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Ultrason Sonochem
Ultrason Sonochem
Ultrasonics Sonochemistry
1350-4177
1873-2828
Elsevier

S1350-4177(24)00276-1
10.1016/j.ultsonch.2024.107028
107028
Original Research Article
Evaluation of ultrasound-assisted tomato sour soup marination on beef: Insights into physicochemical, sensory, microstructural, and flavour characteristics
Zheng Huaisheng a
Li Lilang bc
Huang Chaobin a
Liu Shuhong a
Chen Xinghua a
Wang Xiaoyu d
Hu Ping phu1@gzu.edu.cn
a⁎
a School of Liquor and Food Engineering, Guizhou University, Guiyang 550025, China
b State Key Laboratory of Functions and Applications of Medicinal Plants, Guizhou Medical University, Guiyang 550014, China
c Natural Products Research Center of Guizhou Province, Guiyang 550014, China
d College of Life Science, Guizhou University, Guiyang 550025, China
⁎ Corresponding author. phu1@gzu.edu.cn
13 8 2024
11 2024
13 8 2024
110 10702822 5 2024
10 7 2024
13 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
This study evaluated the quality attributes of tomato sour soup marinade and investigated the effects of ultrasound-assisted marination on the physicochemical properties, microstructure, texture, sensory quality, and flavour profile of beef. The results showed that tomato sour soup significantly increased the marinade absorption rate and improved beef tenderloin’s physicochemical properties, texture, and flavour attributes compared to static brine (P < 0.05), with organic acids playing an essential role in the marinade tenderisation process. Compared to static sour soup marination, ultrasound treatment significantly accelerated the marination process, reducing beef’s shear force, hardness, and chewiness while increasing its tenderness. Microstructural observations revealed that sour soup marination induced a fragmented and irregular muscle fibre structure. Furthermore, sour soup marination significantly increased the relative concentrations of volatile flavour compounds, including alkanes, organic sulphides, alcohols, aldehydes, and aromatic compounds. Appropriate ultrasound treatment positively affects the texture and flavour characteristics of beef marinated with tomato sour soup, and the optimal approach was 320 W ultrasound treatment for 60 min. Overall, tomato sour soup improved beef’s textural and flavour attributes, while ultrasound-assisted marination is an effective processing method to improve the quality of meat products.

Keywords

Tomato sour soup
Ultrasound
Beef
Physicochemical attributes
Flavour
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pmc1 Introduction

Marination is an effective means of improving the organoleptic quality and culinary properties of meat, as well as being an essential method of extending the shelf life of foods and affecting the microbiological properties of meat [1]. Seasoned or flavoured mixtures of components are used to improve and enhance the flavour, colour, texture, tenderness, juiciness, and palatability of meat and meat products, including mineral salts, organic acids, chemical tenderisers, wine, vinegar, essential oils, pickle juice [1], [2]. The marinade composition and pH affect the physicochemical properties of the meat. Traditional marination methods, such as adding ions (sodium and phosphate), improve the water-holding capacity and tenderness of the meat by increasing the binding of proteins and ions [3].

In recent years, marinades derived from natural ingredients have gained attention in the field of meat marinating, as they have shown potential and broad prospects [1], [4]. Marinades based on natural ingredients are rich in antioxidants, such as phenolic acids and flavonoids in the extracts, which can inhibit lipid peroxidation and protein oxidation, reducing microbiological and safety risks in meat products by quenching or scavenging free radicals, thus improving the safety and shelf life of meat products [5]. Many reports have shown that marinated ingredients （phenolic acids） exert various beneficial effects, such as anti-inflammatory, antibacterial, anti-allergic, and anti-thrombotic [2]. Marinades made from homemade fermented pickle juice [6], citrus juice [7], organic fruit vinegar [8], [9], wine [10], beer [11], buttermilk, acid whey, and essential oils extracted from plants enhance the sensory quality of meat, improve its cooking characteristics, effectively inhibit the growth of food-borne pathogens, and prolong the shelf life and safety of meat and meat products [1], [2]. A variable pressure-assisted immersion process was used to treat meat by adding 0.2 mM/L (−)-epicatechin. This treatment effectively inhibited the formation of polycyclic aromatic hydrocarbons without altering the flavour characteristics of roasted beef [12]. Moreover, organic acids in marinades also significantly impact meat quality, enhancing colour development and suppressing microbial growth in marinated meat [13]. The addition of organic acids (acetic and citric acid) to pork meat inhibited the heat-induced formation of advanced glycation end-products (Nɛ-carboxymethyllysine and Nɛ-carboxyethyllysine) [14]. Furthermore, compared to single organic acids, complex organic acids exhibited a softer flavour and enhanced antimicrobial effects [15].

Tomato sour soup(TSS) is a traditional Chinese vegetable fermented food of the Miao nationality. TSS is rich in organic acids, as well as vitamins, lycopene, minerals, and phenolic compounds, which is regarded as a functional food, including anti-fatigue, intestinal flora regulation, immunomodulatory function and nonalcoholic fatty liver disease prevention [16], [17]. Originally, sour soup’s distinctive sour and spicy flavour was used as a substitute for salt in Guizhou Province, China, due to the region’s limited salt production and remote geographical location. Today, its unique flavour and health benefits have made TSS an increasingly popular seasoning and base for meat, fish and vegetable dishes [18]. TSS, with its organic acids, phenolic components, vitamin C, lycopene, and other bioactive compounds, not only enhances the flavours of foods but also has significant antimicrobial and antioxidant effects. Therefore, it could be speculated that TSS is a potential meat-curing ingredient that can improve the quality and flavour of marinated meat.

Marination is a slow process, and the rate of marinade penetration can be influenced by various factors, including the medium, marinade composition, and concentration [1]. Additional treatments exemplified by tumbling, injection, high pressure, and ultrasound are commonly used to enhance the marinating process. In particular, as a non-thermal physical tenderising technology, ultrasound has been widely used in the curing process of meat products for its high efficiency, cost-effectiveness, and environmental friendliness, accelerating the mass transfer process and reducing curing time by inducing cavitation, which breaks down muscle fibres, cell membranes, and connective tissue [19], [20]. Meanwhile, ultrasonic treatment can significantly improve the meat products’ tenderness, water-holding capacity, ionic permeability, and flavour [21]. Bai et al. found that ultrasound facilitated the widening of the intermyofibrillar spaces during the marination process of sea bass, enhanced the penetration of NaCl, shortened the marination duration, and increased the presence of volatile flavour compounds such as aldehydes and esters, thus elevating the quality of marinated sea bass meat [22]. However, TSS has a thicker consistency and can be unevenly distributed throughout the marinade system, affecting the consistency and stability of the curing solution, causing it to adhere to the surface of the meat, thereby requiring a large number of marinades and time for the marinade to penetrate the meat product thoroughly. Therefore, we are exploring the potential of ultrasonic treatment as a marinating technique that dramatically enhances the meat’s marinating rate and plays a vital role in improving the quality and flavour of the meat post-marination.

To date, numerous studies have been carried out on the effects of sour soup fermentation on processing, quality, and flavour. In contrast, limited research has been conducted on the use of sour soup in meat processing, particularly as a curing liquid. Naturally occurring compounds such as organic acids and phenolic compounds in sour soup marinade ingredients are acceptable to consumers. Ultrasound treatment can break down muscle fibres through cavitation and improve the permeability of the marinade, making it a promising technique for reducing marinade time and improving meat quality. This study aimed to explore the effects of sour soup combined with ultrasound-assisted marination on the efficiency and quality properties of beef, including microstructure, tenderness, texture, and flavour, as well as a potential way to achieve “clean-label” of meat products and meet consumer demand [23]. Additionally, the study sought to develop an innovative and effective method for marinating with sour soup. Overall, by expanding the use of traditional sour soup fermented products, this research provides a theoretical foundation and novel recommendations for the diversified application of sour soup.

2 Materials and methods

2.1 Materials

The fresh beef tenderloin (Guizhou local yellow beef breed, China) was supplied by Paoshanlai Agricultural Products Sales Co., Ltd. (Guizhou, China), shipped to the laboratory in ice boxes, and visible connective tissue and fat were removed for subsequent use. Fresh tomatoes and other edible raw materials were purchased from the local farmers market (Huaxi, Guiyang, China). De Man Rogosa Sharpe (MRS) broth was purchased from Bo Microbial Technology Co., Ltd. (Shanghai, China). 2,2-Diphenyl-1-picrylhydrazyl (DPPH), 2,2′-casino-bis (3-ethylbenzothiazoline-6-sulfonate (ABTS), gallic acid, rutin, and lactic acid, acetic acid, malic acid, citric acid, tartaric acid, succinic acid, oxalic acid standards (purity ≥ 98 %) were obtained from Beijing Solarbio Science & Technology Co., Ltd. (Beijing, China). Chromatography-grade methanol was obtained from SEPSERV Berlin Analytik GmbH. Folin-Ciocalteu reagent (1 mol/L) and 2,6-dichloroindophenol were purchased from Shanghai Yuanye Biotechnology Co., Ltd. (Shanghai, China). Other chemicals and reagents were bought from Chengdu Jinshan Chemical Reagent Co., ltd. (Chengdu, China), including N-(1-Naphthyl) ethylenediamine dihydrochloride, potassium persulfate (K2O8S2), sodium hydroxide (NaOH), aluminium nitrate (Al(NO3)3), sodium nitrite (NaNO2), sodium carbonate (Na2CO3), potassium dihydrogen phosphate (KH2PO4). All chemicals used in this study were at least analytical reagent.

2.2 Methods

2.2.1 Preparation of tomato sour soup

The tomato sour soup was fermented according to the team’s previous research methods [17]. Briefly, fresh tomatoes were removed from their pedicels and rinsed with clean water. Then, the tomatoes were crushed into tomato paste and sterilised at 90 °C for 15 min (LS-B75L-I, BINJIANG, Jiangsu, China). After cooling to room temperature, 2 % salt, 2 % liquor and 3 % sugar (based on the mass of beaten tomatoes) were added to the sterilised tomato paste and mixed well. Lacticaseibacillus casei H1 (CCTCC NO: M2016524) and Lactobacillus rhamnosus H3 (CCTCC NO: M2016525) (1:1) were inoculated into the tomato paste at a ratio of 2 %. The above operations were performed on a clean bench (SW-CJ-1FD, Suzhou, China). The tomato paste samples were sealed and fermented in a constant temperature incubator at 28 °C for 5 d. Finally, the samples were bottled and placed in a −20 °C refrigerator for subsequent test analysis.

2.2.2 Organic acids, titratable acidity and pH

The method for determining organic acids was based on Zheng et al. [17] with minor modifications. Briefly, a high-performance liquid chromatography (HPLC) system (Agilent 1260) was used to determine the organic acids in the sour soup. The HPLC parameters were as follows: Agilent 1260 VWD detector, Agilent Z0RBAX SB-AQ column (4.6 mm × 250 mm, 5 μm, Agilent), mobile phase (0.02 mol/L KH2PO4, pH 2.0): methanol = 96:4, detection wavelength 210 nm, injection volume 10 μL, column temperature 35 °C, flow rate 0.8 mL/min. Titratable acidity was determined based on a standard from the Chinese National Food Safety Standard GB 12456-2021. The pH of the tomato sour soup was determined at room temperature using a pH meter (PHS-3C, LEICI, Shanghai, China) with the pH probe immersed in the sample. For meat pH analysis, a 5 g beef sample was homogenised with 20 mL of distilled water for 30 s (10,000 rpm, 4 °C) (FSH-2A, Jingfei Instrument., Jiangsu, China). The pH value was measured when the pH probe reading was stable. Before each measurement, the pH meter was calibrated with pH 4.00, 6.86 and 9.18 buffer solutions.

2.2.3 Nitrite, salt content and

Nitrite was determined according to the method of Ding et al. [24] with some modifications. A 10.0 g of tomato sour soup was homogenised with 12.5 mL of saturated borax solution using a high-speed homogeniser set at 12,000 rpm for 30 s (FSH-2A, Jingfei Instrument., Jiangsu, China). The homogenate was then transferred to a centrifuge tube and centrifuged at 5000 rpm for 10 min (LC-LX-L500, Lichen Bangxi, Shanghai, China). Following this, 1 mL of the supernatant, 2 mL of 2 mg/mL sulfanilic acid solution, and 1 mL of 2 mg/mL N-(1-Naphthyl) ethylenediamine dihydrochloride solution were combined in a 10 mL volumetric flask, and the volume was adjusted to the mark with water. The mixture was vortexed and allowed to stand for 15 min before measuring the absorbance at 538 nm (Multiskan GO 1510, Thermo, Finland). A standard curve for nitrite ions was generated using a nitrite standard solution, and results were expressed as mg/kg of sample. Salt content: 10 g of tomato sour soup was centrifuged at 3500 rpm for 5 min (LC-LX-L500, Lichen Bangxi, Shanghai, China). The supernatant was extracted, and the salt content of the sour broth was measured using a digital probe salinometer (ATAGO, PAL-SALT PROBE, Japan). For beef salinity, 5 g of beef samples were mixed with 20 mL of distilled water and homogenised for 30 s. The salt content was expressed as a percentage.

2.2.4 Antioxidant capacity analysis

The antioxidant capacity of tomato sour soup was determined with DPPH, ABTS+, and FRAP, as described by previous studies [6], [8] with minor modifications. The sample was homogenised twice at 12,000 rpm for 30 s and centrifuged at 5000 rpm for 10 min (LC-LX-L500, Lichen Bangxi, Shanghai, China), and the supernatant was taken for subsequent determination of antioxidant capacity. For the determination of DPPH, 100 μL of the supernatant was vortexed with 1.90 mL of 0.1 mM DPPH solution and incubated in the dark for 30 min, and the absorbance values of the samples were measured at 517 nm (Multiskan GO 1510, Thermo, Finland). For ABTS determination, 20 μL of the acid broth was mixed with 200 μL of the ABTS solution and incubated for 5 min, followed by measuring the absorbance at 734 nm (Multiskan GO 1510, Thermo, Finland). The level of ABTS in the samples was determined from a calibration curve of ascorbic acid. ABTS and DPPH were expressed as mg ascorbic acid equivalent antioxidant capacity (AEAC) per litre (mg AEAC/L). For FRAP, 100 μL of supernatant and 1.8 mL of TPTZ working solution were mixed and incubated at 37 °C for 10 min, and the absorbance was measured at 595 nm (Multiskan GO 1510, Thermo, Finland). The level of FRAP in the samples was determined by reference to a calibration curve of FeSO4, and the results were expressed as mM FeSO4.

2.2.5 Total phenolic content and vitamin C analysis

The total phenolic content (TPC) of tomato sour soup was determined using the Folin–Ciocalteu method described by Silva et al. [25] with modifications. Briefly, 1 mL tomato sour soup supernatant, 1 mL Folin–Ciocalteu solution, and 5 mL distilled water were mixed and left to stand at room temperature for 3 min, then 3 mL 20 %Na2CO3 solution was added and mixed, and then incubated at 36 °C in a water bath for 1 h, and the absorbance taken was measured at 765 nm, the results were expressed in mg GAE/L. The total flavonoid content (TFC) was determined by a colourimetric method [25], 1 mL of sample was mixed with 0.5 mL of 5 % NaNO2 and allowed to react for 5 min, and then 0.5 mL of 10 % Al(NO3)3 solution was added and then left to stand for 5 min. Subsequently, 4 mL of 4 % NaOH solution was added and allowed to stand for 3 min. Finally, the absorbance was measured at 510 nm (Multiskan GO 1510, Thermo, Finland), and the results were expressed as mg RE/L. The results were obtained using the rutin standard as a control standard and expressed as mg RE/L. The 2,6-dichloroindophenol titration was used to determine vitamin C.

2.3 The preparation process of marinated beef in sour soup

The meat was cut into cubes (5 cm × 4 cm × 1 cm) weighing 80.0 g ± 5.0 g. The final marinade was prepared by diluting the tomato sour soup above 1:1 (v/v). The curing process will be carried out as follows: Briefly, the beef and sour soup marinades were packed in self-sealing bags at a ratio of 1:2 (w/w) and sealed. The control and static marinade groups were not sonicated. The samples of the treatment group were sonicated at 20 °C with an ultrasonic frequency of 40 kHz (KQ-800DE, Kunshan Ultrasonic Instrument Co., Ltd, Jiangsu, China). To prevent the temperature of the marinade from increasing during sonication, ice was added to the ultrasonic bath to maintain a constant temperature. All the samples were divided into six groups: Two control groups: Unmarinated treatment (NT) and 2 % static sodium chloride treatment (ST); Four treatment groups: Static marinated group: (30, 60, 90 min); Ultrasonic group: 100 W (30, 60, 90 min), 320 W (30, 60, 90 min), 640 W (30, 60, 90 min). The treatment groups were named according to the power output and treatment time: N30, N60, N90; 100-30, 100-60, 100-90, 320-30, 320-60, 320-90, 640-30, 640-60, 640-90. Specific procedures for Design factors were added to the Supplementary materials. Strict hygienic procedures were followed in this study to avoid microbial contamination during processing.

2.4 Marinade absorption and cooking loss

The marinade absorption was calculated from the difference in weight before and after the marinading process [26]. The samples were cleaned and weighed (W0). After marination, the residual surface moisture was absorbed with absorbent paper, the meat was weighed, and the meat’s weight was recorded for W1. The marinade absorption rate was determined using the following equation.Marinadeabsorption(%)=W1-W0W0×100

where W0 is the initial weight before marinating (g), W1 is the final weight after marinating (g).

Cooking loss was calculated from the difference in weight before and after cooking [26]. The meat samples were placed in the cooking bag and sealed under vacuum, then placed in a water bath at a constant temperature of 85 °C and left to rest for 10 min. After cooking, the meat samples were cooled to 25 °C, wiped with absorbent papers to remove excess water and weighed immediately. The following equation calculated the cooking loss:Cookingloss%=W1-W2W1×100

where W1 is the initial weight after marinating (g), W2 is the final weight after cooking (g).

2.5 Water-holding capacity (WHC)

A 1.50 g meat sample was placed between filter papers and between 10 × 10 cm glass plates and pressed for 5 min at a constant weight of 5 kg and weighed [18]. The water-holding capacity was determined using the following equation:WHC%=100-Ws-WfWs×100

where Ws is the initial weight (g) before pressure application; Wf is the final weight (g) after pressure application.

2.6 Colour difference

The colour parameters of the samples were measured using a digital WSC-3B CIE Lab (Shanghai Inesa Optical Instrument Co., LTD., Shanghai, China) at room temperature as described by [22]. Measurements were made at least five different points on the sample surface. The colourimeter was calibrated with a white standard plate before using (L* = 98.39, a* = 0.20, b* = −0.68). The L*0, a*0 and b*0 values of non-marinated beef were used as reference values to calculate the total colour difference (ΔE) according to the following equation:ΔE=L∗-L0∗2+a∗-a0∗2+b∗-b0∗2

2.7 Moisture and water activity

Moisture and water activity were determined according to Bao et al. [27]. An intelligent water activity tester (HD-6, Wuxi Huake, China) equipped with WSC-5 sensors was used to determine the water activity of marinated beef. The water content of meat was determined at 105 °C with a constant weight.

2.8 Texture profile analyses (TPA)

TPA was accomplished by using a CT3 texture analyser (CT3, Middleboro, MA, USA) according to the method of Bai et al. [22]. The samples were compressed twice using a TA11/1000 cylinder probe (25.4 mm diameter, 35 mm long) with the following parameters: pre-test speed, 2 mm/s; test speed, 2 mm/s; post-test speed, 2 mm/s; compression distance, 50 %; induction power, 8 g; and interval for the second pressing, 2 s.

2.9 Shear force

A TMS-PRO texture analyser (FTC Co., Sterling, VA, USA) equipped with a Warner-Bratzler blade (60° triangular opening) was used to determine shear force [28]. Meat samples were heated in a water bath at 85 °C to a central temperature of 75 °C, allowed to cool naturally to room temperature (25 °C), and then cut into 10 × 10 × 20 mm strips in the direction of the muscle fibres, avoiding as far as possible connective tissue visibly, and perpendicular to the muscle fibres for the shear force test. The parameters set were a trigger force of 0.1 N, an experimental measuring speed of 200 mm/min and the maximum shear force value was recorded. Each sample was measured six times, and the results were expressed in Newton (N).

2.10 Scanning electron microscopy (SEM) analyses

The microstructure analyses were performed according to the method described by Bai et al. [22] with minor modifications. First, the samples were cut into 1.0 × 1.0 × 0.5 cm blocks and fixed in 2.5 % glutaraldehyde for 24 h. Subsequently, the fixed samples were rinsed three times in 0.1 mol/L phosphate buffer (pH 7.0), blotting paper was used to absorb excess water, and the samples were dehydrated at the graded ethanol series (25 %, 50 %, 75 %, 85 %, and 90 %) for 10 min, and then in 100 % ethanol for 30 min. The samples were freeze-dried in a freeze dryer (CTFD 12S, Creatrust, Qingdao, China) for 48 h. The surface of the samples was sprayed with gold and observed under an electron microscope (SU8010, Hitachi, Japan).

2.11 Haematoxylin-eosin (H-E) staining

H-E staining was performed following the method of Guo et al. [19] with modifications. The samples were cut into cubes (1 cm3) and fixed in 4 % paraformaldehyde solution. The fixed samples were then embedded in molten paraffin and sliced into 4 µm thick cross sections. Subsequently, the sections were dyed with haematoxylin-eosin and observed using an optical microscope (Eclipse E100, Nikon, Japan).

2.12 Sensory evaluation

A consumer sensory test was conducted following the procedure described in [27]. Briefly, the sensory evaluation of marinated and non-marinated beef was carried out regarding colour, odour, taste, texture, palatability, and overall acceptability. The experiment was conducted at the Animal Product Processing Laboratory of Guizhou University (Gui Zhou Province). Specific procedures for sensory evaluation were added to the Supplementary materials (Table S1). The evaluation team consisted of ten sensory-trained students (5 male and 5 female) trained in the sensory evaluation of meat products. Sensory evaluation was performed using a 7-point descriptive scale (1 = low intensity, 7 = high intensity) for each sample index. Each sample was calculated by averaging the scores of each panellist.

2.13 Electronic nose (E-nose) analysis

The E-nose (PEN3, Airsense Analytics GmbH, Germany) analysis was used to estimate the aroma profile between all meat samples according to the method of Xu et al. [29] with some modifications. The PEN3 portable electronic nose was equipped with 10 sensor probes for different categories of detectable specific volatile compounds, as shown in Table S2. Pattern recognition software (WinMuster, v. 1.6.2.13) was used to record and process the data. A 1.5 g sample was placed in a 5 mL headspace vial with a cap, and the electronic nose was carried out after equilibration for 30 min in a 40 ± 0.5 °C water bath. Electronic nose measurement conditions: sensor cleaning time 90 s, sample preparation time 5 s, data acquisition time 120 s, carrier gas flow rate 600 mL/min. The values corresponding to the three-time points with stable response values were selected and averaged. Each group of samples was measured in three replicates.

2.14 Statistical analysis

All measurements were conducted in triplicate, and the experimental data are presented as mean ± standard deviation. Statistical analysis was performed using SPSS 26.0 (SPSS Inc., Chicago, IL, USA). Differences between means were assessed by one-way analysis of variance (ANOVA), followed by post-hoc analysis using Duncan’s multiple comparison test, with significance set at P < 0.05. Origin2021 (OriginLab Corporation, Massachusetts, USA) and Adobe Illustrator 2021 were employed to generate all figures.

3 Results

3.1 Physicochemical indexes of tomato sour soup

In this study, the titratable acidity and pH of the tomato sour soup used were 15.72 ± 1.60 g/kg and 3.36 ± 0.04, respectively. The type and content of organic acids were important indicators for evaluating the quality of tomato sour soup. Tomato sour soup was rich in organic acids such as lactic and acetic, citric, tartaric, and malic acids but also in phenolic compounds, minerals, and vitamins [30]. As shown in Table 1, lactic acid, acetic acid, succinic acid, and oxalic acid were dominant organic acids, which were assayed to be 11.479 ± 0.026 mg/mL, 5.064 ± 0.017 mg/mL, 2.844 ± 0.073 mg/mL and 1.558 ± 0.002 mg/mL, respectively. Phenolic compounds and flavonoids are rich in bioactive groups, contributing to antioxidant capacity and antibacterial activity [31]. Furthermore, phenolics have been reported to be effective in slowing the oxidation of meat proteins and lipids [32]. As shown in Table 2, the total phenolic and flavonoid contents of sour soup were 284.30 ± 22.34 mg GAE/L and 206.29 ± 19.57 mg RE/L, respectively. The high content of total polyphenols in sour soup exhibited vigorous free radical scavenging activity, with DPPH, ABTS and FRAP scavenging capacities of 198.02 ± 5.96, 99.33 ± 4.18 mg AEAC/L and 0.54 ± 0.07 mmol/FeSO4. The salt content of the tomato sour soup product was 1.85 %, and the nitrite content was 1.28 ± 0.10 mg/kg, which complies with the standard of less than 20 mg/kg (as NaNO2) of nitrite content in Chinese National Food Safety Standard GB2762-2017. In living organisms, nitrite can react with amines to form carcinogenic nitrosamines, potentially threatening human health [33]. The addition of sodium chloride to sour soup during fermentation improves the taste, flavour, and texture of tomato sour soup and has also been shown to inhibit the growth of pathogenic E. coli. [34]. Acidic marinades increase meat tenderness by lowering the pH value of the meat, which in turn leads to weakening of the muscle structure, increased protein hydrolysis by cathepsins, and increased conversion of collagen to gelatin [9]. Therefore, in this study, sour soup was applied as an acidic marinade for meat curing to investigate further the effect of sour soup on beef texture, microstructure, and flavour.Table 1 The content of organic acids in tomato sour soup.

Organic acids	Concentration (mg/mL)	
Lactic acid	11.479 ± 0.026	
Acetic acid	5.064 ± 0.017	
Succinic acid	2.844 ± 0.073	
Oxalic acid	1.558 ± 0.002	
Citric acid	0.430 ± 0.013	
Tartaric acid	0.023 ± 0.010	
Malic acid	0.210 ± 0.002	
Total acid	21.610 ± 0.133	

Table 2 The characterisation of physicochemical properties of tomato sour soup.

Physicochemical indexes	Concentration	
pH	3.36 ± 0.04	
Titratable acidity (g/kg)	15.72 ± 1.60	
Salt content (%)	1.85 ± 0.04	
Nitrite (mg/kg)	1.28 ± 0.10	
Total flavonoid (mg RE/L)	206.29 ± 19.57	
Total phenolic (mg GAE/L)	284.30 ± 22.34	
Vitamin C (mg/100 g)	66.67 ± 4.76	
DPPH (mg AEAC/L)	198.02 ± 5.96	
ABTS (mg AEAC/L)	99.33 ± 4.18	
FRAP (mmol/FeSO4)	0.54 ± 0.07	

3.2 Physicochemical quality of marinated beef

The organoleptic attributes of meat determine its eating quality, such as tenderness, juiciness and flavour [2]. Marinating with salts or organic acids can help to improve meat tenderness by increasing ionic strength, altering pH, modulating endogenous enzyme activity [35], and possibly indirectly influencing tenderness by regulating the water-holding capacity of the meat [36]. Marinade uptake depends on many factors, including meat type, marination methods, time, ion diffusion rate and marinade formulation [2], [6]. The marination effect and beef quality of beef tenderloin before and after marination in tomato sour soup are shown in Table 3. After marination, the salt and organic acids present in the solution were impregnated into the muscle tissue, resulting in a decrease in pH. Under conventional static marination (0 W), both marinade uptake and water content of the meat tended to increase with increasing marination time (P < 0.05). This may be attributed to the fact that when the pH was lower than the muscle isoelectric point (pH 5.3), there was a direct effect on the physicochemical and textural properties of the meat samples. In this environment, the marinade solution penetrates the muscle fibres, leading to swelling and retention of water in the muscle fibres, thus increasing the marinade uptake [8]. However, there was no significant correlation between water activity and marinade uptake rate in the ultrasound-assisted marination group.Table 3 Effect of marination on the essential physicochemical quality of beef in sour soup.

		Marinade absorption (%)	Cooking loss (%)	Shear force(N)	Water content (%)	Water activity	Water-holding capacity (%)	Salt content (%)	pH	
Control	NT	−	38.37 ± 0.31h	76.95 ± 5.18a	54.67 ± 1.10i	0.920 ± 0.011c	87.46 ± 1.53b	0.09 ± 0.01e	5.84 ± 0.01a	
ST	1.85 ± 0.12g	43.23 ± 0.09c	73.15 ± 4.60b	55.78 ± 0.52hi	0.935 ± 0.002b	85.99 ± 1.02bcd	0.10 ± 0.01bc	5.82 ± 0.02a	


	
0 W	N-30	2.35 ± 0.27f	40.04 ± 1.60fg	63.67 ± 4.31c	57.94 ± 0.60g	0.924 ± 0.002c	91.17 ± 2.05a	0.09 ± 0.01e	5.41 ± 0.01c	
N-60	2.61 ± 0.20ef	42.5 ± 0.40cd	58.33 ± 2.21d	56.76 ± 0.71h	0.943 ± 0.003a	90.90 ± 0.53a	0.09 ± 0.01e	5.24 ± 0.01e	
N-90	2.84 ± 0.22de	45.44 ± 0.89a	53.73 ± 1.07e	58.74 ± 0.85g	0.945 ± 0.005a	90.55 ± 1.63a	0.09 ± 0.00de	5.21 ± 0.02f	


	
100 W	100-30	2.82 ± 0.06de	41.47 ± 0.35de	52.28 ± 1.55e	60.86 ± 0.86f	0.947 ± 0.001a	92.72 ± 1.72a	0.10 ± 0.01bc	5.30 ± 0.02d	
100-60	3.41 ± 0.28bc	42.81 ± 0.08c	46.80 ± 1.32f	62.27 ± 0.79e	0.949 ± 0.001a	93.47 ± 0.64a	0.10 ± 0.01bc	5.23 ± 0.05e	
100-90	2.75 ± 0.07def	44.70 ± 1.32ab	42.73 ± 3.63g	62.70 ± 0.94de	0.949 ± 0.001a	90.88 ± 1.22a	0.12 ± 0.01a	5.03 ± 0.01h	


	
320 W	320-30	2.68 ± 0.49def	40.34 ± 0.52efg	42.40 ± 2.66g	61.57 ± 0.42ef	0.948 ± 0.006a	87.04 ± 3.47bc	0.10 ± 0.00bc	5.49 ± 0.01b	
320-60	3.37 ± 0.21bc	40.03 ± 0.69fg	34.15 ± 2.08h	63.56 ± 0.42cd	0.947 ± 0.001a	84.36 ± 1.61cd	0.11 ± 0.01b	4.94 ± 0.01i	
320-90	3.41 ± 0.28bc	42.93 ± 0.98c	30.48 ± 1.88i	64.8 ± 0.43b	0.948 ± 0.002a	86.1 ± 0.55bcd	0.12 ± 0.01a	4.89 ± 0.01j	


	
640 W	640-30	3.07 ± 0.34cd	39.41 ± 0.44gh	26.95 ± 3.50j	64.31 ± 0.57bc	0.946 ± 0.002a	85.62 ± 1.60bcd	0.10 ± 0.00bc	5.31 ± 0.02d	
640-60	3.72 ± 0.27b	40.86 ± 0.32ef	17.90 ± 2.59k	63.49 ± 0.31cd	0.947 ± 0.003a	83.93 ± 0.65d	0.10 ± 0.01cd	4.92 ± 0.02i	
640-90	4.28 ± 0.07a	43.65 ± 0.61bc	16.07 ± 1.43k	67.38 ± 0.67a	0.950 ± 0.007a	85.44 ± 0.78bcd	0.12 ± 0.01a	4.87 ± 0.01j	
Note: a–i means within the same column with different letters differ significantly between treatments (P < 0.05). Control: NT and ST represent non-marinated and 2 % sodium chloride static marinating, respectively. N-(30, 60, 90), 100-(30, 60, 90), 320-(30, 60, 90), 320-(30, 60, 90) represent the ultrasonic-assisted marination of sour soup for 30, 60 and 90 min under the condition of 0 W, 100 W, 320 W, 640 W ultrasonic power according to the power output and treatment time, respectively.

Water-holding capacity (WHC) was one of the primary indicators used to evaluate the texture quality characteristics in meat products. The WHC and moisture content of meat and meat products were related to their juiciness and tenderness, and increasing WHC is beneficial to the product’s organoleptic properties [37]. Static curing (0 W) and low-intensity ultrasound (100 W) assisted sour soup curing significantly increased the water-holding capacity of sourdough beef compared to uncured and 2 % brine-cured, with the 100-60 treatment group having the highest water-holding capacity. Li et al. reported that ultrasound could promote the binding capacity between chicken muscle fibres and water, thereby increasing the muscle's water-holding capacity [38]. In addition, it is worth noting that the water-holding capacity of the high-intensity ultrasound (320 W, 640 W) was significantly reduced, which may be due to the decrease in pH leading to the contraction of the polypeptide chain network, thus reducing the water-holding capacity. Changes in myofibrillar proteins and sarcoplasmic proteins lead to a decrease in WHC when pH decreases relatively rapidly [39].

The cooking loss rate tended to increase significantly with ultrasound time, which may be attributed to the excessive disruption of beef tissue by high-intensity ultrasound, which reduced the water retention capacity of myofibres, leading to an increase in cooking loss of marinated beef after excessive ultrasound, accompanied by a loss of some valuable nutrients in the exudate. Chang et al. found that ultrasound treatment increased the cooking loss and decreased the WHC of meat products with growing time, which was attributed to the fact that protein denaturation or excessive degradation of the tissue structure in meat products leads to a decrease in WHC of the muscle fibres, which ultimately leads to higher cooking losses [40]. However, research has found that an increase in ultrasonic intensity does not significantly affect cooking loss during the ultrasound-assisted marination of lamb. This may be due to the variation in the conditions of ultrasonic treatment and the marinade components, leading to different trends in cooking loss [41].

Shear force was often used as a primary indicator to assess tenderness, and sour soup marination showed a significant reduction in meat shear force while increasing meat moisture content compared to the non-marinated control. This improved moisture retention, reduced protein fibre density, and effectively improved meat tenderness. In addition, ultrasound-assisted marination further reduced beef’s pH and shear force in the sour soup treatment, resulting in enhanced tenderness compared to conventional static curing in sour soup. This effect was attributed to ultrasound’s cavitation and mechanical action, which disrupted myofibrillar proteins and connective tissue and increased intermyofibrillar permeability [26]. Moreover, the cavitation induced by ultrasound can cleave the muscle cell membrane and release intracellular substances, such as lysosomal cathepsins, or intracellular Ca2+ to activate calpains, which can degrade proteins to increase the tenderness of the meat [40].

3.3 Colour variation analysis

Meat colour is primarily determined by the redox state of myoglobin and the formation of complexes between myoglobin and exogenous chemicals [42]. As shown in Fig. 1, sour soup curing significantly altered the colour characteristics (ΔE) of beef, with the sour soup-marinated group showing higher values for brightness (L*), yellowness (b*) and lower values for redness (a*) compared to the non-marinated group. Previous studies have shown that this change may be related to the acidic environment at low pH (pH < 5.3), leading to the conversion of myoglobin to high iron myoglobin, susceptibility of myoglobin to heat denaturation, contributing to the conversion of myoglobin to metmyoglobin, resulting in a darker colour [8]. The ultrasound treatment group was able to influence the change in meat colour after marination, with a significant decrease in b* value in the low-intensity ultrasound group (100 W) (P < 0.05). In contrast, a* value was significantly higher, and ΔE was considerably lower in the high-intensity ultrasound group (320 W, 640 W) (P < 0.05) compared to the static marination group. This is due to the reddish colour of the tomato sour soup, which may affect the final product’s colour. In addition, we found that low-intensity ultrasound increased the L* value, whereas high-intensity ultrasound decreased it, which is consistent with the findings of Bai et al. [22]. This may be because high-intensity ultrasound caused water loss in muscle tissue, thereby decreasing surface refractivity.Fig. 1 L*, a*, b*, ΔE values of the non-marinated and marinated with tomato sour soup in beef at different marinated conditions. L*, lightness; a*, redness; b*, yellow; ΔE, total colour differences. a–i: Different superscripts indicate statistical significance (P < 0.05).

3.4 Texture profiles analysis

Recent studies have found that acid marinades significantly improve meat texture, digestibility and flavour by using natural products such as lemon juice, kimchi juice and fruit vinegar to marinate meat [6], [8], [43]. Tenderness was a crucial indicator of beef’s eating quality and a key factor influencing consumer satisfaction. Processes such as soaking in acidic solutions or applying heat treatments can disrupt myofibrils, form gel matrices, and induce muscle swelling to facilitate tenderisation [6]. As shown in Fig. 2, marinating in sour soup significantly reduces the hardness and chewiness of beef while increasing its gumminess, springiness, cohesiveness, and resilience. Sengun et al. [9] reported that organic acids effectively improved connective tissue and beef texture by weakening muscle structure, increasing tissue protease protein hydrolysis and converting collagen to gelatin. Unal et al. [44] reported that lemon juice marinade reduced the hardness, cohesiveness, and resilience values of chicken breasts compared to non-marinated samples. In addition, weak organic acids and NaCl curing significantly affected connective tissue collagen and meat texture properties, with a reduction in fibre diameter and perimysial thickness and disorganisation of collagen fibre arrangement after weak organic acid curing [45]. In our study, low-intensity ultrasonic treatment (100 W) considerably decreased the hardness and chewiness of beef compared to static marination. High-intensity ultrasonic assistance (320 W, 640 W) increased chewiness and gumminess compared to the 100 W ultrasonic-assisted marination. However, elasticity, cohesiveness, and resilience differences were not statistically significant (P > 0.05). Research has shown that low-frequency ultrasonic treatment results in a more uniform and ordered arrangement of the gel structure in chicken breasts, creating a complex and structured network that enhances the water-holding capacity of the gel and improves the tenderness of chicken breasts [46]. The elasticity of ultrasonically assisted marinated beef is improved compared to non-ultrasonicated groups, possibly due to the cavitation effect of ultrasound, which disrupts muscle integrity, increasing the interstitial spaces within myofibres and further enhancing the myofibrillar water-holding capacity [47]. Different ultrasonic powers have a significant impact on the microstructure of meat. High-intensity and prolonged ultrasonic treatments reduce the cohesiveness and resilience of beef, which can be attributed to excessive ultrasound causing protein denaturation and deterioration of gel properties [22]. In conclusion, tomato sour soup can be used as a natural marinade for beef, and low-intensity ultrasonic-assisted marination is an effective strategy for improving the textural characteristics of meat.Fig. 2 Textural changes under different treatment conditions for sour soup beef. Hardness (A), chewiness (B), gumminess (C), springiness (D), resilience (E) and cohesiveness (F). a–h: Different superscripts indicate statistical significance (P < 0.05).

3.5 Correlation analysis of physicochemical properties

Correlation analysis was performed on these parameters to further evaluate the relationship between the basic physicochemical parameters, colour change, and TPA parameters between the different treatment groups. As shown in Fig. 3, marinade absorption was significantly and positively correlated with water content, water activity, cooking loss, salt content, cohesiveness and ΔE, while there was a significant negative correlation with shear force, pH, hardness, and chewiness (P < 0.05), indicating that higher marinade absorption was associated with lower shear and hardness (higher tenderness). Water content showed a significant negative correlation with pH, water-holding capacity, hardness and chewiness; and a highly significant positive correlation with water activity and salt content (P < 0.05). Gumminess, springiness, cohesiveness, and resilience showed highly significant positive correlations (P < 0.05). Furthermore, pH showed a highly significant positive correlation with hardness and chewiness, possibly due to muscle structure disruption at low acidity, which can effectively improve beef texture, increase release of endogenous muscle proteolytic enzymes and promote beef tenderisation [45]. The results of this study indicated that tomato sour soup had a positive effect on the organoleptic properties of beef, in which organic acids played an important role. In addition, the conversion of myoglobin to metmyoglobin was accelerated in acidic environments, and this conversion may cause a darker change in the colour of the meat [8].Fig. 3 Heat map of Pearson's correlation analysis between data on basic physicochemical parameters, colour change and TPA parameters under different treatment conditions for sour soup beef, with red representing positive correlation and blue representing negative correlation. The asterisk (*) represents the correlation coefficient, which is marked as significant (*P < 0.05, **P < 0.01).

3.6 Changes in microstructure of marinated beef

We conducted HE staining observations to achieve a more comprehensive understanding of the effects of acid marination on muscle tissue. The histological sections of marinated beef muscle tissue in acid brine are depicted in Fig. 4. In the non-marinated group (a), the myofibres are neatly and closely arranged, enveloped by the observed endomysium and relatively intact connective tissue. However, in the static marinated (0 W) and low-intensity ultrasound-assisted marinated (100 W) groups, the myofibres appear fragmented, and their structural integrity is compromised. Although the tenderness of the beef increases, prolonged marination time gradually widens the gaps between myofibres, accompanied by cracks in the endomysium and connective tissue. Studies have demonstrated that acidic marinades can facilitate protein dissolution and degradation of muscle structure, resulting in the formation of a gel matrix that enhances the water retention capacity and tenderisation of meat [48], aligning with the findings on beef tenderisation, textural characteristics, and water holding properties under low-intensity ultrasound-assisted acid brine marination as presented in Table 3 and Fig. 2. In an acidic milieu, the heat denaturation temperature of muscle proteins decreases. Besides damaging the connective tissue, the cooking process helps to sustain the hydration capacity of myofibres induced by the decrease in pH, contributing to tenderisation [36].Fig. 4 The transverse images of hematoxylin-eosin (H-E) staining of non-marinated and sour soup marinated beef (a–n). a: non-marinated; b: 2 % sodium chloride static marinating; c–e: tomato sour soup static marinating for 30, 60, 90 min; f–h: 100 W ultrasound-assisted sour soup marinating for 30, 60, 90 min; i–k: 320 W ultrasound-assisted sour soup marinating for 30, 60, 90 min; l–n: 640 W ultrasound-assisted sour soup marinating of the beef samples for 30, 60, 90 min, magnifications were 200×, and the scale bar was 100 μm.

The high-intensity ultrasound groups (320 W, 640 W) exhibit more pronounced fibre twisting and breakage, along with an increase in inter-myofibril spaces, dissolution of connective tissue, rupture of the endomysium, and a gradual roughening of the myofibres surface with increasing ultrasound power. Research indicates that low pH levels and the absorption of marinades can induce an expansion effect on myofibrils, reducing the gaps between them [8]. However, under conditions of high-intensity ultrasound, cavitation and mechanical forces result in the formation of micropores in myofibrils, causing a certain degree of deformation in muscle fibre bundles and a relative increase in the gaps between fibres and cross-sectional area.

Muscle tissue contains abundant myofibre bundles, composed of aligned muscle cells enveloped by the endomysium to maintain structural stability [49]. Fig. 5 illustrates myofibres’ cross-sectional morphology and the muscle membrane changes between the different groups. From Fig. 5(a), it can be observed that the cross-section of myofibres in the non-marinated group is regular and compact, consistent with the HE results in Fig. 4(a). The myofibres are neatly arranged in the statically marinated (0 W) and low-intensity ultrasound-assisted (100 W) groups, retaining more delicate connective tissue between the fibres. Ultrasound treatment increases the gaps between myofibres, allowing the expanded myofibres can retain more moisture [22]. The increase in marination rate and moisture content in Table 3 confirms this observation. Different ultrasound powers have a significant impact on the microstructure of beef. For the high-intensity ultrasound groups (320 W, 640 W), the muscle membrane and endomysium are thinner than those in the non-marinated group, and the myofibrils exhibit disordered and noticeably intertwined patterns. This phenomenon may be due to the mechanical effects of ultrasound waves. The more significant deformation of the muscle membrane results in irregular folds and twists on the surface of myofibres [50]. Zhu et al. observed that after static marination, the myofibrils in lamb were neatly aligned with intact Z-lines. However, following ultrasonic treatment, there was increased damage to the myofibrils, with Z-lines gradually breaking [41]. Similarly, Kang et al. found that ultrasound treatment resulted in disruption and swelling of Z-lines and myofibrils, which correlated positively with the degree of meat tenderisation [51]. These changes in microstructure may further explain the decrease in shear force values. In conclusion, from a structural point of view, organic acids in the sour soup group promote the swelling of beef muscle fibres and connective tissues compared to the single brine-marinade group, whereas ultrasonic treatment further enhances the penetration of organic acids and brine marinading, contributing to beef tenderisation.Fig. 5 Scanning electron microscopy (SEM) images of non-marinated and sour soup-marinated beef. a: non-marinated; b: 2 % sodium chloride static marinating; c–e: tomato sour soup static marinating for 30, 60, 90 min; f–h: 100 W ultrasound-assisted sour soup marinating for 30, 60, 90 min; i–k: 320 W ultrasound-assisted sour soup marinating for 30, 60, 90 min; l–n: 640 W ultrasound-assisted sour soup marinating of the beef samples for 30, 60, 90 min, magnification were 100 and 150×.

3.7 Sensory assessment

The results of the sensory evaluation (colour, flavour, taste, texture, palatability and overall acceptability) of beef marinated in sour soup are shown in Fig. 6. Compared to single salt treatment (ST), sour soup marination improved the meat colour of the beef, and the colour of the beef increased significantly (P < 0.05) with increasing ultrasonic power and marinating time. Flavour and taste scores significantly increased with increasing marinating time and ultrasonic power, and both scores were higher in the 320-60 treatment. This can be attributed to the fact that high-intensity ultrasound accelerates the penetration of flavour and aroma compounds from the marinade into the meat, such as organic acids and alcohols, resulting in marinated beef with a better flavour. There was also a significant difference in sensory texture between the treatment groups (P < 0.05). In contrast, the acceptance and palatability of the beef marinated in sour soup under the 320 W ultrasound conditions were better compared to the other groups, which was attributed to the ability of the organic acids in the sour soup to promote softening of the muscle tissue, and the mechanical and cavitation effects of ultrasound treatment, which promoted myofibre rupture, reduced the shear force of the beef and tenderised the meat. Beef marinated in sour soup achieved higher palatability under the 100 W and 320 W treatment conditions. The overall sensory acceptability of the ultrasound treatment group was higher than that of the static treatment group. In particular, the overall acceptability of the 320-60 treatment group was the highest, indicating better consumer recognition. Overall, sour soup marination improved beef texture and added flavour, while ultrasound-assisted marination produced superior meat texture and flavour.Fig. 6 Effect of different marination treatments on the sensory evaluation of sour soup marinated beef.

3.8 The E-nose analysis

An electronic nose has emerged as a rapid and convenient tool for effectively analysing olfactory characteristics, capable of distinguishing subtle variations in volatile compounds [12]. To further understand the effect of tomato sour soup marination on beef flavour, the E-nose was employed to analyse aroma characteristics beyond sensory evaluation. Response values from 10 sensors for beef samples, both non-marinated and marinated under varying ultrasonication times and intensities, as shown in Fig. 7(A–D), indicated more robust and distinct responses for W1S, W1W, W2S, W2W, and W5S sensors compared to the non-marinated group. The result demonstrated that marinating with tomato sour soup significantly affected the intensity of flavour substances in the beef. Among them, W1S, W1W, W2S, W2W, and W5S sensors exhibited higher signal intensities in all groups, indicating an increase in the content of methane, organic sulfides, alcohols, aldehydes, ketones, aromatic compounds, and nitrogen oxides in the beef post-marination (P < 0.05). Conversely, W1C, W3C, W3S, W5C, and W6S sensors showed lower response intensities between groups. However, there were still differences in response values between each sample group (Supplementary material), which could be attributed to variations in ammonia, hydrides and hydrocarbons. Additionally, significant differences in sensor response intensities were observed under various ultrasonic intensity conditions (P < 0.05), indicating that ultrasound is a condition affecting the flavour of beef. The Principal Component Analysis (PCA) results shown in Fig. 7(E) indicated that PC1 and PC2 accounted for 76.89 % and 18.69 % of the variance, respectively, with a total contribution rate of 95.58 %, reflecting the overall characteristics of the original data. Based on the radar plot, PCA score plot and electronic nose loading plot, significant differences in the response intensities of different samples to each electronic nose sensor were clearly observed, suggesting that different marination conditions significantly affect the formation of volatile flavour compounds in beef. The PCA loading plot (Fig. 7F) revealed that W1C, W3C, and W5C were clustered on the negative axis of PC1. In contrast, ultrasound groups (320 W, 640 W) were primarily positioned on the positive axis of PC1 and correlated with W1S, W1W, W2S, W2W, and W5S sensors, indicating some variation in flavour characteristics with some overlap. This suggested that high-intensity ultrasound-assisted marination can detect a multitude of volatile compounds contributing to meat flavour, including alkanes, organic sulphides, alcohols, aldehydes, ketones, aromatic compounds, and nitrogen oxides. Furthermore, static marination (0 W) and low-intensity ultrasound (100 W) groups were positioned on the positive axis of PC2 and correlated with W3S and W6S sensors, indicating an increase in the content of long-chain alkanes, hydrides, and other flavour substances in sour soup beef. This may be attributed to ultrasound’s cavitation and mechanical action, which promotes the penetration and absorption of flavour substances, such as organic acids from the tomato sour soup during marination. Notably, the improvement in beef flavour after sour soup marination could also be related to the bioactive components in the soup. Tomatoes are rich in polyphenolic substances such as quercetin [52], and quercetin can impact the flavour and nutrient release from the beef into soup during the stewing process [53]. Moreover, ultrasound was also shown to promote increased protein oxidation, increased free amino acid content and promoted lipid degradation, generating a greater quantity of volatile compounds such as aldehydes, ketones, and esters to enhance pre-flavouring substances in the muscle, thereby enhancing the flavour of sour soup beef [21]. Similar flavour characteristics were reported by [41], where ultrasonic treatment improved the flavour of lamb by facilitating the oxidation of fats and proteins to produce flavour-related compounds, including aromatic compounds, alcohols, aldehydes, ketones, and short-chain alkanes. Studies found that traditional marination increased malodorous compounds such as hexanal and 1-octane-3-ol in salted Culter alburnus fish, but a significant improvement was observed following a 200 W ultrasonic treatment [54]. Ultrasound treatment at an appropriate power (320 W) reduced substances such as long-chain alkanes and hydrocarbons, which were usually less favourable to flavour while increasing the content of alcohols, aldehydes and ketones, which significantly improved the aroma and taste of the product. It can be seen that ultrasonic technology appears to be an effective method for enhancing the overall flavour of sour soup beef.Fig. 7 The radar chart(A–D), principal component analysis (PCA) score plot (E) and PCA loading plot (F) of electronic nose data for non-marinated and sour soup marinated beef with different ultrasound treatments.

4 Conclusion

This study demonstrated that tomato sour soup is a natural acidic marinade with wide application prospects, which can effectively improve the physicochemical, textural, and microstructural properties of beef, increasing its tenderness. This improvement might be related to the organic acids in the tomato sour soup. Additionally, ultrasound-assisted marination promoted myofibrillar fragmentation, increased marinade penetration into the muscle, and shortened marination time. Meanwhile, ultrasonic treatment significantly improved the texture of sour soup beef and expanded the variety and content of flavour compounds in the meat products, including alkanes, organic sulphides, alcohols, aldehydes, and aromatic compounds, thereby enhancing the flavour. The optimal treatment conditions for marinating with tomato sour soup were 320 W ultrasound for 60 min. In conclusion, the combination of ultrasound technology with meat marination accelerated marination efficiency, resulting in unique physicochemical effects and improving the overall quality of marinated meat products. However, the effect of tomato sour soup on the digestive properties and nutritional value of marinated meat products, including its health-promoting (functional) aspects, needs to be followed up with further studies.

CRediT authorship contribution statement

Huaisheng Zheng: Writing – original draft, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Lilang Li: Methodology, Investigation, Formal analysis. Chaobin Huang: Validation, Formal analysis. Shuhong Liu: Visualization, Investigation. Xinghua Chen: Investigation, Formal analysis. Xiaoyu Wang: Writing – review & editing, Funding acquisition. Ping Hu: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.

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.

Appendix A Supplementary data

The following are the Supplementary data to this article:Supplementary Data 1

Supplementary Data 2

Acknowledgements

This work was funded by the 10.13039/501100001809 National Natural Science Foundation of China (No. 31960485 , 31260379 ) and the Science and Technology Support Program of Guizhou Province, China (QKHZC-2021-178 ).

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2024.107028.
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