CONICET | Buscador de Institutos y Recursos Humanos

Crystallization of amorphous sugars is of practical and economic importance in food and pharmaceutical industries. Dehydrated lactose, dried products and dairy powders are hygroscopic. Water sorption often promotes reactions (including crystallization) that unleash both physical and chemical deterioration resulting in quality lost. The seeds of vinal tree (Prosopis ruscifolia) have an interesting galactomannan gum that could be used as emulsifier, thickening or stabilizing agent to replace the actually imported guar and locust bean gums. The aim of this work was to analyze the behaviour of galactomannan gum obtained from vinal (VG) to affect the crystallization kinetics of lactose or trehalose freeze-dried systems subjected to humidification to critical relative humidity (RH). A comparison of a similar and commercially available gum was conducted. The systems were prepared by dissolving 20%w/v of lactose or trehalose with 0.1 or 0.5%w/v of guar gum (GG) or VG. They were agitated (1 h), sonicated (15 min), filtered, frozen (24 h at -20°C) and finally freeze-dried. Lactose systems were exposed (15 days) at different saturated salt solutions with RH values between 11 and 97% and trehalose systems were exposed (1-3 days) at 43% RH. Water content, thermal transitions (glass transition temperature -Tg-, sugar crystallization and crystal melting) were determined by differential scanning calorimetry (DSC) in isothermal (50-70 °C) and dynamic runs. X-ray diffraction was conducted on lactose samples. Avrami equation was used to describe the kinetics of lactose isothermal crystallization. As expected, lactose monohydrate crystallization was promoted above 33% RH. Trehalose dihydrate crystallization occurs during humidification at 43% RH since the amount of water and the Tg value (15-17°C) allows the formation of the crystals. The amount of both monohydrate lactose and dihydrate trehalose crystals in the freeze-dried systems humidified at their critical RH were reduced by VG and GG, according to the enthalpy values of melting for trehalose in dynamic runs and of crystallization for lactose in isothermal runs. However, earlier and sharper peaks appeared in lactose thermograms in presence of gums, and taking into account the higher Avrami n value obtained for both gum-containing systems, it can be proposed that gums favour crystallization kinetics of at least a fraction of the system by acting as a nucleation agent, but do not allow lactose monohydrate complete crystallization. It is also interesting to note that both gum-lactose systems showed much smaller crystals than the lactose system, avoiding the formation of macroscopic crystals. Present work reveals that the presence of VG and GG reduce the amount of both lactose and trehalose crystals in freeze-dried systems humidified at their critical RH. The application and use as a sugar crystallization-modifier agent of a novel gum obtained from local sources with similar properties to a well-known and established commercial product like guar gum was promising. Acknowledgments: CONICET (PIP 100846), ANPYCT (PICT 0928) and UBA (2002100100397)

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