Abstract: The effect of multifrequency ultrasonic-assisted vacuum cooking on the muscle fiber structure and water-holding capacity of stewed marinated beef was studied. Changes in the muscle fiber structure, texture properties and moisture content of stewed marinated beef prepared from beef shank under different ultrasonic frequencies (40/52/68, 40/52, 40/68, 52/68, 40, 52, 68 and 0 kHz) were measured. The results showed that with increasing the number of ultrasound frequencies, myofibrillar fragmentation index (MFI), the contents of β-turn and random coil increased, and the texture attribute of elasticity tended to increase, while the contents of α-helix and β-sheet decreased, and so did the hardness, chewiness, and cohesiveness of stewed marinated beef. Meanwhile, the solubility of connective tissue on the surface of muscle fibers increased, the separation and fracture of muscle fibers became more apparent. Low field-nuclear magnetic resonance (LF-NMR) results showed that ultrasound treatment caused a leftward shift of the transverse relaxation time (T2) and shortened it, thus changing the water distribution. As the number of ultrasonic frequencies increased, the shifting range of transverse relaxation time to the left increased, and the contents of bound water and free water also increased, while the content of quasi-bound water correspondingly decreased. Cooking loss was significantly lower in the triple-frequency group than in the dual-frequency and single-frequency groups, while the product yield showed an opposite trend, the highest value being found in the triple-frequency group. In summary, ultrasound treatment destroyed the muscle fiber structure and improved the water-holding capacity of stewed beef; the more the number of ultrasonic frequencies, the more pronounced the effect.

Key words: stewed marinated beef; multifrequency ultrasound; vacuum cooking; muscle fiber structure; water-holding capacity