Introduction
The increasing global population poses a major challenge to the sustainable production of seafood. Although plant-based protein sources are commonly used in aquafeeds, their sustainability is limited by competition with the human food supply and the high carbon footprint associated with their production and transport. In this context, the circular economy concept, particularly emphasized by the European Union’s Farm to Fork strategy, promotes the valorization of agro-industrial by-products. These by-products are generated in large quantities and are often discarded, thereby contributing to environmental impact. Europe imports around 800.000 tons yearly, which corresponds to 32.000 tons of peanut peel generated by-products (FAOSTAT, 2022; Manrich et al., 2022). Due to their composition, these by-products may be used as feed additives or alternative ingredients for aquafeeds. Among these, peanut peel as a major by-product of the peanut butter industry, typically discarded, representing an additional cost to the industry. Despite being undervalued as a feed ingredient, peanut peel contains moderate protein and lipid levels and a high concentration of bioactive antioxidant compounds that may promote fish wellbeing. To the best of our knowledge, this is the first study to evaluate the use of peanut peel in aquaculture. Therefore, this study aims to evaluate the apparent digestibility coefficients (ADCs) of the peanut peel by-products and to assess the effects of its dietary inclusion at increasing levels on the zootechnical performance of juvenile gilthead seabream (Sparus aurata).
Materials and methods
The apparent digestibility of peanut peel by-products (PP) ingredient was evaluated. Two diets were formulated to determine ADCs; a control diet similar to a current commercial diet, and a test diet, containing 30% PP and 70% of the control diet. Apparent digestibility coefficients (ADCs) of dry matter, protein, lipid, and energy for both diets and the PP ingredient were determined according to Bureau et al. (1999). The digestibility trial was performed in a RAS system with 6 fiberglass tanks (60 L) equipped with fecal sedimentation columns. Triplicate groups of 15 gilthead seabream juveniles (IBW: 50g) were fed twice a day, and feces were collected daily, before the morning meal, for 30 days.
A growth trial was also conducted to evaluate the zootechnical effects of dietary inclusion of PP. Five isoproteic (44%) and isolipidic (18%) diets: a control diet similar to a current commercial diet, and 4 diet including an increased level of PP. Two diets included 0.5% and 1% (0.5PP and 1PP), to evaluate PP as functional ingredients, and 2 other diets including 5% and 10% peanut peel (5PP and 10PP), replacing up to 56% of soybean meal to evaluate PP’s potential as an alternative ingredient. The growth trial was conducted for 12 weeks in a RAS system maintained at 24 ºC. Triplicate groups of 15 gilthead seabream juveniles (initial body weight: 36g) were fed until apparent satiation, six times per week. At the end of the trial, fish’s growth performance, feed intake, feed efficiency, protein efficiency ratio, nitrogen, and lipid retention were evaluated. At the end of the trial, 3 fish per tank were sampled, and liver and intestine were collected for oxidative stress analysis and lipid peroxidation, and muscle for lipid peroxidation analysis.
Digestibility data were analyzed using one-way ANOVA. Growth performance data were evaluated by polynomial contrast analysis and one-way ANOVA, followed by Tukey’s post hoc test. A significance level of p < 0.05 was used for all statistical analyses.
Results
Apparent digestibility coefficients (ADCs) for dry matter, protein, lipid, and energy were statistically similar between the control and peanut peel diets, indicating that the inclusion of up to 30% peanut peel in aquafeeds does not compromise overall nutrient digestibility. The apparent digestibility coefficient for protein in peanut peel was approximately 38.5% ± 20.1, whereas lipid digestibility reached 73.5% ± 6.8.
During the growth trial, mortality was low and not affected by the dietary inclusion of PP. Weight gain, daily growth index, feed intake, feed efficiency, protein efficiency ratio, and lipid retention were not influenced by increasing levels of PP. Nitrogen retention was also unaffected by PP inclusion; however, a quadratic tendency was observed, with increased retention in diets containing 0.5%, 1%, and 5% PP compared to the control diet, and a decrease at the 10% inclusion level. Whole body dry matter, lipid, and ash contents were not influenced by PP inclusion. However, whole-body protein content showed a quadratic trend to increase in the PP included diets compared to the control. Visceral and hepatosomatic indexes were not affected by the inclusion of PP.
Conclusion
Peanut peel showed moderate digestibility as an ingredient, but when included at 30% in the diet, its digestibility was comparable to that of the control diet. The dietary replacement of soybean meal by PP up to 10% (representing a replacement level of 56%) did not affect growth performance, feed utilization or whole-body composition. Oxidative stress and lipid peroxidation results are being analysed and will be presented.
These findings highlight the potential of peanut peel as a sustainable alternative ingredient in aquafeeds and open new avenues for future research on its functional and nutritional applications in aquaculture.
References
Bureau, D. P., Harris, A. M., & Cho, C. Y. (1999). Apparent digestibility of rendered animal protein ingredients for rainbow trout (Oncorhynchus mykiss). Aquaculture, 180(3), 345-358. doi:https://doi.org/10.1016/S0044-8486(99)00210-0
FAOSTAT. (2022). Crops and livestock products. Retrieved from https://www.fao.org/faostat/en/#data/QCL
Manrich, A., Martins, M., & Mattoso, L. (2022). Manufacture and performance of peanut skin cellulose nanocrystals. Scientia Agricola, 79. doi:10.1590/1678-992x-2021-0010
Acknowledgments
This research was supported by national funds through FCT - Foundation for Science and Technology within the scope of 2023.02516.BD.