Valorization of Agro-food By-products in the Rearing of Tenebrio molitor and Zophobas morio

The demand for food in the coming decades is expected to grow, leading to an increase in agro-food by-products. To optimize their use and promote a circular production model, some of these by-products can be used as feed for insect rearing, which provides an alternative source of protein for human consumption and animal feed. Moreover, the use of such substrates can help to reduce the production costs of edible insects. This study, funded by the EU project "ON Foods – Research and Innovation Network on Food and Nutrition Sustainability, Safety and Security - Working ON Foods" (NRRP, No. 1550), evaluates five by-products from the agro-food industry for the growth of Tenebrio molitor and Zophobas morio (Coleoptera: Tenebrionidae): potato waste, okara, brewer's spent grains, hemp waste, and tomato peels. Wheat bran was used as a control for both species. The by-products were provided to the larvae either as such or mixed with wheat bran (1:1), with five replicates for each treatment. In addition to the nutritional composition of the substrates, survival, final weight per larva, development time, final biomass, substrate conversion ratios, and the chemical composition of the larvae were evaluated. The larvae grew on all substrates, whether used as such or mixed with bran. However, T. molitor showed approximately 50% mortality when using whole okara and potato waste, and 28.4% mortality with whole brewer's spent grains. Zophobas morio, on the other hand, showed 59.6% mortality only with okara. In all other cases, no significant differences were found between wheat bran and the mixtures. For T. molitor, developmental time was shorter with the mixtures compared to the 100% by-products (F(48,1)=16.58, p<0.001), and the final weights of the larvae were also higher in the mixtures (T. molitor F(48,1)=7.53, p<0.01; Z. morio: F(48,1)=69.0, p<0.001). In both species, the addition of wheat bran improved the "poor" substrates (potato and hemp waste). The first has a low protein and lipid content and likely did not meet the larvae's amino acid requirements, while hemp waste has a high ash content. The larvae were still able to utilize fibrous substrates, including poor-quality fibers (e.g., tomato peels, which are high in ADL). The crude protein content of the larvae on different substrates ranged from 39.78% to 48.24% for T. molitor and from 37.85% to 59.47% for Z. morio, while the lipid content ranged from 26.41% to 35.56% and from 28.59% to 48.85%, respectively. The results showed that the addition of by-products to wheat bran did not negatively affect the growth of the larvae, or the other parameters considered. The possibility of using some of these by-products from the agro-food industry for rearing insects for edible purposes represents a potential way of valorising them and reintegrating them into the food system, while also helping to reduce production costs.