Abstract: 【Objective】 The influences of ultrasound on chemical forces and water holding capacity of myofibrillar protein (MP) gel were studied. The relationship between MP gel chemical forces and WHC was revealed.【Method】AA type broilers were slaughtered. The MP was extracted from breast muscle. The MP solution and heat induced gel in different ultrasound times were prepared. Total sulfhydryl group (SH) content changes of gel were used to reflect the formation of disulfide bond. Surface hydrophobicity and Zeta potential value of the gel were employed to present the hydrophobic interaction and electrostatic repulsion, respectively. The ratio of I₈₅₀/I₈₃₀ was used to show the hydrogen bonding changes of gel. Water holding capacity (WHC) of MP gel was calculated by high-speed centrifuge.【Result】As the ultrasonic time (UT) increased (0-6 min), the total SH contents of the MP  gel decreased and reactive SH content significantly increased, at stronger UT (6-15 min), the total SH and reactive SH contents all decreased. At short time (0-6 min), the changing trends of MP and MP gel about the total SH and reactive SH content were the same, while the total SH and reactive SH contents of MP all had no significant changes (P＞0.05) over 6 min, suggesting that UT promoted internal SH of MP into disulfide bond for shorter time, and UT and heating gel for longer time work together to promote reactive SH into disulfide bond. Surface hydrophobicity of MP gels increased from 1 194.1 to 1 489.5 (P＜0.05) and decreased from 1 489.5 to 1 230.8 at last, for the cavitation phenomenon induced by UT could bring the hydrophobic regions into the surface, while at UT＞6 min, the hydrophobic group was wrapped in the gel network. Zeta potential value of the gel changed from 6.03 to 7.68（P＜0.05）（0-6 min）, and then decreased over 6 min, which showed that ultrasonic wave made protein molecular chain unfolding, negatively charged by exposure, resulting in the stronger electrostatic repulsion, while excessive expansion of MP molecules were not conducive to the electrostatic force of the gel. After moderate UT (0-6 min), the normalized intensity of I₈₅₀/I₈₃₀ ratio increased from 0.9805 to 1.023 (P＜0.05), which indicated that more phenolic hydroxyl groups of tyrosine were exposed to the aqueous environment, generating hydrogen bonds with water molecules, while the ratio decreased at UT＞6 min, hydrogen bonds between water molecules and protein were weaken. At UT ≤6 min, the WHC of samples increased sharply from 47.5899% to 72.9855% (6 min) (P＜0.05), while at 9 min and above, WHC dropped gradually to 44.356%. Results of the correlation analysis showed that WHC of MP gel was unrelated to total SH content (P＞0.05) and was significantly related to reactive SH content (P＜0.05) and had a significantly correlation with Zeta potential absolute value, the surface hydrophobicity and hydrogen bonding (P＜0.01).【Conclusion】Ultrasound has significant impacts on both chemical forces and water holding capacity of the MP gel. Hydrophobic force, electrostatic repulsion and hydrogen bonding play important roles in holding water in MP gels, disulfide bonds not. Thus a value of 6 min is the optimum time for the water holding capacity of MP gel, under this condition, hydrophobic force, hydrogen bonding and electrostatic repulsion of the gel are maximum, resulting in the gel network structure of uniform density, and retaining water at best.

Key words: myofibrillar protein gel, disulfide bond , hydrophobic force , electrostatic repulsion, hydrogen bond, water holding capacity, microstructure