Exploring Sustainable Protein Alternatives: Physicochemical and Functional Properties of Paper Mulberry (Broussonetia papyrifera (Linn.) L’Hér. ex Vent.) Proteins

Plant leaves are emerging as sustainable protein sources due to their renewability and high nutritional value. This study systematically investigated the proteins in paper mulberry (Broussonetia papyrifera (Linn.) L’Hér. ex Vent.) leaves, including a protein extract with 45.44 ± 2.47% purity and its predominant albumin fraction with 30.63 ± 5.51% purity. Amino acid analysis revealed total amino acid contents of 21.60% (protein extract) and 31.92% (albumin), with essential amino acids meeting FAO/WHO recommendations for adults but falling slightly short of sulfur amino acid requirements for children (2–5 years). Secondary structures were dominated by β-sheets, conferring structural stability. Differential scanning calorimetry identified two distinct thermal transitions: the protein extract denatured at 84.70 ℃ and 150.40 ℃, while albumin demonstrated superior thermal stability with denaturation temperatures of 101.67 ℃ and 154.90 ℃. Functional properties demonstrated pH-dependent solubility (U-shaped curves, minimum at pH 4.0), with the protein extract showing superior water absorption capacity (5.77 ± 0.11 g/g) and albumin exhibiting higher oil absorption capacity (3.32 ± 0.01 g/g). Emulsification characteristic mirrored solubility trends, peaking under alkaline conditions (EAI: ~ 6 m²/g, ESI: ~ 25 min). The protein extract (14% w/v) exhibited stronger gelling ability than albumin (16% w/v). However, both proteins demonstrated relatively low foaming capacity across all pH ranges tested. Despite challenges like low protein purity and limited foaming performance, paper mulberry leaf proteins offer potential for food applications, particularly in systems requiring thermal stability and hydration capabilities. This study underscores the need for optimized extraction methods to enhance purity and functional properties for commercial utilization.