Inhibition of lactose crystallisation in the presence of galacto-oligosaccharide

The stabilization of lactose in the form of amorphous (i.e. non-crystalline form) is the basic requirement to maintain the quality of relevant food and pharmaceutical products. The physiochemical properties of amorphous lactose mixed with galacto-oligosaccharide (GOS) were investigated. Water sorption, glass transition temperature, and crystallisation behaviour of lactose in the present of GOS (1:1 w/w) were measured at various water activity (0.11–0.75 a_w, 25 °C) and lactose mutarotation was also evaluated. All of them were compared with the physiochemical properties of trehalose-lactose (1:1 w/w). The addition of GOS to lactose increased the hygroscopicity of the mixture, as well as slightly increased the glass transition temperature compared to lactose alone. The critical water activity (at 0.68 a_w) of lactose crystallisation was increased by the addition of GOS as compared to that of trehalose-lactose (1:1 w/w) (at 0.58 a_w) or lactose alone (at 0.44 a_w). The dramatical inhibition of lactose crystallisation with a lower crystallisation kinetic constant and the alternation of lactose crystal forms in the presence of GOS was observed as compared to the crystallisation behaviour of trehalose-lactose (1:1 w/w) and pure lactose at 0.68 and 0.75 a_w, 25 °C. Without affecting its T_g, the significantly delayed crystallisation of lactose in GOS-lactose mixture (1:1 w/w) was more likely due to the change of lactose mutarotation. As comparing to trehalose that is an effective inhibitor, GOS has a stronger ability to prevent lactose from crystallisation in hydrous matrices.