Confirmation of the phenomenon of sugar crystallization
caused by the water loss in freeze-dried mango powder during
storage was attempted by means of SEM and XRD analysis.
3.2.1. X-ray powder diffraction
Due to the fact that the amorphous materials show no X-ray
patterns that are typical of crystalline materials (Roos, 1995), the
XRD patterns for dehydrated mango powders were measured for
crystallization of sugar (Fig. 2). The XRD patterns clearly revealed
a sharp peak in the samples stored at 57.6% and 68.9% RVP and
thus, disclosed the crystalline morphologies in the samples. As expected,
the sample taken prior to storage showed amorphous characteristics
and no crystalline peak was observed. This indicated
that freeze drying resulted in amorphous mango powder. This finding
coincided with Haque and Roos (2005), who found that freeze
drying results in amorphous lactose powder. After storage for 7 d
at 11.3% RVP, the mango powder still showed amorphous characteristics.
This meant that there was no change in the physicochemical
properties of the freeze-dried mango powder under such
storage conditions. At such an RVP value for storage, samples are in
a glassy state and some phenomena, such as sugar crystallization,
are not expected to occur. Samples stored at 43.2% RVP are expected
to show crystalline characteristics; however, no crystalline
peaks as 57.6% or 68.9% RVP were observed. This was possibly because
the amount of formed crystals in the sample at 43.2% RVP
was not sufficient for detection by XRD analysis. Moreover, the
intensity of 68.9% RVP samples was determined to be substantially
elevated in comparison to the 57.6% RVP samples. Haque and Roos
(2005) and Jouppila et al. (1997) elaborated on the intensity of the
XRD pattern as an indication of crystal formation in their samples.