Ultrasonication is a valuable technique and has diverse applications in food items, such as mechanical cell distortions in order to enhance the retrieval of bioactive components, micro-organism inactivation and immiscible liquid emulsification
[31][42][43]. Recently, low frequency (16–100 kHz), high frequency (HIU, 10–1000 Wcm
−2) ultrasonic techniques have been developed as a fast and secure application to alter protein structures, functional and physicochemical properties
[44]. Most importantly, ultrasonication is widely encouraged by researchers to enhance food quality. Ultrasonic techniques have special characteristics that make them more effective, such as microstreaming currents, turbulence, high pressure, cavitation bubbling, leading to modification of protein structural properties
[45]. Gülseren, et al.
[46] reported that adding phenolic compounds to proteins and treating them with ultrasonic techniques could reduce protein cross-linking and oxidation, both of which play important roles in the gelling and textural properties of seafood proteins. He, et al.
[47] reported that the application of ultrasonication combined with high salt concentration enhanced the textural properties (hardness and springiness) of silver carp surimi. Meanwhile, it prevented the change in secondary structural changes by inhibiting the unfolding of α-helix content. Pan, et al.
[48] reported that the addition of phenolic compounds to MPs and treatment with ultrasonication reduced the protein oxidation (surface hydrophobicity and carbonyls) by enhancing the hydrogen and hydrophobic interactions. A big rise in the yield of soy protein isolate (SPI) catalyzed by mTGases led to an improvement in WHC and textural properties after the ultrasonication technique. Therefore, ultrasonication is also helpful in raising the SPI and wheat gluten-based hydrogels by assisting in protein structural modifications
[49]. Xu, Lv, Zhao, He, Li, Yi and Li
[45] reported that the ultrasound treatment with diacylglycerol (DAG) enhanced the gelling attributes of golden thread surimi. In addition, ultrasound combined with (DAG) improved the structural and microstructural properties of golden thread surimi by enhancing the intermolecular interaction, hydrogen and hydrophilic bindings. The use of short-term sonication speeds up water leakage rates because of the existence of poor structural matrix protein gels
[50]. In addition, the production of low-porosity, homogenous protein structures for water absorption at higher sonic times can be related to two distinct mechanisms, including: (I) an exposition through modifications in the molecular conformation of proteins to internal active polar groups on the surface and (ii) a more fitting dispersion of these functionary groups into a sonic reaction environment
[44]. It has previously been observed that high-intensity ultrasonic treatment could enhance the gel strength of SPI-set gels caused by adding glucono-β-lactone
[51] and calcium sulfate
[52]. Gao, et al.
[53] reported that the polysaccharide-added surimi protein had higher Ca2
+ATPase and sulfhydryl content after ultrasonication, which reduced oxidative changes in the myosin globular head by stimulating hydrogen and hydrophobic interactions. Thus, this results in a more stable gel texture and structure. Ultrasonic application appears to alter the function of the protein matrix by expanding the amount of interfacial covalent interactions. Typically, hydrophobic associations in the protein gel matrix are intensified following ultrasonication
[51]. Hu, et al.
[54] have demonstrated that the secondary SPI textural properties of mTGase-based gels were not altered at 20 kHz and 400 W of ultrasonic treatment. However, Gharibzahedi, Roohinejad, George, Barba, Greiner, Barbosa-Cánovas and Mallikarjunan
[26] reported modifications of the secondary structure of the wheat gluten-SPI gels caused by ultrasonic technique, leading to increased β-sheets, decreased α-helices and β-turns, which indicates fewer protein oxidative, denaturation and aggregation changes. Cui, et al.
[55] have identified an increase in β sheet count to boost the hydrophobic surface and viscoelastic properties of protein gels. The enhanced configurations of the β-sheets also modified the protein-protein cross-linking of hydrophobic active groups. β-sheets are more capable of hydrating water molecules compared to α-helix during ultrasonic pretreatment and it provides stronger hydrogels
[54]. Ultrasonication can transform the molecular protein structure from β-turns into random coils in order to improve the cross-linkage of amino acid side chains
[31]. Therefore, the larger amount of inter-molecular disulfide bonds in ultrasonic-based surimi gels can also justify the rise in gel strength. The appearance of a polymer matrix with more compact and dense aggregations of multi-molecular cross-links, hydrophobic associations and inter-molecular disulfide bindings, increases the gel strength
[56]. The microstructure of heat-induced gels, however, can be dramatically altered with increasing ultrasonic time and cavitational pressure, from a spongy matrix with wider irregular pores, to a more dense and homogeneous alveolar network with fewer pores by reducing myosin oxidative changes as well as enhancing the intermolecular bonding interactions
[57].
Qin et al.
[49] took a particular approach to explain why the increased ultrasonic approach enhanced the wheat gluten added gel. They observed that the non-covalent interactions in the gluten structure (e.g., hydrogen bonds) were attenuated by the partial expression of protein molecules. The spatial structure of gluten reportedly promotes gel strength by creating novel and different linkages/relationships in the molecular structure, particularly covalent cross-links of inter-(μ-glutamine)-lysine. The use of Na
2SO
3/ultrasonic pretreatment increased the power of wheat gluten gel by up to 67% of the different pretreatments (e.g., alkaline, urea and Na
2SO
3) coupled with ultrasonication
[58]. The use of ultrasonic treatment generally results in higher protein solubility in various solvents, particularly over long periods of time. Ultrasonication increases the sum of electrostatic bonds and other non-covalent connections compared with covalent interactions in protein gel structure. This decreases the molecular weight and increases the solubility rate by hydrolyzing disulfide bonds by adding Na
2SO
3 into the reaction mixture
[52]. Under certain circumstances, the dispersion of protein molecules on the aqueous surface allows exposure of specific functional groups by the cavitation phenomenon during the ultrasonic application
[59][60].