Scanning electron microscopy
The microstructure of the soy donuts was determined using the Scanning Electron Microscope (SEM, JSM-5300, JAPAN ELECTRIC, Tokyo, Japan). Donut samples (10 × 10 × 10 mm) were cut and freeze-dried, and then kept in a vacuum sealed wrap until use within 1 week. Donuts were separately placed on the sample holder with double sided scotch tape, and sputter-coated with gold (100 s, 10 Pa); each sample was then transferred to the SEM where it was observed at 30 kV ×35 magnification.