Introduction
Tenebrio molitor (TM) is one of the recently authorized species for use in aquafeeds. When defatted (d-) is particularly rich in highly digestible protein (up to 93% on a dry matter basis) and have a well-balanced amino acid profile able to meet European sea bass (Dicentrarchus labrax) requirements (Basto et al., 2020). However, dTM has limited long-chain polyunsaturated fatty acids (LC-PUFA), like eicosapentaenoic (EPA; 20:5n-3) and docosahexaenoic (DHA; 22:6n-3), with health benefits for consumers. When a new ingredient is tested in aquafeeds, the flesh nutritional value, sensorial properties and acceptance by consumers are important factors to consider. In this study, we have evaluated the impact of replacing fish meal (FM) with dTM in European sea bass growth performance, nutrient utilization, skin and muscle quality traits.
Material and methods
A FM-based diet with 47% of protein and 20% of fat was formulated and used as control (CTRL). Two other isoproteic and isolipidic diets were formulated to replace 50 and 100% of FM by dTM (TM50 and TM100, respectively). Each diet was assigned to quadruplicate homogeneous groups of 15 fish (69 ± 5 g) fed until apparent satiation for 16 weeks. Fish were subjected to a 12-hour light/12-hour dark photoperiod regime and kept in a recirculating saltwater system (35‰, 22 ± 1 °C). By the end of the trial, feed consumption was determined and all fish were weighed and measured. Ten fish from the initial fish stock, and 4 fish per tank, by the end of the experiment were sampled for whole body (WB) composition, total lipid and FA profile analysis. Muscle from other 4 fish per tank were also collected for nutritional evaluation (lipid content and FA profile analysis), color analysis and textural properties (TPA and histology). Twenty fish per treatment were collected for sensory evaluation using a panel of 60 untrained panelists. After the growth trial, the apparent digestibility coefficients of the experimental diets were determined according to Cho & Slinger (1979), after including 1% Cr2O3, as inert marker, to the experimental diets.
Results
Fish fed TM50 had a significantly lower voluntary feed intake (VFI) and feed conversion ratio (FCR) compared to the CTRL, resulting in similar final body weight and condition factor among treatments. TM50 had the highest N and E retention efficiency values and reduced N and E overall losses. Total phosphorus (P) losses decreased with dietary dTM inclusion. The whole body lipids increased with dietary inclusion of dTM, mainly due to oleic acid (OA; 18:1n-9) gain and retention. Muscle total lipids levels remained similar among dietary treatments. The sum of saturated FA (SFA) increased in muscle of fish fed dTM, whereas monounsaturated FA (MUFA) didn’t change. The dietary inclusion of dTM resulted in a concomitant increase of muscle LA level. Despite the relative percentage (% total FA) of muscle EPA and DHA decreased in fish fed dTM, when expressed in wet weight muscle EPA + DHA final contents were similar among all fish (0.30 g 100 g-1).
Muscle cohesiveness and resilience were higher in fish fed TM50; this fish also had the largest-sized and lowest number of muscle fibers. Fish fed TM100 had a significantly lower number of smaller-sized muscle fibers (<25 µm) than those fed the CTRL. Skin and muscle lightness (L*), redness (a*), yellowness (b*) and Chroma (C*) values were similar among groups. The hue angle (Hº) was significantly lower in skin of fish fed TM100 compared to CTRL, but in muscle remained similar among all dietary treatments.
The panelist could not detect any significant differences in fish overall liking and acceptance that remained high for all fish, but dTM samples were strongly associated with a “pleasant taste” and “juicy texture”.
Discussion and conclusions
The results of the present study demonstrate that the substitution of 50% FM by dTM significantly improves FCR. Diets with dTM didn’t impair European sea bass growth and still resulted in fair levels of EPA and DHA in muscle (0.30 g 100 g-1 of wet weight), which are above the recommended level for human consumption to decrease the risk of cardiovascular diseases (0.25 g 100 g-1 portion of fish; EFSA, 2010). Despite some differences observed in the muscle histomorphometric and instrumental texture measurements, all samples were characterized by their soft and pleasant texture. Altogether, these results evidence a great potential of dTM to fully replace FM in diets for European sea bass. However, the long term impact of feeding dTM diets still needs to be addressed.
Acknowledgements
Work supported by the structured program of R&D&I ATLANTIDA - Platform for the monitoring of the North Atlantic Ocean and tools for the sustainable exploitation of the marine resources (NORTE-01-0145-FEDER-000040), supported by the North Portugal Regional Operational Programme (NORTE2020), through the European Regional Development Fund (ERDF) and project ANIMAL4AQUA, funded by Portugal2020, financed by ERDF through the Operational Competitiveness Program (COMPETE) - POCI-01-0247-FEDER – 017610. AB was financially supported by FCT (SFRH/BD/138593/2018).
References
Basto, A., Matos, E., Valente, L.M.P., 2020. Nutritional value of different insect larvae meals as protein sources for European sea bass (Dicentrarchus labrax) juveniles. Aquaculture. 521, 735085.
Cho, C.Y., Slinger, S.J., 1979. Apparent digestibility measurement in feedstuffs for rainbow trout, Finfish Nutrition and Fishfeed Technology. Halver, J. E. Tiews, K., Berlin, Germany, pp. 239-247.
EFSA, 2010. Scientific opinion on dietary reference values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, trans fatty acids, and cholesterol. EFSA 8, 1461-1566.