Abstract / Summary
D-Tagatose is a rare low-calorie sweetener whose scalable production from dairy whey remains limited by incomplete mechanistic understanding of the isomerization of D-galactose, by stoichiometric amounts of Ca(OH) 2 . This work addresses this gap through a kinetic framework describing D-galactose consumption, D-tagatose, D-talose, D-glucose and degradation product formation under Ca(OH) 2 mediated conditions, extended to mixed-sugar dairy feedstocks. Ab initio thermodynamic calculations for the free D-galactose/D-tagatose isomerization in water predict an equilibrium conversion of 23–30% at 25 ° C (K eq = 0.3–0.44) depending on the conformational treatment, consistent with the below 30% yields reported across conventional catalytic systems. 1 H NMR analysis of the processed intermediate Ca-galactose gel provides evidence consistent with open-chain D-galactose. D-tagatose was not detected in the analyzed pre-acidification sample but was detected following acidification. Initial rate analysis gave apparent reaction orders greater than unity for all product pathways, lumped over the equimolar D-galactose/Ca(OH) 2 pair, consistent with a coordination-based mechanism involving Ca(OH) + and Ca 2+ as Lewis acid species. A chemistry-informed power-law model incorporating both mechanistic pathways was globally fitted across 200–400 mM D-galactose feed (R 2 = 0.995). Extension to equimolar D-galactose/D-glucose mixed feeds through systematic model selection (AIC, BIC, F-tests) across eighty-one candidates identified an optimal six-parameter structure quantifying inhibition induced by D-glucose, with D-tagatose formation rate constants reduced to 23–49% of pure-feed values. Application to commercial whey hydrolysate at room temperature achieved 51% D-tagatose yield and 70% selectivity within 120 min. D-tagatose yields exceeding this free-sugar equilibrium range are observed under Ca(OH) 2 -mediated conditions, consistent with the additional calcium complexation, acidification and salt formation steps not captured by the free-sugar thermodynamic model.