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| Study on the Effect of Oil Extraction Temperature on the Structural
Characteristics of Protein Components in Walnut Cake |
| FU Chao1, 2, 5, XIAO Xuan-rui1, BAI Tong-tong1, ZHU Meng-yu1, WANG Ping1, 2, BAI Bing-yao1, 2, ZHANG Chun-lan1, 2, ZHANG Rui3, XI Xiang1, JIAN Tian-tian4* |
1. College of Food Science and Engineering, Tarim University, Alar 843300, China
2. Production & Construction Group Key Laboratory of Special Agricultural Products Further Processing in Southern Xinjiang, Tarim University, Alar 843300, China
3. College of Horticulture and Forestry, Tarim University, Alar 843300, China
4. Instrumental Analysis Center, Tarim University, Alar 843300, China
5. College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, China
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Abstract Using ‘Wen 185’ paper-skin walnuts as the raw material, walnut albumin, globulin, prolamin, and glutelin were separated using a solubility gradient method. The study investigates the effect of oil extraction temperature on their structural characteristics. Infrared spectroscopy was employed to analyze the changes in the secondary structure of proteins, while fluorescence and ultraviolet spectroscopy were used to examine the changes in the tertiary structure. The results from infrared spectroscopy showed that the secondary structure of albumin was minimally affected by the oil extraction temperature. However, the secondary structure of globulin, prolamin, and gluten underwent significant changes when the extraction temperature reached 130 ℃. Additionally, the total content of α-helix and β-sheet structures in gluten was lower than in the other three protein components, indicating that its secondary structure stability is weaker than that of the other proteins. Fluorescence spectroscopy results revealed that the maximum fluorescence peak of albumin and globulin shifted to a longer wavelength (red shift) after the oil extraction temperature exceeded 130 ℃, indicating that their tertiary structure unfolded, exposing more hydrophobic amino acids on the protein surface. The fluorescence peak of prolamin shifted slightly to a shorter wavelength (blue shift) after the extraction temperature exceeded 100 ℃, suggesting that the hydrophobic amino acids on its surface were buried within the protein molecule. However, when the extraction temperature exceeded 130 ℃, a noticeable red shift occurred, indicating an increase in hydrophobic amino acids on the protein surface. The maximum fluorescence peak of gluten exhibited a red shift within the oil extraction temperature range of 40 to 70 ℃, indicating that changes in its tertiary structure led to increased exposure of hydrophobic groups. However, when the temperature exceeded 130 ℃, a blue shift in the fluorescence peak was observed, suggesting a reduction in the exposure of surface hydrophobic groups. This indicates that the tertiary structure of gluten is relatively more unstable compared to the other three types of proteins. Furthermore, the overall fluorescence intensity of albumin and globulin was higher than that of prolamin and gluten, suggesting that albumin and globulin contain more hydrophobic groups than prolamin and gluten. Ultraviolet spectroscopy results showed that all four protein components exhibited a significant UV absorption around 275 nm. As the oil extraction temperature increased, there was little change in the peak intensity of albumin, globulin, prolamin, and gluten. However, the number of UV absorption peaks in albumin and globulin was greater than in prolamin and gluten, suggesting that albumin and globulin contain more exposed hydrophobic amino acid groups, which is consistent with the conclusion drawn from fluorescence spectroscopy that albumin and globulin have more hydrophobic groups on their surfaces. In conclusion, when the oil extraction temperature reaches 130 ℃, it significantly affects the structure of albumin, globulin, and prolamin. The structural stability of gluten is relatively low, and at an oil extraction temperature of 70 ℃, its structure is already noticeably impacted. This study provides useful insights for understanding the effect of oil extraction temperature on walnut protein structure and for the development of walnut cake protein component products.
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Received: 2024-11-13
Accepted: 2025-06-18
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Corresponding Authors:
JIAN Tian-tian
E-mail: 1816822451@qq.com
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