الملخص الإنجليزي
Milk is a multicomponent mixture containing mainly water, protein, fat, lactose and other minor constituents, thus it is a challenge to trace different state and phase changes from its complex thermogram as measured by Differential Scanning Calorimetry (DSC). In the literature, negligible research works are reported on the thermal characteristics of camel milk power, especially for different components of camel milk. This is mainly due to the complex interactions between different types of complex components present in the milk. Thermal characteristics of freeze-dried whole and skimmed camel milk were measured by DSC. The thermogram showed three endothermic peaks (two for fat-melting and other for non-fat solids-melting) and three shifts. Two shifts at low temperature could be related to the glass transitions. However, it was difficult to identify which components in the milk were providing these transitions. The shift at higher temperature after melting of non-fat solids could be related to structure ordering in the milk. However, it was difficult to trace the glass transitions of each component in the milk due to the complex interactions of the components' phases. For this reason, different components of the camel milk (fat, cream, casein, whey protein, and lactose) were separated and then measured its thermal characteristics. The thermogram of camel milk fat showed two endothermic peaks, one wide and the other sharp. The wide peak at low temperature was due to the melting of different fractions of fatty acid and the sharp peak indicated melting of a significant amount of specific fatty acid. The melting of fat started at -5°C and ended at 52°C, respectively. The melting of fat in cream started at lower temperature -12°C as compared to the pure fat at -5°C. This decrease in melting temperature could be due to the effects of protein content in the cream. Casein showed one endothermic peak due to non-fat solids-melting and two shifts in the thermogram line indicating two glass transitions. The first glass transition started at 38°C, second glass transition started at 77°C and melting onset at 95°C, respectively. Similarly whey protein precipitated by ammonium sulfate and ethanol also showed two glass transitions and one melting endotherm. In the cases of all types of protein, the second glass transition was observed just before melting of non fat solids. In the case of first scan of lactose, only two endothermic melting peaks were observed without any trace of glass transition. However, the second scan with annealing showed two glass transitions and two endothermic peaks. The onset of the first and second glass transitions were at 56 and 114°C, respectively. Similarly the onsets of first and second melting endotherms were at 145 and 213°C, respectively. In case of commercial lactose, the glass transition could not be traced, however two similar melting endotherms were observed, first one at 141°C and second one at 215°C, respectively.
