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Article

Physicochemical and Microbiological Changes Associated with Processing in Dry-Cured Tuna

by
Mónica Sánchez-Parra
1,
José L. Ordóñez-Díaz
1,*,
Jesús Pérez-Aparicio
1 and
José M. Moreno-Rojas
1,2,*
1
Department of Agroindustry and Food Quality, Andalusian Institute of Agricultural and Fisheries Research and Training (IFAPA), Alameda del Obispo, Avda. Menéndez Pidal, S/N, 14004 Córdoba, Spain
2
Foods for Health Group, Instituto Maimónides de Investigación Biomédica de Córdoba (IMIBIC), 14004 Córdoba, Spain
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2023, 13(10), 5900; https://doi.org/10.3390/app13105900
Submission received: 5 April 2023 / Revised: 5 May 2023 / Accepted: 9 May 2023 / Published: 10 May 2023
(This article belongs to the Special Issue Chemical and Physical Properties in Food Processing)
Browse Figure
Versions Notes

Abstract

:
Dry-cured tuna is a traditional product manufactured in the Mediterranean region of Spain, known as mojama. The traditional salting-drying elaboration process attributes new organoleptic characteristics to the final product, changing its flavor, color, and nutritional value. This study aimed to evaluate the changes in physicochemical, biochemical, and microbiological parameters taking place during the process. The physicochemical parameters were affected by the processing steps (salting, salt-washing, and drying), except for total acidity and pH. The water activity value and relative moisture percentage decreased to 0.86 and 33.03%, respectively. Moreover, the addition of salt and the drying step increased the water-holding capacity. The lipid oxidation values increased from raw tuna loins to the final product (1.37 vs. 5.56 mg malondialdehyde/kg). Moreover, the total volatile basic nitrogen values increased in the final product, fundamentally due to the concentration effect caused by the water loss, although may also be due to the degradation of proteins during processing. The microbiological analysis showed that the values obtained in the dry-cured tuna were below the limits established by the reference regulation for dry-cured fish products.

1. Introduction

Fish and fish products are well-known for their nutritional value, being a rich source of protein (15–24%), essential amino acids, vitamins, micro and macro-nutrients, and unsaturated fatty acids [1]. The global fish consumption is about 128 million tons, increasing the amount consumed by a single person from 9 kg in 1961 to 20.3 kg in 2017 [2]. Eating fish can prevent diseases among consumers since fish provides omega–3 highly unsaturated fatty acids (HUFA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids. However, the consumption of fresh fish has its own set of problems, including its vulnerability and perishability, which remain the biggest challenges for its preservation [3]. Post-mortem changes due to microbiological proliferation, and endogenous enzyme activity in the cells and gut, have been observed to cause rapid deterioration of fish after death [3,4]. Therefore, the development and optimization of methods to improve the preservation and processing of fish without affecting its nutritional value is of the utmost importance. The chemical and physical composition of fish is a vital aspect of the processing because it affects its quality [4] which is evaluated by attributes related to its freshness, including appearance, odor, flavor, and palatability [5].
The most commonly used methods for preserving fish for longer periods are freezing, canning, smoking, curing, or pickling [6]. In order to achieve the desired products, one or a combination of these methods is used for preservation. Curing methods based on salting and drying process have been widely used in fishing regions such as the Iberian Peninsula [7].
Dry-cured tuna, called mojama, is probably the most emblematic food product obtained from the traditional processing of tuna on the Mediterranean coast of Spain (Andalusia, Murcia, and Valencia) and Portugal [8]. This is a premium quality and gourmet product, made by salting and drying yellowfin (Thunnus albacares) or bluefin tuna (Thunnus thynnus). Tuna mojama is a great source of protein, providing 43 g per 100 g of the product. Besides, this product has a low content of fat (2 g per serving), making it a great option for consumers looking for a lean protein source. Additionally, tuna mojama contains important minerals, such as iron and calcium, and it is a good source of vitamin B12, an essential nutrient that helps maintain healthy nerve cells and red blood cells and it is involved in DNA synthesis [9,10]. The value of this product was recognized in 2015 by the EU with the registration of two Protected Geographical Indications (PGI), namely “Mojama de Barbate” [11,12] (Cádiz, Spain) and “Mojama de Isla Cristina” (Huelva, Spain) [13,14] in the Andalusia region. The companies belonging to the PGIs must apply the traditional manufacturing process established in the specifications of both PGIs. Consequently, the manner in which the various operations of the process are carried out determines the quality of the final product.
After thawing, the tunas are decapitated and eviscerated, and then washed with drinking water. Progressively, the tunas are manually cut by specialized people following a traditional process called “ronqueo”, which results in two upper (black) and two lower (white) loins, with the skin and bones removed. The four loins are subsequently cut into strips no thicker than 5 cm and categorized by fat level. Then, the strips are covered in sea salt for a period ranging from 18 to 36 h. The salting step has the capability to reduce water activity (aw) in a short time and inhibits the growth of spoilage microorganisms due to the high content of chloride ions proving toxic to most microorganisms [15]. The salt also causes important osmotic dehydration, which removes water from the product and modifies its properties by brine salting [16]. This standardized processing step is necessary to increase the stability of the final product.
The next step involves washing the salted strips of loins to remove excess salt from the muscle tissue and soaking them in water for around 7 to 9 h. This is performed to achieve the optimum salt concentration. The loins are then compressed, forcing an additional loss of fat and water, and finally the tuna strips are dried on a horizontal surface for at least two days and then hung for a period ranging from 15 to 21 days under controlled microclimatic conditions (T 16-17 °C; RH 60–70%). This step is largely responsible for the characteristic smell, flavor, and color of the product, modifying the final characteristics of the dry-cured tuna, including changes in the lipid fraction [17]. Both salting and drying steps contribute to the stability of the final product. The dry-cured tuna products are finally vacuum-packed in transparent plastic bags. It is traded under cold conditions (4–6 °C) to preserve its physical, chemical, and organoleptic characteristics. The different steps of the traditional mojama elaboration process are essential to improve the stability and flavor of the product. Thus, the salting process inhibits unwanted reactions in the fish and promotes the desired taste, while the drying process enhances the preservation effect of the salt. Overall, the combination of these two steps is important to produce a high-quality mojama that can be enjoyed for a longer period without compromising its texture and taste.
The purpose of this study was to evaluate the changes in physicochemical, biochemical, and microbiological parameters during the traditional salting-drying process to elaborate tuna mojama.

2. Materials and Methods

2.1. Chemicals and Reagents

Trichloroacetic acid (TCA), ethylenediaminetetraacetic acid (EDTA), propyl gallate, and sodium hydroxide were purchased by Merck KGaA (Darmstadt, Germany). Thiobarbituric acid (TBA) and sulfuric acid were supplied from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). Silver nitrate (AgNO3), sodium acetate, potassium carbonate, 1,1,3,3-tetraethoxypropane (TEP) and deionized water were from Panreac (Barcelona, Spain). All the chemicals and reagents used were analytical grades of the highest purity.

2.2. Tuna Samples

The tuna samples belonged to Thunnus albacares captured in the FAO 34 area (Eastern Atlantic) during the years 2020 and 2021. The samples collected in this study were provided by companies belonging to the Asociación Andaluza de Fabricantes de Salazones Ahumados y Otros Transformados Primarios de la Pesca included in the PGIs “Mojama de Barbate” and “Mojama de Isla Cristina”.
In the industry, 500 g of muscle was sampled in a total of 100 tunas after each of the four steps: raw tuna loins, salting, salt-washing, and drying (Figure 1). The fresh tuna loins were labeled to ensure the traceability of the elaboration. The samples obtained after the four steps were stored on ice during sampling and transportation to the laboratory to prevent spoilage, while the final product samples were vacuum-packed and refrigerated at 4 °C for commercialization After the later subsampling for analysis, the samples were stored at −20 °C until their analysis.

2.3. Physicochemical Parameters

2.3.1. Salt Content (NaCl)

The salt content was performed according to described by Chen et al. [18] using the Mettler Toledo T70 automatic titrator (Mettler Toledo, Switzerland). Briefly, Tuna flesh samples (10 g) were homogenized with 40 mL of distilled water by a homogenizer (Ultraturrax®, Stauten, Germany) at 500 rpm for 2 min and then, the mix was titrated with standard silver nitrate solution.

2.3.2. Moisture Content, Water Holding Capacity (WHC), and Water Activity

The moisture was determined by the drying method, where one gram of the sample was weighed at 103 ± 2 °C for two hours until reaching a constant weight. The WHC was evaluated according to Szmańko et al. [19] using the filter paper press method. The water activity was determined by an Aqualab PRE at 25 °C.

2.3.3. pH Value and Total Acidity

Ten grams of sample were weighed and homogenized with 15 mL of distilled water in an ultraturrax for 2 min to form a slurry. The pH value was measured using a combined pH glass electrode DGi111-SC (Mettler Toledo, Switzerland). Total acidity was determined by titration with 0.1 M NaOH according to the method of Benjakul et al. [20] in a Mettler Toledo T70 automatic titrator (Mettler Toledo, Switzerland). It was calculated by multiplying of the molarity of the titrant, volume of the titrant used, and molecular weight of lactate and dividing it by the weight of the samples to obtain the percentage of total acidity equivalent of lactic acid. The results were expressed as the lactic acid percentage (corrected with the moisture of each sample).

2.3.4. Color

The CIELAB color space was evaluated using a portable Konica Minolta, CM–700D (Minolta Corporation Ltd., Osaka, Japan). L*a*b* parameters of the samples were determined by conducting three consecutive measurements perpendicularly at three different sites on the surface. Readings were performed using D65 as an illuminant and an observer angle of 10° for the measurement of lightness (L*), redness (a*), and yellowness (b*), and the values of the intensity of color purity (chroma) and hue angle (h*) were estimated. The instrument was calibrated to standard white before analysis.

2.3.5. Total Volatile Basic Nitrogen (TVBN)

The level of TVBN has been widely used as an indicator of the quality and freshness of fish and fish products. TVBN was determined using the reference method described in Regulation (CE) 2074/2005 [21]. The samples (10 g) were ground with 90 mL of 6% HClO4 (w/v) for 2 min and centrifuged at 3000 rpm for 5 min at 4 °C. Then, the supernatant was filtered. After alkalinisation, 50 mL of the filtrate was introduced into an automatic apparatus for steam distillation. The steam distillation was regulated so that around 100 mL of distillate could be produced in 10 min. The distillation outflow tube was submerged in a receiver with 100 mL of 3% H3BO3 solution, and three drops of the Tashiro mixed indicator were added. Subsequently, the TVBN contained in the solution was determined by titration with a 0.01 M HCl. All samples were expressed as mg/100 g sample.

2.3.6. Lipid Oxidation Test

The content of lipid oxidation in food matrices can be determined by measuring the concentration of malondialdehyde (MDA) in the samples using the thiobarbituric acid reactive substances (TBARS) assay. Three grams of sample were weighed and homogenized (Ultraturrax®, Stauten, Germany) with 10 mL of 7.5% TCA solution. Next, the mixture was homogenized at 5000 rpm for 10 min at 4 °C. The supernatant was used for MDA determination according to Du et al. [22] MDA–TBA complex was determined at 530 nm using a microplate spectrophotometer (ThermoFisher Scientific Oy, Vantaa, Finland). The results were expressed as µmols MDA per kilogram of muscle.

2.3.7. Microbiological and Pathogenic Bacterial Analysis

The control of both pathogenic and spoilage microorganisms is critical for fish product quality and safety [23]. The counts of mesophilic microorganisms, molds and the detection of Salmonella spp., Enterobacteriaceae, Staphylococcus aureus, and Listeria monocytogenes were determined following the methodologies ISO collected in Regulation (EC) 2073/2005 [24]. Psychrophilic microorganisms and molds were determined by ISO 17410:2019 and ISO 7954:1987, respectively [25,26].

2.4. Statistical Analysis

All analyses were run in triplicate (except TVBN and microbiological and pathogenic analyses, which were performed in duplicate). The statistical analysis was performed using R software (v. 3.6.3, R Core Team, Vienna, Austria). The influence of the manufacturing process of dry-cured tuna on the physicochemical, biochemical, and microbiological properties of the samples (n = 100) was tested by analysis of variance (one–way ANOVA). Tukey’s honestly significant difference (HSD) post–hoc test was applied within the 95% confidence interval. Statistical significance was considered at p < 0.05 for all analyses.

3. Results and Discussion

3.1. Physicochemical Parameters

The evolution of physicochemical characteristics, including NaCl content, water activity (aw), moisture content, water hold capacity (WHC), total acidity, and pH during the manufacturing process of mojama is shown in Table 1.
Concerning the salt content, the raw tuna fillets contained a low amount of salt, more specifically 0.30%. As expected, the mean values showed significant differences, mainly due to the salting process that occurs in the manufacturing process. The concentration of salt increased significantly after the salting step and decreased after the excess salt was removed during the salt-washing step (6.53% vs. 4.79%). Salting is a very common technique used for food preservation and for flavor enhancement [27]. The mechanism behind salting relies on the concept of diffusion. During the salting step, a simultaneous exchange of flows takes place, such as water loss and salt uptake. NaCl diffuses into the tuna flesh and water diffuses out of the tuna muscle, due to the osmotic pressures [15]. After the drying step, the salt content of the samples reached 5% (on average) in accordance with the range established in the specifications for the manufacturing process for the PGIs “Mojama de Barbate” and “Mojama de Isla Cristina” (3–9%) [11,12,13,14].
The average aw found in the raw tuna fillets was 1.01 (Table 1). This result was similar to that of Sánchez-Zapata et al. [28] for the muscle of yellowfin tuna (0.99). During the salting step, a significant decrease was observed (0.94), mainly due to the salt bonds with the muscle tissue and the water loss that reduces the available free water. The samples analyzed after the salt-washing step showed a significant increase in aw (0.98), due to the hydration of the product in the water. The lowest values of aw (0.86) were measured in the samples obtained after the drying step. These results are in accordance with those reported by Lin et al. [29] for salted fish products (0.73–0.86). The decrease in water activity is very important for the quality and safety of the mojama since it is a way of controlling microbial growth, lipid oxidation, and the formation of non-desirable compounds such as biogenic amines [30].
Regarding the moisture content, the raw tuna presented a high content, with an average of 62.89%. This value did not significantly change for the samples analyzed after the salting and salt-washing steps. These results in yellowfin tuna agree with the literature [31,32]. The final product samples (after drying) presented a significant decrease for this parameter (33.03%). This value was in accordance with the standards for this product, which ranged from 35 to 45% [33].
The WHC of fish products is an important quality parameter since it represents the capacity of flesh to retain its internal water [34]. The WHC values increased from 77.25% in the raw tuna samples to 97.29% in the final product. During processing, factors such as the type of salt used and the salting method may affect the WHC [35], which leads to decreased water content in the fish muscles due to alterations in the properties of proteins [36]. Most free water that can be easily released lies between the actin and myosin filaments of myofibrils in fresh tissue due to the structural change of muscle [37]. This can directly influence the consumers’ perception of appearance and texture. Therefore, having a high WHC is one of the major aims of fish processing as it relates to the quality and sensory attributes of the product [38].
Another important factor is the pH. The method of preservation has an impact on this parameter since the salting and drying may alter the functionality of the proteins of flesh. Lauritzsen et al. [35] reported that pH value was considered of major importance for the quality of salt-cured fish products. The pH of the raw tuna samples was in the range between 5.78–6.38, the values obtained for the samples analyzed during the different steps of the manufacturing process not changing significantly (Table 1). The results were consistent with those of the published studies regarding tuna species and their subsequent dry-cured products [8,28,39,40]. Changes in the pH of fish depend on different factors, such as the state of the fish, the type and amount of salt used, and the type of salting process [41]. Furthermore, pH levels are correlated with lactic acid bacteria growth and lactic acid formation in fish muscles after the catch [42]. Lactic acid bacteria (LAB) are responsible for the production of lactic acid in the case of food contamination. LAB are Gram-positive and even though they are not considered a main microorganism group usually present in fish, some species as Carnobacterium, Vagococcus, Lactobacillus, Enterococcus, Lactococcus, have been found in aquatic environments [43]. The role of LAB in fish and fish products is complex and has not been established yet. In some cases, the presence of LAB can cause sensory degradation [43,44]. In this study, the total acidity values measured as lactic acid did not show significant differences among the different stages of the processing (approximately 0.50%). Those values were in accordance with the values found for pH. This is evidence of the absence of microbial activity both in the raw fresh tuna and during the manufacturing process, assuring the integrity of the whole process.

3.2. Color Parameters

Color is a basic sensory parameter in the evaluation of food quality because it is the first factor that triggers consumer perception of acceptability [45]. The lightness (L*), red and green scale (a*) and yellow and blue scale (b*) were measured in each processing step and the results are shown in Table 2.
Significant differences were found in the samples for the L* coordinate during the manufacturing process of mojama. The L* value was significantly affected by the salting process, with the flesh gaining a lighter color from 33.98 to 40.75 in the raw tuna and after the salting step samples, respectively. Hemoglobin and myoglobin are the basic pigments of the dark muscle of fish. NaCl is reported to enhance the removal of heme proteins such as hemoglobin (Hb) and myoglobin (Mb), resulting in a less pigmented color [46]. The salt-washing step increases the removal of heme proteins as well since they are water soluble, thus an increase in the L* value was noted (no significant) for the samples collected after this step. The tuna flesh samples obtained after the drying step were darker (29.94) because the drying step reduces the amount of free water on the surface, causing a decrease in the value of this coordinate [47]. The forced aeration during the drying step involved oxidation phenomena that could also contribute to that decrease. The red-green coordinate (a*) significantly decreased during the manufacturing process, with a later increase in the final product. Generally, the desired color in fresh fish is bright red, which is related to the content of Hb and Mb in the muscle [28]. As stated, the salting and salt-washing steps remove heme proteins so the a* values decreased significantly. Finally, the drying step could favor the development of oxidative phenomena, where the red color gradually becomes different shades of brown due to the conversion of oxymyoglobin to metmyoglobin (MMb) [48]. Pérez-Alvarez et al. [49] reported that the values of a* decrease when MMb content increases. During mojama manufacturing, the b* coordinate (blue-yellow) of the samples analyzed increased after the salting and salt-washing steps, followed by a significant decrease for the samples obtained after the drying steps. Although this coordinate is the least studied color component in foods, particularly in fish products, it has been reported that it is influenced by the concentration of pigments and the state of the flesh (oxidized, reduced, or modified by chemical and/or biochemical reactions) as well as the structural changes that the muscle undergoes during the manufacturing process. Besides, an important characteristic to consider is that the yellow component of the color of tuna muscle decreases when it is subjected to the salting process [50] due to modifications/degradation of the carotenoids and the hemopigments occurring in the flesh [51].
Moreover, chroma (C*) and hue angle (h*) values were estimated using the L*, a* and b* parameters. It was observed that the C* coordinate decreased during the mojama manufacturing process, which indicates that the color of the tuna loins gradually becomes duller. The h* coordinate data increased for the samples obtained from raw tuna (59.82) to salt-washing (91.37) involving a higher contribution of the yellow tone ranges. This can be explained by the incorporation of salt during the salting step. After drying, this parameter decreased, with an average value of 67.53, and the mojama samples became a darker red color.

3.3. Biochemical Parameters

Table 3 shows the changes in the total volatile base nitrogen (TVBN) and lipid oxidation test values during the mojama elaboration process.
The TVBN is one of the most widely used measurements for evaluating fish quality, this being one of the most common chemical indicators for fish spoilage [52]. This parameter includes the measurement of trimethylamine (produced by spoilage bacteria), dimethylamine (produced by autolytic enzymes), ammonia (produced by the deamination of amino acids and nucleotide catabolites), and other volatile basic nitrogenous compounds associated with fish spoilage [53]. In our study, the values of TVBN in raw tuna were 34 mg/100 g on average. The significant decrease in this value for the samples obtained after the salt-washing step (28.42 mg/100 g) could be explained by the fact that some of the TVBN compounds diffused into the water together with the other nitrogen fractions. An increase in this value was observed for the samples obtained after the drying step (52.67 mg/100 g), resulting from the concentration effect caused by the water loss.
The degradation of proteins due to protein oxidation during the manufacturing process has been identified as a contributing factor to the accumulation of TVBN in fish products [54,55]. To ensure the safety and quality of mojama, it is important to control the processing stages that can affect TVBN levels. For example, parameters such as salting and drying time, salt concentration, and storage temperature, and humidity can help minimize the increase in TVBN levels during processing. Proper storage and handling practices can also prevent the growth of spoilage bacteria and ensure the safety and freshness of tuna products. Several studies have reported a correlation between TVBN levels and fish freshness [56,57,58], but there is controversy regarding the use of TVBN as the sole indicator of fish freshness [59,60].
While TVBN levels are a useful indicator of the degree of spoilage in fish, they do not provide a complete picture of the freshness of fish. Other factors, such as the presence of specific volatile organic compounds (VOCs), color, and pH, can also be important indicators of freshness [60]. Furthermore, the relationship between TVBN levels and the sensory quality of fish may vary depending on the fish species, storage conditions, and other factors. For instance, some fish species may have high levels of TVBN naturally occurring in fresh fish, while other species even during spoiling may present low levels of TVBN [61], in conclusion, the TVBN data should be evaluated together with other complementary parameters for checking the freshness of the fish.
On the other hand, MDA is commonly used as a marker for oxidation in fish, this being a product of the oxidation of polyunsaturated fatty acids. The development of lipid oxidation is an indicator of the enzymatic reaction and changes in non-enzymatic free radicals [62]. As can be seen in Table 3, in the raw tuna samples, the TBARS values were below the acceptability limit of 2 mg of MDA/kg fish sample (1.37 mg MDA/kg), which is usually regarded as the limit for which the fish will develop an awful odor and/or taste [63]. In the salting step, the TBARS value was 4.04 mg MDA/kg. Several studies describe salt as a pro-oxidative factor, but the exact mechanism is not yet fully understood [64]. Salting leads to the shrinkage of the cells and the degradation of muscle proteins, making it easier for oxidizing agents to enter the cells [65]. Furthermore, salt inhibits the antioxidant activity of enzymes such as catalase, glutathione peroxidase (GPx), and superoxide dismutase (SOD), which limit lipid oxidation [66]. Another explanation for this pro-oxidative effect of salt could be attributed to its capacity to detach iron ions from molecules, i.e., heme proteins [67]. On the contrary, other studies support that salt has no effect on lipid oxidation [68], while other researchers even proven salt to slow down oxidation [69,70]. Certainly, the oxidative stability of fish during the salting process depends on several factors, such as salt composition and concentration, fatty acids composition, size and weight of fish fillets, and pH [71]. In mojama, a significant increase in this parameter was observed (5.56 mg MDA/kg). This data could be explained by the exposition of tuna samples to the forced air process for a longer period (15–21 days) in the drying step. To attenuate the further oxidation of the lipids, the final product is packaged under vacuum and stored at 4 °C for commercialization. These measures have been proven to protect the product [72]. Lipid oxidation can lead to quality problems like off-flavors and odors, shorter shelf life and losses of important nutrients [73].

3.4. Microbiological Parameters

The microbiological quality characteristics of dry-cured tuna are crucial in determining the safety of the manufacturing process. All the microbiological results demonstrate the significant impact of the salting step (Table 4).
The main antimicrobial mode of action of salt is creating a low aw environment (<0.9), which forces microorganisms to experience a prolonged lag phase and eventually enter the death phase. The aerobic mesophilic counts in every step were far below the maximum limit established by the reference regulation (1 × 105 cfu/g) in the dry-cured fish product [74]. Hence, the salting and salt-washing stages did not promote the growth of mesophilic bacteria. After drying, the mojama samples showed the lowest values (4.9 × 102 cfu/g), which could be explained by the lowest aw in the final product. These analyses (mesophilic and psychrophiles flora) are used as an indicator of hygiene and good manufacturing practices. Hence, the manufacturing process can be viewed as acceptable, in accordance with good microbiological standards.
Mold counts in the samples of raw tuna (1.2 × 103 cfu/g) were significantly higher than those obtained after the salting and salt-washing steps (3.8 × 102 and 4.2 × 102 cfu/g, respectively). However, the values for the samples obtained after the drying step were 1.1 × 103 cfu/g on average, with a slight increase compared to those obtained after the salting process. These results are in accordance with those reported by Santos et al. [71] for fresh tuna and mojama samples from Thunnus obesus, where the mold counts decreased after the salting stage (4 × 10 cfu/g) and increased after the drying step (3 × 104 cfu/g).
All the samples analyzed during the different steps of the elaboration of mojama were free from Salmonella and S. aureus. Salmonella does not occur naturally in marine waters and its occurrence is usually due to unhygienic handling, infected water, or fresh fish itself containing it [75]. Hence, Salmonella is frequently cited in food safety problems. In the same way, Staphylococcus aureus is found in the skin of animals and its presence could be an indicator of contamination since it can survive for prolonged periods in extreme environments such as salt, dry-cured products, and low aw products [76]. This result agrees with those reported by Medveďová et al. [77], who reported that halophilic bacteria, such as S. aureus, could not grow at aw below 0.9. Besides, Listeria monocytogenes and Enterobacteriaceae were not detected during the entire process.
All in all, the microbiological evaluation of the fresh tuna loins during the manufacturing process of mojama indicates that they were of high quality and safe for consumption as fresh fish and for further processing and use, such as mojama.

4. Conclusions

The present study evaluated different quality parameters during the different steps of the manufacturing process of dry-cured tuna (mojama). A significant decrease in aw and moisture with respective final values of 0.86 and 33.3% were associated with the salting and drying steps. The bright red color of the raw tuna loins gradually became darker and duller, eventually turning into the characteristic brownish color of mojama. The increase of TVBN observed after the drying step could be due to the concentration effect caused by the water loss and may be due to the degradation of proteins as a result of lipid and protein oxidation. Final products with an aw below 0.9 do not pose a risk by pathogen bacteria to consumer health since they cannot grow in those conditions.
Based on the results obtained, we can conclude that the salting step during the manufacturing process of mojama improves its stability in the initial stage of the process, inhibiting undesirable reactions and promoting the flavor of the product. Finally, the drying step enhances the preservation effect of the salt and is necessary for achieving a lower aw and further increasing the stability of the final dry-cured tuna.
The control of time is important to prevent the proliferation of spoilage bacteria before the salting step. Thus, the raw tuna must be handled by specialized operators for short periods of time, and they must observe strict hygiene protocols.
As the production of mojama is of great importance to the economy of the regions in which it is produced, and its fame is attributed to its high quality, it is of crucial importance for the producer companies to keep optimizing their elaboration techniques and follow good manufacturing practices to ensure safety and quality in their product.

Author Contributions

Conceptualization, J.L.O.-D., J.P.-A. and J.M.M.-R.; methodology, M.S.-P., J.L.O.-D., J.P.-A. and J.M.M.-R.; investigation, M.S.-P. and J.L.O.-D.; resources, M.S.-P., J.L.O.-D. and J.M.M.-R.; data curation, M.S.-P., J.L.O.-D., J.P.-A. and J.M.M.-R.; writing—original draft preparation, M.S.-P., J.L.O.-D. and J.P.-A.; writing—review and editing, M.S.-P., J.L.O.-D., J.P.-A. and J.M.M.-R.; supervision, J.P.-A. and J.M.M.-R.; project administration, J.L.O.-D. and J.M.M.-R.; funding acquisition, J.P.-A. and J.M.M.-R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Andalusian Institute of Agricultural and Fisheries Research and Training (IFAPA) and the European Rural Development Fund (ERDF, EU) through the Project PR.PEI.IDF2019.003. J.L.O.-D. was supported by Junta de Andalucía for his postdoctoral contract within the PAIDI 2020 program (POSTDOC_21_00914). M.S.-P. was granted by a research contract funded by the Andalusian Institute of Agricultural and Fisheries Research and Training, Spain (IFAPA), and the National Youth Guarantee System funded through the European Social Fund, European Union (ESF) and the Youth Employment Initiative, Spain (YEI).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data are contained within the article.

Acknowledgments

The authors thank the Consejo Regulador de las Indicaciones Geográficas Protegidas “Mojama de Barbate” and “Mojama de Isla Cristina” and the “Asociación Andaluza de Fabricantes de Salazones Ahumados y Otros Transformados Primarios de la Pesca”. We thank Manuel Becerra for his availability during samplings and the coordination with the companies. We thank the companies and workers: Juan Vázquez, José Vázquez, José Luis Trufero and Lourdes Fernández (Usisa, S.A.); Manuel Columé, Fidel Columé and María de las Mercedes Rodríguez (Pescatún Isleña, S.L.); Francisco Pacheco and Francisco Javier Rodríguez (Herpac) and José Luis Gómez (Salpesca, S.L.).

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 05900 g001 550
Figure 1. Flowchart of the industrial production process of dry-cured tuna.
Figure 1. Flowchart of the industrial production process of dry-cured tuna.
Applsci 13 05900 g001
Table
Table 1. Physicochemical characteristics of fresh yellowfin tuna (Thunnus albacares) and the respective dry-cured tuna (mojama), as affected by salting and salt-washing.
Table 1. Physicochemical characteristics of fresh yellowfin tuna (Thunnus albacares) and the respective dry-cured tuna (mojama), as affected by salting and salt-washing.
Raw TunaSaltingSalt-WashingDry-Cured Tunap-Value
Salt (% NaCl)average ± SD0.30 ± 0.19 c6.53 ± 2.51 a4.79 ± 1.52 b5.09 ± 1.05 ab***
min0.053.192.493.69
max0.529.507.197.01
awaverage ± SD1.01 ± 0.00 a0.94 ± 0.03 c0.98 ± 0.02 b0.86 ± 0.03 d***
min1.000.900.930.82
max1.011.001.000.89
Moisture (%)average ± SD62.89 ± 2.37 a56.70 ± 6.99 a62.75 ± 4.85 a33.03 ± 8.87 b***
min59.4445.8454.0222.04
max66.2967.2768.9649.66
WHC (%)average ± SD77.25 ± 1.36 c84.46 ± 3.05 b75.03 ± 2.62 c97.29 ± 0.87 a***
min75.0180.9271.7295.89
max79.2189.5679.2098.25
pHaverage ± SD6.04 ± 0.226.07 ± 0.186.05 ± 0.145.93 ± 0.18ns
min5.785.655.745.49
max6.386.356.356.22
Total acidity (%)average ± SD0.55 ± 0.030.50 ± 0.080.51 ± 0.030.54 ± 0.15ns
min0.510.360.470.38
max0.590.590.580.84
Different letters indicate significant differences by Tukey’s test. ***, ns—significant at p < 0.001 and not significant, respectively. SD—standard deviation
Table
Table 2. Changes in L*, a*, b*, C* and h* (mean ± SD) parameters and function during the manufacturing process of mojama.
Table 2. Changes in L*, a*, b*, C* and h* (mean ± SD) parameters and function during the manufacturing process of mojama.
Raw TunaSaltingSalt-WashingDry-Cured Tunap-Value
L*33.98 ± 3.70 b40.75 ± 5.24 a41.74 ± 4.61 a29.94 ± 2.64 b***
a*4.12 ± 1.30 a1.39 ± 1.47 bc0.23 ± 2.02 c2.31 ± 1.06 b***
b*7.87 ± 1.29 b9.91 ± 1.25 a9.00 ± 3.00 ab5.71 ± 1.21 c***
C*11.98 ± 2.24 a10.16 ± 1.23 ab9.23 ± 4.63 ab8.02 ± 1.30 b**
h*59.82 ± 5.03 b81.79 ± 8.65 a91.37 ± 14.48 a67.53 ± 9.96 b**
Different letters in the same row indicate that there were significant differences. **, ***—significant at p < 0.01 and p < 0.001, respectively.
Table
Table 3. Changes in malondialdehyde (MDA) and total volatile basic nitrogen (TVBN) content of yellowfin tuna during the mojama elaboration process.
Table 3. Changes in malondialdehyde (MDA) and total volatile basic nitrogen (TVBN) content of yellowfin tuna during the mojama elaboration process.
Raw TunaSaltingSalt-WashingDry-Cured Tunap-Value
TVBN (mg/100 g)mean ± SD34.00 ± 0.85 b31.00 ± 1.71 bc28.42 ± 2.97 c52.67 ± 5.68 a***
range33.00–35.0028.00–34.0025.00–34.0045.00–57.00
TBARS (mg MDA/kg)mean ± SD1.37 ± 0.98 b4.04 ± 1.87 ab2.05 ± 1.18 ab5.56 ± 4.11 a*
range<LOD–3.36<LOD–7.52<LOD–4.06<LOD–15.37
Different letters in the same row indicate significant differences by Tukey’s test. *, ***—significant at p < 0.05 and p < 0.001, respectively. SD—standard deviation. LOD—limit of detection.
Table
Table 4. Microbiological count in the different steps of the manufacturing process of mojama.
Table 4. Microbiological count in the different steps of the manufacturing process of mojama.
Raw TunaSaltingSalt-WashingDry-Cured Tunap-Value
mesophilic flora (cfu/g)average3.0 × 103 a3.1 × 103 a2.2 × 103 a4.9 × 102 b***
min–max3.6 × 102–4.8 × 1032.5 × 102–6.5 × 1038 × 102–4 × 1033 × 10–1.2 × 103
psychrophiles flora (cfu/g)average1.8 × 102n.d.0.0030.06ns
min–maxn.d.–1.7 × 103n.d.n.d.–2 × 10n.d.–3 × 102
Molds (cfu/g)average1.2 × 103 a3.8 × 102 c4.2 × 102 bc1.1 × 103 ab**
min–max4 × 10–2.6 × 103n.d.–2.4 × 103n.d.–9.6 × 102102–2.8 × 103
Enterobacteriaceae n.d.n.d.n.d.n.d.
Staphylococcus aureus n.d.n.d.n.d.n.d.
Listeria monocytogenes n.d.n.d.n.d.n.d.
Salmonella n.d.n.d.n.d.n.d.
Different letters in the same row indicate significant differences by Tukey’s test. **, ***, ns—significant at p < 0.01, p < 0.001 and not significant, respectively. n.d.—not detected.
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Sánchez-Parra, M.; Ordóñez-Díaz, J.L.; Pérez-Aparicio, J.; Moreno-Rojas, J.M. Physicochemical and Microbiological Changes Associated with Processing in Dry-Cured Tuna. Appl. Sci. 2023, 13, 5900. https://doi.org/10.3390/app13105900

AMA Style

Sánchez-Parra M, Ordóñez-Díaz JL, Pérez-Aparicio J, Moreno-Rojas JM. Physicochemical and Microbiological Changes Associated with Processing in Dry-Cured Tuna. Applied Sciences. 2023; 13(10):5900. https://doi.org/10.3390/app13105900

Chicago/Turabian Style

Sánchez-Parra, Mónica, José L. Ordóñez-Díaz, Jesús Pérez-Aparicio, and José M. Moreno-Rojas. 2023. "Physicochemical and Microbiological Changes Associated with Processing in Dry-Cured Tuna" Applied Sciences 13, no. 10: 5900. https://doi.org/10.3390/app13105900

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