(342d) Sustainable Production of Tagatose Via CaO-Promoted Galactose Isomerization with CO2 Neutralization Under Ambient Conditions

The use of low-calorie sugars has garnered significant interest due to their potential to facilitate a reduction in individuals’ overall caloric consumption, hence leading to weight loss. The market for allulose (0.4 kcal/g), a low-calorie sugar alternative, has been expanding rapidly, driven by growing consumer awareness around health concerns like obesity and diabetes. In 2023, the global allulose market was valued at $128 million, with projections estimating a compound annual growth rate of 15%, leading to a market size of $440 million by 2032. This growth reflects the rising demand for healthier, low-calorie sweeteners, particularly among keto and diabetic diets, as allulose minimizes blood sugar levels. Despite this promising trend, the production of allulose through enzymatic epimerization of fructose remains costly ($19.8 48.5/kg). This cost burden limits consumer access and is a key barrier to broader market adoption. Additionally, allulose has only about 70% sweetness as sucrose, meaning larger quantities are required to match the sweetness level in food products. This can complicate formulation and affect the texture of various products. These findings suggest that low-cost, low-calorie sweeteners with a taste similar to sucrose are essential in addressing the obesity epidemic.

Tagatose, with a glycemic index of around 3 and a caloric value of 1.5 kcal/g, offers around 90% sweetness of sucrose. It has minimal blood sugar spikes, unlike sucrose, which has a glycemic index of around 65 and a caloric value of 3.9 kcal/g. Therefore, it is an ideal choice for individuals with diabetes and aids in weight management. Most importantly, it has been approved for use in various food and beverage products as a “generally recognized as safe (GRAS)” ingredient. The growing global demand for tagatose has prompted an increasing exploration of its production methods. Lactose from whey (a byproduct in the dairy industry) can be enzymatically split into glucose and galactose using lactase or acids. Research on the enzymatic isomerization of galactose to tagatose, facilitated by L-arabinose isomerases, is of great interest. However, it encounters several challenges, including the poor thermal stability of the enzymes, the low galactose conversion, and the lengthy processing time required. In addition, research on the chemical and catalytic isomerization of galactose to tagatose is still in its infancy, with less than a dozen studies conducted.

This study explores a green process for tagatose production, involving the isomerization of galactose using CaO at room temperature and subsequent neutralization with CO₂. Increasing the CaO/galactose molar ratio (from 1/10 to 1/1) and galactose concentration (from 0.1 to 0.5 M) significantly improved galactose conversion, reaching up to 86.65%. However, the increase in CaO replacement (from 20 to 100 mol%) with CaCl₂ resulted in a substantial decrease in galactose conversion, with only 14.47% conversion achieved. Interestingly, the same phenomenon was observed when CaO was replaced with triethylamine (17.81% galactose conversion and 47.22% tagatose selectivity with 80 mol% replacement) or arginine (32.88% galactose conversion and 49.13% tagatose selectivity with 80 mol% replacement). These findings suggest that OH⁻ and Ca²⁺ derived from CaO play distinct yet essential roles in facilitating the formation of a gel complex and isomerizing galactose to tagatose. Neutralizing the gel complex with H₃PO₄ resulted in higher galactose conversion (89.10% vs. 69.11%) and lower tagatose selectivity (53.59% vs. 73.20%) compared to neutralization with H₂SO₄. A techno-economic analysis revealed that a minimum selling price (MSP) of tagatose ($0.80/kg) for the galactose (0.5 M) isomerization with CaO (0.5 M) and subsequent neutralization with H₂SO₄ was comparable to that achieved with CO2 ($0.81/kg) but lower than that achieved with H₃PO₄ ($0.85/kg). Sensitivity analysis identified plant capacity and internal rate of return (IRR) as the key factors influencing the MSP of tagatose. Furthermore, a life cycle assessment demonstrated that using CO₂ for neutralization exhibited the lowest environmental impacts. This study proposes a cost-effective and environmentally friendly approach to produce tagatose from galactose.