Powder properties
The material properties including moisture, water activity, densities
and particle size of both powders were determined by the method described
in Ng et al. [8]. Moisture was determined by oven drying in vacuumat
70 °C for 24 h.Water activity of the powderwas measured using
the water activity meter (GDX Instrumentation Scientifique, France).
Different types of densities including absolute, tap and bulk densities
of powders were measured. The values of bulk density and tap density
were used to calculate the Hausner ratio [11] and Carr index [12]. The
Hausner ratio and Carr index were used to calculate flowability. A gas
pycnometer (AccuPyc II 1340, Micromeritics, Norcross, USA) was used
to determine the absolute density. The particle size of the guava and
the pitaya fruit powder was measured using a particle size analyzer
(Malvern Mastersizer 2000, Malvern Instrument Ltd, UK). A Scanning
Electron Microscope (Philips XL 30 ESEM; MEMS and Nanotechnology
Exchange, Virginia) was used to analyze the particle size and shape of
the powders. Samples for the SEMwere prepared firstwith Karnovsky's