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An equal amount (5 g) of fresh fish (FF), fish with salt treatment (FS), and Kimchi (KC) were sampled and placed inside a 100 mL (needless throwaway) syringe as shown in Fig. 1. Each syringe was then filled with air for a total volume of 60 mL, under the condition of not restricting air exchange and to allow for the collection of gas samples with Tedlar bags. For each food type, a total of five samples were prepared at the beginning of the experiment to allow for measurements of the odorant concentrations at five different intervals (0, 1, 3, 7, and 14 days). For the analysis of each carbonyl, a 25 mL air sample was withdrawn from the syringe. It was then mixed with N₂ by injecting the sample into a 10 L Tedlar bag. Then, air samples from each Tedlar bag were passed through Lp DNPH cartridges (Supelco Inc., USA) at a normal set-up value of 10 min (at a fixed sampling flow rate, 0.8 L min^-1) via a Sep-Pak ozone scrubber (Waters, USA). After sampling, each cartridge was wrapped in aluminum foil and stored at 4°C before injection into the HPLC.

Fig. 1. 
A list of photographs showing the progress of the decay experiment at 0, 1, 3, 7, and 14 days.

Emissions of odorants are an important parameter used to assess the impact of chemical substances on air quality. The target analytes of this study are listed in Table 1. The analysis of the carbonyl compounds was performed using HPLC (Lab Alliance 500) equipped with a UV detector and dsCHROM software for peak integration. To initiate the analysis, the cartridges were eluted slowly with acetonitrile and filtered through 0.45 μm, mini spike, 13 mm, GHP Acrodisc filters (PALL, NY, USA) into 5 mL capacity borosilicate glass volumetric flasks. The eluate was manually injected into the HPLC system, equipped with a 20 μL sample loop. Carbonyl-hydrazones were separated on a Hichrom 250×4.6 mm ODS (octadecyl silane), 5 μm reverse phase C₁₈ column using a mobile phase of acetonitrile+ water (7 : 3 by volume) at a flow rate of 1.5 mLmin^-1 at a wavelength of 360 nm. Quantification of the carbonyls was performed against five point calibration curves drawn at 0.15, 0.3, 0.6, 1.2 and 4.8 ng μL^-1 (at 20 μL injection volume). Liquid phase standards were prepared from the carbonyl-DNPH mix (Supelco, USA) at a wavelength of 360 nm.

The basic quality assurance for this research is provided in terms of detection limits (DL). The DL values for all of the carbonyl species were estimated by multiplying the standard deviation (SD) values of the least detectable quantities (in absolute mass or concentration unit) by a factor of 3. The DL values, if expressed in terms of mixing ratios (assuming a total sampling volume of 15 L), fell in the range of 1.46 (benzaldehyde) to 1.65 ppb (acrolein). The precision of analysis, if assessed in terms of the relative standard (RSE) value, tended to vary in the range of 1.06% (valeraldehyde) to 2.52% (isovaleradehyde).

In this study, the emission concentrations of carbonyl compounds were measured during the decaying process of three food types at room temperature: Kimchi (KC), fresh fish (FF), and fish with salt treatment (FS) (Table 2(a)). Our analysis of environmental samples based on the HPLC system allows us to quantify at least 13 carbonyl compounds. However, only three carbonyl compounds were measured above their DL from these decaying experiments : formaldehyde (Form-A), acetaldehyde (Acet-A) and acetone.

Acetaldehyde has a fruity smell and occurs naturally as an intermediate product in the respiration of higher plants. Acetaldehyde can be found in ripening fruit and vegetables, including apples, broccoli, coffee, grapefruit, grapes, lemons, mushrooms, onions, oranges, peaches, pears, pineapples, raspberries and strawberries (Acetaldehyde fact sheet), and it has been detected in the essential oils of alfalfa, rosemary, balm, daffodil, bitter orange, camphor, angelica, fennel, mustard and peppermint (Acetaldehyde fact sheet). Acetaldehyde also naturally occurs as a result of forest fires, volcanoes, animal waste and insects. The health impact of acetaldehyde is well known, as it is an irritant of the skin, eyes, mucous membranes, throat and respiratory tract (CEPA, 1993). Hence, exposure to acetaldehyde can cause such symptoms as nausea, vomiting, headache, dermatitis and pulmonary oedema (fluid in the lungs). As such, it is a substance which may reasonably be anticipated to be a carcinogen (Acetaldehyde fact sheet).

is a flammable, colorless gas with a pungent, suffocating odor. It is released into the environment from both anthropogenic and natural sources. It occurs naturally in fruits and some foods, while it is formed endogenously in mammals, including humans, as a consequence of oxidative metabolism of many xenobiotics (IRIS, 2007). Combustion processes from automobile exhaust, power plants, incinerators, refineries, wood stoves, kerosene heaters and cigarettes account for the dominant fraction of its release into the environment (ATSDR, 1999). The primary routes of potential human exposure to formaldehyde are inhalation and dermal contact. If exposed to low levels of formaldehyde, irritation of the eyes, nose and throat can occur. Such exposure can also cause allergies by affecting the skin and lungs. At higher exposure levels, people can experience throat spasms and a build up of fluid in the lungs which can lead to death. Contact with formaldehyde can also cause permanent damage and severe eye and skin burns. It is also known as the cause of an asthma-like respiratory allergy. Hence, repeated exposure to formaldehyde may cause bronchitis, with coughing and shortness of breath (IRIS, 2007). In 2004, the National Occupational Health and Safety Commission classified formaldehyde as a potential carcinogen.

Acetone is a colorless compound with a distinct smell and taste. If skin is exposed to acetone, the compound can go into the blood supply and be carried into organs. However, the liver can break down small quantities of acetone into a less harmful form and utilize its energy for normal bodily functions. If a human breathes moderate-to-high levels of acetone for short periods of time, the following symptoms are expected to occur: nose, throat, lung and eye irritation, headaches, light-headedness, confusion, an increased pulse rate, effects on blood, nausea, vomiting, unconsciousness and shortening of the menstrual cycle in women(ACETONE, 1995).

If the emission patterns of the different food types are compared in terms of magnitude, the results of the KC appears to be the least variable, with the mean values of all compounds near 15 ppb (Fig. 2). However, if their differences are compared in absolute terms, their mean values differ by approximately two orders of magnitude. In the case of the FF sample (Table 2(b)), the emission patterns vary from 2.75 ppb (Acet-A) to 135 ppb (Form-A). In the case of FS, the concentration differences are moderate between three compounds, in decreasing order of Form-A>Acetone>Acet-A.

Fig. 2. 
Comparison of the mean emission concentrations of carbonyl compounds during the decaying process of Kimchi (KC), fresh fish (FF), and fish with salt (FS).

The results derived using three food types showed that there are highly unique patterns for each compound in relative terms, but not between the different food types (Fig. 2). The concentrations (ppb) of formaldehyde formation decreased fairly consistently through time for all three types of food: from 33.8 (d=0) to 2.70 (d=14) in KC, 450 to 8.59 in FF and 107 to 5.98 in FS. In contrast, the concentrations of acetaldehyde increased in KC from 7.77 (d=0) to 29 ppb (d=7), and from 0.71 to 62.9 ppb in FS, as time advanced. However, in the case of the FF sample, the patterns of Acet-A emissions were moderately different from the others. More specifically, acetaldehyde initiated its emission after 7 days at concentration levels of 4.44 (d=7) and 1.06 (d=14) ppb. The emission concentrations of acetone also showed an upward trend, but the values tended to peak either by 1 day (FS) or 7 days (KC or FF).

As food begins to decay, the emission concentrations of an odor can vary as a function of time. Fig. 3 shows the emission patterns of these three carbonyl compunds for all three food types as a function of time. In all three cases, the values of Form-A were the highest at the beginning stage. In a number of previous studies, the highest concentrations of formaldehyde naturally occurring in foods are found in some fruits, up to 60 mg/kg (Tsuchiya et al., 1975; Möhler and Denbsky, 1970), and some marine fishes (Tsuda et al., 1988; Rehbein, 1986). Formaldehyde develops postmortem in marine fish and crustaceans through the enzymatic reduction of trimethylamine oxide to formaldehyde and dimethylamine (Sotelo et al., 1995). Although formaldehyde can be formed during the aging and deterioration of fish flesh, it does not accumulate in fish tissue, due to subsequent conversion into other chemical compounds (Tsuda et al., 1988). Formaldehyde formed in fish reacts with protein and can subsequently cause muscle toughness (Yasuhara and Shibamoto, 1995). It is thus suggested that fish containing the highest levels of formaldehyde (e.g., 10-20 mg/kg) may not be considered palatable as a human food source. It should also be noted that formaldehyde is often used as a preservative in some foods, such as some types of cheeses, dried foods and fish (Toxicological Profile for Formaldehyde, 1999).

Fig. 3. 
Temporal changes in emission concentrations of carbonyl compounds during the decay process of Kimchi (KC), fresh fish (FF), and fish with salt (FS): the results are compared for each of the three compounds (a) Acet-A, (b) Form-A, and (c) Acetone.

As shown from our study, the emission of acetaldehyde is a significant process during food decay. Considering the environmental toxicity of acetaldehyde, its production pattern from decaying foods needs to be carefully evaluated. Acetaldehyde is used as a preservative for fruit and fish, as well as a flavoring agent. Hence, acetaldehyde emission can be expected from various food types. Our experimental results show that emission concentrations of acetaldehyde increased over time, especially from salted fish. Although acetaldehyde is produced by fish foods during decay, its harmful impact on live fish is also well known. Based on measured concentrations of acetaldehyde, the 96-hour LC₅₀ (Lethal Concentration 50) was 30.8 mg-L^-1 for fathead minnow (Pimephales promelas) (Brooke et al., 1984). Other short-term LC₅₀ values for fish, including the guppy (Poecilia reticulata), the pinfish (Lagodon rhomboides), and the bluegill (Lepomis macrochirus), range from 33 to 140mg-L^-1 (Von Burg and Stout, 1991; Geiger et al., 1990; Deneer et al., 1988; Grahl, 1983; Juhnke and Luedemann, 1978; Daugherty and Garrett, 1951). For microorganisms, the most sensitive effect of acetaldehyde is found with the protozoan Chilomonas paramecium, with a 48-hour EC₅₀ (effective concentration) of 82 mg/L (Von Burg and Stout, 1991). Five-day LC₅₀ values for the diatom Nitzchia linearis range from 237 to 249 mg/L (Patrick et al., 1968). A 25-minute EC₅₀ of 303 mg/L was reported for Photobacterium phosphoreum in a Microtox test (Chou and Que Hee, 1992).

Likewise, acetone concentration also increased as time advance during the experiment. Approximately 97% of the acetone released to the atmosphere comes from natural sources. It is a natural product of metabolism in the body (virtually every organ and tissue). Natural sources of acetone generally include forest fires, volcanoes, and the metabolism of vegetation, insects and animals (Acetone VCCEP Submission, 2003). Acetone also occurs naturally as a biodegradation product of sewage, solid waste, alcohols and as an oxidation product of humic substances. Acetone has been detected in a variety of plants and foods, including onions, grapes, cauliflower, tomatoes, morning glories, wild mustard, milk, beans, peas, cheese and chicken breast (Lovegren et al., 1979; Grey and Shrimpton, 1967; Day and Anderson, 1965). The results of our study thus suggest that acetone may belong to a major carbonyl that is released during the food decay process.