NARO finds that inoculating decomposing food waste with insect larvae can help control odor

Senior Researcher Takuya Uehara and Assistant Group Leader Tetsuya Kobayashi of the Insect Design Technology Group, Institute of Agrobiological Sciences, NARO; Professor Masami Shimoda of the Graduate School of Agricultural and Life Sciences at the University of Tokyo; and Associate Professor Seiichi Furukawa and graduate student Rena Michishita of the University of Tsukuba announced on April 24th at NARO (Tsukuba City, Ibaraki Prefecture) that they had developed a technology to suppress the odor of food waste with the help of insects.

When the American black soldier fly (Hermetia illucens) was inoculated on organic waste, production of methyl disulfide and methyl trisulfide (mainly responsible for bad odors) was drastically reduced. Furthermore, there were notable changes in the types of bacteria present in the organic waste. Addition of the black soldier fly inoculated substrates to the organic waste in advance can reduce the production of spoilage‐generated odors. Utilization of the black soldier flies as a protein source for livestock and aquaculture fish feed is expected to facilitate the recycling of organic waste and expansion of insect protein use. The results were published in the international journal Scientific Reports.

The American black soldier fly is an insect native to North and Central America, which was introduced to Japan in 1950. Currently, it reproduces naturally in Honshu, Shikoku, Kyushu, and other regions of Japan. The larvae feed on decaying organic matter, such as grass, fruit, animal carcasses, and feces, and do not sting humans. In Europe and the U.S., food waste is being treated to rear black soldier flies, which are then used as a protein source for livestock feed production.

With the global population exceeding eight billion, demand for animal‐derived protein is expected to surpass supply. This poses a challenge due to the large amount of food loss in developed countries. In Japan, 5.22 million tons of food waste is generated annually, but it is either incinerated or discarded in landfills. This study aimed at the domestication of black soldier flies using food residues (e.g., domestic food waste and food factory‐derived waste) as a substrate, and the research group noted that the flies emitted less odor as they grew. This led to the investigation of the relationship between these flies and odor reduction.

During the mass production of black soldier flies, lids of the inoculation containers were usually removed to improve operational efficiency and promote water evaporation. However, the issue of foul odor emission by the decomposing food residue of the feed still persisted.

First, the group prepared a bait mixture composed of homogenized food residues that had been formulated to resemble food scraps from a typical household. They then compared the amounts of methyl disulfide and methyl trisulfide in the samples with (inoculated plot) and without (non‐inoculated plot) larvae, respectively. These chemical substances are regulated by the Offensive Odor Control Law. The amount of methyl disulfide and methyl trisulfide decreased to the detection limit in both the 3‐ and 10‐larvae inoculated areas. These chemicals remained high in the non‐inoculated plots. Therefore, the group considered the possibility that the intestinal bacteria were being discharged through the feces of the black soldier flies into the bait, where they multiplied and decreased the foul odor.

Metagenomic analysis was performed on the bacterial flora of the residue of each plot after the experiment. The results showed that the bacterial flora changed notably between the non‐inoculated and inoculated plots. There was a decrease in Enterococcus spp. and an increase in Lactobacillus spp. in the inoculated samples. A decrease in the diversity of the microbiota due to inoculation was also observed.

These results confirmed that the mixing of the rearing residues (including feces, etc.) with food residues, could suppress methyl disulfide by up to one‐seventh after seven days. Prior mixing of the rearing substrates with the food residue that will be the new feed may suppress the generation of offensive odors during decomposition. A patent has already been filed for this technology.

Depending on the moisture content, feeding on 100 kg of food residue will yield approximately 15 kg of larvae (live weight) in about 20 days. Assuming the production of easy‐to‐process larvae, these can be used as feed for farmed fish, and rearing substrates can be used as organic fertilizer. In the future, these studies could lead to the development of deodorants and products that prevent production of bad odors from a wide range of waste material.

Kobayashi explained, "In the future, we would like to identify the type of bacteria that could potentially contribute to suppress the production of foul odors. Additionally, only a few food waste recycling plants are currently using black soldier flies in Japan. However, we have received inquiries from companies that want to set up a test plant. We would like to pursue joint research with such companies."