Tagatose Production from Dairy Waste: Chemical Isomerization and Process Economics

Catalytic lactose hydrolysis into glucose–galactose syrup (GGS) offers a sustainable approach to valorize over 60 million metric tons of lactose-rich whey waste produced annually in the United States, reducing environmental impacts from waste disposal. Galactose can be isomerized into tagatose, a low-calorie sweetener with a high commercial value. This study investigates the chemo-isomerization of galactose using 11 metal-based reagents, identifying Ca(OH)₂ as the most effective due to its high yield, ambient temperature operation, and low cost. A tagatose yield exceeding 60% was achieved within 60 min at ambient temperature, which is two–four times higher than yields obtained with other reagents. Reaction time, temperature, Ca(OH)₂ dosage, and initial galactose concentration were systematically studied. However, under our lab-scale experiments, glucose present in GGS suppressed tagatose formation during coisomerization, necessitating a selective glucose removal step. A front-end simulated moving bed (SMB) separation was engineered to isolate galactose prior to isomerization, improving the tagatose yield and reducing downstream purification burden. A conceptual process that integrates catalytic hydrolysis, chemo-isomerization, purification by activated carbon and ion-exchange resins, and sugar separation through simulated moving bed (SMB) chromatography was developed. This integrated approach reduces the process complexity while maintaining high product selectivity and carbon efficiency. A complete mass balance and technoeconomic analysis (TEA) for a 10,000 tons/year facility demonstrated an internal rate of return (IRR) of 58.21% and a net present value (NPV) of $234.64 million. While Ca(OH)₂-driven isomerization leads to CaSO₄ precipitation during neutralization, solid–liquid separation must be incorporated to prevent downstream fouling. Unlike enzymatic routes that require immobilized biocatalysts, cofactor regeneration, and extended residence times, our nonenzymatic process provides a commercially scalable, rapid, and low-cost alternative for tagatose production from galactose. This work establishes the first technoeconomically validated route to produce tagatose from lactose-derived galactose with high yield and industrial relevance.