Amino Acid Composition Analysis
Amino acid (AA) composition is an important chemical property of protein. The reason is that it can determine the nutritional value of protein [26]. AA composition of the untreated and heat-processed samples of walnut kernel is exhibited in Table 1. The data of experiments revealed that the protein in different WKF samples included 16 amino acids, including 7 essential amino acids (EAA) and 9 nonessential amino acids (NEAA) (Table 1). No significant difference was found in the total content of AA between untreated, roasting and steaming samples for 15 and 20 min (P> 0.05). The AA content of the sample was significantly higher than that of other samples when it was steamed for 30 min (P < 00.05). From the Table 1, it was observed that the amount of acidic AA in samples was higher than alkaline AA, which suggested that the WKF had the acidic characteristic [27]. These findings were in agreement with those previously reported for walnut meal flour [27]. Glutamic (Glu), aspartic (Asp), and Arginine (Arg) in all WKF were relatively abundant (Table 1), Asp contents in untreated, S15-S30 and R15-30 were 7.13, 5.77-7.24 and 6.04-5.10 g 100g-1, respectively; Glu contents were 19.15, 15.83-15.50 and18.60-17.66 g 100g -1, respectively; Arg contents were13.26, 13.48-15.18 and 13.90-13.31 g 100g-1, respectively.
In addition, EAA content could reflect the nutritional value of protein [28]. All samples contained EAA contained (except Tryptophan). The total EAA contents in untreated, S15, S20, S30, R15, R20 and R30 were 26.72, 26.93, 27.89, 30.19, 27.07, 26.11 and 25.74 g 100g-1, respectively, which occupied 30.42, 33.33, 32.12, 33.23, 31.08, 31.12 and 31.2 %of the total AA content, respectively. Therefore, the walnut kernel obtained by steam heating could be regarded as high quality of natural proteins due to being composed of the most of the EAA (Table 1). After different heat treatments, the content of EAA in protein of walnut kernel changed to a certain extent compared to the untreated sample. The steam treated samples were significantly higher than the untreated and roasted samples (P < 0.05). After steaming, the total amount of total EAA in protein increased significantly (P < 0.05) with the increase of heating time. Roasting treatment had no significant effect on the total EAA content of walnut protein (P > 0.05). The results could be explained by their different processing methods. The thermal stability of protein was comprehended by the composition of AA.17 The higher the content of hydrophobic AA in the protein, the more stable the protein was, which suggested that the more stable protein contained the higher total hydrophobic AA content. The hydrophobic AA content of the steaming treated walnut kernel gradually increased compared with the untreated, while the roasting treated walnut kernel gradually decreased, which was due to a thermodynamically less stable for the protein of walnut kernel. However, there was the small difference for total hydrophilic AA among samples.
It is generally believed that the closer the Amino Acid Score (AAS) is to 100, the closer the AA pattern in samples is to the recommended value,18 which suggests that the nutritional value of sample is higher. AAS of untreated and heated walnut kernels is observed in Table 1. For walnut kernel samples before and after heat treatment, the AAS of rest AA except lysine was closest to 100 or more. It was illustrated that these amino acids could basically meet the recommended requirements [18]. Lysine was the first limiting amino acid in walnut kernel samples according to the AA score (Table 1) [18]. It was worth noting that the lysine AAS by steaming treatment increased from 39.78 to 47.56%, while roasting treatment decreased from 39.78 to 37.11%. This showed that the steaming treatment could improve the nutritional value of protein in kernel. These results proved that the AAS of walnut kernels by steaming processing significantly increased compared to the untreated and roasting processing (P < 0.05). Although the AAS of histidine, valine, leucine, isoleucine, phenylalanine and threonine had decreased after roasting treatment, it was still close to the recommended value of WHO/FAO/UNU Expert Consultation [18], and AAS of methionine by roasting treatment for 15 min increased from 71.76 to 77.65%. These data indicated that the two kinds of heat treatment could improve the nutritional value of protein in walnut kernel. The amino acid content of each component of protein in walnut kernel changed after different heat treatment, but most EAA could still meet the recommended requirements. Therefore, the walnut protein by different thermal processing could be used as a high-quality protein ingredient in food [29].
In addition, the different heat treatments also changed the amount of AA composition of the protein in walnut kernel. The results showed that the content of other AA contents except for glutamic gradually increased with the extension of steaming treatment time. It indicated that the steaming processing led to a significant increase in major AA contents, which might be contributed to the release of bound amino acids during steaming with water [30]. The increase in the amino acid levels after the steaming of walnut kernel could inspire utilization in food formulations. It was worth noting that the content of AA components changed little when the roasting time was 15 min; when the roasting time was extended to 20 and 30 min, other AA contents except for arginine reduced. This phenomenon could interpret the higher concentration of hydrophobic AA in steamed walnut kernel. Compared with the untreated and steaming processing samples, the major AA content and total AA of samples in roasting processing from 15 to 30 min gradually reduced. This might be due to little water in roasting processing, which would lead to the partial degradation or loss of protein or the result of Maillard reaction between amino acids and sugars [31].