As a typical salt-soluble protein, myofibrillar protein (MP) is essential for the production of a three-dimensional gel matrix after heating and subsequent chilling [1,2]. Treatments that alter the physico-chemical characteristics and functionalities (e.g., solubility, emulsifying properties, gelation and rheological behavior) of MP have an appreciable impact on the fat-holding capacity, water-holding capacity (WHC), textural properties, adherence and sensory attributes of the meat products, especially emulsified meat products [3,4]. However, the strengths and elasticities of heat-induced MP gels are influenced by the various intrinsic conditions (e.g., quality of raw meat, levels of sodium chloride (NaCl) or phosphates, pH value and the addition of non-meat proteins) and processing parameters (e.g., heating temperature and heating or cooling rate) [5,6]. To address this issue, various functional ingredients (e.g., native/modified starches, cereal flours, hydrocolloids, plant or animal-based proteins, polyphenols and phenolic acids) [7,8], as well as some novel processing techniques (e.g., high-pressure processing, pulsed electric field treatment, magnetic field treatment and ultrasound (US)-assisted heating) [[9], [10], [11], [12], [13]] have been applied to MP to enhance the gelling properties, texture and rheological characteristics of MP gels.

From a practical perspective, the incorporation of hydrocolloids (e.g., carrageenan, flaxseed gum, curdlan, konjac glucomannan, chitosan, xanthan, locust bean gum, sodium alginate, dietary fibre and gelatine) has been considered as the most effective, feasible and economical strategy to enhance the stiffness and water-entrapping capacity of MP gels [14]. Numerous studies have indicated that the various modes of intermolecular interactions between MP and hydrocolloids play a crucial role in determining the quality profiles (e.g., texture, mouthfeel, stability and overall acceptability) of meat products [15,16]. In the review by Wijaya et al. [17] they reported that synergistic interactions between proteins and hydrocolloids in their mixed systems could be used to obtain mixed gels with different properties and structures. Cao et al. [18] investigated the effect of κ-carrageenan (KC) doping mediated by different NaCl concentrations on the rheological behavior and microstructure of MP sols and found that the higher NaCl concentration (0.6 M) promoted the intermolecular interactions between MP and KC, leading to a stronger and tighter intermolecular gel-like network structure of the samples. Such interactions may be non-covalent (e.g., electrostatic interactions, hydrogen bonding, hydrophobic interactions and van der Waals) [19], which are mainly affected by some critical factors, such as concentration, ionic strength, environmental pH, charge density, the ratio of protein to hydrocolloid, conformational structure and temperature [17]. In particular, the protein conformation is often the most important factor affecting the WHC and gel properties of meat products. During the last decade, numerous attempts (e.g., physical, chemical and enzymatic processing) have been widely applied to alter the structure of MP to obtain desired functionalities, which subsequently modulated the interactions between MP and hydrocolloids [20]. For instance, results obtained by Villamonte et al. [21] indicated that high-pressure processing could cause extensive unfolding of the molecular structures of MP, as well as increase the number of exposed sites for interacting with xanthan gum, which promoted the quality profiles of meat products. Wang et al. [22] observed that compared with native MP, the incorporation of common hydrocolloids (e.g., konjac glucomannan, carrageenan or xanthan gum) could significantly enhance the gel strength and WHC of heat-induced phosphorylated-MP gels. In our previous work, we indicated that transglutaminase-induced structural modification (covalent cross-linking) promoted the physical entanglement between meat protein and κ-carrageenan (KC), which greatly enhanced the gel properties of meat protein gel after heating [4]. Thus, appropriate structural modification or exposure of active residues could effectively promote the interactions between MP and hydrocolloids, leading to an improvement in the functional properties (e.g., solubility, emulsifying properties and gelation) of MP.

Most recently, US treatment, which is considered a green, safe, superior, efficient and emerging non-thermal technology, has been widely applied in the meat industry to improve the quality profiles and sensorial attributes of the final meat products [23]. Due to its acoustic cavitation effect, previous studies have verified that the US could effectively alter the conformational properties of MP and subsequently promote the functional properties of MP during various processing procedures in the meat industry. For instance, there were numerous studies demonstrating that US treatment could enhance protein-protein molecular interactions and change the secondary or tertiary structure of MPs, resulting in the enhancement of functional properties (e.g., gel properties, rheological properties, etc.) of MPs [12,24,25]. Meanwhile, it had been demonstrated that US pretreatment combined with carrageenan had a synergistic effect on the gelling properties of transglutaminase-mediated MP gels, improving the formation of dense and interpenetrating gel networks to produce composite MP gels with optimal gel strength [26]. Zhao et al. [27] also found that the combination of US treatment and polysaccharide addition resulted in more evenly dispersed agglomerates and higher viscosity of the meat paste system, which improved the gelling properties of low-salt chicken products.

As a typical linear hydrophilic polysaccharide, KC has widespread applications in meat processing, which can effectively improve the gelling properties and structural characteristics of the final product. In our previous study, we found that the addition of 0.2 % KC enhanced the rheological behavior and gelling properties of MP [18]. In this context, we hypothesized that there was a synergy between US treatment and the addition of KC. US-induced structural and physical modifications might affect the molecular interactions between MP and KC, leading to superior gelling properties of MP gels than those treated with US or added KC alone. However, to date, little information is available on optimizing US parameters and elucidating synergistic mechanisms between US and hydrocolloids to improve gel properties. Therefore, the present work aimed to explore the combined effects of US treatment and KC addition on the gel properties of heat-induced MP gels, as well as the rheological behavior of MP during heating and cooling treatments. In addition, the US parameters were optimized and the mechanism by which US synergizes with KC to improve the MP gel properties was elucidated, hopefully providing an effective strategy for improving the final quality of meat products in practical production.