From native tomato cutin to branched polyesters: Structural insights from supercritical-water depolymerisation

Cutin is the structural biopolyester that scaffolds the plant cuticle, forming a thermosetting matrix. It is insoluble in conventional solvents, chemically resilient and presents excellent barrier properties, preventing water loss, regulating gas permeability and protecting plants against environmental hazards. Conventional valorisation relies on batch processing with harsh chemicals that destroy the native architecture, yielding monomers rather than branched oligomers for polyester materials, and limiting mechanistic insight. Here we report a controlled depolymerisation strategy based on continuous supercritical-water hydrolysis that achieves complete solubilisation of the insoluble cutin network into soluble branched polyester oligomers (Mw ≈ 7.35 kDa) within 10.5 s of reaction time. Continued treatment further reduces Mw to ≈ 1.82 kDa at 120 s, approaching the limit of 24 h alkaline hydrolysis (≈ 1.22 kDa). Precise control of residence time reveals a hierarchical cleavage mechanism in which secondary esters at branching points hydrolyse preferentially over primary backbone esters. FTIR, NMR, GPC and GC–MS identify 10,16-dihydroxyhexadecanoic acid as the dominant monomeric unit and reveal 16-hydroxy-9Z-hexadecenoic acid as a novel SCW-specific product, consistent with a carbocation-driven dehydration pathway inaccessible under alkaline conditions. A structurally constrained network model depicts cutin as a heterogeneous crosslinked polyester dominated by 10,16-dihydroxyhexadecanoic acid, with glycerol as a branching node, C18 hydroxy acids and α,ω-diacids as linkers, and fatty acids as chain terminators. With an E -factor of 0.8 against 4.1 for alkaline hydrolysis, this process-intensified approach resolves the native architecture of cutin and establishes a scalable platform for plant polyester valorisation. Problem statement Cutin, considered the third most abundant biopolymer after cellulose and lignin, remains largely underutilised as a renewable polyester feedstock due to its highly cross-linked, insoluble structure, which severely limits processability and scalable valorisation [1] . Conventional depolymerisation strategies such as alkaline, acidic, or enzymatic hydrolysis rely on long reaction times, stoichiometric reagents, and multistep downstream processing [1] , [2] , leading to high material consumption, salt generation, and limited control over product characteristics. Although supercritical water presents a unique opportunity in biomass processing by functioning simultaneously as solvent and reaction medium [3] , its use for the depolymerisation of cutin into tunable polyester fractions remains largely unexplored. As a result, a critical gap exists in the development of fast, catalyst-free, and environmentally efficient processes capable of converting cutin-rich agro-industrial residues into functional polyester building blocks with defined molecular characteristics. Also, a data-driven model is proposed to infer representative structures of depolymerised products from experimental data.