Emulsion Gel Stabilized by Young Bamboo Culm (Phyllostachys edulis) Fiber–Soy Protein Composite: Ball-Milling Effects on Stability and Lipid Digestion

This study aimed to valorize discarded moso bamboo (Phyllostachys edulis) culm tips, a fiber-rich food processing by-product, into a clean-label structuring ingredient with controlled lipid digestibility. Bamboo culm tip insoluble dietary fiber (BIDF) was ball-milled for 0 to 12 h and combined with soy protein isolate (SPI) to stabilize camellia oil-in-water fiber-network-stabilized emulsions at an oil fraction of 0.7; lipolysis was evaluated under INFOGEST 2.0. Ball milling for 9 h reduced BIDF to sub-10 μm particles and raised its three-phase contact angle from approximately 98° to 105° while preserving the cellulose I crystal form, and this milled fiber (SM-BIDF) was selected for emulsification. At 0.1% (w/v) SM-BIDF, emulsions showed the smallest droplet size (8.68 μm, vs. 18.4 μm for SPI-only), the highest |ζ|-potential (53.23 mV), and a gel network that remained stable for 14 days at 4 °C; this formulation also released the lowest free fatty acids (24.76% at 120 min), substantially below SPI-only controls. A concentration-dependent three-stage mechanism of interfacial reinforcement, network optimization, and excessive weakening was proposed to explain these observations. These results demonstrate that ball-milled bamboo culm tip IDF combined with SPI provides a sustainable dual-functional emulsifier that simultaneously stabilizes emulsions and retards lipid digestion.