Comprehensive Textural Characterization of Tuna Muscle to Drive Development of Plant‐Based Analogs

ABSTRACT

This study presents a comprehensive textural characterization of raw bluefin and yellowfin tuna muscle to establish reference parameters for developing plant‐based analogs (PBAs) designed for raw consumption. Five instrumental texture tests were performed on samples from both groups of species (n = 64): Texture Profile Analysis, surface compression, puncture, shear, and tensile tests. The anisotropic nature of tuna muscle was evaluated by applying forces both perpendicular and parallel to connective tissue layers. Results revealed minimal textural differences between bluefin and yellowfin species, though significant anisotropy was observed across all mechanical tests depending on test methodology and attribute. Force‐distance curves from puncture tests demonstrated a strong correlation (R ² > 0.80) between force peaks and the physical location of myocommata, confirming the structural importance of connective tissue layers. Raw tuna muscle exhibited low cohesiveness (15%–21%) and moderate firmness with fracturability under compression, particularly when force was applied perpendicular to connective tissue layers. These findings establish quantitative reference ranges for multiple texture parameters that are essential for developing authentic plant‐based tuna alternatives, potentially addressing sustainability challenges in vulnerable tuna fisheries while meeting consumer expectations for raw fish applications.

This study establishes key textural, instrumental, and sensorial references of raw tuna muscle, highlighting anisotropic properties and connective tissue impact, to inform the development of authentic plant‐based tuna alternatives. Findings support sustainable seafood innovation while aligning with consumer expectations of plant‐based foods.

1. Introduction

During the last decade, the plant‐based sector has experienced significant growth, with a focus on replicating conventional animal products and appealing to both mainstream “flexitarian” consumers and niche markets. The seafood analog industry comprises approximately 29 companies worldwide in 2017, 87 in mid‐2021, 143 in 2023, and 150 in 2025, including companies that follow plant‐based, fermentation, and cell‐based approaches (Alternative Protein Company Database ²⁰²⁵). However, it should be noted that this is a relatively new field in food science, which still faces numerous challenges that have yet to be solved.

Given the greater challenges associated with the replication of whole cuts of meat compared to processed meats (Clayton and Specht 2021), the majority of fish plant‐based analogs (PBA) companies concentrate on emulating the sensorial properties of processed fish products with regard to appearance, texture, smell, and taste (Kazir and Livney 2021). For instance, several companies have developed processed fish PBAs, such as plant‐based canned tuna and salmon, or fish patties, nuggets, or sticks. Other companies have developed seafood PBAs that do not have a complex structure like that of fish muscle, such as crustacean PBAs (i.e., shrimp, crab, or lobster), cephalopods PBAs (i.e., calamari or cuttlefish), caviar, or scallops.

Tunas, along with tilapias and salmonids, is considered the most commercially viable fish species for analog production, according to data from McKinsey & Company (2023). Of these species, tunas represent an economic market volume between three and four times higher than the other species, with the majority of this coming from wild capture (Brennan et al. 2023).

Tuna comprises a diverse group of commercially important species, each varying in size, nutritional composition, and sensory attributes (Jumilla‐Lorenz et al. ²⁰²⁴). Bluefins (Atlantic, Pacific, and southern) and yellowfin tuna are the most vulnerable categories of tuna because they are larger, in high demand, and take the longest to reach reproductive maturity (ISSF ²⁰²²). Given their widespread use in raw applications and their vulnerability, these two species groups are the targets for developing a tuna analog specifically designed for raw consumption.

The primary challenge in developing PBAs is simulating the animal reference appearance, texture, flavor, and mouthfeel, which stem from the muscular structure of fish. Current PBAs face the challenge of closely mimicking the overall texture and quality of fish (Liu et al. 2023). As highlighted by Coppes et al. (2002), texture is a primary sensory factor influencing the quality of consumed raw fish. While some companies have initiated the development of seafood PBAs that resemble raw whole‐cut tuna, the level of similarity to the consumer's expectations remains unmet. The structural organization of fish muscle largely determines its sensory attributes, particularly texture (Listrat et al. 2016), making it essential to understand how to replicate these qualities in plant‐based alternatives.

Fish muscle is a hierarchical structure composed of muscle sheets, called myotomes, connected in a W‐shaped pattern by connective tissue, called myocommata (Hyldig and Nielsen 2001). The myotomes are formed from muscle fibers, which are bundles of myofibrils are made up of actin and myosin filaments (Venugopal and Shahidi 1996). Muscle fibers are elongated, multinucleated, spindle‐shaped cells with a diameter of approximately 10–100 μm and a length that ranges from a few millimeters in fish, compared to several centimeters in terrestrial animals (Listrat et al. 2016). Muscle fibers are composed of 1000–2000 myofibrils packed together into bundles, each up to 5 μm in diameter. The muscle fibers in the myotomes run parallel to the longitudinal direction of the fish (Venugopal and Shahidi 1996). The most abundant protein in the myocommata is collagen, which plays a critical role in maintaining the fillet structure and muscle cohesiveness (Aussanasuwannakul et al. 2012). Compared to terrestrial animals, fish have a simpler connective tissue organization and lack a tendinous system that connects muscle bundles to the skeleton (Huss 1995; Lampila ¹⁹⁹⁰). Fish also have lower collagen content because their aquatic environment does not require as much structural support (Hultin 1984). In addition, fish collagen is more soluble than that of terrestrial animals, contributing to the softer texture of fish fillets.

The seafood industry has traditionally relied on practical methods that allow for a rapid quality assessment. In many scenarios, such as during processing or at the point of sale, quick and easy methods are used to assess the quality of the fish, mainly based on its texture. For instance, a commonly used approach to determine freshness is the “finger method”, which consists of subjectively assessing the hardness of the fish muscle when a finger is pressed on it and the mark or the hole left after pressing (Sigurgisladottir et al. 1999). Instrumental methods can be excessively time‐consuming for these environments, require specialized expertise to operate the instruments and interpret the results, and have high equipment costs. These reasons limit the widespread use of instrumental methods in the fish. However, a descriptive and in‐depth exploration of texture beyond conventional quality parameters is required to replicate the textural characteristics of raw fish in PBAs. For that purpose, a detailed analysis of the physical attributes of raw tuna muscle is needed.

The Texture Profile Analysis (TPA) test is widely used to evaluate food texture for industrial and research purposes because of its versatility, standardization, and ease of use. TPA is designed to simulate the eating experience by mimicking the action of biting and chewing by compressing the food sample twice. While TPA offers many advantages, it is important to complement it with other tests—both instrumental and sensory—to gain a complete understanding of the raw tuna muscle texture (Jumilla‐Lorenz et al. ²⁰²⁴). Some of the other instrumental texture tests used in fish have been reviewed by Hyldig and Nielsen (2001), Coppes et al. (2002), and later by Cheng et al. (2014). Some books such as Rehbein and Oehlenschläger (2009) and Cruz et al. (2014) also review the tests and basics of fish muscle texture analysis. This literature includes methods used for both raw and cooked fish and covers a variety of fish species.

The objective of this study was to characterize the texture of the two primary commercial groups of tuna used for raw applications: bluefin tuna (Atlantic, Pacific, and Southern species) and yellowfin tuna. TPA, along with four additional texture tests (surface compression test, puncture test, shear test, and tensile test), was selected to provide a broader understanding of tuna texture and structure from an instrumental perspective and to have a tailored texture characterization useful to guide the development of raw tuna PBAs.

2. Materials and Methods

2.1. Sample Collection

In June and July of 2022, approximately 500 g of bluefin tuna ( Thunnus thynnus , T. orientalis , and T. maccoyii , n = 31) and yellowfin tuna ( Thunnus albacares , n = 33) were sourced from the local fish pier (3 fishmongers) and Japanese supermarkets (2 stores) in San Francisco, CA, USA. According to the sellers, the fish were wild‐caught, and the portions had been stored in ice since capture. These tuna specimens were obtained from various geographical locations, as detailed in Table 1. The study focused on the dorsal ordinary muscles of these tuna species, as these muscles are commonly used in raw tuna dishes due to their lower fat content compared to other parts of the fish. The fattier parts of the fish are considered a delicacy and are used for more specific and high‐end applications. This selection of muscle aligns with the objective of better understanding the qualities most relevant to conventional raw tuna consumption.

TABLE 1.

Geographical origins of tuna samples.

Species		n	Origins
Bluefin	Pacific bluefin	15	Mexico and USA
	Atlantic bluefin	5	Spain
	Southern bluefin	11	New Zealand, Philippines and Australia
Yellowfin		33	USA, Mexico, Philippines, Australia, Tahiti and Marshall Islands

Following the acquisition of the samples, they were transported under refrigeration to the laboratory located in San Francisco, CA, USA. Thereafter, they were immediately processed.

2.2. Sensory Evaluation

A sensorial texture evaluation of the raw tuna muscle samples was performed by the Addison Sensory Research Services of Mérieux Nutrisciences (Addison, IL, United States) employing the Spectrum Methodology for texture evaluation, as described in Jumilla‐Lorenz et al. (2024). The results of this assessment are referenced throughout this paper.

2.3. Instrumental Texture Analysis

A mechanical texture analyzer (TA.XT plus, Texture Technologies, Hamilton, MA, United States) fitted with a 50 kg load cell was used to perform the tests presented in Table 2. The Texture Exponent Connect software (Stable Microsystems) was used to control the texture analyzer and to analyze the force–time deformation curves.

TABLE 2.

Selected mechanical texture tests for tuna muscle analysis.

Test	Probe a	Rationale	References
Double compression test (TPA)	TA‐30	To study basic textural properties	Cruz et al. (2014)
Surface compression test	TA‐18B	To mimic the “finger method”, widely used in the fish industry	Jonsson et al. (2001); Sigurgisladottir et al. (1999)
Puncture test	PN‐2	To study the inner structure	Ando et al. (1991), Roth et al. (2010)
Shear test	TA‐46MORS	To study the behavior under a shear force	Jonsson et al. (2001); Novaković and Tomašević (2017); Sigurgisladottir et al. (1999); Wheeler et al. (1997)
Tensile test	TA‐226mini	As a model of a pulling force, to study deformability and product structure	Ashton et al. (2010)

^a TA.XT plus, Texture Technologies (https://texturetechnologies.com/accessories/probes‐and‐fixtures).

In order to address certain limitations associated with fish texture analysis, texture measurements were conducted in two orientations: connective tissue layers fibers parallel or perpendicular to the force direction, as illustrated in Figure 1. This approach was undertaken to account for the non‐uniformity in the structure of fish, which is characterized by its anisotropic nature.

FIGURE 1.

Testing orientations for texture analysis of raw tuna muscle. Red represents the muscle and white represents the connective tissue.

All tests were performed with both orientations, with the exception of the surface compression and tensile tests, which were conducted exclusively with the perpendicular orientation. All texture measurements were conducted in triplicate.

To calculate the anisotropy ratio in the tests that included texture evaluation in both orientations, the following formula was used: $\text{Anisotropy ratio}=\frac{\text{Parallel}}{\text{Perpendicular}}$, as established by Krintiras et al. (2014).

2.3.1. Texture Profile Analysis (TPA)

Samples (15 × 15 × 15 mm) were submitted to a two‐cycle compression test up to 75% strain at room temperature (22°C–23°C) using an aluminum cylindrical probe with a diameter of 76.2 mm (TA‐30, Texture Technologies, Hamilton, MA, USA). The parameter settings during the test were as follows: trigger force 5 g, pre‐test speed 1 mm/s, test and post‐test speed 5 mm/s, time elapsed between cycles 5 s. The texture parameters considered were hardness (N), fracturability (N), springiness (dimensionless), and cohesiveness (dimensionless), as defined by Bourne (2002). TPA was performed with normal force both perpendicular to and parallel to the connective tissue layers.

2.3.2. Surface Compression Test

A spherical probe was selected to replicate the sensorial “human finger method” on tuna samples (50 × 30 × 20 mm). Deformation depth of 5 mm into the fillet was selected as the maximum distance that could be applied without breaking the muscle fibers and affecting the muscle structure by erupting it and leaving a mark on the fillet. The probe used was the TA‐18B (Texture Technologies, Hamilton, MA, USA), which had a diameter of 25.4 mm. The double compression test was performed as follows: the probe moved at a speed of 2 mm/s until a deformation depth of 5 mm was reached. Subsequently, the force was reduced, and the sample was allowed to relax for 15 s with the sphere in contact with the surface. The probe then pressed the sample a second time. The parameters considered were: hardness (N) and springiness (dimensionless), calculated in the same way as the TPA parameters according to Bourne (2002). This test was performed with normal force perpendicular to the connective tissue layers.

2.3.3. Puncture Test

A 2‐mm‐diameter needle probe (P‐N2, Stable Micro Systems, UK) was used to perform puncture tests on tuna samples (50 × 30 × 20 mm). The probe penetrated the sample 15 mm at a 2 mm/s speed, with a pre‐test speed of 1 mm/s and a post‐test speed of 10 mm/s. The parameters measured were maximum force of insertion (N) and the distance at which force peaks were observed.

The puncture test was performed both with normal force perpendicular and parallel to the connective tissue layers. Moreover, when testing samples in the perpendicular orientation, the distances at which sample rupture peaks were observed were correlated with physical measurements of the distance between connective tissue layers.

Calipers (01407A, Neiko, China) were used to measure the perpendicular distance between myocommata. These measurements were taken at the at sushi cut, as shown in Figure 2

FIGURE 2.

Preparation of raw tuna muscle for myocommata spacing measurement.

2.3.4. Shear Test

The blade used was a TA‐46MORS (Meullenet‐Owens Razor Shear, Stable Micro Systems, UK) with a sharp edge. The blade cut 15 mm into the samples (50 × 30 × 20 mm) at a speed of 2 mm/s, with a pre‐test speed of 1 mm/s and a post‐test speed of 10 mm/s. The parameters measured included initial stiffness (N/s), resistance to shear (N), and total stiffness (N/s). Stiffness was calculated as a gradient from the starting point until the first peak, in the case of initial stiffness, and until the highest peak, in the case of the total stiffness. Resistance to shear was calculated as the force value at the highest peak. The shear test was performed both with normal force perpendicular and parallel to the connective tissue layers.

2.3.5. Tensile Test

The probe used was the TA‐226 mini (Texture Technologies, Hamilton, MA, United States), which consists of two sets of six sharpened steel spikes mounted on aluminum blocks. One block is fixed to the base unit, and the other is attached to the arm of the texture analyzer. At the start of the test, the two blocks were spaced 1 mm apart. The samples (40 × 20 × 10 mm) were placed on the spikes. The arm of the texture analyzer was then moved away from the base at a speed of 2 mm/s, covering a total travel distance of 70 mm. These settings were sufficient to tear the sample apart. The parameters measured included the deformability modulus (N/s), which indicates the gradient of a straight line drawn from 0 to the highest point on the curve and corresponds to the elasticity of the sample. Initial extensibility (mm) is defined as the distance traveled to when the first tear of the sample is detected, while total extensibility (mm) is the distance traveled to completely separate the sample into two parts. The tensile test was performed with a normal force perpendicular to the connective tissue layers.

2.4. Statistical Analysis

Results were expressed as mean ± SD. SPSS software package version 25.0 for Mac (IBM SPSS Statistical Software Inc., Chicago, IL, USA) was used for statistical analysis. For comparisons of a single variable between two groups (bluefin tuna and yellowfin tuna groups), Student's t‐tests or, alternatively, the nonparametric Mann–Whitney U‐test were used to evaluate differences between means. Shapiro–Wilk and Levene's tests were used to assess normality and homogeneity of variances, respectively. For analyses involving comparisons between two variables in these groups, two‐way analysis of variance (ANOVA) tests were performed for parametric data, with the Kruskal‐Wallis test as the nonparametric alternative. The Shapiro–Wilk test was used to test for normality, and Tukey's HSD test was used for pairwise comparisons when significant effects were observed. The significance level for all tests was set at p < 0.05.

3. Results and Discussion

3.1. Texture Perception

Mastication is key to many food sensations such as texture perception. As per the results published by Jumilla‐Lorenz et al. (2024), the expert panel scored all texture‐related parameters with virtually identical values for bluefin and yellowfin samples (Figure 3). The sensory analysis panel described the tuna flesh as soft, dense but not compact, not sticky, and requiring extensive chewing before being swallowed.

FIGURE 3.

Sensory analysis: Texture attributes of the raw muscle from bluefin and yellowfin tuna, rated from 0 to 15 using a 150‐point scale, except for some descriptors with alternative scales: “number of chews to bolus” and “presence of fibers” (1 = present, 2 = absent) (Jumilla‐Lorenz et al. ²⁰²⁴).

Although sensory hardness is a relatively simple attribute that is mostly assessed at first bite, other instrumental texture attributes such as chewiness, springiness, and cohesiveness are much more complex. For some of these, multiple physical measurements may be required to explain sensory perception. For example, the complex manipulation of food during oral processing is not a simple compression but a mixed mode of compression and shear.

Fibrousness was assessed directly through the attribute “fibrous between teeth” and indirectly through “cohesiveness of the mass”. These attributes were rated around 2.3 and 5.1, respectively, for both tuna species, indicating a slight sensory fibrousness. It is important to note that a relatively soft fibrous network, which can be broken down easily, may not be perceived as fibrous in the mouth. Other attributes, such as moisture release/juiciness and oily mouthcoating, provide insight into characteristics not clearly defined by instrumental texture analysis.

3.2. Instrumental Texture

Five instrumental techniques were used to measure and evaluate the texture of tuna: TPA, surface compression, puncture, shear, and tensile. Representative force‐time deformation curves obtained from each of the instrumental tests and, where applicable, for the different orientations, are presented in Figure 4

FIGURE 4.

Representative graphs for the texture tests performed. (A) TPA (perpendicular), (B) TPA (parallel), (C) Puncture test (perpendicular), (D) Puncture test (parallel), (E) Shear test (perpendicular), (F) Shear test (parallel), (G) Surface compression test (perpendicular), (H) Tensile test (perpendicular).

Raw tuna muscle is inherently anisotropic due to its fibrous molecular structure and its tissue structure organized into myotomes and myocommata. For this reason, studies assessing the muscle of fish usually specify the orientation in which the measurements were taken, such as Ocaño‐Higuera et al. (2011) or Roy et al. (2012). For all tests performed in both orientations (TPA, puncture, and shear tests), different profiles of time‐force curves are obtained depending on whether the force is applied perpendicular or parallel to the connective tissue layers, confirming the anisotropic structure of the fish muscle. In this regard, the results showed that more force is required to deform, cut, or perforate the muscle when the force is applied perpendicular to the connective tissue layers. In the same sense, in the TPA curves (Figure 4A,B), a fracture point in the first compression cycle is observed earlier in the graph corresponding to the perpendicular orientation, and later or not at all in the parallel orientation. The TPA plots revealed no negative work between the two cycles, which is consistent with the lack of adhesiveness that was observed by the sensory panel.

The force‐time curve of the puncture test is another direct indication of the complexity of the internal structure of the fish muscle.

The puncture test is highly sensitive to slight changes in texture due to the small surface area of the probe, which was a 2 mm needle probe. Several fracture peaks are observed in the graphs, which in the samples analyzed in perpendicular orientation are thought to be related to the penetration of the needle through the connective tissue layers of the raw tuna muscle (Figure 4C). To confirm this assumption, the successive force peaks in the force‐time curves obtained in the perpendicular orientation were correlated with the physical measurements of the location of the connective tissue layers (Figure 5).

FIGURE 5.

Correlation of location of force peaks with physical measurements of bluefin (A) and yellowfin (B) tuna samples in the puncture test.

The correlation between these data sets (R ² > 0.80) confirms that the peaks observed in the curves correspond to the needle passing through the connective tissue layers or myocommata. Furthermore, these peaks were not observed in the force‐time curves obtained from samples analyzed in the parallel orientation, longitudinally to the connective tissue layers (Figure 4D).

In the surface compression test with a spherical probe, which aims to imitate the “finger test”, the maximum force of the two cycles is very similar (Figure 4G), which could indicate of a good recovery capacity of the muscle after a slight deformation (about 16%). In the tensile test graph (Figure 4H), the maximum value indicates the force required to cause the first tear of the sample, while the subsequent peaks correspond to the subsequent fractures that precede the complete separation of the sample into two parts (force = 0 N).

3.3. Specific Texture Attributes

The following sections present the results for the most relevant texture attributes (i.e., firmness, elasticity, cohesiveness, fracturability, shear strength, and deformability) which were derived from each test of the instrumental analysis.

3.3.1. Firmness

Several of the texture tests applied (TPA, surface compression, puncture, and shear tests) to the tuna muscle samples yielded information related to their firmness (Table 3).

TABLE 3.

Comparative firmness measurements and anisotropy ratios of bluefin and yellowfin raw tuna muscle using TPA, surface compression, puncture, and shear tests in two sample orientations (mean ± SD, n ≥ 23).

		TPA	Surface compression	Puncture	Shear
		Hardness	Hardness	Maximum force of insertion	Resistance to shear
Bluefin	Perpendicular (N)	28.49 ± 6.63 a	2.81 ± 0.97 A	0.88 ± 0.35 a	4.29 ± 1.84 a
	Parallel (N)	23.74 ± 7.79 b	—	0.65 ± 0.23 b	1.76 ± 0.52 b
	Anisotropy ratio	0.83 ± 0.21	—	0.79 ± 0.28	0.50 ± 0.25
Yellowfin	Perpendicular (N)	29.71 ± 5.45 a	4.00 ± 2.10 B	1.12 ± 0.61 a	5.48 ± 2.37 a
	Parallel (N)	27.04 ± 6.09 b	—	0.71 ± 0.24 b	1.95 ± 0.47 b
	Anisotropy ratio	0.93 ± 0.20	—	0.68 ± 0.23	0.44 ± 0.20

Note: Different superscript letters in the same column denote significant differences (p < 0.05) between species (capital letters) and/or sample orientations (small letters).

The firmness values observed in all tests were generally higher for yellowfin compared to bluefin. However, the statistical analyses for most of the mechanical tests showed no significant differences between the species in terms of firmness (p ≥ 0.05), which aligns with the conclusions of the sensory evaluation panelists. This suggests that both species exhibited similar levels of firmness. However, the surface compression test revealed significant differences between the two species (p < 0.05), suggesting that yellowfin tuna flesh exhibited greater surface firmness compared to bluefin tuna. This opposes earlier findings, such as those reported in a 1992 NOAA study (NOAA ¹⁹⁹²), which indicated that bluefin tuna muscle was firmer than yellowfin tuna. However, it should be noted that the NOAA study lacks details regarding the conditions under which the assessment was made, such as the origin of the fish, the muscle examined, and the techniques used to assess the various attributes.

A shear test replicates the actions of cutting with a knife or the first bite taken when food is placed between the front teeth. The shear force was higher for the perpendicular orientation (approximately 5 N) than it was for the parallel orientation (around 1.8 N). As already mentioned, this can be explained by the presence of myocommata, which were cut by the probe in the perpendicular orientation, but not in the parallel orientation. The connective tissue, or myocommata, exhibits greater resistance to cutting compared to muscle fibers, resulting in elevated shear forces when the blade cuts perpendicularly through these resilient layers (Lepetit and Culioli 1994).

As expected, all the tests revealed significant differences in firmness depending on the orientation of the sample during the analysis (p < 0.05), attributable to the anisotropic structure of the fish muscle. Figure 6 shows the anisotropy ratios calculated for each test performed in both orientations based on the firmness data of all the samples analyzed, compiling both species groups as no species‐specific differences were observed.

FIGURE 6.

Firmness anisotropy ratios of tuna, including bluefin and yellowfin, by test (mean ± SD, n ≥ 47). Different letters indicate significant differences (p < 0.05) between test methods.

The anisotropy ratios, indicative of directional firmness variations, displayed significant variability across the different mechanical tests applied. Defined on a unitary scale, the anisotropy ratio quantifies the firmness disparity between orientations, with a lower score indicating a greater difference in firmness. Our data revealed that the shear test resulted in the lowest anisotropy ratio, establishing it as the most sensitive indicator for detecting firmness anisotropy within the muscle tissue samples tested. The connective tissue, or myocommata, exhibited greater resistance to cutting compared to muscle fibers, resulting in increased shear force when the blade cuts perpendicularly through these tougher layers (Lepetit and Culioli 1994).

These results seem to indicate that the presence of the connective tissue layers (myocommata) gives the tuna muscle a greater resistance to cutting (shear test), followed by an increase in the force required for penetration (puncture test) but not a significantly different capacity to deform in response to a compressive force (TPA).

3.3.2. Stiffness and Springiness

Stiffness or rigidity is the extent to which an object resists deformation in response to an applied force. For the TPA test, stiffness was determined by measuring the gradient up to the fracturability peak, marking the first fracture under compression. In the shear test, stiffness was calculated from the start to the highest peak since this test is performed with a narrow blade, and fracture of the material due to compression was not observed. The results are shown in Table 4.

TABLE 4.

Stiffness and springiness values for bluefin and yellowfin tuna raw muscle performed in two sample orientations (mean ± SD, n ≥ 23).

		Bluefin		Yellowfin
		Perpendicular	Parallel	Perpendicular	Parallel
Stiffness (N/s)	Shear test	0.62 ± 0.24a	0.26 ± 0.08b	0.79 ± 0.31a	0.26 ± 0.06b
	TPA	13.72 ± 3.41a	10.99 ± 3.16b	14.22 ± 2.95a	11.70 ± 2.33b
Springiness (%)	TPA	24.13 ± 3.24a	28.43 ± 6.24b	24.18 ± 2.65a	25.83 ± 3.12b
	Surface compression	55.57 ± 4.60	—	58.49 ± 6.40	—

Note: Different superscript letters indicate significant differences (p < 0.05) within the same test.

Stiffness measurements from both TPA and shear tests were significantly higher for samples measured in the perpendicular orientation (p < 0.05), indicating that raw tuna muscle exhibits greater resistance to deformation when a force is applied perpendicular to the myocommata or connective tissue layers. No significant differences were observed for stiffness between species (p ≥ 0.05).

Springiness is calculated from double compression tests and quantifies the extent to which a product recovers its original form after being deformed during the initial compression, followed by a specified waiting period between tests. Thus, it serves as a textural parameter that reflects the sample elasticity. Not much literature was found focused on determining fish muscle springiness. Sigurgisladottir et al. (1999) reported that springiness measured from TPA curves provides limited information in discerning fish quality. However, considering that high springiness means more masticatory energy is required (Shafiur Rahman and Al‐Mahrouqi ²⁰⁰⁹), the springiness value of fresh fish muscle might be an interesting parameter to be used as a reference for the development of a tuna PBAs.

Springiness values calculated from TPA and surface compression test curves indicated that raw tuna muscle is less elastic in the perpendicular orientation, as also suggested by the results from the stiffness. It is important to note that springiness values generated with probes smaller than the product, such as the spherical probe used in the surface compression test, can sometimes be misleadingly low. This is due to the potential for smaller probes to break through and penetrate the product. However, in this instance, we deemed it acceptable to calculate springiness values from the surface compression test, as the test conditions did not impede the sample's ability to effectively spring back.

Statistical analyses revealed no significant differences in springiness across different fish species, as determined by both the surface compression test and TPA (p > 0.05). However, when muscle orientation was considered, significant differences were observed in the springiness values derived from TPA. This finding reinforces the previously mentioned anisotropic nature of fish muscle.

3.3.3. Fracturability

During the TPA test, muscle fracture under compression was observed. The force at this fracture is referred to as fracturability or initial break force, and it was measured as the force at the first significant peak (where the force falls off) on the force–time curve. The distance traveled by the probe at the breaking force was also recorded. The results are shown in Table 5.

TABLE 5.

Fracturability measurements of bluefin and yellowfin tuna raw muscle using the TPA test in two sample orientations (mean ± SD, n ≥ 23).

	Bluefin		Yellowfin
	Perpendicular (N)	Parallel (N)	Perpendicular (N)	Parallel (N)
Fracturability (N)	20.00 ± 7.42	20.33 ± 7.95	20.95 ± 5.78	23.61 ± 6.14
Distance to Initial Break (mm)	8.60 ± 1.38a	11.95 ± 2.65b	8.72 ± 1.77a	11.52 ± 1.63b

Note: Different superscript letters indicate significant differences (p < 0.05) between orientations.

Fracturability showed similar values for both species and orientations tested, except for yellowfin tuna tested in the parallel orientation, which showed a slightly higher average fracturability. However, statistical tests do not reveal significant differences in these categories (p ≥ 0.05). The distance the probe traveled to the rupture point was similar for both species, but it differed depending on the orientation (p < 0.05). Samples tested in the perpendicular orientation fractured at 72% ± 12% of the compression distance, whereas samples in the parallel orientation fractured at 89% ± 10%. These values indicate that sample fracture is observed earlier in the compression in perpendicular orientation and later or not present at all in the parallel orientation. In comparison, a higher percentage indicates more elasticity, and a lower percentage indicates more brittleness. Therefore, it is confirmed that the samples are more elastic in the parallel than in the perpendicular orientation. Based on the structure of the fish muscle, one explanation is that the myocommata oriented in perpendicular provide an extra layer of resistance, but also a weak point in the structure at the connective tissue layer since the composition of the myotomes and the myocommata is different.

3.3.4. Cohesiveness

Cohesiveness is a controversial textural property, as its meaning varies depending on the scientific community using it and the food matrix being characterized. For example, the term is interpreted differently by sensory scientists and researchers concerned with physical measurements (Rosenthal and Thompson 2021).

In the present study, we hypothesized that “cohesiveness” can serve as a structural integrity indicator of a product when subjected to external forces. By leveraging instrumental analysis, our objective was to quantitatively assess the product's resilience to second deformation, relative to its capacity to resist the first deformation. Therefore, cohesiveness was calculated from TPA by comparing the areas under the force‐time curves of the two compression cycles (Table 6).

TABLE 6.

Cohesiveness values for bluefin and yellowfin tuna raw muscle using the TPA test in two sample orientations (mean ± SD, n > 23).

	Bluefin		Yellowfin
	Perpendicular (%)	Parallel (%)	Perpendicular (%)	Parallel (%)
TPA	18.32 ± 2.97a	21.58 ± 5.66b	18.34 ± 2.65a	21.82 ± 3.47b

Note: Different superscript letters indicate significant differences (p < 0.05) between samples.

Cohesiveness was lower in the perpendicular orientation (around 18%) than it was in the parallel orientation (around 21%) (p < 0.05). In general, foods that exhibit fracturability are products that possess low cohesiveness and some degree of hardness (Bourne 2002). This is consistent with our findings, where fractures were observed in the samples. Higher hardness and lower cohesiveness values were obtained in the perpendicular orientation, indicating a greater likelihood of fracture compared to when the samples were compressed in the parallel direction.

Some authors, such as Hyldig and Nielsen (2001), have criticized the calculation of cohesiveness from samples that fracture after being subjected to high compression in the TPA test. Nonetheless, we consider that these values can be used as an additional texture reference for the development of a PBA.

3.3.5. Deformability and Fracture in Tension

Deformability, defined as the extent to which a food product can undergo deformation under an applied force without breaking, has been investigated in raw tuna muscle using the tensile test.

Tensile tests have been primarily used to assess quality aspects of squid, as evidenced by the following studies: squid ( Illex illecebrosus and Loligo pealei) (Kuo et al. 1990); shortfin squid (Ilex coindetii) (Collignan and Montet 1998); oval squid ( Sepioteuthis lessoniana ), Japanese common squid ( Todarodes pacificus ), and arrow squid (Heterololigo bleekeri) (Kagawa et al. 2002). They have also been used in Japanese clams (Fulvia mutica) (Yoneda et al. 2002), Chinook salmon ( Oncorhynchus tshawytscha ) (Jerrett et al. 1996), and Atlantic salmon ( Salmo salar L.) (Ashton et al. 2010).

The deformability modulus was calculated as the slope from the start to the highest peak of the force‐time curve, as in Ashton et al. (2010). This peak indicates the first significant fracture, suggesting that until that point, the product deforms under the pulling force without fracturing. Initial extensibility is defined as the length the sample was stretched before the first tear, and total extensibility is defined as the length at which the sample completely separated. The deformability values are shown in Table 7.

TABLE 7.

Deformability of bluefin and yellowfin tuna raw muscle using the tensile test in the perpendicular orientation (mean ± SD, n ≥ 23).

	Bluefin	Yellowfin
Deformability modulus (N/s)	0.86 ± 0.55	0.77 ± 0.55
Initial extensibility (mm)	6.49 ± 1.82	6.37 ± 1.88
Total extensibility (mm)	31.63 ± 8.16	31.27 ± 9.37

Note: Different superscript letters indicate significant differences (p < 0.05) between samples.

The deformability modulus coming from the tensile test showed high variability, and the statistical test did not reveal significant differences between species (p ≥ 0.05).

The tensile test requires meticulous sample preparation. Samples must be prepared with great care to ensure they are in the correct shape and as uniform as possible; otherwise, there may be scatter in the tensile parameters, slippage, or premature breaking of the sample (Bourne 2002). Due to the limitations in mounting the samples on the probe, the tensile test was only performed in the perpendicular orientation.

Raw tuna muscle extends approximately 21.50% of its total extensibility before the first tear. No significant differences were found between species (p > 0.05). The article by Segars et al. (1981) devises a model for the tensile deformation of raw fish muscle, in which muscular tissue behaves like a set of springs bound together. It is to note that the tensile test also provides an insight into the fibrous nature of the muscle, as evidenced by the tearing patterns depicted in Figure 7.

FIGURE 7.

Tuna muscle sample extending during the tensile test.

Mechanical methods typically measure only one dimension of texture (Muñoz et al. 1986). For that reason, the complex texture of raw fish cannot be captured with a single instrumental test (Hyldig and Nielsen 2001). By integrating multiple testing methods, the multidimensional nature of fish muscle texture can be better understood and replicated in PBAs, enhancing the authenticity and consumer acceptance of these innovative seafood products. It should be noted that PBAs that closely resemble their animal counterparts tend to be well received by consumers due to their familiarity (He et al. 2020). Developing a comprehensive model of animal meat is crucial to replicate these products holistically and prevent consumer dissatisfaction with PBAs (Cordelle et al. 2022).

The characterization of the animal model enabled us to determine the ranges of texture parameters that could be considered acceptable in the development of an analog as similar as possible to raw tuna. Considering the minimal differences between the analyzed bluefin and yellowfin tuna groups, and the relatively high variability of some characteristics within each group, the intervals were set using the value of the mean and standard deviation (mean ± SD) of all data, integrating the results from both types of tuna. Table 8 presents the ranges of texture attributes, including anisotropy ratios for tests conducted in both orientations.

TABLE 8.

Ranges for texture attributes of raw tuna PBAs.

Attribute	Test	Parameter	Perpendicular	Anisotropy ratios
Firmness (N)	Puncture	Maximum force of insertion	0.58–1.43	0.47–0.99
	Shear	Resistance to shear	2.71–7.09	0.29–0.87
	Surface compression	Hardness	1.68–5.22	—
	TPA	Hardness	23.11–35.14	0.66–1.05
Stiffness (N/s)	TPA		10.82–17.14	0.62–1.00
	Shear		0.40–1.06	0.23–0.65
Springiness (%)	Surface compression		51.33–62.96	—
	TPA		21.24–27.08	0.90–1.38
Fracturability	TPA	Fracturability (N)	13.92–27.06	0.73–1.31
		Distance to break (mm)	7.08–10.24	1.01–1.81
Cohesiveness (%)			15.55–21.11	0.98–1.49
Deformability	Tensile	Deformability modulus (N/s)	0.27–1.36	—
Extensibility		Initial (mm)	4.60–8.26	—
		Total (mm)	22.74–40.15	—

The variability observed in the study for raw tuna texture—influenced by factors such as age, size, seasonal changes, and post‐mortem handling practices—presents a challenge to standardizing these textural attributes. This variability is further compounded by differences in the sampling methods, as highlighted by Nollet and Toldrá (2010).

4. Conclusions

This work has presented a comprehensive characterization of tuna muscle texture, serving as a reference for the development of a tuna PBA for raw applications. The most comprehensive evaluation of fish texture is achieved by combining various textural tests.

There were minimal differences between the two groups of tuna (bluefin and yellowfin tuna), and some characteristics showed great variability within each group. The muscle of raw tuna shows low cohesiveness and a certain degree of toughness, which contributes to its fracturability under compressive stress. Significant anisotropy was determined, with ratios ranging from around 0.6 to 0.8 in TPA, shear, and puncture tests, respectively. The complex fibrous structure of raw tuna, evident from notched shear profiles and sensory analysis, and the detection of connective tissue layers during puncture tests are also essential features to mimic in PBAs.

Author Contributions

Daphne Jumilla‐Lorenz: conceptualization, methodology, investigation, data acquisition, formal analysis, writing – original draft, writing – review and editing. Dolors Parés: conceptualization, methodology, interpretation of data, writing – review and editing, validation, supervision. All authors have read and agreed to the published version of the manuscript.

Ethics Statement

This study does not involve any human or animal testing. Sensory data was referenced from Jumilla‐Lorenz et al. (2024), which was performed at the Addison Sensory Research Services of Mérieux Nutrisciences (Addison, IL, United States), and the panel participants expressed informed consent.

Conflicts of Interest

Daphne Jumilla‐Lorenz reports financial support was provided by Current Foods Inc. Daphne Jumilla‐Lorenz reports a relationship with Current Foods Inc. that includes employment. Dolors Parés declares that she has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Jumilla‐Lorenz, D. , and Parés D.. 2025. “Comprehensive Textural Characterization of Tuna Muscle to Drive Development of Plant‐Based Analogs.” Journal of Texture Studies 56, no. 5: e70043. 10.1111/jtxs.70043.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.