Production of tagatose-rich syrup with ion-exchange resin purification

Abstract

The production of rare sugar syrups, particularly tagatose-rich syrups, has attracted increasing interest due to their low glycemic index and potential applications in reduced-sugar food products. Although CaO-promoted alkaline isomerization of galactose offers an efficient route for tagatose production, residual inorganic ions remaining after neutralization hinder achievement of higher quality. In this study, ion-exchange purification of tagatose–galactose syrup produced via CaO-promoted isomerization was systematically optimized. Following isomerization, the reaction mixture was neutralized using either CO₂ or sulfuric acid and purified using combinations of strong-acid cation and strong-base anion exchange resins. The effects of neutralization route, resin type, resin dosage, and contact time on electrical conductivity, mineral removal, pH stability, and color were evaluated. CO₂-neutralized syrups exhibited lower initial ionic strength and superior deionization performance compared with sulfuric-acid-neutralized syrups. The optimal resin system reduced syrup conductivity by approximately 90% to ≤250 µS cm⁻¹ (measured on diluted samples), indicating effective deionization performance. Empirical second-order regression models (R² > 0.87, R² > 0.92); trendline slopes (0.97–0.94) were developed to guide process optimization. The purified syrup was subsequently concentrated to 65 and 75 °Brix leading to low ash content (0.09–0.13%), reduced water activity (∼0.7), acceptable color (90 IU), low HMF formation (4.2 mg kg⁻¹) and no detectable pathogens. This work provides bench-scale ion-exchange design guidelines for CaO-isomerized tagatose–galactose syrups, with mass-balance-confirmed sugar preservation and ionic removal data that can inform future column-scale process development.