Water and Oil Holding Capacity
Because the water holding capacity (WHC) and oil holding capacity (OHC) affect the application of samples in food, it is necessary to study the functionality [51]. In this study, the effect of different heat treatments on WHC and OHC of walnut meal flours were assessed. WHC of untreated and treated WKF is depicted in Table 4. Compare to the untreated and roasting samples, WHC values of steaming samples increased significantly (P < 0.05). The results were supported by finding of Choe et al. [9], they found that moist–heat treatment could significantly increase WHC of whole seeds. The first reason possibly was the structural changes of components in walnut kernel during heating, which would lead to absorbing more water and improving the hydration property [9]. Another reason was that the phenomenon of starch swelling or gelatinization or damaging in walnut kernel occurred during steaming treatment, which would cause the increase of WHC due to more exposed hydroxyl regions with interacting with water [9]. Finally, it also probably was due to the higher content of hydrophilic amino acids in walnut kernel during steam processing [9]. On the contrary, the roasting walnut kernel showed the higher OHC than that of untreated or steaming treatment (Table 4), which was attributed to the protein denaturation as well as the exposure of more hydrophobic regions [52].
However, WHC of R30 was significantly lower than untreated, while there was non-significantly (P > 0.05) between untreated and R15. Additionally, with the extending of heating time, WHC gradually reduced. Untreated sample had WHC of 2.84 g g-1 while as steaming and roasting treated flours had WHC ranging from 4.43 to 3.45 g g-1 and 3.32 to 1.85 g g-1, respectively. It was interesting that the walnut kernel by roasting treatment showed the highest OHC, which was 5.67-5.73 g g-1, the untreated sample exhibited the lowest OHC of 1.21 g g-1. However, no significant differences in OHC were observed between roasting samples for 15, 20 and 30 min. In steaming samples for 15, 20 and 30 min, the decrease in OHC might be related with the low concentration of hydrophobic amino acids (Table 1) [46], which had a high attraction for oil. Therefore, OHC of S15 significantly was higher than that of S20 and S30. Because WHC and OHC altered with heat processing, which should be thought over when the food formulas were developed.