Introduction
Insect meals have been extensively studied as a sustainable alternative ingredient for aquafeeds, particularly the yellow mealworm (Tenebrio molitor). To date, most studies have focused on defatted insect meals, which contain between 4–14% lipid content [1]. Industrial processing of insect larvae often includes defatting, a thermal-mechanical fat removal step [2], due to the high susceptibility of lipids to oxidation, which may compromise shelf-life and hinder pulverization processes. Nevertheless, the defatting process may reduce both the environmental sustainability and economic viability of insect meal production and potentially affect its functional properties [3]. Notably, mealworms have a naturally high fat content, reaching 30% [1]. This study aimed to evaluate the comparative effects of full-fat and defatted mealworm meal as alternative feed ingredients to fishmeal, on growth performances, whole-body composition, fillet fatty acid profile, and quality attributes of rainbow trout (Oncorhynchus mykiss).
Materials and methods
A single batch of mealworm larvae was either subjected or not subjected to a thermos-mechanical defatting process, resulting in two distinct mealworm meals (MM): a full-fat mealworm meal (FFMM, crude protein: 51.9%, crude lipids: 32.8%) and a defatted mealworm meal (DMM, crude protein: 66.7%, crude lipids: 12.9%). Three isoproteic (45%) and isolipidic (22%) experimental diets were formulated for juvenile rainbow trout. A control diet (CTRL) was formulated to mimic the composition of a conventional commercial trout diet. Two other diets were formulated based on the CTRL diet but incorporated 25% of the full-fat and defatted MMs, totally replacing fishmeal (FFMM and DMM diets). All diets included 11.5% of fish oil (dry matter basis), and rapeseed oil was adjusted to maintain consistent lipid levels across all formulations. A growth trial was conducted with juvenile rainbow trout (11.4 ± 1.5 g). Nine homogeneous groups of 20 fish each were randomly assigned to nine fiberglass tanks (120L), within a recirculating aquaculture system kept at a constant 14 °C. Fish were fed to apparent satiation, twice daily, over an eight-week period. At the end of the trial, zootechnical parameters, whole-body composition, and fillet quality and composition were evaluated.
Results
At the end of the feeding trial, rainbow trout exhibit an eightfold increase in body weight, with no significant difference observed in overall zootechnical parameters across dietary treatments. However, both the FFMM and DMM diets led to a reduction in feed intake, while simultaneously improving feed efficiency. Furthermore, the hepatosomatic index was significantly lower in fish fed the FFMM diet compared to fish fed the CTRL diet. Whole-body dry matter and protein contents were unaffected by experimental diet. In contrast, lipid content was significantly higher in fish fed the DMM than in those fed the CTRL diet. Additionally, whole-body energy and ash contents were significantly elevated in fish from both the FFMM and DMM groups relative to the CTRL group. In the fillet, no significant differences were observed in dry matter and protein contents, among the dietary treatments. However, fillet lipid content was higher in the FFMM group compared to the DMM group. No differences were observed in muscle lipid peroxidation, pH, or textural parameters (hardness, adhesiveness, springiness, cohesiveness, chewiness, and resilience). Colorimetric analysis showed higher b* values in the CTRL and FFMM groups compared to those from the DMM group, indicating a more yellow coloration in the former treatments. The fillet fatty acid class profile is summarized in Table 1. SFA and MUFA contents were higher in the FFMM groups compared to both the CTRL and DTM groups. The CTRL diet led to the highest concentrations of n-3 PUFA and the lowest n-6 PUFA contents, followed by DMM and FFMM diets. Consequently, fish fed the FFMM diet exhibited higher atherogenicity and thrombogenicity indexes, whereas fish from the CTRL and DTM groups demonstrated higher hypocholesterolemic / hypercholesterolemic ratios and peroxidation indexes
Conclusion
Overall, the defatted mealworm meal promoted a more favourable fatty acid profile, with higher n-3 and n-3/n-6 ratio, and more beneficial health-related lipid indices. These results suggest that defatted mealworm meal may be a more suitable replacement for fishmeal. However, the use of full-fat mealworm meal could also be considered to reduce the need for added vegetable oils.
Acknowledgements
This work was funded by the SAFE project (SmartAqua4FuturE) within the European Union’s Horizon Europe programme under grant agreement no. 101084549. This research was also supported by national funds through FCT (Foundation for Science and Technology), within the scope of UIDB/04423/2020, UIDP/04423/2020 and D. Amaral doctoral grant (2023.03476.BD).
References
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