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Feed for Thought: From refuse to reuse

Recycling post-consumer food waste into high-quality fish feed

February 8, 2024  By Magida Tabbara

Figure 1 (A) Manual sorting of food waste. (B) Dry ground food waste. (C) Oil extraction from food waste.

Food waste represents a major ethical dilemma, especially when considering the increasing need to produce food to satisfy the needs of the ever-climbing global population. 

Despite aquaculture offering nutritious sources of animal protein for less, some of the industry’s production expenses hamper its ability for further development. Thus, aquaculture needs alternative, cheap ingredients to grow the animals in a cost-effective manner and feed the world. 

One such option by which nutritionists can attempt to kill two birds with one stone is to recycle the available human food waste into adequate aquaculture feed. The waste is mostly comprised of unconsumed edibles and plate scraps. After all, post-consumer food waste retains some nutritional value that, with a bit of processing, can be turned from “refuse” to “reuse”. 

The idea
Human demand for food is not solely based on the need to fill one’s stomach, but rather on obtaining high-quality protein and essential fatty acids and molecules. Fish are an important source of protein (marine fish particularly so) and a great source of omega-3 fatty acids (EPA & DHA) which are necessary for proper brain function and a healthy heart. Aquaculture mostly produces freshwater fish, which can be a source of omega-3s if provided with EPA & DHA in their feed. 

Beirut is a beautiful city located on the coast of Lebanon where a lot of restaurants offer authentic Mediterranean food, rich in omega-6 fatty acids, fish products, and carbohydrates. With sustainability, demand for food, and recycling in mind, our research team at the American University of Beirut decided to make use of restaurant food waste by recycling it into fish feed. 

Knowing that food waste composition is not consistent and that it can be deficient in some nutritional aspects, we suggested using inexpensive and available biological processes and agents to try and improve its nutritional value. 

To increase the protein content of food waste, we chose to use the main byproduct of the brewing industry, brewer’s yeast, as it has high nutritional value and protein content coupled with a good essential amino acid profile that offers a variety of sugars and enzymes. In the presence of oxygen, yeast can ferment sugars into proteins. 

Use of brewer’s yeast in fish feed is not, however, an innovation. Previous research has already demonstrated that yeast can improve fish growth and even promote immune system activity [1,2,3]. Furthermore, food waste in the Mediterranean region is rich in omega-6 fatty acids, as olive oil represents an essential component of Levantine cuisine. If we were to try and produce freshwater fish with EPA and DHA in their fillet, the omega-6s in the food waste must be somehow converted into omega-3s. 

Previous research mentioned that two species of common soil fungus, Aspergillus niger and Trichoderma virens, have enzymes capable of doing just that. Therefore, we decided to use microorganisms to bioprocess the waste into a nutritious feed ingredient.

Table 1 Survival (S; %), final body weight (FBW; g), total length at harvest (TL; cm), feed conversion ratio (FCR), specific growth rate (SGR; % day-1), and Fulton’s condition index (K), values of juvenile O. niloticus offered diets with increasing proportions of fermented food waste. All values are represented as (mean ± SE)

The approach
We collected plate leftovers from a local restaurant, hand-sorted the waste prior to drying and then ground it into powder. We then extracted the oil from the food waste powder using a grain oil press, resulting in a lipid content of six per cent. The obtained waste powder was then used in a screening trial to evaluate whether the microorganisms can improve the protein content of the food waste and increase its omega-3 levels. All microorganisms were cultured in the laboratory and transferred into a liquid suspension prior to being spread on the food waste. Ten days later, the bioprocessed food waste was analyzed for protein and fatty acid content. 

Based on the results of the screening trial, we chose to formulate fish diets to contain various levels of food waste fermented with brewer’s yeast. A feed mixture was prepared, and the fermented food waste was used to replace 0, 15, 20, 25, 30, 35, and 40 per cent of the diet before pelletizing it, allowing for a total of seven diets. 

Nile tilapia was the fish of choice for the trials, owing to its popularity, mild taste, and a meat profile suitable for a plethora of cuisines.

Nile tilapia juveniles from our laboratory were sized and sorted by hand, and 13 fish were stocked into each of 21 glass aquaria in a recirculating aquaculture system, allowing for three replicates per diet. 

Diets were randomly assigned to the aquaria, and the fish were offered feed manually four times a day. The feed ration was adjusted weekly when the fish were weighed, at four per cent of the heaviest replicate. The experiment lasted 42 days, during which water quality parameters, including temperature, salinity, dissolved oxygen, total ammonia nitrogen, nitrite nitrogen, and pH were maintained within adequate ranges for optimal Nile tilapia growth. 

At termination, the fish were group-weighed and individual weight and length were measured. The measurements were used to calculate such parameters as specific growth rate (SGR), feed conversion ratio (FCR), and Fulton’s condition factor, as the latter allows for body condition assessment. 

Afterwards, four fish from each aquarium were euthanized prior to dissection in order to weigh the liver and the viscera. These data were used to calculate the hepatosomatic and the viscerosomatic indices, allowing for body condition evaluation. Finally, the fish were macerated, dried, and analyzed for whole body protein, lipid, moisture, and ash content.

The outcome
Results of the screening trial demonstrated that baker’s yeast increases the protein content of food waste by 15.3 per cent on a per weight basis. However, both species of fungi failed to bio-convert omega-6 fatty acids into omega-3s. Accordingly, only the yeast was used to produce the food waste used in tilapia feed. 

The fish feeding trial made it clear that tilapia growth, survival, and FCR remain the same even when up to 30 per cent of their feed is food waste. Even the metabolic indices of the fish were normal, which very much encourages recycling food waste in Nile tilapia feed.

This study gives us hope that recycling human food waste can be as beneficial for humans as for the environment. Using living microorganisms can help improve the nutritional value of food waste, which in turn can help sustainably and inexpensively improve food security and decrease pollution. 

Full scientific article citation: Tabbara, M., Monzer, S., Zein Eddine, R., Abiad, M., & Saoud, I. (2022). Bioprocessing post consumer food waste for use as a fish feed ingredient. Aquaculture Research, 53(6), 2383-2391. https://doi.org/10.1111/are.15756 

[1] Dimitroglou, A., Merrifield, D. L., Carnevali, O., Picchietti, S., Avella, M., Daniels, C., Güroy, D., & Davies, S. J. (2011). Microbial manipulations to improve fish health and production – A Mediterranean  perspective. Fish & Shellfish Immunology, 30, 1–16. https://doi. org/10.1016/j.fsi.2010.08.009

[2] Leclercq, E., Pontefract, N., Rawling, M., Valdenegro, V., Aasum, E., Andujar, L. V., Migaud, H., Castex, M., & Merrifield, D. (2020).  Dietary supplementation with a specific mannan-rich yeast parietal fraction enhances the gut and skin mucosal barriers of Atlantic salmon (Salmo salar) and reduces its susceptibility to sea lice (Lepeophtheirus salmonis). Aquaculture, 529, 735701. https://doi.org/10.1016/j.aquaculture.2020.735701

[3] Rawling, M., Leclercq, E., Foey, A., Castex, M., & Merrifield, D. (2021). A novel dietary multi-strain yeast fraction modulates intestinal toll like-receptor signalling and mucosal responses of rainbow trout (Oncorhynchus mykiss). PLoS One, 16(1), e0245021. https://doi.org/10.1371/journal.pone.0245021 

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