Circular bioeconomy valorisation of dairy waste: Optimised acid hydrolysis for resource recovery towards biostimulant production and plant response assessment

Dairy industry waste is a stream within the agri-food sector that requires sustainable management solutions. It is also a significant source of valuable nutrients that can be effectively recovered and valorised through sustainable environmental technologies within a circular bio-based economy. This study presents a chemical valorisation approach to produce an amino acid-rich biostimulants from dairy industry waste. Acid hydrolysis parameters, including the proportion of the hydrolysing medium (X₁: 10-30%), acid concentration (X₂: 10-100%), and temperature (X₃: 20°C-100°C), were optimised using response surface methodology with a Box-Behnken design. Neutralised hydrolysates (pH ~5) yielded 15 suspensions, modelled by a second-order polynomial (R² = 0.9989). Optimal conditions (X₁: 10%, X₂: 10% H₂SO₄:H₃PO₄, X₃: 100°C) produced 64.5 g kg⁻¹ amino acids. Physicochemical and microbiological analyses confirmed a high nutrient load and significant microbial reduction (<1.00 × 10¹ CFU g⁻¹), ensuring environmental safety of the hydrolysates for agricultural application. Germination tests on cucumber (Cornichon de Paris) showed enhanced root development and biomass. The best formulation increased seedling fresh mass by 47% over the control and 27% over a commercial product. This valorisation strategy allows for effective recovery of resources while reducing the environmental burden of dairy waste and supporting the integration of industrial by-products into sustainable agricultural practices. This research highlights a path consistent with sustainable waste management in line with the circular economy.