Free Ca²⁺ activity as the missing mechanistic variable in LM/LMA pectin gelation: a whole-field audit and a Minimum Reporting Standard (MRS-Ca)

Ionotropic gelation of low-methoxyl (LM) and amidated low-methoxyl (LMA) pectins is governed by the thermodynamic activity of free Ca²⁺ rather than by the nominal concentration of calcium salts added. Nevertheless, LM/LMA literature almost exclusively parameterises calcium as CaCl₂ dosage or Ca²⁺:pectin ratios, implicitly assuming ideal solution behaviour. Importantly, calcium is also a native structural component of plant cell walls (Ca-pectate), so pectin feedstocks enter extraction with a non-zero calcium background that may carry over into commercial materials. A structured screening of Scopus records (1978–2026; n = 167) focused on food-relevant LM/LMA systems revealed a striking field-level pattern: none of the mechanistic studies reported direct measurement of free Ca²⁺ or Ca²⁺ activity using ion-selective electrodes or complexometric methods. This absence persists despite frequent mechanistic interpretations of junction-zone formation, gelation kinetics and nonlinear rheology. Using electrolyte theory and ion–polymer association equilibria, we demonstrate why nominal CaCl₂ cannot serve as a mechanistic descriptor in real matrices. Intrinsic calcium in commercial pectins, together with competitive binding by polyols, proteins, organic acids, starch surfaces and RG-I domains, decouples added calcium from the free ionic fraction controlling crosslink formation. We propose a Minimum Reporting Standard (MRS-Ca) for LM/LMA gelation studies comprising nominal Ca²⁺ added, measured free Ca²⁺ or activity, aCa²⁺/nominal ratio, ionic strength, pH/buffer conditions and pectin fine-structure descriptors. Routine implementation of MRS-Ca represents a low-cost methodological correction that would immediately improve reproducibility and enable mechanistic comparability across studies.