Measurement and analysis of the mannitol partition coefficient in sucrose crystallization under simulated industrial conditions

Mannitol is a major deterioration product of Leuconstoc mesenteroides bacterial metabolism of sucrose and fructose from both sugarcane and sugar beet. The effect of crystallization conditions on the mannitol partition coefficient (Keff) between impure sucrose syrup and crystal has been investigated in a batch laboratory crystallizer and a batch pilot plant-scale vacuum pan. Laboratory crystallization was operated at 65.5 °C (150 °F), 60.0 °C (140 °F), and 51.7 °C (125 °F) with a 78.0 Brix (% refractometric dissolved solids) pure sucrose syrup containing 0%, 0.1%, 0.2%, 1.0%, 2.0%, 3.0%, and 10% (at 65.5 °C only) mannitol on a Brix basis. Produced mother liquor and crystals were separated by centrifugation and their mannitol contents measured by ion chromatography with integrated pulsed amperometric detection (IC-IPAD). The extent of mannitol partitioning into the crystals depended strongly on the mannitol concentration in the feed syrup and, to a lesser extent, the crystallization temperature. At 65.5 and 60.0 °C, the Keff varied from ∼0.4% to 3.0% with 0.2% to 3.0% mannitol in the feed syrup, respectively. The mannitol Keff was lower than that reported for dextran (∼9–10% Keff), another product of Leuconstoc deterioration, under similar sucrose crystal growth conditions. At 10% mannitol concentration in the syrup at 65.5 °C, co-crystallization of mannitol with sucrose occurred and the crystal growth rate was greatly impeded. In both laboratory and pilot plant crystallizations (95.7% purity; 78.0 Brix; 65.5 °C), mannitol tended to cause conglomerates to form, which became progressively worse with increased mannitol syrup concentration. At the 3% mannitol concentration, crystallization at both the laboratory and pilot plant scales was more difficult. Mannitol incorporation into the sucrose crystal results mostly from liquid syrup inclusions but adsorption onto the crystal surface may play a minor role at lower mannitol concentrations.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Sucrose crystals were grown in supersaturated sucrose solutions containing mannitol. ► A liquid inclusion mechanism for mannitol incorporation into the sucrose crystal is proposed. ► The proposed mechanism is for mannitol concentrations greater than 1%. ► Conglomeration of crystals became progressively worse with increased mannitol concentration.