Influence of ultrasound-assisted tumbling on NaCl transport and the quality of pork

Highlights

• •Ultrasound-assisted tumbling (UAT) treatment significantly increased the NaCl content in pork.
• •The applied NaCl transfer model could accurately describe the NaCl diffusion coefficient.
• •Moderate UAT treatment improved the tumbling yield, water-holding capacity, and textural properties of pork.
• •Moderate UAT treatment facilitated the protein extraction.
• •Excessive UAT treatment led to protein denaturation.

Abstract

The present study aimed to investigate the impact of ultrasound-assisted tumbling (UAT; 20 kHz, 100, 300, 500 and 700 W) with different treatment time (30, 60, 90 and 120 min) on the diffusion and distribution of NaCl as well as the change of pork texture properties during curing. Results showed that in comparison with the single tumbling (ST), the NaCl content and the NaCl diffusion coefficient were increased along with UAT treatment (P < 0.05). The scanning electron microscopy and the energy dispersive X-ray analysis showed that UAT treatment changed the microstructure of pork which may facilitate the NaCl dispersion homogeneously. In addition, the moderate UAT treatment of 300 W with 60 min could significantly improve the tumbling yield, water-holding capacity and textural properties of pork compared with the ST treatment (P < 0.05). Meanwhile, in comparison with the ST group, protein extraction was considerably increased after UAT (300 and 500 W) treated for 120 min (P < 0.05). Our study demonstrated that UAT treatment could effectively promote the penetration and distribution of NaCl and improve pork meat quality via facilitating the extraction of meat protein.

1. Introduction

Curing has been used for preserving food for many years. Among curing ingredients, NaCl is the most important one that could inhibit microbial growth and improve the flavor, texture and other quality attributes of food [1]. The traditional curing approaches, including dry-curing and wet-curing, have been proved to consume too much time in the practical process [2]. Thus, the industry has sought novel process methods for accelerating the curing efficiency of food products [3]. Among which tumbling has been revealed to have positive effects on the curing of meat and meat products [4], [5]. When meat samples are subjected to tumbling, they could be beaten by the tumbler paddles and rubbed through the meat-meat and meat-apparatus. These mechanical effects could disrupt meat cells and facilitate brine distribution into meat [6]. Besides, Gao et al. [5] revealed that tumbling could extract myofibrillar proteins and form protein exudate covered on the meat surface. Consequently, the protein exudate could play the role of “glue” to bind meat pieces, thus improving the sliceability, texture, tenderness, and cohesion of meat and meat products [7]. However, Li et al. [8] pointed that the present single tumbling (ST) is time-consuming and potentially causes uneven distribution of NaCl. Even though the ST could enhance the meat production efficiency compared with the traditional curing method, it could still take 10–16 h when used interval ST [9].

High-intensity ultrasound (US, 20–100 kHz, 10–1000 W/cm²) has been considered as an innovative and green technology applied in mass transport [10], and US could well shorten the curing time and improve the meat quality [11], [12]. Kang et al. [13] used US with different ultrasonic intensities (2.39, 6.23, 11.32, 20.96 W/cm²) and observed that US could significantly increase the final NaCl content compared with static immerse curing. Inguglia et al. [14] found that in chicken breast marination, US treatment (25, 45, 130 kHz) for 6 h could reach a similar NaCl concentration as the control treatment for 16 h (without US). The effects of US could be attributed to the following mechanisms: (1) when US travels through a liquid medium, it causes microturbulence and microstirring in brine; (2) the microjets which result from ultrasonic cavitation could cause erosion on food surface and disruption in meat cell; (3) the sinusoidal pressure of ultrasonic wave could cause meat matrix compression and expansion (“sponge effect”) and thus generate microchannel [15], [16]. All the above effects could diminish the external and internal resistance of meat during the curing process, thus accelerating brine penetration into the meat and enhancing mass transfer. However, no published study has investigated the effects of ultrasound-assisted tumbling (UAT) on meat curing. Thereby, our work aimed to research the impact of UAT treatment during curing on the diffusion and distribution of NaCl and the quality of pork.

2. Materials and methods

2.1. Meat sample and brine preparation

Pork mesoglutaeus was obtained from Sushi Meat Processing Company (Huai'an, Jiangsu province, China) after 24 h of slaughtering. The pH was detected using Consort C831 pH-meter (Consort N.V., Turnhout, Belgium) paralleling to the direction of muscle fiber and the pH of meat ranged from 5.6 to 5.8 was selected for the experiment. After all the visible fat and connective tissue were trimmed, the meat samples were cut into cuboids (100 × 40 × 40 mm³) and vacuum packaged in a plastic bag. The packaged meat samples were kept at −20 °C and thawed completely at 4 °C before the experiment. The brine used for tumbling contained 0.34% (w/v) sodium tripolyphosphate, 0.17% (w/v) sodium pyrophosphate, 0.34% (w/v) sodium hexametaphosphate and 8.6 % (w/v) NaCl. The prepared brine was cooled at 4 °C.

2.2. Tumbling treatment

The tumbling treatment was operated by the redesigned tumbling equipment which was reported in our previous study [17]. For the ST treatment, the brine and meat samples were poured into the tumbling drum at the ratio of 35:100 (v/w). The tumbling parameters were set as follow: the vacuum in the inner tumbling drum was −0.08 MPa, the temperature was 4 °C, the tumbling drum was tilted 35° relative to the horizon, the rotation velocity was 10 rpm, and the tumbling mode was intermittent tumbling with 20 min of work and 10 min of rest in a cycle. The tumbling cycle was set with 1, 2, 3, and 4 times, respectively. Thus, the corresponding total tumbling time was 30, 60, 90 and 120 min, respectively.

The UAT treatment was used with the same tumbling parameters as ST treatment, particularly the work and the rest of the ultrasound unit were set to be synchronized with tumbling. Therefore, the UAT treatment time was 30, 60, 90 and 120 min, respectively. In addition, the ultrasonic power was set as 100, 300, 500 and 700 W, respectively.

2.3. NaCl content

The NaCl content of each sample was determined in triplicate using a digital salt meter (ES-421, ATAGO, Tokyo, Japan) according to the method of Contreras-Lopez et al. [12] with a slight change. After the experimental treatment was completed, all samples were immediately taken out of the tumbler drum and the adhered surface brine was washed with deionized water. It is worth noting that meat samples were cut 1 cm strip from every side to eliminate the edge effects [18]. Accurately, 5 g samples were cut from the center of the meat to the surface. Then, these samples were diluted by 5 times using deionized water and homogenized (PD 500-TP, Prima Technology Group Co., Ltd., UK). After that step, the homogenate was placed for 1 h to dissolve aqueous extracts and then the supernatant was placed in the sample cell. The actual NaCl content was expressed as g/100 g meat.

2.4. Determination of NaCl diffusion coefficient

The NaCl diffusion coefficient was determined based on Fick's second law, where the equation (1) was according to Siró et al. [19].

$$\frac{C_{s,0}-C_{s,t}}{C_{s,0}-C_{s,eq}}=1-\sum _{n=0}^{\infty }{\frac{8}{{\left(2n+1\right)}^2{\pi }^2}e}^{\frac{-D_s{\left(2n+1\right)}^2{\pi }^{2}t}{L^2}}$$ (1)

where C_s,0 is the initial NaCl content (g/100 g) in pork, C_s,t represents the NaCl content at a given time t (s), C_s,eq indicates the NaCl content at brining equilibrium. D_s corresponds to the NaCl diffusion coefficient (m²/s) and L is the thickness of meat samples (m).

The percentage of explained variance (% VAR) was used to evaluate the fitness of the model by equation (2)

$$\%VAR=1-\frac{S_{\mathit{est}}^2}{S_{\mathit{sam}}^2}$$ (2)

where S²_est and S²_sam are the variance of the estimation and sample, respectively.

2.5. Scanning electron microscopy (SEM) with energy dispersive X-ray analysis (SEM-EDX)

The microstructure of meat samples was observed by SEM with an accelerating voltage of 3 kV (SU8010, Hitachi, Tokyo, Japan). The slices of meat samples (1 × 3 × 3 mm³) was cut from 1 cm below the meat surface and fixed in 2.5% (v/v) glutaric aldehyde acid. Then the slices were operated by a series of post-fixation, washing and dehydration according to Wang et al. [20]. As for the SEM–EDX spectrometric analysis, the slice samples were fixed by oven drying instead of being immersed in 2.5% (v/v) glutaric aldehyde acid to prevent NaCl dissolving. Then the X-ray detector (X-Max 80, Oxford, UK) was used for observing the distribution of Na⁺ and Cl⁻ inside the meat samples.

2.6. Tumbling yield

The tumbling yield was calculated according to Steen et al. [7] using the following equation (3):

$$\mathit{Tumbling}yield=\frac{w_1-w_0}{w_0}\times 100\%$$ (3)

where w₀ and w₁ refer to the weight of meat samples before and after tumbling, respectively.

2.7. Cooking loss

The cooking loss of meat samples was determined following the methods of Inguglia et al. [14], [21] with some changes. Briefly, after meat samples were cut off 1 cm strip, the new cuboid was cut into 5 × 3 × 2 cm³ and cooked at 90 °C until the center temperature reached 72 °C. The cooking loss was calculated by the percentage of weight loss in relation to the initial weight before cooking.

2.8. Protein concentration in the brine

The methods of protein determination were followed by McDonnell et al. [22] with a slight change. After tumbling, the brine was collected and centrifuged at 10,000×g for 10 min at 4 °C to precipitate meat crumbs (Beckman Avanti J-E, Beckman Coulter, Fullerton, CA, USA). The protein content of the supernatant was measured by the Biuret method with the BSA as the standard protein [23].

2.9. Texture profile analysis (TPA)

The samples after the cooking loss analysis were cut into cubes (1 × 1 × 1 cm³) for TPA (TA XT Plus, Stable Micro Systems, Surrey, UK) according to Zou et al. [24]. The cylindrical probe was selected as P50, the compression rate was modified to 75%, and the test, pre-test, and post-test speed was 1 mm/s, 5 mm/s, and 5 mm/s, respectively. Five samples of each treatment replicate were determined for hardness, springiness, cohesiveness, and chewiness.

2.10. Statistical analysis

The software of SAS 9.2 was used. The main effects of US power, treatment time, and their interactions on the NaCl content, tumbling yield, cooking loss, protein content, and TPA indexes were analyzed via Fisher's LSD (Least Significant Difference) with Bonferroni correction. As for the NaCl diffusion coefficient, it was analyzed through one-way ANOVA and Duncan's multiple range test. The results were exhibited as means ± standard error. The significant difference was considered when P < 0.05.

3. Results and discussion

3.1. NaCl content

The NaCl content of meat is the vital index to assess the degree of meat brining, and the influence of different treatments on NaCl content is presented in Table 1. The US power and treatment time obviously influenced the NaCl content (P < 0.05), while their interaction had no significant impact on NaCl content (P > 0.05). Notably, at the treatment time of 30 min, the NaCl content of UAT groups (300, 500 and 700 W) was apparently higher than that of ST groups (P < 0.05), reaching the highest value at UAT-700 W group. Similarly, at the treatment times of 60, 90 and 120 min, a considerable enhancement in NaCl content was observed after UAT treatment (500 and 700 W). These results indicate that UAT treatment could accelerate the diffusion of NaCl into meat compared with ST treatment. Thus UAT treatment had the potential to shorten curing time. The superiority of UAT treatment might be due to the ultrasonic cavitation, which produces the microjets attacking myofibrils and thus generating microchannels in the meat boundary, which is conducive for NaCl to penetrate into the meat [25]. Our results are consistent with previous studies which have proved the positive effects of the US on meat brining. Mcdonnell et al. [11] used the US (4.2, 11 and 19 W cm⁻²) for pork brining and found that US-assisted brining groups significantly gained more NaCl content than control groups.

Table 1.

Effect of different tumbling methods on NaCl content (g/100 g) of pork.

	Treat time (min)		Ultrasound power (W)				SE	P-Values
		ST	UAT-100	UAT-300	UAT-500	UAT-700		Power	Time	Power × Time
NaCl Content (g/100 g)	30	0.90cz	0.93cz	1.13by	1.20by	1.39ay	0.15	<0.001	<0.001	0.986
	60	1.15by	1.17byz	1.39abxy	1.51ax	1.59ax
	90	1.31bx	1.36bxy	1.49abx	1.61ax	1.65ax
	120	1.37bx	1.53abx	1.58abx	1.70ax	1.75ax

¹P-Values indicate the level of significance including high-intensity ultrasound power, treatment time and their interaction.

² SE: standard error.

³ a-d in the same row indicate a significant difference between ultrasound power (P < 0.05).

⁴ x-z in the same column indicate significant difference between treatment time (P < 0.05).

3.2. NaCl diffusion

NaCl diffusion coefficient could reflect the intrinsic dynamics of NaCl transfer. As shown in Table 2, all %VAR of the treatments was more than 95% suggesting great fitness for the experimental NaCl kinetics and NaCl transfer model. During curing, the physical properties of meat tissue could be changed [26]. Thus, the pseudo-binary system (solute-tissue) is used in determining the NaCl diffusion coefficient in meat tissue based on Fick's second law [26]. In this case, meat samples are regarded as a slab that could keep a stable shape during curing without any shrinkage, the mass transport is unidirectional, and the NaCl diffusion coefficient is a constant value [13]. Many researchers have modified Fick's second law for different immerse curing conditions [27], [28]. However, to the best of our acknowledge, only one study from Siró et al. [19] used equation (1) to evaluate the NaCl diffusion coefficient when meat samples were treated by the ST treatment. Nevertheless, it is unclear whether equation (1) could describe the NaCl diffusion coefficient in UAT treatment. In this study, the high %VAR indicated that the equation (1) was well adequate to describe the NaCl diffusion coefficient in UAT treatment. In terms of the NaCl diffusion coefficient, the UAT treatment owned the higher value than that of the ST treatment. Moreover, the NaCl diffusion coefficient was notably increased with the increase of US power (P < 0.05) and reached the highest value in the UAT-700W group. The enhancement of the NaCl diffusion coefficient is consistent with the result of NaCl content. When the cavitation bubbles asymmetric collapse near the meat surface, it could generate the microjets and possess instantaneously high speed (100 m/s), thus destroying meat tissue [15]. Besides, the turbulence and agitation produced by ultrasonic waves could further enhance the NaCl transfer.

Table 2.

Calculated diffusion coefficients of NaCl in pork at 4 °C for various tumbling methods.

Tumbling Methods	Diffusion coefficients (10−8 m2/s)	%VAR
ST	2.23 ± 0.51d	97.99
UAT-100	2.69 ± 0.62d	99.63
UAT-300	3.66 ± 0.88c	98.91
UAT-500	4.98 ± 0.60b	98.83
UAT-700	7.54 ± 0.99a	97.24

a-d indicate a significant difference between different groups (P < 0.05).

3.3. SEM micrographs and SEM-EDX analysis

As shown in Fig. 1, the muscle fibers of untreated meat were intact and compact, which was the typical microstructure of pork meat. Compared with untreated meat, meat samples subjected to ST treatment showed that the connective tissue fibers were slightly disrupted and dispelled. However, the gap between fibers was still small. As for the UAT-300 W group, the muscle fibers were significantly damaged. With the destruction of the perimysium structure, some obvious microfissures were identified in myofibrils. Pan et al. [29] reported similar results that ultrasonic cavitation could cause microholes on myofibrils of pork because the microjets collided with the meat tissue. The destruction of myofibrils could reduce the external resistance of mass transfer, which was beneficial for NaCl diffusion. In addition, the more significant gap between muscle fibers was generated in the UAT-300 W group which could further allow more brines to be penetrated into the meat. However, when meat samples were treated by excessive UAT treatment (700 W), some granular assemble appeared on the muscle fibers. According to Kang et al. [30], the US could decompose water molecular and generate highly reactive free radicals which could oxidize meat proteins and cause protein aggregation. Siró et al. [19] also reported that cavitation could produce localized microscale heat leading to protein denaturation. Therefore, the change of microstructure under excessive UAT treatment might be related to protein denaturation. In addition, the connective tissue changes need to be further investigated to more specifically evaluate the effect of UAT treatment on the microstructure of meat.

Fig. 1.

Longitudinal section observed by SEM of mesoglutaeus muscle of pork tumbling for 60 min with different tumbling methods (magnification, 1000×): A: untreated meat; B: ST; C: UAT-300; D: UAT-700. CV: impact of cavitation bubble erosion on myofibrils.

The SEM-EDX mapping images showed the distribution of NaCl of different tumbling treatment groups (Fig. 2) with the yellow dot representing the Na⁺ and the red dot indicating the Cl⁻. Compared with the untreated meat, more dots of Na⁺ and Cl⁻ in ST treatment suggested that ST treatment could promote brine absorption. Regarding the UAT treatments, as the US power increasing, both the dots of Na⁺ and Cl⁻ were distributed more uniformly and the number of dots was increased. The results illustrated that UAT treatment could remarkably promote the NaCl penetration and dispersion evenly in meat compared with the ST treatment. Additionally, the results further proved that UAT treatment could lead the meat samples to gain more NaCl during tumbling, thus improving the curing effect of tumbling.

Fig. 2.

Effect of different tumbling methods on NaCl dispersion of pork. Longitudinal section observed by SEM-EDX of mesoglutaeus of pork meat tumbling for 60 min with different tumbling methods.

3.4. Tumbling yield

The tumbling yield could reflect the bine absorption capacity of pork. As shown in Table 3, the US power, treatment time, and their interaction had a significant impact on the tumbling yield (P < 0.05). At the treatment time of 60 min, the tumbling yield had no significant difference between the ST group and UAT-100 W group (P > 0.05), which might be due to that the US power of 100 W was lower than the threshold. When US power was at 300 W and 500 W, the tumbling yield was considerably enhanced in comparison with the ST group (P < 0.05). On the contrary, when US power was at 700 W, the tumbling yield was decreased compared with that of the UAT-300 W group. Similarly, at the treatment time of 120 min, compared with the ST group, the change of tumbling yield was increased at the US power 300 W and 500 W (P < 0.05), and then decreased at the US power of 700 W (P < 0.05). The increase of tumbling yield indicated that moderate UAT treatment could promote brine uptake. According to the results of SEM-EDX, once the moderate UAT treatment was applied to the meat, more NaCl entered the meat. The Cl⁻ could in turn bind to the side chain of myosin and increase the electrostatic repulsive force between myofibril filaments [31]. Consequently, the interfilament spaces were increased and allowed more brine uptake [19]. In addition, the “sponge effect” also expanded the interfilament spaces and caused microchannels on interfaces which was also conducive for brine absorption [32].

Table 3.

Effect of different tumbling methods on tumbling yield (%), cooking loss (%) of pork and protein content (mg/mL).

	Treat time (min)		Ultrasound power (W)				SE	P-Values
		ST	UAT-100	UAT-300	UAT-500	UAT-700		Power	Time	Power × Time
Tumbling Yield (%)	60	9.28by	10.17bx	13.39ax	13.22ay	9.25by	0.77	<0.001	<0.001	0.0044
	120	12.79cx	11.88dcx	14.24bx	17.55ax	11.20dx
Cooking loss (%)	60	22.04ax	20.95abx	19.71bcx	18.93cx	15.05dx	1.21	<0.001	0.0027	0.0018
	120	18.93ay	18.15aby	17.68abx	16.55by	17.65abx
Protein Content (mg/mL)	60	7.91aby	9.36ax	8.20aby	8.92ay	6.50by	0.62	0.013	<0.001	0.073
	120	9.84bx	10.04bx	10.92ax	11.38ax	10.08bx

¹P-Values indicate the level of significance including high-intensity ultrasound power, treatment time and their interaction.

²SE: standard error.

³a-d in the same row indicate a significant difference between ultrasound power (P < 0.05).

⁴x-z in the same column indicate significant difference between treatment time (P < 0.05).

On the other hand, there are several mechanisms for the reduction of tumbling yield under the highest US power treatment. McDonnell et al. [25] pointed out that the extracellular spaces turned to be more viscous as the solubilization of myosin which could impede water uptake. In addition, the direction of water flow was dependent on the structure of meat tissue [28]. As the SEM micrographs showed, the microstructure of meat samples under the UAT-700 W treatment became granular aggregation which might prevent water transfer. Graiver et al. [26] found similar images when pork meat was immersed curing in 330 g/L NaCl solution. Under this NaCl concentration, they also observed that the water-holding capacity (WHC) showed more water loss rather than water uptake.

3.5. Cooking loss

Table 3 shows the cooking loss of various groups applied by different US power and treatment time. The US power, treatment time, and their interaction notably impacted the cooking loss (P < 0.05). Particularly, compared with the ST group, the cooking loss was considerably reduced when the US power ranged from 300 W to 700 W for 60 min (P < 0.05), and the cooking loss was significantly decreased along with the increase of US power (P < 0.05). Similarly, when meat samples were treated for 120 min, the cooking loss was significantly decreased at the group of UAT-500 W compared with that of the ST group (P < 0.05). On the contrary, when meat samples were treated by UAT-700 W, the cooking loss was subsequently increased with no significant difference with the ST group (P > 0.05). Therefore, the reduction of cooking loss suggested that the moderated UAT treatment could improve the WHC of pork meat. The previous study by Zou et al. [24] has observed similar results that the US-assisted cooking treatment improved the WHC of beef. Furthermore, our previous study has revealed that the US could exert the cavitation force to reduce the protein particle size and expose the active region, which increased the protein-water interaction to bind more water thus improving the WHC [17].

3.6. Protein content

The tumbling treatment could cause cellular disruption in muscle tissue by applying mechanical energy [33] and facilitating the extraction of the meat proteins to form protein exudates [34]. The total protein extracted from meat to brine is shown in Table 3. Both the US power and the treatment time significantly impacted the protein content (P < 0.05), while the interaction of US power and treatment time had no significant effect on the protein content (P > 0.05). It was apparent that the protein content was considerably increased when UAT treatment was applied (300 and 500 W) for 120 min compared with ST treatment (P < 0.05). However, the protein content of the UAT-700 W group was oppositely reduced to a lower level than that of the UAT-500 W group regardless of the treatment time (P < 0.05). The brine in this study did not contain any protein before usage. Thus, the increase of protein content in brine indicated that moderate UAT treatment could further promote protein extraction and solubilization to brine compared with ST treatment. The results are consistent with McDonnell et al. [22], who employed a series of ultrasonic intensities (4.2, 11, and 19 W cm⁻²) to salting pork meat and found that the protein content in brine was increased with the ultrasonic intensity increase. In addition, as the NaCl content in the UAT group increased, the higher ionic strength could increase myofibrillar proteins extraction, which could bind more external water and form the protein gel to retain water during cooking [35]. Therefore the results of protein content in brine are consistent with the results of tumbling yield and cooking loss. Also, the reduction of protein content under excessive UAT treatment confirmed the occurrence of protein denaturation.

3.7. Texture profile analysis (TPA)

Table 4 shows the TPA indexes of hardness, springiness, cohesiveness and chewiness of pork meat after various treatments. The US power and treatment time had a significant impact on these indexes (P < 0.05), while the interaction of US power and treatment time only significantly influenced the cohesiveness and chewiness (P < 0.05). As for the hardness, at the short treatment time of 60 min, the hardness of each UAT group was higher than that of ST group (P < 0.05). However, only the UAT-500 W group had higher hardness than the ST group when meat samples were treated for 120 min. Moreover, the excessive UAT treatment (700 W; 120 min) significantly reduced the hardness compared with the UAT-500 W treatment. The increase of hardness under moderate UAT treatment might be related to the fact that the extracted myofibrillar proteins could form a compact thermal-induced gel network which could improve the hardness. Zhang et al. [36] also implied that the moderate US could aid myofibrillar protein gel to form a uniform and dense network, thus strengthening the gel properties. As the result of protein content showed (Table 3), moderate UAT treatment could improve the myofibrillar protein extraction which allowed the formation of protein gel with great structure during cooking, thus improving the hardness of pork meat. In addition, the protein content also proved that excessive US could cause protein denaturation which partly explained the decrease of hardness under long treatment time and excessive UAT treatment.

Table 4.

Effect of different tumbling method on the TPA profile of pork.

	Treat time (min)		Ultrasound power (W)				SE	P-Values
		ST	UAT-100	UAT-300	UAT-500	UAT-700		Power	Time	Power × Time
Hardness (N)	60	105.06cy	112.77bx	113.55bx	119.29ay	116.11abx	3.16	<0.001	0.11	<0.001
	120	114.64bx	115.03bx	112.21bx	125.27ax	90.52cy
Springiness	60	0.54cdy	0.58bx	0.61ax	0.56cx	0.53dx	0.014	<0.001	0.25	<0.001
	120	0.59ax	0.59ax	0.53by	0.54bx	0.53bx
Cohesiveness	60	0.52dy	0.56bx	0.58ax	0.56bcx	0.54dcx	0.011	<0.001	0.027	<0.001
	120	0.56ax	0.54bcy	0.55abx	0.53cdx	0.52dy
Chewiness (N)	60	25.45cy	32.06bx	36.84ax	32.88bx	30.89bx	1.35	<0.001	0.0046	<0.001
	120	30.00bx	30.01bx	30.84by	33.77ax	25.79cy

¹P-Values indicate the level of significance including high-intensity ultrasound power, treatment time and their interaction.

²SE: standard error.

³a-d in the same row indicate a significant difference between ultrasound power (P < 0.05).

⁴x-z in the same column indicate significant difference between treatment time (P < 0.05).

Compared with the ST group, the change in springiness was on an upward trend when treated by UAT treatment, especially in UAT-100 W and UAT-300 W for 60 min. However, the long treatment time (120 min) combined with strength UAT treatment (300–700 W) declined the springiness. In addition, the trends of cohesiveness and chewiness coincided well with the springiness. In short, compared with the ST group, the mild UAT treatment (100–500 W) accompanied by a short treatment time (60 min) could significantly increase the cohesiveness and chewiness (P < 0.05). In comparison, the excessive UAT (700 W) treated meat samples for a long time (120 min) could obviously decrease those indexes (P < 0.05). This phenomenon might be due to the increased content of myofibrillar proteins under moderate UAT treatment ensured enough formation of protein-protein interaction which could improve the texture of protein gel [37] and the protein denaturation under strength UAT treatment might deteriorate the protein gel [38], thus decreasing the springiness, cohesiveness and chewiness.

4. Conclusion

In this study, the gradual enhancement of NaCl content and the NaCl diffusion coefficient after UAT treatment implied that UAT treatment could accelerate the NaCl diffusion into pork. Furthermore, the images of SEM-EDX identified that compared with ST treatment, UAT treatment was able to facilitate the NaCl to enter into the meat and distribute uniformly. The results of SEM explained the advantage of UAT treatment that moderate ultrasonic cavitation (300 W) could destroy the meat fibers and enlarge the interfilament spaces which could uptake more brine, thus significantly increasing the tumbling yield. However, the SEM images showed that excessive UAT treatment could cause the formation of granular aggresome. In addition, the results of protein content in the brine indicated that moderate UAT treatment could improve the extraction of protein which further resulted in the improvement of WHC and texture profile. Therefore, UAT treatment could enhance the curing efficiency and improve the WHC and texture profile under moderate US conditions.

CRediT authorship contribution statement

Ruyu Zhang: Conceptualization, Data curation, Methodology, Software, Formal analysis, Writing – original draft, Writing – review & editing, Investigation. Jian Zhang: Data curation; Methodology. Lei Zhou: Writing – review & editing, Data curation; Methodology. Lin Wang: Data curation; Methodology. Wangang Zhang: Supervision, Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.