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Systematic four?dimensional mapping of zeolitic imidazolate framework biocomposites reveals how precursor ratios, total concentration, and washing define crystalline phase, protein loading, and release kinetics. This comprehensive study identifies conditions yielding record loading (?85%) and precise phase–property correlations. Phase mixing enables tunable multi?step release, providing predictive guidelines for designing efficient protein?based drug?delivery biocomposites.Metal–organic framework (MOF) biocomposites consist of MOF matrices into which biomacromolecules (e.g., proteins/enzymes) are immobilized for applications in drug delivery and biocatalysis. Zeolitic Imidazolate Frameworks (ZIFs) based on Zn2+ and 2?methylimidazole are the most studied MOFs for protein encapsulation. How varying the Zn2+:2?methylimidazole:protein ratio, total precursor concentration, and washing procedure yields distinct Zeolitic Imidazolate Framework (ZIF) phases (ZIF?C, sod, dia) and amorphous forms is systematically investigated. Each phase is found to strongly influence crucial properties, including encapsulation efficiency (EE%), loading capacity (LC%), and release kinetics. Notably, unprecedented LC values (e.g., ?85%) are achieved with negligible presence of unreacted MOF precursors, ensuring a minimal carrier fraction while enabling high protein content. Using bovine serum albumin as a model protein, the relationships between precursors, crystallographic phase, EE%, LC%, and release profiles are established. ??1?antitrypsin, a protein?based biotherapeutic, is further encapsulated in ZIF?C, sod, and dia, and retained inhibitor activity upon release is examined. Moreover, it is shown that blending different phases enables multi?step release profiles, which are highly desirable for controlled drug delivery. These results highlight the importance of systematic phase control to tune protein loading and release, offering structure?property guidelines for the rational design of ZIF?based biocomposites as drug?delivery platforms.