Ethyl formate, formic acid and ethanol in air, wheat, barley and sultanas: analysis of natural levels and fumigant residues

Abstract

Ethyl formate and ethanol in air, eg in fumigant studies, were readily detected by gas chromatography (GC) (¯ame ionisation). Residues in wheat, barley and sultanas were analysed by GC, after extraction in polar solvents (eg methanol, aqueous propanol). Both natural levels and levels resulting from fumigation with ethyl formate were measured. Formic acid was extracted from commodities with polar solvents (eg methanol, water) and analysed by GC after esteri®cation. Solvent extracts of commodities were concentrated after addition of disodium hydrogen orthophosphate, and an aliquot added to acidi®ed alcohols (several combinations of acids and alcohols were tested) in a sealed container. Formic acid esters were determined by GC, from headspace sampling over the esteri®cation solvent. Esteri®cation was faster with strong acids than with boric acid. However, esteri®cation with boric acid/butanol gave the least interference of all tested methods. Product identity was con®rmed by GC/mass spectrometry. High natural levels of formic acid, and low natural levels of ethyl formate and ethanol, presented problems in identifying residues arising from fumigation. These natural levels are relevant to food regulations for ethyl formate, especially those based on`total formic acid, free and combined'. Polar columns (eg FFAP, carbowax) were useful for measurement of formic acid esters, which eluted before the alcohols used for esteri®cation or extraction, whereas elution followed the molecular mass on non-polar columns, such as GS-Q or DB-624.

Modelling the kinetics of ethyl formate sorption by wheat using batch experiments

“…Based on this coefficient, wheat with an initial concentration of 100 mg kg −1 will decrease to 1.0–0.1 mg kg −1 in approximately 13–19 days (first‐order kinetics). This is broadly comparable with industrial measurements of grain EF concentration decrease in the literature,4–7 which indicate that approximately 2–5 weeks is required, depending on grain moisture and temperature. Specific laboratory measurement of the rate of reaction for the adsorbed EF fumigant within various grains has not been performed; this would provide independent determination of this rate coefficient and enable the rate of intrakernel diffusion to be determined from that of intrakernel reaction.…”

“…However, data suitable for defining the kinetics and specific reactants for such reactions are lacking, and the order of any reactions unknown. The reaction of adsorbed EF with grain has been observed for different grains, and residues were found to decrease slowly over 2–5 weeks,4–7 at rates dependent on grain moisture content (Mahon D and Ren YL, unpublished) 4. Production of ethanol was observed throughout the sorption of EF by wheat (Mahon D and Ren YL, unpublished), indicating that hydrolysis was one of the reactions occurring.…”

“…Most fumigants are observed to react with grain constituents,1, 2 yet desorption studies indicate that the reaction kinetics is slow, as significant amounts of unreacted fumigant exist after several weeks 1, 2. Sorption of ethyl formate also exhibits these characteristics 3–7…”

“…Adsorbed EF has been reported to react slowly with grain constituents,4–7 with rates dependent on moisture and temperature 4. However, data suitable for defining the kinetics and specific reactants for such reactions are lacking, and the order of any reactions unknown.…”

Postharvest management of New Zealand flower thrips on export apricots using ethyl formate

“…The quality of apricots treated with mean EF concentrations of 0.54-1.7% + CO 2 at 15°C for 1 h was not affected. EF breaks down quickly into ethanol and formic acid (Desmarchelier et al 1999) and is exempt from residue limits when applied as a postharvest fumigant (MPI 2013). In the presence of apricots 72-73% EF degraded over a 1-h fumigation, boxes degraded 40-42% EF, and a combination of fruit and boxes degraded 74-77% EF, compared with that in empty chambers.…”