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.