Calcium-enriched substrates to improve the nutritional composition of mealworm

Mealworms (Tenebrio molitor) can exploit food by-products and former foodstuffs, transforming them into products with enhanced nutritional profile. Although mealworms are a good source of protein and fats, they lack in calcium (Ca) content. This research aimed to increase the Ca levels in mealworms by rearing them on calcium-enriched substrates, incorporating different percentages of Ca (4%, 8% and 12%) into diets based on brewery spent grains and bread. Larvae near their last instar were reared for a week and sampled after one, three and seven days (T1, T3 and T7). Additionally, part of the larvae were subjected to a fasting period of 48 h. Both non-fasted and fasted larvae were analysed to assess their growth performances, proximate composition and their elemental composition through X-Ray Fluorescence analysis. The results showed that the average weight of larvae and the ether extract (on dry matter, DM) content decreased with higher dietary calcium, while crude protein (DM) content increased. Specifically, dietary Ca levels (8% and 12%) associated with long exposure (7 days) led to lower average larval weights, but the mortality rate remained unaffected. The fasting led to a significant reduction in the Ca content of mealworms, even though larvae previously fed Ca-enriched diets retained a substantial amount of Ca. Notably, the effect of substrates, time and fasting also affected the concentration of the other elements in larvae. Among the different supplementation levels tested, the Ca 8% emerged as the most effective compromise, optimising the elemental composition with a mild effect on growth performance.