Effect of Carbonation of Supersaturated Lactose Solution on Crystallisation Behaviour of Alpha-Lactose Monohydrate

The effect of dissolution of carbon dioxide (CO2) in lactose solution on the particle size, morphology, yield and densities of crystallized alpha-lactose monohydrate (α-LM) was investigated. Dry ice was added into a 50% w/w aqueous lactose solution (relative supersaturation SR = 4.55) at 0, 500 and 1000 ppm in batch mode and closed system at 40°C. Agitation and sonication then were applied to the carbonated aqueous lactose solution for creating CO2 bubbles to assist the nucleation process. For ultrasound (US) treatment, the aqueous lactose solution was sonicated at 50% amplitude for 1 min. All samples (CO2 + agitation and CO2 + US + agitation) were then agitated for 3 h at 25°C upon crystallisation process. Regardless of mechanical treatment, prior addition of CO2 at higher concentration tended to induce formation of smaller crystals with triangular and trapezoidal-shaped morphologies whilst non-carbonated lactose favoured generation of bigger tomahawk-shaped crystals. A remarkable increase in α-LM yield was found in 1000 ppm carbonated sample prior to sonication as against non-carbonated counterpart. The crystal size d(0.5) was also found significantly lower (~32 μm) for the carbonated sample as compared to those of non-carbonated sample (~56 μm). The bulk and true densities of α-LM powder obtained were independent on CO2 concentration added but addition of 1000 ppm CO2 affected the packed densities of α-LM. Carbonation of lactose solution can be considered as an additional tool to manipulate crystal size, shape and yield of α-LM that may suitable for pharmaceutical and food applications.