Exploring the quality change mechanism of tea polyphenol-assisted cured grass carp from perspectives of physicochemical and flavor

ABSTRACT

In this study, the quality changes of grass carp during tea polyphenol-assisted curing were studied from physicochemical and flavor perspectives. The addition of tea polyphenols has a positive effect on maintaining the water content of fish as the curing time increases (p < 0.05). The tea polyphenol-assisted treatment maintains the brightness of grass carp meat. The addition of tea polyphenols also slowed down the rate of pH growth and enhances the texture of grass carp (p < 0.05). The addition of tea polyphenols reduced His content, which had a positive effect on the taste of the cured grass carp products. When the pickling time reached 60 min, the inosine monophosphate content of tea polyphenol-treated groups was higher than that of the group soaked in salt only. Combined with equivalent umami concentration and taste activity values, it was found that the addition of 0.1% TP could improve freshness of grass carp cured products.

KEYWORDS:

• Pickling
• tea polyphenols
• physicochemical properties
• flavor

1. Introduction

Grass carp (Ctenopharyngodon idella) is one of the four major Chinese carps and the most important freshwater aquaculture fish in China. In 2020, the total amount of grass carp (Ctenopharyngodon idella) farmed worldwide was approximately 5791.5 thousand tonnes, representing 11.8% of the world’s major inland aquaculture species (FAO, 2022). Grass carp is one of the ways for humans to obtain high-quality protein. Its amino acid composition meets the needs of the human body and is suitable for all kinds of people (Y. Zhang et al., 2007). Therefore, it is widely praised in the market and has a huge market space. Among them, food flavor is one of the main indicators that influences customers’ choice of fish products. Flavors of fish substances are mainly divided into odor substances and taste substances, which are volatile compounds and non-volatile compounds (water-soluble substances) that stimulate the olfactory and taste cells, respectively. Flavor can also reflect the maturity and deterioration of foods (Cavanna et al., 2019). Grass carp is mainly sold fresh. Because of its high water content and soft muscle tissue, fish meat is more prone to spoilage in a short time than other meat products, and its sensory and nutritional aspects quickly drop to an unacceptable level, thereby losing its edible value (J. B. Zhang et al., 2019).

In recent years, quite a lot of research has focused on how to improve the quality of fish, so people use different types of processing methods to prolong the shelf life of fish, such as drying, smoking, pickling, fermentation, freezing, modified atmosphere packaging, etc. (Amaral et al., 2021; Kim et al., 2020; Martinez et al., 2010; Moriya et al., 2021; Tsironi & Taoukis, 2014; Y. S. Xu et al., 2018). Among them, pickling is one of the oldest processing methods. During the pickling process, the salt water forms a high osmotic pressure on the surface of the fish, which makes the water and some tissue components in the fish seep out, and then the salt diffuses into the interior of the fish (Andreetta et al., 2016). Lower moisture levels inactivate endogenous enzymes, and the microorganisms in the fish lose their optimal living conditions, thereby slowing the rate of fish spoilage. With the improvement of living standards, people are more and more pursuing a healthy diet. Therefore, low salt products are widely favored by people. Studies have found that excessive salt amount is directly related to the occurrence of hypertension, heart disease, kidney disease, and inducing cerebral hemorrhage (Y. P. Li et al., 2021).

However, the curing process is often accompanied by the occurrence of oxidation. As fish contains unsaturated fatty acids, ferrous haemoglobin, metal catalysts, and other oxygen-promoting factors, it is highly susceptible to reaction with oxygen, which in turn can lead to deterioration of organoleptic properties such as color, texture, and flavor and loss of nutritional value (Guyon et al., 2016). Therefore, it is of great concern how to improve the physical and chemical properties of cured products during processing to prevent deterioration of texture and loss of nutritional value.

Tea polyphenols (TP) are a complex of polyhydroxy phenolic compounds extracted from tea leaves, consisting of more than 30 phenolic substances, which are non-toxic and odorless natural antioxidants. Its main component is catechin and its derivative, epigallocatechin gallate (EGCG). In addition, tea polyphenols also contain EGC (epigallocatechin), EC (epigallocatechin), GCG (gallocatechin gallate), ECG (epigallocatechin), and other catechin-like substances. The catechins in tea polyphenols all have significant antioxidant and antibacterial effects, with the total extract of tea polyphenols and their main component EGCG being the most widely used in aquatic products. It has been shown that tea polyphenols are each effective in maintaining the quality of aquatic products (Xie et al., 2019; Zhou et al., 2019). X. B. Nie et al. (2018) found that after the Japanese sea bass (Lateolabrax japonicas) was soaked with 0.5% TP for 10 min at 4°C, the number of microorganisms, volatile salt nitrogen, lipid oxidation, and protein decomposition levels were reduced. Yi et al. (2011) found that the total number of bacteria in the Collichthys fish balls with 0.25% TP was 6.7 log₁₀CFU g⁻¹ after storage at 0°C for 17 days, which was significantly lower than that of the group without TP, and both TVB-N and TBAR values were found to drop. Y. Li et al. (2022) found that the melanosis and lipid oxidation of Pacific white shrimp (Litopenaeus vannamei) increased after 6 days of frozen storage, while the melanosis and lipid oxidation of the above samples soaked in 1% TP were alleviated.

At present, there are relatively few studies on the effects of tea polyphenol-assisted marination on the physicochemical properties and flavor changes of fish. In this study, the effects of different concentrations of tea polyphenol-assisted pickling on the quality of grass carp were investigated. This study aims to provide theoretical and technical support for the processing and quality control of grass carp.

2. Materials and methods

2.1 Preparation of fish samples

Live grass carp, weighing 2600–3000 g each and 55–60 cm in length, were purchased and quickly transported back to the laboratory with water and oxygen. The fish was killed by a heavy blow to the head, head removed, tail removed, gutted, washed with water to remove blood stains, sliced into two pieces along the spine, the back meat taken and cut into 3 cm × 3 cm × 2 cm pieces in size for subsequent curing. According to the results of the previous pre-experiment, three different curing solutions were prepared (1) group C: 12% NaCl solutions (W/V); (2) group E₁: 12% NaCl solutions with 0.1% TP (W/V); (3) group E₂: NaCl solutions with 0.2% TP (W/V). The grass carp pieces were left to marinate for different times (10, 20, 30, 40, 50, and 60 min) in the above configured marinade at 4°C and a material-to-liquid ratio of 1:3 (W/V). After the marinade was completed, the grass carp meat was removed from the marinade jars and wiped with kitchen paper to remove the surface water and stored in the refrigerator at −80°C pending subsequent determination of the index.

2.2 Determination of water content

The water content was measured according to Wang et al. (2022). The weighing bottle was dried in an oven to a constant weight. Two grams of fish meat (accurate to 0.0001 g) was weighed in the weighing bottle and dried in an oven to constant weight at 105°C. After the above steps are over, take out the weighing bottle, put it in a desiccator, and cool it for 0.5 h before weighing. The moisture content is calculated according to the following formula:

(1) $\mathrm{W}\left(\mathrm{\%}\right)=\frac{{\mathrm{m}}_2-({\mathrm{m}}_3-{\mathrm{m}}_1)}{{\mathrm{m}}_2}\times 100$(1)

Where W (%) refers to the moisture content; m₁ and m₂ represent the mass (in grams) of the weighing bottle and the sample, respectively; m₃ represents the mass (in grams) of the weighing bottle and sample dried to constant weight.

2.3 Determination of NaCl content

The salt content was measured according to Jiang, Jia, et al. (2019). 1.00 g (m₄) of grass carp meat was homogenized with 10 ml of distilled water, left to filter, and then the NaCl content of the filtrate was measured using a PAL-ES1 salinometer (Atago Co., Ltd., Japan) with a reading of X. The formula for calculating the NaCl content of the sample is as follows:

(2) $\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\left(\mathrm{\%}\right)=\mathrm{X}\times \left({\mathrm{m}}_4+10\right)\times 100$(2)

2.4 Determination of pH

The determination of pH was based on the method of T. T. Li et al. (2012). 2.00 g of grass carp meat was mixed with 18 mL of deionized water, homogenized for 60 s and then centrifuged at 10,000 r/min for 15 min at 4°C. The filtrate was filtered at rest, and the pH of the filtrate was determined using a pH meter (FE20, Mettler Toledo Co., China).

2.5 Measurement of color characteristics

The brightness values (L*), redness values (a*), and yellowness values (b*) of grass carp meat were measured using a colorimeter (CR-40, Konica Minolta Sensing Inc., Japan), which calibrated the lens of the colorimeter against the built-in white board before measurement. Ten parallels were set up for each group.

2.6 Measurement of texture properties

The measurement of texture was according X. P. Li et al. (2011) with slight modification. Grass carp pieces were cut to a size of 2 cm × 2 cm × 1 cm, and the hardness (g), elasticity, cohesiveness, and chewiness (g) of grass carp meat were determined by a texture analyzer (TA. XT plus, Stable Micro System, UK). The measurement mode was TPA, using a P50 flat-bottomed column probe with a pre-test speed of 3.00 mm/s, a test speed of 1.00 mm/s, a return rate of 1.00 mm/s, a compression level of 50%, a time dwell of 5 s between compressions and a data acquisition rate of 200 pps. Ten parallels were set up for each group.

2.7 Measurements of free amino acids

The method of Yimdee and Wang (2016) was used with minor modifications. 2.00 g of grass carp was weighed, added to 15 mL of pre-cooled trichloroacetic acid (TCA) solution (15%, W/V), homogenized for 1 min, left to stand for 2 h and centrifuged at 10,000 r/min for 15 min at 4°C. The supernatant was filtered, and 5 mL of the supernatant was adjusted to pH 2.2 with 3 mol/L and 1 mol/L NaOH solution. The solution was fixed to 10 mL with ultrapure water, filtered through a 0.22-μm membrane, pending subsequent determination. The conditions of the automatic amino acid analyzer (L-8800, Hitachi, Tokyo, Japan) were referenced to Fang et al. (2018).

2.8 Measurements of ATP-related compounds

The method of determination was referred to Mu et al. (2022) with slight modifications. Adenosine triphosphate (ATP)-related compounds were analyzed using W2690–2998 HPLC (Waters Co., Milford, MA, U.S.A). 3.00 g of samples were weighed, 10 mL of 10% (V/V) pre-cooled perchloric acid was added, homogenized at high speed for 1 min and then centrifuged (10,000 r/min, 4°C, 15 min) and the supernatant was filtered. Wash the precipitate with 5 mL of 5% (V/V) perchloric acid and centrifuge under the same condition, repeat the operation twice, and combine the supernatants. The pH was adjusted to 6.5 with KOH solutions at concentrations of 10 mol/L and 1 mol/L, respectively, left for 30 min, and then fixed to 50 mL with ultrapure water, mixed, and passed through the 0.45-μm filter membrane, and left to be determined on the machine. The HPLC conditions were referred to the method of Kong et al. (2012).

2.9 Calculation of Taste Activity Value (TAV) and Equivalent Umami Concentration (EUC)

Taste Activity Value (TAV) is the ratio of the concentration of each flavor-presenting substance in a sample to the corresponding threshold value and reflects the extent to which the flavor-presenting substance contributes to the overall flavor of the sample. TAV>1 means that the substance contributes significantly to the overall taste of the sample, and the larger the value, the greater the contribution; TAV<1 means that the substance does not contribute significantly to the overall taste of the sample (Wang et al., 2022). The formula for calculating TAV is as follows:

(3) $\mathrm{T}\mathrm{A}\mathrm{V}=\mathrm{C}/\mathrm{T}$(3)

where C is the concentration of the flavoring substance (mg/100 g) and T is the threshold value of the corresponding substance (mg/100 g).

Equivalent Umami Concentration (EUC) is the amount of umami intensity equivalent to that of a single monosodium glutamate (MSG) produced by the synergistic action of umami amino acids (Aspartic acid and Glutamic acid) and odour-presenting nucleotides (inosinic acid and adenosinic acid) (D. W. Chen & Zhang, 2007). EUC is calculated as follows:

(4) $\mathrm{E}\mathrm{U}\mathrm{C}=\hspace{0.17em}\sum \mathrm{a}\mathrm{i}\mathrm{b}\mathrm{i}+1218\times \left(\sum \mathrm{a}\mathrm{i}\mathrm{b}\mathrm{i}\right)\times \left(\sum \mathrm{a}\mathrm{j}\mathrm{b}\mathrm{j}\right)$(4)

where EUC is the MSG equivalent (g MSG/100 g); ai is the concentration of the fresh tasting amino acid; bi is the freshness factor of the fresh tasting amino acid relative to MSG (Asp: 0.077, Glu: 1); aj is the concentration of the tasting nucleotides 5ˊ-IMP, 5ˊ-AMP; bj is the freshness factor of the tasting nucleotides relative to IMP (IMP: 1, AMP: 0.18). The threshold of EUC is 30 mg/100 g.

2.10 Statistical analysis

The results were analysed using SPSS 24.0 software, and multiple comparisons were performed using the Duncan method based on One-Way ANOVA with a significance level of p < 0.05. The results were expressed as mean ± standard deviation. Plots were made using Origin 2018 software.

3. Results and discussion

3.1 Changes in the water and NaCl content of grass crap during pickling

From Figure 1, it can be seen that water content of cured grass carp in the three groups showed a gradually decreasing trend as the curing time was extended. When the marination time was 60 min, the water contents of group C, E₁, and E₂ were 74.26%, 75.25%, and 75.34%, respectively. The water contents of groups E₁ and E₂ were significantly higher than those of group C (p < 0.05), but there was no significant difference between the moisture contents of groups E₁ and E₂ (p > 0.05). It has been shown that the addition of appropriate tea polyphenols can inhibit oxidative denaturation of proteins, avoid protein degradation, and promote protein cross-linking. As a result, the network structure was strengthened and water loss was reduced (X. H. Nie et al., 2015).

Figure 1. Effects of tea polyphenols assisted pickling on water and NaCl content of grass carp.

As shown in Figure 1, the NaCl content of all three groups showed a gradual increase as the curing time increased. The NaCl content of groups E₁ and E₂ was slightly lower at the end of curing compared to group C, where the NaCl content at the end of curing was 2.86%, 3.23%, and 3.41% for groups C, E₁, and E₂, respectively. The reason for this phenomenon is that the tissue cells of the fish were disrupted by the internal osmotic pressure and external pressure, resulting in the leakage of water from the cells and the brine thus entering the interstitial space of the fish (Shen et al., 2020), thus achieving an equilibrium between the internal and external osmotic pressure.

3.2 Changes in pH of grass crap during pickling

Changes in pH of meat products are caused by the dual action of physiological and biochemical reactions of the organism and the growth and reproduction of microorganisms and are closely related to the tenderness, color, water retention, and flavor of meat, which can reflect the degree of quality change (Liu et al., 2022). pH is closely related to the freshness of meat products.

The changes in pH of grass carp meat during the marination time are shown in Table 1. As the soaking time increased, the pH values of the three groups showed similar trends with smaller changes, but an overall decreasing trend. This was consistent with the findings of Jiang et al. (2023) There was a significant difference between groups E₁ and E₂ and group C (p < 0.05). The decrease in pH was due to the gradual breakdown of lactic acid, ATP, and phosphocreatine produced by anaerobic enzymolysis of glycogen in the fish to produce phosphate, or because of the hydrolysis and denaturation of proteins, which in turn released acidic groups (Yang et al., 2020). The increase in pH may be due to the accumulation of alkaline nitrogenous substances caused by protein degradation. In this experiment, no decrease and then increase in pH was observed, probably because the short curing time did not cause the accumulation of alkaline substances. Compared with group C, it can be found that the curing assisted by tea polyphenols can effectively inhibit the rise in pH of grass carp meat, because tea polyphenols can effectively control the enzyme activity and microbial activity, thus delaying the spoilage of fish (Fan et al., 2008).

Table 1. Effects of tea polyphenols assisted pickling on the pH value of grass carp.

Pickling time/min	C	E1	E2
10	6.88 ± 0.03^aA	6.72 ± 0.01^aB	6.64 ± 0.02^bcC
20	6.87 ± 0.03^aA	6.69 ± 0.01^cB	6.68 ± 0.02^aB
30	6.88 ± 0.04^aA	6.72 ± 0.02^aB	6.69 ± 0.01^aB
40	6.89 ± 0.06^aB	6.69 ± 0.01^bcB	6.67 ± 0.01^abB
50	6.88 ± 0.07^aA	6.66 ± 0.01^dB	6.62 ± 0.02^cB
60	6.85 ± 0.06^aA	6.71 ± 0.01^abB	6.64 ± 0.01^bcB

Different lower case letters indicate significant differences between data in the same column (p < 0.05) and different capital letters indicate significant differences between data in the same row (p < 0.05). The same as below.

3.3 Changes in color characteristics of grass crap during pickling

Color has become one of the important indicators to evaluate the quality of fish. Color directly determines the appearance of the product and is one of the most important factors influencing consumers’ desire to purchase (Ocano-Higuera et al., 2011). The L* values are brightness values, and it can be seen from Table 2 that the L* values of all three groups showed a trend of increasing and then decreasing with the extension of the curing time, indicating that curing has a certain effect on the L* values of grass carp, which is consistent with the trend of L* values measured by Hao et al. (2016). This is because during the curing process, there is an osmotic pressure difference between the surface and the interior of the fish, and water loss occurs through osmosis, leading to a decrease in the refractive index of light, which in turn leads to a decrease in the L* values (Q. M. Chen et al., 2023). All three groups reached the minimum value at 60 min of curing time, with higher L* values in groups E₁ and E₂ than in group C. This is in agreement with the variation in water content described above. The color a* values represent redness and greenness, with positive values representing an increase in redness values and negative values representing an increase in greenness values. The a* values were all negative in this study, probably because the NaCl content in grass carp flesh increased with the increase of curing time, which may be related to the ring-opening degradation of some myoglobin in the dark flesh of the fillets to bilirubin under the effect of certain salt concentration after curing (Jiang, Nakazawa, et al., 2019). The a* values in groups E₁ and E₂ were significantly higher than those in group C (p < 0.05), probably because the high concentration of brine causes water loss in the fish flesh, making the water-soluble proteins containing hemoglobin to exude, which in turn leads to discoloration of the fish (Yang et al., 2020). The b* values represent yellow-blueness. Higher b* values indicate that the samples are less fresh (Yao et al., 2022). The b* values of cured grass carp fillets showed a decreasing trend with the extension of curing time. b* values were found to be related to fish fat oxidation by Hong et al. (2012). The b* values of both E₁ and E₂ groups were lower than those of group C, suggesting that tea polyphenols have a positive effect on delaying the fat oxidation of grass carp fish flesh.

Table 2. Effects of tea polyphenols assisted pickling on the color of grass carp.

color	pickling time/min	C	E1	E2
L*	10	41.83 ± 0.60^bB	43.27 ± 0.15^bA	43.10 ± 0.70^bcA
	20	45.50 ± 1.71^aA	45.63 ± 1.56^aA	42.13 ± 0.76^cdB
	30	45.20 ± 0.95^aA	44.03 ± 0.38^bA	43.97 ± 1.18^abA
	40	41.90 ± 1.15^bAB	43.77 ± 1.07^bA	41.10 ± 0.61^deB
	50	41.77 ± 1.37^bB	44.47 ± 0.67^abA	44.67 ± 0.76^aA
	60	41.33 ± 1.67^bA	41.57 ± 0.35^cA	40.13 ± 0.32^eA
a*	10	−2.60 ± 0.10^aA	−2.47 ± 0.21^aA	−2.50 ± 0.17^aA
	20	−2.83 ± 0.06^abA	−2.73 ± 0.25^aA	−2.53 ± 0.21^aA
	30	−2.77 ± 0.06^abB	−2.63 ± 0.12^aAB	−2.27 ± 0.32^aA
	40	−3.10 ± 0.46^bA	−2.90 ± 0.44^aA	−2.37 ± 0.23^aA
	50	−3.17 ± 0.25^bB	−2.83 ± 0.31^aAB	−2.43 ± 0.21^aA
	60	−2.77 ± 0.06^abB	−2.57 ± 0.15^aA	−2.47 ± 0.06^aA
b*	10	−0.73 ± 0.15^aA	−0.87 ± 0.21^aA	−0.83 ± 0.15^aA
	20	−0.80 ± 0.35^aA	−1.40 ± 0.10^bB	−0.90 ± 0.20^aA
	30	−1.13 ± 0.12^aA	−1.50 ± 0.26^bcA	−1.23 ± 0.15^bA
	40	−1.80 ± 0.26^bAB	−1.83 ± 0.21^cB	−1.40 ± 0.10^bA
	50	−1.73 ± 0.21^bA	−1.63 ± 0.15^bcA	−1.47 ± 0.21^bA
	60	−1.93 ± 0.12^bB	−1.47 ± 0.06^bA	−1.30 ± 0.20^bA

Different lower case letters indicate significant differences between data in the same column (p < 0.05) and different capital letters indicate significant differences between data in the same row (p < 0.05).

3.4 Changes in texture properties of grass crap during pickling

Texture can reflect the tissue state of meat products and is the most important quality characteristic for evaluating muscle foods, which is usually expressed as hardness, springness, cohesiveness, and chewiness (Jiang et al., 2023). Hardness is the force required to deform the food to a certain degree when chewed by the teeth and is one of the most direct indicators of taste; elasticity is the property of the food to recover its original size and shape after deformation. Cohesion refers to the relative resistance to a second compression after the first compression and deformation of the food; mastication indicates the resistance to the force of pressing, which is calculated according to the hardness (Yilmaz et al., 2012).

As can be seen from Table 3, the overall hardness and chewiness of grass carp meat showed an increasing trend as the curing time increased. With longer curing time, the fish meat was dehydrated due to internal and external osmotic pressures, and salt gradually accumulated in the fish meat, causing the salt-soluble muscle proteins in the fish meat to leach out, changing the muscle fiber morphology and leading to an increase in hardness and thus chewiness. The hardness and chewiness of E₁ and E₂ groups were significantly higher than those of C group (p < 0.05), presumably because tea polyphenols inhibited the oxidative denaturation of proteins and protein degradation is slower, making the myofibrillar protein meshwork tighter (Jiao et al., 2022); or tea polyphenols bind to myofibrillar protein, which in turn induces changes in protein conformation and enhances the connectivity of the meshwork in fish proteins (Y. L. Xu & Xu, 2021).

Table 3. Effects of tea polyphenols assisted pickling on the texture of grass carp.

	picking time/min	C	E1	E2
Hardness	10	1920.12 ± 145.74^bC	2149.78 ± 73.03^cB	2506.84 ± 40.79^bA
	20	1410.61 ± 78.43^cdB	2564.43 ± 71.33^bA	2435.14 ± 145.63^bA
	30	1780.73 ± 126.91^bcB	2671.07 ± 87.17^abA	2391.22 ± 144.06^bA
	40	1293.25 ± 97.46^dC	2817.79 ± 72.57^aA	2140.40 ± 105.82^cB
	50	2493.63 ± 100.00^aB	1730.82 ± 93.10^dC	2796.11 ± 42.54^aA
	60	2571.63 ± 99.03^aA	2170.77 ± 74.68^cB	2953.97 ± 43.04^aA
Springness	10	0.92 ± 0.02^aA	0.74 ± 0.01^aB	0.49 ± 0.00^abC
	20	0.88 ± 0.01^bA	0.62 ± 0.01^bB	0.50 ± 0.01^aC
	30	0.88 ± 0.01^bA	0.55 ± 0.01^cB	0.48 ± 0.02^abC
	40	0.89 ± 0.02^bA	0.46 ± 0.00^fB	0.45 ± 0.01^dB
	50	0.88 ± 0.00^bA	0.51 ± 0.02^dB	0.47 ± 0.01^bcC
	60	0.88 ± 0.00^bA	0.49 ± 0.00^eB	0.47 ± 0.00^cC
Cohesiveness	10	0.52 ± 0.01^bAB	0.53 ± 0.01^aA	0.51 ± 0.01^aB
	20	0.54 ± 0.01^aA	0.52 ± 0.01^aB	0.51 ± 0.02^abB
	30	0.52 ± 0.02^bA	0.46 ± 0.01^cB	0.50 ± 0.00^abcA
	40	0.52 ± 0.00^bA	0.50 ± 0.01^bB	0.48 ± 0.01^cdB
	50	0.50 ± 0.01^aA	0.49 ± 0.01^bAB	0.48 ± 0.01^dAB
	60	0.46 ± 0.01^aB	0.49 ± 0.01^bA	0.49 ± 0.00^bcA
Chewiness	10	695.01 ± 19.17^bcB	731.02 ± 16.53^bcAB	751.9 ± 21.84^aA
	20	635.21 ± 20.30^cB	737.159 ± 5.21^bcA	756.04 ± 39.92^aA
	30	790.34 ± 53.70^aA	758.33 ± 23.49^abA	798.66 ± 43.21^aA
	40	759.01 ± 33.07^abA	711.83 ± 18.75^cA	742.02 ± 17.11^aA
	50	758.96 ± 39.98^abA	727.87 ± 21.12^bcA	771.77 ± 38.19^aA
	60	731.11 ± 29.32^abA	777.481 ± 11.23^aA	774.26 ± 38.86^aA

Different lower case letters indicate significant differences between data in the same column (p < 0.05) and different capital letters indicate significant differences between data in the same row (p < 0.05).

As can be seen from Table 3, the change in elasticity during marination showed an opposite trend to the change in hardness. Elasticity in group C changed significantly (p < 0.05) at 10 min of marination compared to 20 min and not significantly (p > 0.05) after 20 min, but the overall change was not significant. The elasticity of the tea polyphenol treated group changed significantly (p < 0.05) with time. The change in cohesiveness of the three groups also showed a similar trend and reached a maximum at 10 min of marination. This may be due to the decrease in water content in the fish during the marination process and denaturation of the proteins, which reduced their gel properties and manifested as a decrease in fish elasticity and cohesiveness.

3.5 Changes in FAA and TAV values of grass crap during pickling

Free amino acids are the main products of protein degradation, and they are thought to make an important contribution to flavor. Flavor evaluation in muscle is related to the composition and content of free amino acids, and the threshold and content of free amino acids will influence its flavor profile (Yin et al., 2022). The different free amino acids exhibit different taste characteristics depending on their structural characteristics, which influence the acceptance of the product. The amino acids presenting the fresh taste are asparagine (Asp) and glutamic acid (Glu), the sweet substances are threonine (Thr), serine (Ser), glycine (Gly), alanine (Ala), and proline (Pro) and the bitter substances are cysteine (Cys), valine (Val), methionine (Met) isoleucine (Ile), leucine (Leu), tyrosine (Tyr), phenylalanine (Phe), histidine (His), and arginine (Arg).

According to Tables 4–6, 17 free amino acids were detected in all three cured groups. Compared with the fresh samples, the total amount of free amino acids in all three groups showed a certain degree of decrease during the curing process (p < 0.05), probably because some of the free amino acids were freed into the curing solution due to water loss during the high salt curing process, which is consistent with the results of Wang^[26] et al. After curing, the contents of Gly, His and Pro in the meat of the three groups of grass carp were relatively high. The content of His was well above the threshold value of 20 mg/100 g. It was studied that histidine could produce the characteristic flavor of fish meat (Qin et al., 2022). It is worth noting that the histidine content of both E₁ and E₂ groups was lower than that of C group during the curing process, indicating that the addition of tea polyphenols could reduce the bitterness and improve the taste of grass carp cured products. For sweet amino acids, the glycine content of grass carp increased after curing, from 113.76 mg/100 g (0 min) to 115.02 mg/100 g (group C), 114.62 mg/100 g (group E₁), and 181.50 mg/100 g (group E₂). For the freshness amino acids, the aspartic acid content of grass carp increased after curing, from 0.66 (0 min) to 1.07 mg/100 g (group C), 1.21 mg/100 g (group E₁), and 0.90 mg/100 g (group E₂). This indicates that curing has a positive effect on the fresh sweet taste of grass carp cured products.

Table 4. Changes in the composition and content of free amino acids during curing in group C.

		Contents (mg/100g)
Types	Threshold (mg/100g)	0min	10min	20min	30min	40min	50min	60min
Asp*	3	0.66 ± 0.02^b	1.13 ± 0.11^a	1.27 ± 0.19^a	1.15 ± 0.10^a	1.15 ± 0.13^a	1.14 ± 0.12^a	1.07 ± 0.04^a
Thr*	260	19.04 ± 0.08^a	15.60 ± 1.21^b	16.73 ± 0.40^b	16.78 ± 0.77^b	16.75 ± 1.46^b	17.24 ± 1.02^b	17.13 ± 1.32^b
Ser*	150	6.91 ± 0.19^b	7.07 ± 0.21^b	7.43 ± 0.36^ab	7.76 ± 0.32^ab	7.20 ± 1.06^ab	6.81 ± 0.69^b	9.11 ± 1.79^a
Glu*	30	1.75 ± 0.05^c	1.82 ± 0.03^d	2.57 ± 0.12^c	2.78 ± 0.06^c	3.02 ± 0.14^bc	3.31 ± 0.15^ab	3.77 ± 0.58^a
Gly*	130	113.76 ± 4.58^ab	105.48 ± 5.97^b	109.72 ± 3.76^ab	115.20 ± 1.51^a	117.04 ± 4.49^a	116.41 ± 3.70^a	115.02 ± 3.84^a
Ala*	60	33.35 ± 1.55^b	33.30 ± 3.40^b	33.15 ± 2.68^b	37.56 ± 2.92^ab	37.07 ± 3.32^ab	42.44 ± 3.99^a	43.03 ± 4.75^a
Cys	ND	1.64 ± 0.12^a	1.59 ± 0.11^a	1.38 ± 0.38^a	0.60 ± 0.05^b	0.62 ± 0.14^b	0.63 ± 0.14^b	0.74 ± 0.14^b
Val^▲	40	10.98 ± 0.28^a	6.38 ± 0.05^a	6.05 ± 0.55^a	4.50 ± 0.34^b	4.19 ± 0.21^b	4.53 ± 0.38^b	4.68 ± 0.17^b
Met^▲	30	2.45 ± 0.09^a	2.07 ± 0.03^c	2.35 ± 0.17^c	1.20 ± 0.19^d	3.13 ± 0.44^ab	3.00 ± 0.17^b	3.65 ± 0.43^a
Ile^▲	90	7.69 ± 0.28^a	4.85 ± 0.13^b	4.82 ± 0.68^b	4.90 ± 0.38^b	4.69 ± 0.31^b	4.70 ± 0.49^b	4.60 ± 0.45^b
Leu^▲	190	11.13 ± 0.63^a	6.40 ± 0.26^b	6.50 ± 0.78^b	7.12 ± 0.52^b	6.61 ± 0.33^b	7.12 ± 0.41^b	6.73 ± 0.22^b
Tyr^▲	ND	10.17 ± 0.74^a	9.81 ± 1.61^a	8.33 ± 0.53^b	2.58 ± 0.17^c	2.79 ± 0.65^c	2.55 ± 0.38^c	2.27 ± 0.41^c
Phe^▲	90	5.14 ± 0.15^a	3.42 ± 1.48^a	2.34 ± 0.38^b	4.09 ± 0.16^a	3.99 ± 0.29^a	4.30 ± 0.11^a	4.44 ± 0.26^a
Lys^▲	50	15.93 ± 0.81^a	13.52 ± 1.41^ab	13.25 ± 1.54^b	14.76 ± 0.70^ab	15.14 ± 0.43^a	14.31 ± 0.35^ab	14.35 ± 0.05^ab
His^▲	20	301.67 ± 3.29^b	329.59 ± 8.29^ab	343.38 ± 7.89^a	310.53 ± 7.93^bc	304.77 ± 13.76^c	325.53 ± 6.28^ab	310.67 ± 10.81^bc
Arg^▲	50	5.72 ± 0.35^a	4.47 ± 0.23^c	4.86 ± 0.22^bc	5.43 ± 0.28^a	4.84 ± 0.41^bc	5.15 ± 0.20^ab	4.90 ± 0.20^abc
Pro*	300	33.95 ± 1.06^a	15.45 ± 0.91^b	18.09 ± 0.89^a	16.96 ± 0.27^a	17.68 ± 0.40^a	13.42 ± 0.75^c	13.25 ± 1.24^c
TFAA		581.93 ± 10.01^a	561.90 ± 8.37^bc	582.22 ± 4.39^a	553.90 ± 11.62^bc	550.68 ± 7.49^c	569.94 ± 6.40^ab	561.07 ± 9.53^bc

Different letters in the same row indicate significant differences (p < 0.05). *denotes sweet amino acids; ^▲denotes bitter amino acids; ND denotes threshold not found. TFAA denotes total free amino acids.

Table 5. Changes in the composition and content of free amino acids during curing in group E₁.

		Contents (mg/100g)
Types	Threshold (mg/100g)	0min	10min	20min	30min	40min	50min	60min
Asp*	3	0.66 ± 0.02^c	1.43 ± 0.06^a	1.14 ± 0.09^b	1.21 ± 0.19^ab	1.23 ± 0.09^ab	1.22 ± 0.23^ab	1.21 ± 0.05^ab
Thr*	260	19.04 ± 0.08^a	12.34 ± 0.56^b	11.85 ± 1.27^b	11.22 ± 0.65^b	12.35 ± 0.52^b	11.25 ± 0.40^b	11.85 ± 1.26^b
Ser*	150	6.91 ± 0.19^a	5.58 ± 0.25^ab	5.05 ± 0.60^b	5.52 ± 0.25^ab	5.87 ± 0.14^a	5.36 ± 0.60^ab	5.06 ± 0.19^b
Glu*	30	1.75 ± 0.05^b	2.66 ± 0.35^a	2.94 ± 0.37^a	2.85 ± 0.15^a	3.21 ± 0.07^a	3.13 ± 0.24^a	3.22 ± 0.65^a
Gly*	130	113.76 ± 4.58^b	129.29 ± 7.21^ab	135.38 ± 3.73^a	138.16 ± 2.15^a	134.08 ± 7.54^a	122.05 ± 5.09^bc	114.62 ± 1.00^c
Ala*	60	33.35 ± 1.55^b	38.38 ± 4.18^a	37.89 ± 2.82^a	38.83 ± 5.21^a	35.11 ± 1.52^b	35.58 ± 3.82^b	34.54 ± 3.80^b
Cys	ND	1.64 ± 0.12^a	0.67 ± 0.08^a	0.51 ± 0.07^bc	0.41 ± 0.12^c	0.65 ± 0.04^a	0.47 ± 0.05^bc	0.59 ± 0.03^ab
Val^▲	40	10.98 ± 0.28^a	5.58 ± 0.61^b	5.34 ± 0.29^b	4.89 ± 0.71^b	5.18 ± 0.76^b	5.00 ± 0.37^b	5.63 ± 0.36^b
Met^▲	30	2.45 ± 0.09^b	2.93 ± 0.61^a	3.18 ± 0.88^a	2.59 ± 0.13^b	3.38 ± 0.26^a	2.67 ± 0.13^b	2.48 ± 0.18^b
Ile^▲	90	7.69 ± 0.28^a	5.83 ± 0.61^a	5.74 ± 0.57^a	4.80 ± 0.14^b	4.90 ± 0.25^b	5.29 ± 0.34^ab	5.71 ± 0.19^a
Leu^▲	190	11.13 ± 0.63^a	10.14 ± 0.56^a	9.81 ± 0.94^a	6.95 ± 0.11^c	8.81 ± 1.70^ab	7.37 ± 0.29^bc	9.48 ± 0.37^a
Tyr^▲	ND	10.17 ± 0.74^a	2.53 ± 0.38^b	2.72 ± 0.28^b	2.21 ± 0.57^b	2.61 ± 0.26^b	2.69 ± 0.25^b	2.83 ± 0.32^b
Phe^▲	90	5.14 ± 0.15^a	5.04 ± 0.47^b	4.68 ± 0.15^b	4.52 ± 0.48^b	4.83 ± 0.55^b	4.32 ± 0.29^b	4.67 ± 0.41^b
Lys^▲	50	15.93 ± 0.81^a	13.95 ± 2.41^b	13.34 ± 1.52^b	12.32 ± 0.85^b	12.20 ± 2.14^b	13.24 ± 2.76^b	12.64 ± 1.85^b
His^▲	20	301.67 ± 3.29^b	314.37 ± 5.47^a	268.72 ± 23.01^b	225.97 ± 19.58^c	243.89 ± 11.78^bc	254.19 ± 3.01^b	242.57 ± 9.46^bc
Arg^▲	50	5.72 ± 0.35^a	3.83 ± 0.43^ab	4.04 ± 0.73^ab	3.69 ± 0.28^b	4.84 ± 0.49^a	4.85 ± 0.89^a	3.37 ± 0.28^b
Pro*	300	33.95 ± 1.06^a	21.17 ± 1.60^b	22.44 ± 0.92^b	23.88 ± 0.96^b	20.91 ± 3.56^c	24.68 ± 3.48^c	23.60 ± 2.82^c
TFAA		581.93 ± 10.01^a	575.71 ± 11.11^a	534.77 ± 33.82^ab	490.01 ± 13.64^bc	504.06 ± 11.05^bc	503.36 ± 18.77^bc	487.42 ± 38.32^c

Different letters in the same row indicate significant differences (p < 0.05). *denotes sweet amino acids; ^▲denotes bitter amino acids; ND denotes threshold not found. TFAA denotes total free amino acids.

Table 6. Changes in the composition and content of free amino acids during curing in group E₂.

		Contents (mg/100g)
Types	Threshold (mg/100g)	0min	10min	20min	30min	40min	50min	60min
Asp*	3	0.66 ± 0.02^c	1.09 ± 0.14^ab	1.24 ± 0.14^a	1.00 ± 0.13^ab	0.91 ± 0.20^b	0.79 ± 0.09^b	0.90 ± 0.28^b
Thr*	260	19.04 ± 0.08^a	13.50 ± 1.28^ab	14.23 ± 0.57^a	12.93 ± 0.81^ab	14.47 ± 1.10^a	12.35 ± 0.45^b	11.74 ± 1.06^b
Ser*	150	6.91 ± 0.19^a	4.97 ± 0.07^b	4.77 ± 0.34^b	4.56 ± 0.22^b	4.52 ± 0.11^b	4.70 ± 0.22^b	4.82 ± 0.49^b
Glu*	30	1.75 ± 0.05^a	1.52 ± 0.09^ab	1.19 ± 0.09^b	1.19 ± 0.28^b	1.71 ± 0.40^a	1.41 ± 0.29^ab	1.38 ± 0.39^ab
Gly*	130	113.76 ± 4.58^c	202.47 ± 4.15^a	201.15 ± 4.93^a	191.32 ± 2.06^ab	192.37 ± 7.47^ab	178.94 ± 12.44^b	181.50 ± 8.90^b
Ala*	60	33.35 ± 1.55^b	38.81 ± 2.10^a	36.36 ± 3.10^ab	36.54 ± 2.15^ab	32.36 ± 1.33^bc	29.10 ± 3.53^c	29.38 ± 3.10^c
Cys	ND	1.64 ± 0.12^a	1.32 ± 0.37^ab	1.27 ± 0.07^ab	1.34 ± 0.25^ab	1.58 ± 0.39^a	1.44 ± 0.13^ab	0.96 ± 0.18^b
Val^▲	40	10.98 ± 0.28^a	3.50 ± 0.24^a	3.47 ± 0.06^ab	3.32 ± 0.13^ab	3.55 ± 0.32^a	3.46 ± 0.23^ab	3.07 ± 0.06^b
Met^▲	30	2.45 ± 0.09^a	1.42 ± 0.20^b	1.68 ± 0.04^b	1.71 ± 0.13^b	1.40 ± 0.19^b	1.69 ± 0.14^b	1.34 ± 0.40^b
Ile^▲	90	7.69 ± 0.28^a	2.48 ± 0.30^b	2.55 ± 0.06^b	2.27 ± 0.11^b	2.55 ± 0.21^b	2.47 ± 0.33^b	2.17 ± 0.08^b
Leu^▲	190	11.13 ± 0.63^a	5.81 ± 0.64^ab	5.97 ± 0.08^a	5.30 ± 0.58^ab	5.89 ± 0.45^ab	5.08 ± 0.44^ab	4.99 ± 0.41^b
Tyr^▲	ND	10.17 ± 0.74^a	1.84 ± 0.37^a	1.34 ± 0.22^ab	1.52 ± 0.41^ab	0.97 ± 0.11^b	1.55 ± 0.38^ab	1.35 ± 0.28^ab
Phe^▲	90	5.14 ± 0.15^a	2.67 ± 0.03^b	2.63 ± 0.06^b	2.67 ± 0.27^b	2.61 ± 0.10^b	2.71 ± 0.28^b	2.47 ± 0.08^b
Lys^▲	50	15.93 ± 0.81^a	8.17 ± 0.78^a	8.26 ± 0.18^ab	8.74 ± 1.05^ab	8.59 ± 0.45^ab	9.50 ± 0.49^a	8.97 ± 0.28^ab
His^▲	20	301.67 ± 3.29^a	265.53 ± 19.21^ab	263.26 ± 16.37^ab	272.86 ± 13.41^ab	246.15 ± 23.60^ab	280.23 ± 9.75^a	241.96 ± 21.96^b
Arg^▲	50	5.72 ± 0.35^a	1.93 ± 0.20^ab	1.61 ± 0.12^c	1.53 ± 0.12^c	1.73 ± 0.08^bc	2.04 ± 0.08^a	1.62 ± 0.20^c
Pro*	300	33.95 ± 1.06^a	22.00 ± 2.13^ab	22.26 ± 1.17^ab	20.42 ± 0.33^b	20.63 ± 1.37^b	24.38 ± 2.84^a	20.30 ± 0.67^b
TFAA		581.93 ± 10.01^a	579.05 ± 16.76^a	573.26 ± 11.44^a	569.23 ± 17.35^a	541.99 ± 26.07^ab	561.82 ± 22.80^a	518.86 ± 7.74^b

Different letters in the same row indicate significant differences (p < 0.05). *denotes sweet amino acids; ^▲denotes bitter amino acids; ND denotes threshold not found. TFAA denotes total free amino acids.

In order to further observe the contribution of each free amino acid to the taste of the product, its contribution to the taste should also be indicated by calculating the TAV value of the amino acid; TAV>1 indicates that the amino acid contributed significantly to the overall taste of the sample; TAV<1 indicates that the amino acid did not contribute significantly to the overall taste of the sample. As can be seen from Table 7, the TAV values of fresh sweet free amino acids in grass carp in group C were all less than 1. After the addition of tea polyphenols, the TAV values of Gly in group E₁ were greater than 1 after 20 min, 30 min, and 40 min of curing, and the TAV values of Gly in group E₂ were all greater than 1 during the curing process (10–60 min), while the TAV values of other fresh sweet amino acids were all less than 1, indicating that Gly contributed more to the taste of grass carp cured products.

Table 7. Effects of tea polyphenols assisted pickling on the taste activity values of fresh and sweet amino acids in grass carp cured products.

		TAV
	Pickling time/min	Asp	Thr	Ser	Glu	Gly	Pro
C	0	0.22 ± 0.01^b	0.07 ± 0.00^a	0.05 ± 0.00^a	0.06 ± 0.00^d	0.87 ± 0.04^a	0.11 ± 0.00^a
	10	0.38 ± 0.04^a	0.06 ± 0.00^a	0.05 ± 0.00^a	0.06 ± 0.00^d	0.81 ± 0.04^b	0.05 ± 0.00^c
	20	0.42 ± 0.06^a	0.06 ± 0.00^a	0.05 ± 0.00^a	0.09 ± 0.00^c	0.84 ± 0.03^ab	0.06 ± 0.00^b
	30	0.38 ± 0.03^a	0.06 ± 0.00^a	0.05 ± 0.00^a	0.09 ± 0.00^c	0.89 ± 0.01^a	0.06 ± 0.00^b
	40	0.38 ± 0.03^a	0.06 ± 0.00^a	0.05 ± 0.00^a	0.10 ± 0.00^bc	0.9 ± 0.04^a	0.06 ± 0.00^b
	50	0.38 ± 0.04^a	0.05 ± 0.02^a	0.05 ± 0.00^a	0.11 ± 0.00^ab	0.89 ± 0.03^a	0.04 ± 0.00^c
	60	0.35 ± 0.01^a	0.07 ± 0.01^a	0.06 ± 0.02^b	0.13 ± 0.02^a	0.88 ± 0.03^a	0.04 ± 0.00^c
E1	0	0.22 ± 0.01^c	0.07 ± 0.00^a	0.05 ± 0.00^a	0.06 ± 0.00^a	0.87 ± 0.04^c	0.11 ± 0.00^a
	10	0.48 ± 0.02^a	0.05 ± 0.00^b	0.04 ± 0.00^b	0.09 ± 0.01^b	0.99 ± 0.06^ab	0.11 ± 0.00^b
	20	0.38 ± 0.03^b	0.05 ± 0.00^bc	0.04 ± 0.00^c	0.10 ± 0.01^b	1.04 ± 0.03^a	0.07 ± 0.01^b
	30	0.40 ± 0.07^ab	0.04 ± 0.00^bc	0.04 ± 0.00^b	0.09 ± 0.00^b	1.06 ± 0.02^a	0.08 ± 0.00^b
	40	0.41 ± 0.03^ab	0.05 ± 0.00^b	0.04 ± 0.00^b	0.11 ± 0.00^b	1.03 ± 0.06^a	0.08 ± 0.00^b
	50	0.40 ± 0.07^ab	0.04 ± 0.00^c	0.03 ± 0.00^c	0.10 ± 0.00^b	0.92 ± 0.06^bc	0.07 ± 0.01^b
	60	0.40 ± 0.02^ab	0.05 ± 0.00^bc	0.03 ± 0.00^c	0.11 ± 0.02^b	0.88 ± 0.01^c	0.08 ± 0.01^b
E2	0	0.22 ± 0.01^c	0.07 ± 0.00^a	0.05 ± 0.00^a	0.06 ± 0.00^a	0.87 ± 0.04^c	0.11 ± 0.00^a
	10	0.36 ± 0.05^ab	0.05 ± 0.00^bc	0.03 ± 0.00^a	0.05 ± 0.00^ab	1.56 ± 0.04^a	0.08 ± 0.00^bc
	20	0.41 ± 0.05^a	0.05 ± 0.00^bc	0.03 ± 0.00^a	0.04 ± 0.00^b	1.55 ± 0.04^a	0.07 ± 0.00^bc
	30	0.34 ± 0.05^ab	0.05 ± 0.00^bc	0.03 ± 0.00^a	0.04 ± 0.00^ab	1.47 ± 0.02^ab	0.07 ± 0.00^bc
	40	0.30 ± 0.06^bc	0.06 ± 0.00^bc	0.03 ± 0.00^a	0.06 ± 0.02^ab	1.48 ± 0.06^ab	0.07 ± 0.0^c
	50	0.27 ± 0.03^bc	0.05 ± 0.00^bc	0.04 ± 0.00^a	0.05 ± 0.01^ab	1.38 ± 0.09^b	0.08 ± 0.01^b
	60	0.30 ± 0.09^bc	0.05 ± 0.00^c	0.03 ± 0.00^a	0.05 ± 0.01^ab	1.40 ± 0.06^b	0.07 ± 0.00^bc

Different lower case letters indicate significant differences between data in the same column (p < 0.05).

3.6 Changes in ATP-related compounds and TAV values of grass crap during pickling

Nucleotides are an important indicator of fish quality and flavor, and ATP is broken down by a series of endogenous enzymes into related nucleotide compounds: adenosine diphosphate (ADP), adenosine monophosphate (AMP), inosinic acid (IMP), hypoxanthine adenosine (HxR), and hypoxanthine (Hx). IMP is an important freshness enhancer in aquatic products and has a synergistic effect with L-glutamic acid (Takashi et al., 2008). AMP can inhibit bitterness, and IMP and AMP play a synergistic role in the freshness and sweetness of aquatic products. HxR and Hx are important bitterness components in fish, and their excessive accumulation can affect the overall flavor of fish (Q. Li et al., 2017).

As can be seen from Table 8, the most abundant in the three treatment groups was IMP, followed by HxR. However, the differences between the three groups for each type of nucleotide compound were not significant (p > 0.05). As a whole, both IMP and AMP showed a decreasing trend with the extension of curing time, while HxR and Hx contents were influenced by IMP and showed an increasing trend. This is similar to the findings of Wu et al. (2020) on grass carp cured products. Hx is the end product of the ATP degradation pathway and has a bitter taste. As the reaction progressed, ATP degradation was greater and HxR and Hx accumulated in the grass carp meat. At 60 min, the IMP contents of E₁ and E₂ groups were 89.72 mg/100 g and 91.20 mg/100 g, respectively, which were higher than those of group C (87.55 mg/100 g), indicating that the addition of tea polyphenols had a positive effect on improving the taste of grass carp marinated products.

Table 8. Effects of tea polyphenols assisted pickling on the nucleotide compounds in grass carp cured products.

		Contents (mg/100g)						TAV
	pickling time/min	IMP	ATP	ADP	AMP	Hx	HxR	IMP	AMP
C	10	106.63 ± 7.42^a	3.47 ± 0.27^a	9.35 ± 0.18^a	1.87 ± 0.12^a	1.11 ± 0.05^c	11.58 ± 1.82^d	4.27 ± 0.30^a	0.019 ± 0.00^a
	20	101.36 ± 5.25^ab	3.35 ± 0.32^a	9.10 ± 0.10^ab	1.72 ± 0.29^a	1.18 ± 0.10^c	13.04 ± 1.54^cd	4.05 ± 0.21^ab	0.017 ± 0.00^a
	30	94.16 ± 0.51^bc	2.64 ± 0.06^b	8.89 ± 0.09^b	1.66 ± 0.14^ab	1.39 ± 0.03^b	14.85 ± 0.96^c	3.99 ± 0.29^ab	0.017 ± 0.00^ab
	40	92.12 ± 1.02^bc	2.44 ± 0.24^b	8.19 ± 0.36^c	1.63 ± 0.06^ab	1.47 ± 0.11^b	15.76 ± 1.12^bc	3.77 ± 0.08^b	0.016 ± 0.00^ab
	50	92.82 ± 2.75^bc	2.79 ± 0.07^b	8.02 ± 0.02^c	1.38 ± 0.10^bc	1.67 ± 0.06^a	17.99 ± 1.12^ab	3.80 ± 0.26^ab	0.014 ± 0.00^bc
	60	87.55 ± 4.71^c	2.69 ± 0.16^b	8.31 ± 0.14^c	1.34 ± 0.06^c	1.74 ± 0.09^a	19.79 ± 1.63^a	3.79 ± 0.06^ab	0.013 ± 0.00^c
E1	10	108.33 ± 4.59^a	3.88 ± 0.21^a	9.36 ± 0.24^a	1.95 ± 0.19^a	1.15 ± 0.08^b	11.80 ± 0.99^c	4.33 ± 0.18^a	0.020 ± 0.00^a
	20	100.12 ± 6.70^b	3.63 ± 0.43^a	9.26 ± 0.33^a	1.84 ± 0.16^ab	1.14 ± 0.06^b	13.11 ± 0.87^bc	4.00 ± 0.28^b	0.018 ± 0.00^ab
	30	97.48 ± 0.32^bc	2.97 ± 0.04^b	9.40 ± 0.25^a	1.58 ± 0.01^c	1.12 ± 0.12^b	14.12 ± 0.09^ab	3.90 ± 0.01^bc	0.016 ± 0.00^c
	40	95.74 ± 2.08^bc	3.58 ± 0.15^a	9.23 ± 0.11^ab	1.58 ± 0.08^c	1.22 ± 0.11^b	14.29 ± 0.43^ab	3.82 ± 0.08^bc	0.016 ± 0.00^c
	50	93.12 ± 3.65^bc	2.09 ± 0.31^c	8.93 ± 0.16^ab	1.71 ± 0.02^bc	1.51 ± 0.05^a	13.88 ± 0.51^ab	3.72 ± 0.15^bc	0.017 ± 0.00^bc
	60	89.72 ± 1.2^c	2.43 ± 0.32^c	8.76 ± 0.32^b	1.78 ± 0.04^abc	1,61 ± 0.09^a	15.31 ± 1.53^a	3.59 ± 0.05^c	0.018 ± 0.00^abc
E2	10	102.54 ± 4.75^a	3.58 ± 0.18^a	9.63 ± 0.41^a	1.62 ± 0.04^ab	1.04 ± 0.08^d	13.33 ± 0.95^b	4.10 ± 0.19^a	0.016 ± 0.00^ab
	20	95.57 ± 4.76^ab	3.47 ± 0.03^a	8.91 ± 0.29^b	1.59 ± 0.15^ab	1.23 ± 0.09^c	13.38 ± 1.14^b	3.82 ± 0.19^ab	0.016 ± 0.00^ab
	30	95.46 ± 4.42^ab	3.07 ± 0.27^b	9.10 ± 0.18^b	1.65 ± 0.04^a	1.31 ± 0.08^c	14.97 ± 0.53^ab	3.82 ± 0.18^ab	0.016 ± 0.00^a
	40	92.36 ± 4.25^b	2.95 ± 0.28^b	8.25 ± 0.08^c	1.50 ± 0.03^b	1.33 ± 0.06^c	15.99 ± 0.75^ab	3.69 ± 0.17^ab	0.015 ± 0.00^b
	50	94.15 ± 5.02^ab	2.93 ± 0.17^b	8.36 ± 0.39^c	1.57 ± 0.02^ab	1.49 ± 0.07^b	14.71 ± 2.83^ab	3.77 ± 0.20^ab	0.016 ± 0.00^ab
	60	91.20 ± 5.58^b	2.81 ± 0.19^b	8.69 ± 0.06^bc	1.26 ± 0.02^c	1.82 ± 0.03^a	16.82 ± 2.56^a	3.65 ± 0.22^b	0.013 ± 0.00^c

Different lower case letters indicate significant differences between data in the same column (p < 0.05).

3.7 Changes in EUC and TAV values of grass crap during pickling

The amount of monosodium glutamate (EUC) is the amount of monosodium glutamate produced by the synergistic action of two amino acids (Glu and Asp) and two amino acids (IMP, AMP). As a taste evaluation index, this index can directly reflect the contribution of umami synergistic effect between umami substances to taste. The changes of EUC and TAV in the curing process of grass carp are shown in Figures 2 and 3. The TAV values of EUC values in all groups were greater than 1, indicating that the synergistic effect of two free amino acids and three flavor-enhancing nucleotides had a significant effect on the overall taste of grass carp during the curing process. The MSG equivalents of group C increased significantly from 1.01 g MSG/100 g to 1.57 g MSG/100 g (p < 0.05), indicating that the curing method can improve the freshness of the fish. The EUC values in group E₁ and group E₂ showed a trend of decreasing first and then increasing. The minimum value of group E₁ was 1.39 g MSG/100 g after curing for 30 min, and the minimum value of group E₂ was 0.59 g MSG/100 g after curing for 20 min. During the curing process, the EUC content of group E₁ was higher than that of group C, while that of group E₂ was significantly lower than that of groups C and E₁. TAV values also showed the same results. It is conjectured that the addition of the right amount of tea polyphenols can effectively maintain the freshness of the fish, while too many tea polyphenols may adversely affect the freshness of the fish. Therefore, combining EUC and TAV, the addition of 0.1% TP to the marinade could increase the freshness of grass carp marinated products.

Figure 2. Effects of tea polyphenols assisted pickling on EUC of grass carp Different lower case letters indicate significant differences between data in the same column (p < 0.05).

Figure 3. Effects of tea polyphenols assisted pickling on TAV of grass carp Different lower case letters indicate significant differences between data in the same column (p < 0.05).

4 Conclusion

This paper investigates the effect of adding different concentrations of tea polyphenols to the curing solution on the quality of grass carp cured products. The main findings were as follows: (1) With the extension of curing time, the water content of groups E₁ and E₂ was higher than that of group C, while the salt content was lower than that of group C. The addition of tea polyphenols has a positive effect on maintaining the water content of fish as the curing time increases (p < 0.05). The pH of fish meat decreased slowly with the increase in pickling time, and the pH of E₁ and E₂ groups was slightly lower than that of C group. The hardness and chewiness of the grass carp meat increased and the elasticity and cohesiveness decreased in the group with the addition of tea polyphenols. (2) The total free amino acid content of all groups decreased as the curing time increased, and the His content of groups E₁ and E₂ was lower than that of group C, indicating that the addition of tea polyphenols could reduce the bitterness and improve the taste of the cured grass carp products. IMP and AMP in all treatment groups showed a decreasing trend with the increase in marination time, while HxR and Hx contents showed an increasing trend. At 60 min of curing, the IMP content in groups E₁ and E₂ was higher than that in group C. The TAV values of all treatment groups were greater than 1, indicating that the synergistic effect of fresh flavor amino acids and flavor-presenting nucleotides contributed significantly to the taste.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Key R&D Program of China (Grant No. 2018YFD0901003) and the Yantai science and technology innovation development plan project (Grant No. 2022YT0620061).