler and Peterson 1985), the roots The work presented here was a joint effort by many and shoots of plants (Piluk et al. 2001), manure (Clanton people over 10 years, particularly by Jim Desmarchelier, and Schmidt 2000), compost (Derikx et al. 1991), and YongLin Ren, Peter Annis, Victoria Haritos, Jonathan micro-organisms (Blezinger et al. 2000). The natural Banks, Sylvia Allen, Le Vu, Gaye Weller and Daphne occurrence of COS is linked to that of carbon disulfide Mahon. I thank YongLin Ren, Andrew Bartholomaeus, and the environmental sulfur cycle. The primary natural Victoria Haritos, Gaye Weller, Colin Waterford and sources of atmospheric sulfur are the oceans (primarily Michael Tichon for assistance in preparation of this paper.
The work on COS was supported financially by the CSIRO, the Grains Research and Development Corpora- tion, AWB Ltd, Grainco Australia, GrainCorp Operations, importance, the major sulfur compounds released into the Ausbulk Ltd., Co-operative Bulk Handling of WA and atmosphere as a result of biological processes are CH3SCH3 (dimethyl sulfide, or DMS), H2S, CS2, COS,CH3SH, and CH3SSCH3. Biogenic and anthropogenic References
production of COS is not a major source of atmosphericsulfur in comparison to H2S, DMS and CS2, however ACGIH (American Conference of Governmental Industrial COS is the major sulfur moiety in the atmosphere (Turco Hygienists) 2001. 2001 TLVs® and BEIs®, threshold limit et al. 1980; Khalil and Rasmussen 1984; Kjellström values for chemical substances and physical agents and bio- 1998). Ambient COS levels have been determined to be of logical exposure indices, 2001. Cincinnati, ACGIH.
the order of 0.5 ppb (Sze and Ko 1980). COS is also Annis, P.C., Ren, Y.L., Desmarchelier, J.M. and Johnston, F.M.
actively taken up by some plants and converted to CS 2000. Fate of C-labeled carbonyl sulfide on grains and grain fractions. Journal of Agriculture and Food Chemicals, the reverse of the atmospheric pathways (Ren 1999), and soils may act as both a net source or a net sink for COS, Aston, J.W. and Douglas, K. 1983. The production of volatile depending on the concentration of COS and the character- sulphur compounds in cheddar cheeses during accelerated istics of the soil (Conrad and Meuser 2000).
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