3. Results and discussion
3.1. Powder properties
The major particle and nutrient properties of pitaya and guava powder are presented in Table 2. The mean particle sizes of pitaya and guava
powders were very small having D50 of 85 μm and 158 μm, respectively
(Table 2). Comparatively, particle sizes of guava powder were almost
two times larger than that of pitaya powder. This was due to presence
of broken hard seeds and soluble polysaccharides. The large variation
in particle sizes hardly influenced the flowability of the fruit powders.
Hausner ratios of pitaya and guava powders were 1.53 and 1.37, respectively, which indicate them as ‘very difficult’ and ‘difficult’ flowing powders. Smaller particle sizes of both powders might be the major reason
for the poor flowability. Generally particle size influences the powder
compaction, flowability, segregation and other factors [18]. According
to Davor et al. [19], powders with smaller particles exhibit poor flow
properties and usually cause handling problems. With decreasing particle size, powders become more cohesive and are sometimes associated
with caking problems. In case of the mixed fruit powder, the average
mean particle size was increased upon mixing. This might due to the filling of the existing large void spaces between particles as well as the effect of contact surface area upon size distribution [20]. However, in spite
of the increased particle size, Carr index and Hausner's ratio of the
mixed powder were higher and the powder exhibited as ‘difficult’ to
flow. This may be due to the moisture content and hygroscopicity of
individual fruit powders. Pitaya powder was found to be hygroscopic
during powder production, which may be due to the presence of excess
amorphous materials like mono-sugars, pigments and micro-compounds
[3]. Hence, moisture content as well as water activity of the mixed fruit
powder was reasonably higher compared to individual powders probably
due to the longer exposure in air during mixing. Kyu et al. [20] reported
that as the water activity of model food powders increased, all the tested
properties also increased, except for loose bulk density and internal friction angle, and the powders emerged with poor flowing property. Vitamin C content (wb) in the powder was found to be relatively higher
than that in the pulp. This is attributed to the concentration of ingredients
in the dried form. On a wet basis (wb) solid content in powders was much
higher than in the pulp.