State diagram of temperature vs date solids obtained from the mature fruit.

Abstract

Mature dates were dried achieving an increase in the level of solids from 70.8% to 92.4% (wet basis). Small deformation dynamic oscillation was employed to identify changes in the viscoelastic properties of dates as a function of solids. Samples were cooled or heated at a constant scan rate of 1 degrees C/min. At high temperatures, e.g., 70 degrees C, the storage and loss modulus (G' and G' ', respectively) remained relatively independent of the time or frequency of observation, thus delineating the plateau zone. Cooling resulted in rapid development of both moduli with G' ' overtaking G'. This is known as the glass transition region and was used to define the rheological T(g). Eventually, the glassy state is reached where the storage modulus becomes dominant once more and approaches values of 10(9.5) Pa. The values of T(g) were used to determine the metastable glass transition curve of mature dates. At lower levels of solids, i.e., between 11.7% and 64%, freezing experiments were employed to identify the equilibrium melting curve of ice. The state diagram yielded a maximally freeze-concentrated solute at 70% solids with the characteristic temperature of glass formation being -50 degrees C. Data on equilibrium and kinetic events were modeled using the Chen and Gordon-Taylor equations yielding the rheological T(g) for date solids at their effective molecular weight.