Oyster Shell Powder as a Calcium-Based Buffer for Stabilizing Acidic Fruit and Vegetable Waste During Black Soldier Fly Larvae Bioconversion

The rapid accumulation of fruit and vegetable waste and oyster shell residues presents increasing environmental challenges, particularly in regions with intensive agricultural and aquaculture production. This study evaluated the use of oyster shell powder as a calcium-based buffering additive to stabilize acidic fruit and vegetable waste during rearing of black soldier fly larvae (BSFL, Hermetia illucens). Five substrates containing 0, 2, 4, 6, and 8% oyster shell powder (fresh weight basis) were prepared and used for larval rearing under controlled conditions for 12 days. Substrate pH dynamics, larval growth performance, substrate utilization efficiency, and nutritional composition of larval biomass were assessed. Oyster shell supplementation significantly increased substrate pH in a dose-dependent manner, shifting the substrate from strongly acidic conditions toward the range favorable for larval development. Moderate supplementation levels (2–4%) resulted in the highest larval biomass, survival rate, dry matter reduction, and bioconversion efficiency, whereas excessive supplementation reduced performance. Protein content increased at moderate supplementation levels, while lipid content decreased with higher oyster shell inclusion. Calcium concentration in larval biomass increased proportionally with supplementation, whereas essential amino acid composition remained stable. These results demonstrate that oyster shell powder can be used as an effective buffering material to improve the stability of acidic organic waste substrates and enhance BSFL-based bioconversion. The combined utilization of fruit and vegetable waste and oyster shell residues represents a practical approach for integrated waste valorization and supports the development of circular bioeconomy strategies for sustainable protein production.