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].