English abstract
Date fruit plays a vital role in the economic and social lives of the people of Arab world. The date-pits are considered as a waste product from date fruit processing industry and it contains mainly complex carbohydrates and oil. The wide utilization of the date-pits powder requires understanding of its physico-chemical and structural characteristics. This thesis presents the studies of polyphenol contents, moisture sorption isotherm and thermal characteristics of date-pits. The polyphenol contents varied from 21-62 mg gallic acid equivalent/g date-pits when acetone, ethanol, methanol and water were used as extraction solvents at 22, 45, and 60 C. The measured moisture isotherm was modelled by BET and GAB models. The BET and GAB monolayer values of date-pits were estimated as 4.3 and 4.1 g/100 g dry-solids. BET monolayer indicated the amount of bound water to the specific polar sites, however GAB model could be used to predict the complete isotherm. The DSC thermogram of freeze-dried date-pits (moisture: 6.7 g/100 g date-pits) showed two endothermic peaks, a shift and an exothermic shift at high temperature after solids melting. The first endothermic peak was due to the melting of oil and the second one for solids-melting, while first shift indicated the glass transition and second exothermic shift after solids-melting was due to the interactions of melted solid components. At the heating rate 5°C/min, the melting peaks of oil and solids were observed at 2.7 and 197°C, respectively, and the exothermic shift was observed at 290°C. The melting peak temperature was decreased and melting enthalpy was increased with the increasing water content. The decrease of peak temperature could be due to the plasticization of solids with water and the increase of enthalpy could be due to the formation of hydrogen bond density requiring more energy. The peak temperature was modelled with Flory-Huggins model and the water-solids interaction parameter was estimated as 6.8x10'. Heating rate (5-100°C/min) and annealing condition were also used to explore the structural characteristics of the date-pits. The melting peak temperature increased exponentially with increasing heating rate and the data was fitted with Mehl-Johnson-Avrami and Kissinger models. The activation energy was observed as 5.6x10", and 9.2x10 J/g, respectively at moisture contents 6.7 and 17.2 g/100 g date-pits. These values are one order lower than the rice starch reported earlier in the literature, and the process with lower activation energy revealed lower interactions of the components in the pits. In the case of annealing, the melting peak temperature increased exponentially with increasing annealing time, which indicates more crystalline (i.e., ordered structure) region formation or further perfection of the existing crystalline regions. The melting enthalpy decreased with the increasing annealing time also an indication of the formation of more crystalline regions. Thermal characteristics have the potential to identify the structural characteristics which is important for the stability determination and would benefit for the modification of the components.
