Morphological Characterization of Heat-treated Walnut Kernel
The samples had shown a significant variation in microstructure and they
varied in the sizes and shapes (Fig. 1). SEM micrographs indicated that
the thermal processing could modify the microstructure of samples, which
varied in the sizes and shapes compared to the untreated sample (Fig.
1). The morphology of untreated walnut kernel flour presented a complex
spherical to oblong/elliptical granular structures together with the
smaller particles and irregularly shaped cellular fragments. The
granular structures might be starch granules, while the protein bodies
were displayed by the shape of smaller spheres particles [24].
Generally, the protein, starch, cellulose molecules etc. were partly or
completely embedded together, for example the protein body substance was
surrounded by the smaller particles with spherical or irregular in shape
[24]. Here, the roasting and steaming at 100 ÂșC for 15, 20 and 30
min had marked influence on the surface of granules compared to the
untreated sample. The clumping phenomenon in samples of thermal
treatment began to appear, and the change extent varied with heating
time (Fig. 1). The part granule integrity in WKF was kept after thermal
treatment for 15 min, whereas it was observed to significantly change at
20 and 30 min (P < 0.05). Both flours treated by
steaming and roasting showed aggregated microstructure with irregular
surfaces. However, the aggregate of sample processed by steaming was
obviously larger than that of roasting (P < 0.05). The
results indicated that the steaming had maximally influence on walnut
kernel compared to roasting treatment, which was likely attributed that
the gelatinization was easy to occur due to the presence of water in
steaming [25].