Food-Waste Odor: The Most Abundant Compound Is Not Always the Main Player

When food-waste odor is analyzed, the compounds producing large instrument signals do not necessarily match those that make a person’s nose recognize the smell as food waste. In a 2020 study of samples collected from seven households, smelling the compounds one by one identified diacetyl and other compounds as possible contributors to the odor.

Moreover, the sample containing the most diacetyl was not the one that smelled strongest. While searching for compounds shared across households with different food ingredients, the study also revealed the complexity of odor mixtures.

Collecting liquid draining from dinner waste in seven households

The study examined vegetable scraps and cooking residues produced between the start of dinner preparation and cleanup in seven households in the Tokyo metropolitan area, each on one day between May 14 and 17, 2018. Vegetable scraps, meat, used tea leaves, and other waste were collected in straining nets, and only the liquid draining from them was collected in containers. After overnight refrigeration, the samples were taken to the laboratory and analyzed as samples A–G.

No restrictions were placed on dishes, ingredients, seasonings, or whether the waste had been touched by hand. The aim was to retain household differences while looking for odor compounds that were nevertheless detected in common. Sample A, for example, included vegetables, fruit, meat, and used tea leaves, while D included ginger, leek, lettuce, cucumber, carrot, and pork.

One milliliter of the drained liquid was placed in a bag with two liters of air and left at 25°C. After one hour and 24 hours, the bag’s odor was assessed by smelling, and compounds were collected for analysis. These conditions differ from directly measuring all the air in a household garbage bin.

Examining the strongest-smelling sample, A, with instruments and the nose

When three people assessed the samples, A, B, D, F, and G after 24 hours reminded them of food waste, with A smelling strongest among them. The researcher therefore examined A in detail.

Conventional gas chromatography–mass spectrometry showed large peaks for ethanol and ethyl acetate at both one hour and 24 hours. At 24 hours, the number of compounds detected and the sum of peak areas had also increased. The author suggested that microbial fermentation or decomposition might have changed the compounds, but the experiment did not measure the microorganisms themselves.

The two compounds with large peaks alone did not adequately explain the sour, fermented, food-waste-like impression. The researcher therefore used gas chromatography–olfactometry, in which a person smells the compounds separated by the instrument in sequence.

One person conducted this assessment, first memorizing the original sample’s odor and then recording where similar characteristics were perceived among the separated compounds. Diacetyl, 3-methyl-1-butanol, acetic acid, and isobutyric acid emerged as possible contributors to food-waste odor. Ethanol and ethyl acetate, which had produced large analytical signals, were not detected in this olfactory record.

Considering not just quantity, but how little is needed to smell a compound

Next, the four candidate compounds and the two compounds with large peaks were quantified. In A after 24 hours, ethyl acetate was present at 80 ppm and ethanol at 55 ppm, compared with 1.7 ppm for diacetyl. Diacetyl was therefore much less abundant than the other two.

However, the concentration at which a smell becomes detectable varies greatly between substances. The study calculated a threshold dilution ratio by dividing the measured concentration by an olfactory threshold from the literature. This provides an indication of how readily each compound might be smelled at its measured concentration.

For diacetyl in A, this value was 34,000, compared with about 110 for ethanol and about 90 for ethyl acetate. Because diacetyl can be smelled at very low levels, the ranking of seemingly important compounds changed when more than concentration was considered. The value 34,000 does not mean that the whole sample smelled 34,000 times stronger: it indicates how many times the adopted detection threshold the diacetyl concentration was.

A shared compound did not identify the strongest-smelling sample

When the compounds highlighted in A were examined in all seven samples, diacetyl exceeded the published detection threshold in every sample at both one hour and 24 hours. 3-Methyl-1-butanol and acetic acid were also found in many samples, but were undetected at some time points. Isobutyric acid was detected only in A after 24 hours.

Diacetyl had a high threshold dilution ratio in many samples, leading the author to consider it a possible common basis for food-waste odor. It did not, however, have the highest ratio in every sample. In C after 24 hours, the value for 3-methyl-1-butanol exceeded that for diacetyl.

The comparison between A and D is particularly important. After 24 hours, diacetyl was present at 1.7 ppm in A but 6.2 ppm in D, making it more abundant in D. Yet the three assessors selected A as having the strongest food-waste odor. In A, the threshold dilution ratios for 3-methyl-1-butanol and acetic acid were higher than in D, and isobutyric acid was also present.

The author inferred that the coexistence of other compounds with diacetyl altered overall odor intensity. The findings suggest that knowing the concentration of any one compound is insufficient to determine the strength of an odor mixture. Because the study did not remove compounds or reconstruct and compare the same composition, the causal role of each compound remains a question for further research.

The same molecule is not always a bad smell

The paper’s discussion also notes that diacetyl is considered a desirable aroma in fermented dairy products. The point here is not that diacetyl always smells bad, but what role it plays within a mixture from food waste. The author suggested that its common occurrence might be explained by food carbohydrates and microbial metabolism, but did not trace that pathway in this study.

Some parts of the report require care in interpretation. The conclusion says that the four candidate compounds could not be detected by conventional GC/MS, but Table 3 lists signals for three of them. The remaining compound, 3-methyl-1-butanol, is listed as undetected at 24 hours in Table 3, yet Tables 4 and 5 provide quantitative values. It is therefore not justified to say that the instrument missed all four compounds and only the nose discovered them. The reliable takeaway is that the candidates were assessed by combining smelled characteristics and thresholds rather than looking only at large analytical peaks.

This was a small study of liquid drained from one day’s waste in each of seven households, with three people assessing overall odor and one person smelling individual compounds. Its findings do not necessarily apply unchanged to all food waste across different seasons and ingredients.

Summary

In this study of household food-waste drainage, small amounts of readily detectable compounds such as diacetyl emerged as possible contributors to the characteristic smell, alongside ethanol and ethyl acetate with their large analytical signals. Diacetyl exceeded its detection threshold in every sample and drew attention as a compound shared across households.

Meanwhile, A, which contained several candidate compounds, was judged to smell stronger than D, which contained the most diacetyl. Understanding an odor therefore requires considering compound quantities, how readily they can be smelled, and the impression they produce together.

At the time of the study, the author was affiliated with the Tokyo Metropolitan Industrial Technology Research Institute. No explicit funding information or separate conflict-of-interest statement was found in the paper.

Reference

Sasaki, N. (2020). Components contributing to garbage odor in living environment. Indoor Environment, 23(2), 67–74. Original paper